A: I mentioned in an earlier post that 6061 alluminum alloy is a popular material for FBS shells. It has good strength and machnineability, it's easy to find in a wide range of forms, it welds reasonably well, and it's relatively inexpensive. Although not the strongest aluminum alloy, it is strong enough for the purpose and the other factors make it attractive to builders.

A: Robot combat isn't a beauty contest. Spend your time and effort on things that will help you win matches.

Note that the anodizing process adds a hard aluminum oxide layer to the surface, which does have some benefit. The basic anodized coating is colorless and does not highlight damage -- stick with that.

A: There's no way to tell if a shaft collar is suitable for your application without the bore diameter. Try a different supplier to find what you need. Shaft collars are a common enough item that the hardware rack at your local home center may yield something useable.

A: Mark J. here: from the diameter and tooth count, your gear is likely a 'mod 0.6' metric pitch (sometimes called a 'metric 48P'). Some hobby-grade R/C cars use metric gears; you may find something suitable at a well stocked R/C hobby supplier, but that bore diameter could be a problem. Hunting down a replacement gear can be very frustrating. Best luck.

A: You can buy individual gears and make your own housing, or you can buy a complete gearbox. There are just too many possible gear combinations to offer a range of housings to accomodate them all. Industrial suppliers like McMaster Carr sell complete right-angle gearboxes, but it's unlikely that you will find the combination of size, capacity, and ratio you require. Belt drives are much more adaptable, more robust, and generally much lighter.

Q: Building off of the miter gear question, how could you get a horizontally mounted motor to turn a vertically mounted shaft with a belt drive?

A: Mark J. here: it's called a quarter-turn belt drive (see diagram - discussion here). It isn't terribly compact and it cannot transmit as much power as a conventional parallel axle arrangement. You've stumbled upon the reason why there are few right-angle drive spinners: it's often easier to design a robot than it is to find the components needed to build it.

A good machinist can pull off all sorts of interesting designs. If you're forced to pick components 'off the shelf' your spectrum is more limited.

A: A torque limiter is a mechanical device that will 'slip' when torque reaches a specific level, preventing damage to components that can only handle a certain amount of torque. Many designs exist -- a toothless belt that slips at high torque, tires that 'break traction' before the motor stalls, friction clutches, ball detent mechanisms, etc. The first two examples are commonly used in robot combat.

A: Your local hobby shop may have a selection of small gears. You'll need to find gears that have suitable bore diameters to match your motor and drive shafts. Also, 10:1 in a single reduction stage will require a large diameter spur or crown gear that may be too large for your selected wheels -- a two-stage reduction may be required to get the drivetrain compact enough to be useful.

Most antweights use a pre-matched motor and gearbox (gearmotor) to simplify their gear reduction needs. Tamiya make inexpensive configurable gearboxes in a wide range of reduction ratios that might suit your project.

A: See #16.

A: Mark J. here: material for a spinner killer scoop needs two attributes: hardness and resilience. Aluminum has neither. The commonly available 'aircraft grade' alloys have a Brinell hardness less than a third that of S7 steel and a fracture toughness about a third that of titanium alloy. A proper spinner impact tooth would dig in, grip, and hurl an aluminum scoop.

The material of choice for a spinner killer scoop is 6Al-4V titanium alloy. A quick check at Titanium Joe shows they currently have a scrap piece of 0.120" 6AL-4V big enough to make about three ant scoops for less than $25. Form your scoop, then heat treat to 36HRc. If the titanium is too expensive, good old chrome moly steel heat treated to 39HRc will get the job done.

A: You have plenty of things to work on without adding new materials to your list, Anthony. It sounds like the materials you had held up well enough to get you thru the tournament. Improve the things that didn't work before you tamper with the things that did.

A: Cutting or drilling carbon fiber composite (CFC) is very much like working with wood - it won't shatter, but it can splinter around the edges of your cut. Use very sharp drill bits, fine-toothed saws, and take your time. Don't breathe the dust or get it in your eyes. Seriously!

For more tips on working with CFC read the Society of Robots carbon fiber page.

A: That depends on how serious you are and what the rules allow at the competition you plan to enter. Serious sumo competitors cast their own tires from polyurethane resin or silicone rubber. Search the archive for 'effective design' for tire links and options.

A: That would depend on how large your 'bot is, wouldn't it? See the previous question on spikes in this archive - search for 'steel spikes'.

Thanks so much for the advice!

A: You don't see carbon fiber composite used much in the heavier weight classes because... well, have you priced the stuff? It is a wonderfully strong, light, and stiff material. When it does fail it shatters like glass, but the force needed to cause that failure would twist comparable weight structures of most other materials like a pretzel.

The common problem leading to failure is that builders confuse the extreme stiffness for high yield strength and use material that is too thin. If you correctly design with adequate thickness for the stress loading, you won't need to worry about brittleness.

If the price for CF proves too much for your budget, you may want to consider garolite as a more affordable substitute. Nearly as strong and stiff, but its failure mode is a little less dramatic. Seach this archive for 'garolite'.

A: Plastics vary a great deal in their material properties. UHMW Polyethylene has enormous capacity to absorb shock loading and is not particularly subject to the drill hole weakness that is common with other plastics; the stuff simply will not crack. UHMW clamp mounts for fairly large motors are commercially available.

UHMW mounts do need to be larger and of a different design than mounts made from aluminum would be in order to provide strength and rigidity, but the material is so light that you can still save considerable weight. Take a look a the motor mounts available from 'Team Tiki' for an example of correct UHMW mount design.

A: All the textured sheet, extruded angles, and flashing that you buy at the hardware store is '3003' aluminum alloy - soft, easy to bend, and unsuitable for robot armor. Some specialty hardware stores may have flat sheets of '6061' aluminum; it will be clearly labeled with its alloy type. Although not the strongest aluminum alloy, 6061 is far, far better than 3003. See our armor guide for some guidance on acceptable alloys. There are also multiple on-line sources for high strength alloys.

A: I'm guessing that this is a paintball CO₂ tank. It will weigh more than a tank built for lower pressure, but if it works well with your design you can certainly use if for compressed air.

Warnings against use of tanks intended for other gasses apply to larger tanks that are color coded for the gas they contain. Confusion about what gas the cylinder contains may create safety concerns in industrial or medical applications.

A: UHMW polyethylene is a very 'slick' material - comparable to teflon. It is also soft. UHMW bearings can be used for relatively low pressure, low temperature applications.

Q: Is it suitable to be use in a 3kg sumo? In terms of using UHMW as both bearing and frame?

A: I know nothing about your design and so cannot comment on the suitability of a specific material. See #17.

A: That previous post was from several years ago and was a 'flip' answer to far too general a question. It was an 'ideal' material because nobody could get into too much trouble with a chunk of 5/16" thick 6AL-4V strapped on the front of their lightweight, irrespective of the design or angle. It was a safe, overkill answer - but I no longer consider that type of answer to be helpful.

Like it says in #17, "We aren't an engineering service. Even if we were, no competent engineer would spec material or thickness before knowing a great deal more about the design than you have told us." Read the rest of FAQ #17 for some advice that is helpful.

A: I can't give you a general answer. It depends on the type of belt used and whether the idler was designed to go on the inside or outside of the belt path.

A: Exposed screws sticking out of your tires works really well to increase traction -- if the arena has a floor made of ice or compacted snow. You might also get some additional traction in heavy turf. For other conditions forget about the screws.

A: Really? Those Yaks weigh 22 ounces each. That's 10 pounds of wheels on a 30 pound robot -- extreme overkill! A set of six Colson 4" wheels would give comparable performance and weigh only 1.5 pounds.

Speed and power with two DeWalt Powerdrives would not be a problem. The Team Tentacle Torque & Amp-Hour Calculator reports a top speed over 22 MPH at 24 volts with the gearboxes in 'high' range. Acceleration is good, reaching about 17 MPH in a 16' by 16' arena. That's plenty quick and fast for a featherweight rammer.

Q: When you say high gear do you mean the 15.3125:1 gear ratio? I used the 45.9375:1 in the Enigma Electric Drivetrain Simulator and the robot was slow. But the 45.9375:1 is listed as the high gear ratio at robot marketplace.

A: I'm referring to 'high' as it is listed in the referenced Team Tentacle Torque & Amp-Hour Calculator: that's the 'high speed' 15.3:1 ratio. Some sources use 'high' and 'low' to refer to the gear reduction ratio rather than the output speed. This does cause confusion!

A: We've discussed working with titanium previously in this archive. Briefly: use very sharp tools, reduced tool speed, firm pressure, and a cutting lubricant.

A: There are many different types of Loctite threadlocker in purple, green, black, blue, and red colors! For most combat robot applications you'll probably want to use the standard and widely available medium strength Loctite 242 blue threadlocker.

High strength red threadlockers create a 'permanent' bond which may require heating the fastener with a torch to remove. Don't use this on anything you might want to take apart in a hurry!

Low strength purple threadlockers are generally for small, fine thread fasteners. There's even a special low strength version for plastic screws.

For full information, download selection guides for Loctite and Permatex threadlocker products.

A: Machine screws are designed for strength along their long axis and have poor strength when exposed to impact shear forces across their diameter. They are intended to hold things together, not to resist the motion of two things sliding relative to each other. Common techniques for preventing hubs from turning on shafts include pins, splines, and keyways.

A hardened pin is a short length of steel rod that has been case hardened to create a very hard surface while retaining impact toughness in the deeper metal. These pins are made just a bit oversize for the hole they fit into so that they must be pressed into place, which holds them secure. The pin typically passes thru the shaft and engages the hub on either side.

Case hardened steel pins can be purchased in a variety of sizes thru most suppliers of mechanical fasteners, such as McMaster-Carr. Keyed shafts and key material are available thru the Robot Marketplace.

Note that keys work well with relatively large shafts. Pins can be used with smaller shafts but drilling a hole thru a very small shaft is difficult and it weakens the shaft. The smaller the shaft, the more difficult the problem of preventing hub rotation becomes. Pretty much every robot builder has learned for themselves that 'set screws suck'.

A: Mark J here: The alloy is most likely 3003, which is not heat-treatable and is about 1/3 as hard and tough as 6061-T6 aluminum. It's intended to be bendable for use as decorative trim. I don't recommend this material for combat robotics.

There are several on-line suppliers of proper aircraft grade aluminum alloys in small quantities.

Q: Okay thanks. Its just that Online Metals is in Seattle and I'm in NC so shipping is huge! But I will do it anyway... One last question. My dad told me about this foam they make to seal crevices in windows that then hardens into a solid. He said it is strong like wood but a lot more lightweight. Could I use that underneath my wedge for support? I still want 2 or 3 small, narrow wood triangle peices to hold the wedge down, but could I fill the gaps in between with that stuff?

A: There is nothing magical about aluminum alloy for your wedge. If it's becoming difficult and/or expensive to obtain the correct aluminum alloy, consider switching to something else. A kitchen cleaver from a dollar store or thrift shop can be cut up to make a great ant wedge.

That insulation foam is gooey, difficult to work with, and it would make any needed repairs to the wedge structure difficult. The place your wedge needs support is along the leading edge -- gap filling between the wooden supports isn't going to add much strength there. The foam doesn't weigh much, but I think I'd use that weight someplace else.

Q: How about a spatula or, as it was called at the Dollar Tree, a "cookie turner?" I didnt see any cleavers, or other large knives...Also, what are cleavers made out of, stainless steel? Thanks.

A: Inexpensive kitchen knives are usually made from 430 stainless steel. This alloy is not as hard as high carbon steel and will not hold an edge as well, but it is fairly tough and resilient. The 'cookie turner' is likely made from the same material, but thinner. I'm a little worried about it being too thin, but if the size is right it could be worth a try.

This Question regards hobbyweight armor, I have read this question website for a while so I know that I should "make it as thick as possible" but I am wondering if 4130 Steel would be good armor in a hobbyweight? this would be a drum bot so the steel would pretty much be the basic structure (containing everything, and put the bearings in for drum), I would put UHMW, and Ti around it eventually. I will make it as thick as possible but since it is quite heavy the thickness range would be from .1" to .2", I belive it should be good enough re-enforced but I would like to get a second opinion from an experienced robot builder. Thank you!

A: See #17. I really can't offer guidance on the suitability of a material for a design about which I know so little. I can tell you that steel is not widely used as chassis or armor material in hobbyweights. The advantage 4130 'chrome moly' steel has over other inexpensive steel alloys is its ability to be 'surface hardened' for durability while retaining good toughness. If you're not going to have it hardened I would suggest other alloys. Look to materials used by other hobbyweight builders for guideance.

Q: As to more info, the bot will be a hobbyweight drum bot. It will be as simple as possible, so it kinda looks like the letter "H" just with one end covered and the drum in/on the other end. The side rails (right and left, longer), will be 10in by 1.5in long Not sure how thick.... the two others (one between and other at end) will be 7in by 1.5in. One of the smaller pieces will 7inches in from the armored end leaving 3in for drum space. so pretty much a 7in by 7in box with two three inch arms at one end. Thats the design, I browsed an online metals supplier and found "hot", and "cold" roll. In my other bots I have only used AL so I have little no experience with steel. So I would like to have a welded steel frame this time. I also tried to look at other bots but I just cant find that much info on them... Could you please recommend a solid, strong weldable alloy of steel? Thank you very much!

A: Mark J here: like it says in #17, "We aren't an engineering service." Aaron didn't ask you for more info. If you sent us complete design drawings we still couldn't recommend specific materials or thicknesses.

I can tell you the differences between 'hot rolled' and 'cold rolled'. Cold rolled has a nicer surface finish, tighter dimensional tollerences, and a sleight increase in hardness from being 'cold worked'. It is also about twice as expensive as hot rolled. Some types of steel are available as either hot or cold rolled while others come only one way.

I'm having trouble following the description of your design, but it sounds like too complex a shape to heat treat without the risk of serious warping. Even welding a structure like this may cause warping. You'll want a steel with low distortion characteristics as well as good weldability and high toughness without heat treating.

Your final decision may come down to availability. Small quantities of a specific steel in the form and thickness you require can be difficult to find. Something like 1025 steel would be a good choice, but a quick search of on-line suppliers didn't turn up any sheets in the thickness you're thinking about. Chrome moly 4130 is more expensive and kind of a waste since you're not going to be able to have the structure hardened, but it is widely available and has many of the properties you're looking for.

As Aaron pointed out, steel is not commonly used for hobbyweights. Sub-light builders have found drawbacks to the use of steel and you're likely going to re-discover those same problems. I think the hamburger is bad, but if you really want to build a steel hobbyweight don't let me stop you.

Thank you! I Just ordered about $10 worth of 1018, so I will try it and see what happens. I can always replace it as I am not using a lot of it. Thanks again!

The axle will be supported within 1/32" of each side of the pulley/wheel (which has a solid bushing through it) so there will be little to no bending forces, only shear. 3/8" plastic should hold up under these conditions for an axle, right?

A: Builders generally use steel for axles. You have a special case: a stationary axle with a wheel/pulley assembly turning freely on it and supported closely on both sides. I'd still go with steel axles for several reasons:

• For all the plastics for which I could find data, the coefficient of friction for plastic on plastic is no less than (and sometimes greater than) polished steel on plastic. Add a spot of silicone grease: a well-lubricated bushing is a happy bushing.

• Those shear forces you mention are not insubstantial. If the axle does not break, it could still stretch, deform, and bind or crack the hub for lack of support.

• I've seen a lot of materials that 'should hold up' fail spectacularly in combat. If I used plastic and it failed, I'd feel really stupid.

A: I can't recommend a battery based on weight and the number of wheels driven! The Team Tentacle Torque & Amp-Hour Calculator can estimate both peak amperage consumption and total amp-hour requirement for a robot based on weight, motors, voltage, gearing, and match length. Enter the values for your robot, then click on 'Battery Chooser'. Write down the needed capacities and go shopping!

A: I don't have a set to test, but I'd guess 'no'. The unspecified 'rubber' is fairly hard (Shore A 45) and is not likely to compare well to the specialized traction compounds used in sumo. The treads should be fine for general robotics, but I wouldn't get into a pushing contest with them.

Q: Then any recommendation for a suitable tank track?

A: Treads (as commonly implemented) do not offer any real traction advantage on an even and clean surface like a sumo dohyo. If they did, they would be seen much more often than they are.

If you are determined to try a set, the Lynxmotion treads look like a good start. You'll need to replace the rubber compound on each segment with custom molded polyurethane or silicone to stand a chance against real sumo tires. There are no competitive 'off-the-shelf' sumo treads out there, AFAIK.

Q: Okay, but oyher than lynxmotion treads, is there any other possible solution for tank treads? the sumo ring is covered with hard rubber.

A: I REALLY DON'T RECOMMEND TANK TREADS! Unless you have a method to evenly load weight over the entire footprint of the tread, you will not have any advantage over wheels. There are some treads available thru Robot Marketplace, but nothing well suited to sumo.

A: I didn't see the demonstration of how tough the soy plastic was, but I feel safe in saying that it's not tougher than polycarbonate. Properly mounted, polycarbonate makes great armor -- and so does UHMW polyethylene. See our Armor Guide for more info.

A: It's approximately 2.25" diameter by 4" deep, including the key.

A: 'Inexpensive' isn't the same thing as 'cheap'. The questioner didn't say why he wanted an inexpensive servo and didn't ask for an opinion on a specific servo. There are applications where a standard $10 servo will work just fine -- I hacked a pair to power my antweight 'Rat Amok' and they worked out great. A pair of $90 titanium geared super-servos would not have been any better in that application. Like it says in #16, with experience you'll learn where you can save money and where you need to buy the good stuff.

A: Mark J here: you aren't getting it. I can't tell you if a specific material of given thickness would be 'good' or not. I can think of designs for which that piece of aluminum would make a good baseplate, and I can think of designs for which it would be unacceptable. I will comment that 12" by 12" is quite small for a 60 pound lightweight, and that 7075 aluminum is a very strong alloy that is entirely suitable for a wide range of robot construction and armor applications.

A: NO! The carbon fiber is by far the stiffest material and would take all the load until it failed. Only then could the other materials come into play.

See earlier questions on composite armor in this archive.

A: See gas spring at Wikipedia.

A: Polyethylene cutting boards may be made from various grades of material, usually HDPE (High Density Polyethylene) or UHMW (Ultra High Molecular Weight polyethylene). There are variances in mechanical properties with different manufacturing processes, so I can't promise 'exact' duplication even assuming that the cutting board in question is made of UHMW.

Also in a somewhat tangential direction: do weight-classes effect the prefered tire type?

A: Pneumatic tires don't have any particular grip advantage over solid, foam, or semi-pneumatic construction for typical combat arena use. They are generally lighter than an equivalent sized solid tire but have the disadvantage of being 'shredable'.

Adjustment of tire pressure to match conditions is not much of an issue in combat robots as the tires are loaded to only a small fraction of the tire rating. More than just a couple pounds of pressure will make the tires too 'bouncy' when so lightly loaded (we leaned this the hard way). Many teams use 'pneumatic' tires bonded to the wheels with no air pressure at all, and perhaps a donut of foam rubber inside.

The selection of a tire type is usually a simple matter of what can be found in the desired size range. Some weight classes have more choice than others. The tire compound and durability are generally more important than the construction design.

A: A 6mm keyed shaft is a challenge. Have you considered going to 1/4" shaft instead? The 3mm bore Finger Tech pulleys can be drilled out to 1/4" just as well as 6mm, the BaneBots hubs are available in 1/4", and McMasterCarr has fully keyed 1/4" shaft in stock.

If you're determined to go 6mm, try asking around the on-line forums to see if someone has a couple of dead 28mm BaneBots gearmotors with good shafts they are willing to sell.

A: Mark J. here: hard anodizing is an electrochemical process that creates a wear resistant oxide coating on the surface of aluminum that is thicker than the oxide layer that naturally forms. This thicker oxide layer resists corrosion and increases surface hardness, but the layer is only a few micrometers thick and does not appreciably modify the overall strength or stiffness of the material.

The purpose of R/C magazines is (IMHO) to hype advertiser products and sell you stuff. You would do well to ignore what you see there.

A: From what - the Power Wheels riding toys? Entering "Fisher-Price gearbox" into your favorite web browser will give multiple sources. There are several offered on EBay.

A: Colson wheels are rugged, inexpensive, and offer good (not great) traction on clean firm surfaces.

Q: Then which types of wheels offer great traction on rubber surface?? What wheels will you recommend?

A: It's really difficult to recommend a robot component when I haven't been told the details of your competition or design. Am I recommending for an antweight or a heavyweight? Is this a sumo or combat application? Will the surface be very clean or the usual dusty and gritty slop? You have to give me a chance at a reasonable answer.

Q: The robot is a sumo robot, 3kg. The field is a wooden ring with a hard rubber coating on the surface. the surface is supposedly to be a bit dirty.

A: We don't compete in sumo, but I know a couple of things about traction.

1. Dirty is a problem. Very grippy tires will pick up dirt and quickly become less effective than more conventional tires.

2. The top sumo competitors mold their own urethane formulations to match surface conditions. See the tire molding article at Roko.ca for tips and links.

A: Mark J. here: we are not an engineering service, but I can tell you that steel and aluminum have different strengths and weaknesses which makes a simple 'resilience' comparison difficult. The thicker 6061 T6 aluminum has greater tensile and yield strength than the 4130 steel -- but the thinner steel is stiffer, more shear-resistant, and has greater surface hardness. Additional differences will depend on the heat treatment used on the 4130 steel.

A: We don't compete in sumo, but I do know that the sumo guys use custom molded urethane traction compounds that really push the limits of traction. The IFI wheels also look too heavy/rugged for sumo use -- the smallest size weighs more than half a pound.

Q: then is there anyway to get one of those treads used on the beast build by carlo ?

A: As I recall, Carlo Bertocchini made the treads on his sumo robot 'Beast' out of pure gum rubber; one-off custom items. They may not be legal under current sumo rules on allowable traction compounds.

Q: IS the wheels sold by team whyachi offer good traction on wooden surface covered by 3mm thick of hard rubber?

A: We have no experience with either the Team Whyachi wheels or hard rubber surfaces. Suggest you check your local library for a copy of Robot Sumo: The Official Guide by Pete Miles for general information on sumo traction solutions.

A: We have no experience in welding titanium, so cannot offer advice on how your repairs would best be done. You might consider taking this opportunity to convert to bolt-on steel impact teeth. Spinners in heavier weighclasses generally have bolt-on impactors that can be replaced easily.

A: Inconel is a specialized corrosion and heat resistant alloy designed for extreme environments. If you were fighting a robot armed with a plasma torch and an acid spray it would be your material of choice. Its physical properties are similar to a high-strength aluminum alloy, but it weighs three times as much. Verdict: too heavy to be good robot armor.

A: What publication might that be? Did they also tell you about using Cheeze Whiz to glue crackers to your robot for armor? Garden hose section tires might be useable for a hobby robot that crawls around in your back yard, but don't even consider it for combat.

Q: Hi, Garden Hose question guy here. The publication was the Robot Wars Technical Manual - I didn't mention the name out of respect for your bulletin. Can you recommend any better way to make wheels from scratch, because I don't have any lawnmowers or electric wheelchairs that I can strip the wheels off of.

A: I'm really hesitant to recommend scratch-built wheels. Tell me more about your robot so that I can have some idea of the stress these wheels are going to have to put up with. I've seen a great many combat robots fail due to poor wheels and hubs, and there is no shortage of sources for workable wheels.

A: You're in the right ballpark, but I can't comment further because I do not know the details of your design. See #17.

A: Mark J. here: have you stopped to wonder why nobody uses carbide impact surfaces on combat robot weapons? Tungsten carbide is extremely hard and wears very little when used to cut thru materials under constant pressure -- but it shatters on hard impact. It is not suitable for the applications you propose -- choose another material.

A: Team JuggerBot fabricated their own. I suspect you'll need to do the same. It might be worth an email to Mike Morrow at Team JuggerBot -- last time I was there Mike was trying to clean out some excess stuff from his workshop and there might be some spikes lying around.

A: You'll need to ask Christian Carlberg. I don't have current contact info for him.

A: Sorry, we aren't experts on cardboard. There were two versions of 'The Judge', but no other combat robots the Mechanicus team.

A: String-powered cardboard and wood robots? I like it! No point in going hi-tech with your lifter materials - wood can stand up to wood. You don't mention how big these robots are, but something like K-nex construction pieces could also work well and just snap together.

Q: Thanks, I [don't] think K-nex would work because the bots are about the size of UK heavyweights. would it work to use a 4-bar,but have the scoop on the back so it scoops them up with the lifter up, then fire it down?

A: Ahhh... BIG cardboard robots. I can't quite picture the 4-bar system you are describing, so can't comment.

A: The spatula came from an end-of-season clearance in the barbeque department at a local department store. I shortened the steel blade by about four inches.

Q: Can you send more pictures of 'Zpatula'?

A: If you have specific questions that can be answered best by a photo I will post that photo here.

A: No!

With all the easy-to-obtain good materials, why do some builders insist on armoring their robots with something they find next to sandwich bags at the supermarket?

A: Yes it's wet here in Oregon, but don't fear the rust! If you're starting with rust-free steel, a wiped-on coating of oil will keep the rust away. If your steel is already rusty, some steel wool or a wire brush will quickly remove the surface rust. You can leave the rust in place for a 'scrapyard' look if that suits you.

A: Mark J. here: first, large plates of S7 tool steel are not available. The heat-treating process the material must go thru would warp large plates. S7 is made for small impact points.

Second, S7 is relatively brittle. A large plate would be prone to shatter. Do not confuse 'hardness' with 'toughness'.

Last, Newtonian physics says that there is no such thing as 'transferring back all of the energy'. Equal and opposite reaction is the rule -- your robot will take as much of a hit as you can transfer back to your spinner opponent. An effective spinner killer does not seek to subject itself to such destructive Newtonian reaction. The idea is to let the wedge/scoop deform to absorb energy and deplete the spinner so you can shove it around without harm. Titanium alloy is the boss for this application.

A: Not even close. Each component of composite armor must be carefully selected to add to the overall strength properties of the material. What do scotch tape and tin foil add?? See our comments about composite armor in this archive.

A: Mark J. here: Grant is correct - Lexan is amazing. The problem is that many builders do not understand how to properly use the material. If you treat it like you would a chunk of metal by drilling a few mounting holes in it and bolting it down firmly, it will fail at the mounting points. For this reason it has gained a poor reputation. If built into a structure by chemical welding, the result is a chassis that will absorb huge impacts and come back for more.

The Riobotz tutorial authors take a traditional engineering approach to their evaluation of materials which looks for rigidity as a measure of strength. This approach overlooks the enormous energy absorbtion potential Lexan provides when allowed to flex. There are a number of things in their tutorial with which I disagree.

Proper use of Lexan polycarbonate requires special techniques not famillar to metalworkers. If you are willing to discover and use those techniques, Lexan can be used to great advantage.

Q: [Chinese Forum] Why do Team Hurtz take so much inetrest in using Lexan Armour? Cause they think Lexan is potential or just the weight problem?

A: Team Hurtz knows how to use Lexan. They know how to weld the structure together, they know how to design for flexibility, and they know where Lexan can and cannot be well used. They also know where to use other materials and how to combine them with the polycarbonate structure. This has served them well.

A: Pit photos show 'Megabyte' running four BattlePacks from Robotic Power Solutions. Their website text hints that they ran the Etek at 60 volts, but the battery set-up looks like a more standard 48 volts. Several sources will build custom NiMhd or NiCad battery packs to your specifications.

A: Mark J. here: I'm not comfortable giving out that information. If you knew enough to safely build such a potentially dangerous system, you'd already know where to get the pieces. It's the same reason I don't give info on how to build flame weapons.

P.S. - I'd feel a little better if you knew how to turn off your Caps Lock key.

A: I make them better than graham crackers but not as good as a soup can. ABS plastic: far too thin, way too flexible.

A: I'll assume the gearbox spins freely when the motor is removed? Possible problems:

• wrong pinion gear on new motor shaft;
• new motor shaft too long;
• improper motor mounting hardware interfering with gear rotation;
• foreign object fell into gearbox when replacing motor.

A: Carefree Tires are made of centrifugally molded polyurethane foam which produces a lightweight, flatproof tire with a firm tread and flexible sidewalls. They take a great deal of abuse. We used them for front tires on heavyweight 'Run Away' in the second Robot Wars Extreme Warriors. Their downside is relatively poor traction compared to a conventional rubber tire of the same size. Some teams buff down the outer tread to get to a spongy layer with greater traction. I wouldn't call them 'perfect', but they do have their uses.

A: Mark J. here: neither Mecanum wheels nor omniwheels are known for a lot of traction, and neither design has an apparent theoretical advantage. The Fingertech Mecanum wheels have actual rubber rollers instead of the hard urethane rollers commonly found in omniwheels, so I'd give them a potential traction advantage. Clean the rollers before every match!

A: Go have a look at the How Stuff Works page on gears for some useful diagrams showing spur and planetary gear arrangements. Planetary gearboxes spread the load over more gear contacts than a comparable spur gear drivetrain and are generally better able to put up with the abuse of robot combat. The down side is that a planetaty gearbox is less efficient than a spur gearbox, wasting more power to friction.

A: Very small linear actuators do exist, but you may get better power and speed by using a high torque servo or gearmotor in a 4-bar lifter design.

A: You'll be spinning that drum too fast to rely on just aluminum running against your axle material. Most builders like to use ball bearings for drum support, and that will work well. Oilite bushings are another option; they are less expensive than bearings and a little more sturdy under heavy impact loading.

A: Just to clarify, the density of the various titanium alloys are almost the same so the real issue here is your availability. Grade 2 has a yield strength less than 1/3 that of 6AL-4V -- it is far inferior for armor. Go with 6AL-V4.

A: Composite armor (two materials used together) is very tricky to get right -- see the discussion further down in this archive. You'd likely do better to leave the balsa off as it adds neither toughness nor impact resistance to the aluminum.

Look at what other builders are using for armor. I don't think you'll find anyone using aluminum/balsa composite.

Q: About the balsa composite, the balsa would be the frame and the 2024 aluminum would be the armour -not both for armour. Thanks.

A: I'd re-think the whole balsa wood concept. Balsa is strong for its weight, but it has low shear resistance and does not absorb impact well. Consider what a balsa glider looks like after a crash.

A: I can't comment on 'thick enough' without knowing the size, design, and mounting/support method of your wedge. I can say the .071" 2024 is very sturdy material and would be thicker than most ant wedges I see.

Bending 2024 is a little tricky. Trying to bend too sharp an angle will cause it to crack. To retain maximum strength, a .071" thickness should be bent around a .25" radius forming tool. Do not heat the material to aid in bending: you will loose that T3 temper.

Q: The wedge we be at a 35 degree (more or less) angle and it will drag the floor. It will be roughly .85 inch tall at the tallest point, making it about 1.3 inches along the bottom. I might support the inside of it with balsa wood, but do I need a brace?

A: I don't have enough information to answer the question. I'd need to know specifics on the mounting method, the number and type of fasteners used, and the depth onto the chassis to which you will fasten. Even then it would be a guess since I don't know the capability of the competition you'll be going against. If I say 'no' and some uber-spinner tears the wedge in half, you'll think I gave you bad advice.

It sounds like your wedge will be pretty sturdy. Balsa wood isn't going to help, unless you plan to fill the entire void between the wedge and the chassis. Build your wedge and test it. Go run it into walls at full speed. If it shows any signs of distortion, reinforce it. Very few robots are 'right' the first time they compete. Be prepared to make changes.

A: Sure. Do your bending in as few blows as possible - alloy aluminum can 'work harden' and become brittle at the bend if you work it too much.

A: Mark J. here: yes, although MIG is preferred for thicknesses greater than 0.125".

A: Your theory is fine -- if you could call 'time out' in the middle of a match to replace the crummy component that just failed on your robot. As noted in #16:

Think of it this way: you spend your time, effort, and considerable money to design and build your robot. You spend more of the same to travel to a tournament and compete. One shoddy piece of equipment fails and your opponent trashes your entire robot and sends you home. You'll wish then that you spent the extra money instead of buying some crap from EBay.

With experience you will learn where you can cut expenses and where you can't. If in doubt, think about that long ride home with a failed robot and buy some decent parts.

A: The Internet Archive can give you access to an archived copy of their old website, but the company has been out of business for a couple of years. The only slim hope to find any of their products would be to ask at the RFL forum if anyone happens to have what you're looking for in their parts bin.

A: There's aluminum and then there's ALUMINUM. An aircraft alloy like 7075-T6 can come close to some titanium alloys in strength, and its lower density lets you go about 50% thicker without a weight penalty. I suspect that MechaVore had well designed, thick, aircraft alloy armor.

A: There are a lot of inexpensive materials I'd suggest before brass. Brass is more expensive than steel, heavier, and not as hard or strong. Go to a dollar store and buy a toughened steel kitchen cleaver for a front wedge that will put up with enormous abuse. While you're there, buy a small UHMW polyethylene cutting board to make awesome side armor. A resourceful builder beats a rich one any day.

A: Mark J. here: as noted in our Armor Guide, the adequacy of a structural piece depends as much on the size, shape, angle, and support that piece has as it does on the material and thickness. I can't get specific knowing only what you have told me.

A few general comments:

• I often suggest that builders look at comparable successful robots for design parameters. I don't think you'll find many UHMW polyethylene front wedges.

• UHMW is a soft and flexible material, not well suited to form a sharp angle on the front of a wedge. This area would suffer damage quite quickly.

• UHMW has very low density; the stuff will actually float. A pound of UHMW, properly used, can provide greater energy absorption than a pound of steel -- but the UHMW would be eight times as thick! A 1/4" UHMW wedge is likely far too thin for a hobbyweight.

Q: What would you think of Lego pieces for armour. Not that I'd just snap them in, I'd hot glue them together or something.

A: Legos are made of acrylonitrile butadiene styrene (ABS) plastic -- reasonably tough and resilient but not equal to the strength of more commonly used plastic armor. Legos can be welded together with solvent-based glue made for polystyrene model kits, but the exposed walls are not very thick and assembled mass is not going to be nearly as strong as a single sheet of polycarbonate or UHMW polyethylene plastic.

If you're just building robots for fun, it sounds like a fine idea. If you enter a competition there'll just be a cloud of Legos where your robot used to be.

Q: Aaron, sorry to bug you with this again, but I'm on a small budget. Would the Lego armour be sufficient for an antweight? Aren't the ant weapons less damaging? Thanks!

A: I really don't see Lego armor working. I don't know about the ants where you will be competing, but high-power spinners are common ant weapons pretty much everywhere. Don't count on low-damage weaponry.

Take a trip thru your local dollar store. You can find kitchen storage containers made out of polypropylene ('PP' in the recycling symbol) that are tough and flexible, and you might find a thin polyethylene cutting board that would be perfect ant armor.

Q: It's been a while, but I am the Lego armor guy. The Inertia Labs Antweight kit comes with an ABS plastic lid, so still confused.

A: Mark J. here: a convertible automobile has a cloth top. Does that mean it's a good idea to use cloth for a front bumper?

Inertia Labs used to offer titanium lids for that chassis, but they found that the top of an antweight takes very little abuse. A sheet of ABS is adequate for top armor only because it is unlikely to be exposed to a high energy weapon. That sheet of ABS is better (and lighter) armor than a bunch of thin-walled ABS Lego blocks glued together.

If you're intent on using Legos, go for it. However, the current Robot Fighting League Judging Guidelines count every visible scratch or gouge as points for your opponent. You can expect to pay dearly if you go cheap on your armor.

A: Bulletproof vests are made of Kevlar, but you don't make bullets out of Kevlar to shoot at them. Armor and impact weapons have very different material requirements:

• Robot armor must be resilient and tough in order to absorb impact without permanent deformation. Armor also needs a high strength to weight ratio to provide the greatest protection for a limited weight allowance. Titanium alloy meets these requirements well, but is quite expensive.

• Robot impact weapons must be hard and unyielding to efficiently translate force into damage. High density is not a drawback as it allows high kinetic energy storage in a compact form. Hardened tool steel alloys fit the bill here and have a much lower cost than titanium.

A: No, Antony. The BattleSwitch is a 'Single Pole Double Throw' (SPDT) switch that can be operated via hobby R/C gear. One BattleSwitch can turn on and off a single motor in a single direction.

To control a single motor 'forward / off / reverse' requires a switch arrangement called an H-bridge, and you would require a total of four switches for two motors. The RCE220 RC Dual Relay Switch can be used as a single H-bridge, but a pair of small H-bridges are more expensive, heavier, and less useful than an Electronic Speed Controller.

Secondly, I have no experience machining titanium. I have worked with mild steel and aluminum, and have plenty of tools in my shop to handle those. Can you advise me on any special saw blades or drill bits I'll need to work the material? I really appreciate all the help.

A: Mark J. here: second issue first - you won't need any special tools to drill or cut thin sheets of titanium alloy. Make sure your bits and blades are very sharp, reduce the drill or saw speed by about half, maintain firm tool to material pressure, and use an appropriate cutting fluid (WD-40 should be fine for your purposes).

The strength of an armor panel depends on size, shape, and the support given to it as much as on thickness, but in general your 0.041" 6AL-4V should be quite adequate for most antweight armor applications. I would consider a double-ply for a front spinner-killer scoop or ram.

The offensive capability of antweights varies a great deal. Take a look at ants active in your area and plan accordingly. Like Team JuggerBot says, "damage is weakness leaving the robot." Make everything strong; if it breaks make it stronger.

Team Run Amok prefers UHMW polyethylene for insect class side/rear armor and polycarbonate for the top. Front impact zones are heat-treated steel -- the weight saved by titanium would be small. We usually have a section of wood somewhere out of respect for our first combat 'bot.

A: Mark J. here: the physical properties of T6 and T651 are the same -- the '51' suffix indicates only that the material has been stress-revieved by a stretching process. Go for it.

A: Mark J. here: I knew somebody else would figure this out eventually. The coefficient of friction between common armor materials and common weapon material is already pretty low, but there are other advantages to slicking-up your armor. I've used 'Armor All' protectant on plastic armor for years -- it makes the armor very slick and can cause traction problems for robots that try to climb over your 'bot by transferring onto their tires. Use it in moderation, and don't blame me when your robot slips out of your hands and lands on your foot.

A: I don't provide advertising for products I haven't tested or at least know something about.

A: Pykrete has properties similar to concrete; do you see anyone using concrete for robot armor? High on the list of properties required for good robot armor is 'fracture toughness'. Materials that can flex rather than break have high fracture toughness. Pykrete has much higher toughness than ice, but much lower than common armor materials. Think about how much better Lexan would have performed in the same tests that Pykrete was put thru by the Mythbusters. A good spinner would turn Pykrete into pulpy slush.

A: British Robot Wars teams were well known for exaggerating the material specifications of their robots. I am unable to verify that Mortis actually used any form of silicon carbide (SiC) armor, although it is possible that they had a thin SiC coating on some more common armor material.

SiC ceramic has been used as a component of military vehicle composite armor, but only for its ability to resist specific weapons not available to robot combatants.

Silicon carbide is extremely hard, rigid, and heat-resistant, but it is also quite brittle. It's fracture toughness is about 1/10th that of steel and it would shatter under direct impact. A sheet of SiC would be very poor robot armor for any weight class.

Date marker: April 2009

---

Q: I have some ideas for a new system of heavyweight armor, but I don't have enough money or experience to build the actual robot, any ideas on how I could test the armor?

A: Mark J. here: without knowing anything about your proposed armor system I can't make much of a suggestion on testing. Simulating a high-speed multi-thousand Joule weapon impact isn't easy. The best test would be to build a full-scale test panel, attach it to a dummy weight, and let a competitive heavyweight spinner take a shot at it.

Incidentally, I can pretty much guarantee that whatever material(s) and mounting technique you're thinking about has already been tried.

Q: My dad has experience in making custom composite parts from resin and carbon fiber cloth. Do you think that this could be a good way to make armor, because you can make it so that the stresses aren't on just one small side piece? Has anyone done this before?

A: Carbon fiber composite armor is common on small robots. It is lightweight, stiff, and highly impact resistant. Robot Marketplace stocks CFC sheets, rods, and angles.

Use of CFC on larger robots has been limited by the expense of large and thick sections of CFC and because CFC shatters when it fails. Judges give your opponent a good score for putting a big dent in your armor, but they give a huge score for ripping chunks of your armor off and tossing them around the arena.

Also, please note section 12.6 of the Robot Fighting League rules:

"Hazardous or dangerous materials are forbidden from use anywhere on a robot where they may contact humans, or by way of the robot being damaged (within reason) contact humans."

Carbon fiber dust as generated when abrasively cutting CFC is dangerous to humans. A specific event organizer may choose to disallow CFC armor if they feel it might present a hazard.

Q: I talked to my dad, and he said that I should consider making the armor out of Kevlar, with a small bit of carbon fiber on the inside. Good idea?

A: Kevlar is a good choice for armor. It is more flexible than CF and does not shatter, even when the resin fails. However, armor made of layers of materials with differing properties can be very tricky. In this case, the Kevlar will flex and transfer most of the impact force to the stiff thin carbon fiber layer which will shatter. I think I'd stick with straight Kevlar. The most effective armor materials are those that can flex to absorb heavy impact.

---

Q: I'm searching for a 12vdc actuated clutch, 0.86" diameter max and max length of 2.5" to support 1 Nm of torque. Any suggestions? Thanks

A: You’ve backed yourself into some pretty specific design and performance criteria. I don't have any sources for such clutches. My only suggestion is that you avoid such narrow constraints in your design next time.

---

Q: I'm thinking of building a beetleweight with a vertical spinner. What type of metal do you think I should use for the structure?

A: Hard to comment since I have no idea what type of structure you plan to build, what your budget is, or what tools and skill level you have. In general, aircraft aluminum alloys like 6061 are popular for structural elements.

---

Q: Several times in your archive you state that you feel that polypropylene is inferior bot armour. Could you elaborate?

A: Mark J. here: I believe I say that only once. I consider it to be inferior to other plastics like polycarbonate or UMHW polyethylene and I have numbers to back that up. Take a look at the table below: the mechanical properties of polypropylene are far below those for polycarbonate. Both polycarbonate and UHMW polyethylene have much greater ability to absorb impact without failing, as shown in the 'Tensile Elongation' and 'Izod Impact' tests that reflect the 'toughness' of the material.

	Polyprop	Polycarb	UHMW-PE
Density (lb/in³)	0.033	0.043	0.034
Tensile Strength (psi)	4,800	9,500	3,100
Tensile Modulus(psi)	195,000	320,000	125,000
Tensile Elongation (%)	23	60	No Break
Flex Modulus (psi)	160,000	375,000	125,000
Compressive Strength (psi)	6,000	12,000	2,000
IZOD Impact (ft-lb/in)	7.5	13	No Break

In thin sections for an insect class robot polypropylene can be put to good use. I've used it myself for my antweight 'Rat Amok' because I happened to have a polypropylene box just the right size for my purpose. If I had a polycarbonate box that same size, I would have used it instead.

---

Q: Can 4130 and 4140 steel be heat treated in the same batch?

A: Mark J. here: there are a lot of different 'heat treatments' that can be applied to alloy steel: annealing, normalizing, spheroidizing, tempering, and hardening -- to name a few. I'll assume that you are interested in hardening and tempering.

Both alloys are oil-quench hardened, but although the temperature requirements for heat hardening and tempering 4130 and 4140 alloys are similar they are not overlapping. I would not suggest treating the two alloys in a single batch. Discuss your specific needs with your heat treater.

Metal Suppliers Online is an excellent source of information on metal properties and treatment requirements.

---

Q: Where can you get McMaster Carr catologs?

A: Industrial supplier McMaster Carr sends out their 3700+ page printed catalogs only to customers who purchase a large volume of products. Their on-line catalog is entirely complete and possibly more useful for most searches than their paper catalog. If you really want a printed catalog, there are often current versions (#114 is the most recent version as of December, 2008) available on EBay.

---

Q: What can you tell me about the Inertia Labs micro pneumatics components?

A: In 2003, Inertia Labs manufactured a line of miniature pnumatics components (archived webpage). These components could be used to make a flipper weapon small enough for an antweight robot. Supplies of these components have run out.

Inertia Labs used these components to build an antweight flipper named 'Pele' that fought from 2003 to 2005 and attained a 10 win, 14 loss record.

The Inertia Labs micro pneumatics system works just like a larger pneumatic system, just on a smaller scale. It's a low pressure system using compressed air at 145 PSI. Answers to specific questions about the micro system can be found at the Micro Pneumatics FAQ.

---

Q: What is good material for antweight lifting arms? How thick should they be? Would 7075 T6 alluminum be good?

A: Mark J. here: many factors interact when designing a robot component. The suitability of a small lifting arm depends on much more than the material used. Other factors include the unsupported length of the arm, the cross-sectional shape of the arm (flat, I-beam, U-channel, tubular...), and the protection the arm has from weapon impacts. I could build a perfectly acceptable lifting arm out of a fast food plastic fork if I was careful with the other factors.

I couldn't give you help on what material you need or how thick the material needs to be without full details about your robot design, but please don't send those details -- I'm not running an engineering service. Look around at other robot designs similar to your own for a starting point. A strong aluminum alloy like 7075 is a perfectly good general robot constructiion material. Build your lifting arm, improve it if it fails, and learn from the process.

---

Q: Where can I buy a bearing block for a 6mm shaft with a 14mm center height?

A: It's really common to get your whole robot design laid out only to discover that you need some very specific part that's difficult or impossible to find. If you have machining skills and a proper shop you can often make the required piece. If not, you either change the design or learn to 'bodge'. You'll probably do best to get a short 6mm pillow block and put a spacer under it to bring it up to the correct height.

---

Q: can u glue lexan with a glue stick and duct tape? it is for an ant chasie.

A: Mark J. here: you might as well use spit and a piece of string. For best strength, Lexan must be bonded with a solvent cement designed for plastics. A solvent cement chemically welds the joined surfaces together into a single piece as strong as the surrounding material. Your local hobby shop may have a suitable cement, but don't let them sell you cyanoacrylate (super glue) or epoxy for this purpose. Here's an eHow article on bonding Lexan

Watch the video on plastic bonding on YouTube.

---

Q: Can 6AL-4V titanium be welded?

A: 6AL-4V titanium is TIG weldable.

---

Q: Is 1/2" premium UHMW strong enough for a beetle? Is 0.05" 6 Al-4V grade 5 titainium strong enough for the wedge and back piece? The wedge and back are 9" long. Thanks.

A: We've said this often: structural performance depends on factors other than just material and thickness. The size and shape of the piece, mounting method, and support from other chassis elements all count toward performance. Just being given the material and length of a piece is too little information. You're not even telling me what you want to make out of the UHMW!

---

Q: Does the Team Delta DeWalt Powerdrive Kit include the motor supports shown in the images attached to the webpage?

A: I wrote to Dan Danknick and asked about the additional suports shown in some of the images:

"You're right, a couple of the images are CAD models including the [RCM505] retainer. [The RCM505 retainer is] no longer available and though I've adjusted the kit contents list and price to reflect that, I forgot to change the images."

So, no -- the long bolts and end plate supporting the motor in some of the images is not included in the current kit and is no longer available.

---

Q: How does '02' tool steel sound for the bottom plate of a superheavyweight?

A: Mark J. here: the various tool steels are not generally suitable for armor. A tool steel is designed to be harder than other steels, but hardness should not be confused with strength. In order to obtain high hardness, other properties are sacrificed. Tool steels tend to be britile -- not a characteristic you want in armor. You are also going to have dificulty finding a plate of tool steel large enough for the bottom plate of a superheavy.

A tool steel bottom plate would be highly resistant to damage from a 'killsaw' style arena hazard. If you really did want to use a tool steel for this purpose, 02 would be a good choice.

---

Q: How come not that many robot builders use tungsten?

A: I don't know of any builders that use metallic tungsten. Tungsten does have high tensile strength, but there are many other properties to consider in material selection. Tungsten is brittle -- it does not hold up well to impact. It also weighs two and a half times as much as steel, which makes it much too heavy for structural members in a combat robot.

---

Q: I'm thinking of using Nylon 6 as a chassis / armour material in a thwackbot featherweight. Do you have any advice on using it, or should I go for something like UHMW polythene or polypropylene instead?

A: Mark J. here: Nylon 6 has high tensile and flexural strength, but it performs poorly under heavy impact: it breaks. Polypropylene is a little better, but still far inferior to the more commonly used plastic or composite armors. Try polycarbonate, UHMW polyethylene, or Garolyte instead.

---

Q: Dear Aaron, I was wondering if the following metals would work for a simple antweight wedge for my Inertia Labs kit:

• 0.05" 2024-T3 Aluminum
• 0.04" 2024-T6 Aluminum

A: Mark J. here: the 2024 and 7075 'aircraft' aluminum alloys are good choices for armor or wedges. Both alloys are strong, hard, and fatigue resistant. The difference between the T3 and T6 tempers will not be noticeable for your purpose. A good summary of aluminum alloy properties can be found at OnlineMetals.com.

You're in the right ballpark on thickness, but just how thick will depend on how well the wedge is supported by the mount and the overall size of the wedge. When in doubt, go thicker.

Other materials you might consider:

• garolite;
• 6AL-4V titanium;
• low carbon spring tempered steel.

Q: How about 0.05" 7075-T6 Aluminum for a 4 centimeter long 10 centimeter wide wedge supported by 8 screws on the top of the robot? (the wedge being bent down and dragging against the floor). Thanks, you're a great help.

A: Hmmmm... that's not what I would consider to be good mounting support. The force applied to the wedge is going to be at a point farthest away from the mount, and the aluminum has already been bent at an angle below the mounting point. You're going to bend the wedge further downward with a good hit. Good engineering practice calls for more support closer to the point where the force will be applied. A full-width brace between the chassis and the wedge would greatly strengthen structure. With that brace, your .05" 7075 wedge should be great.

Q: Do you think that side skirts 2.5 centimeters long and 13 centimeters wide, made out of 0.05" 7075 T6 Aluminum and supported at the top would need extra support near the bottom? Is it a good idea to hinge side skirts or not? Would that tape that you use for your skirts be strong enough for antweight side skirts with the same dimensions as above?

A: Hinged side skirts aren't going to get the same type of stress that might be applied to the fixed front wedge, so no extra support should be needed there. Side skirts should be hinged -- you want them to drop down and ride along the arena floor with zero clearance. We use 3M #396 Super Bond Film Tape for our insect-class skirt hinges. It would be plenty strong enough for your purpose, but it is difficult to find. Aircraft control surface tape (also called hinge tape) from your local hobby shop is very similar.

---

Q: Where can I buy magnets that would be effective in holding a beetleweight to a steel arena floor, not to be used as wheels, but just to be placed in the chassis and give the wheels a bit more grip?

A: A web search for 'surplus neodymium magnets' will turn up a large number of sources for very strong rare-earth magnets. I've had good luck with K&J Magnetics. Why not go with wheel magnets?

---

Q: Where can you buy full pressure CO₂ actuator cylinders? Is it legal in RFl rules to build your own, as shown in the Invertabot construction log?

A: Mark J. here: CAUTION -- full-pressure CO₂ systems are dangerous and definitely not recommended for builders without extensive experience.

For those builders with pneumatic system experience, the usual choice for a high-pressure actuator is a hydraulic cylinder. Hydraulic components are typically rated 1500 to 2500 PSI, well above the maximum pressure available from unregulated CO₂. Hydraulic cylinders are available from industrial suppliers, such as McMaster-Carr.

I would generally discourage anyone from attempting to construct their own high-pressure pneumatic components.The RFL rules call for high-pressure pneumatic components to be rated and/or certified for use at the maximum system operating pressure plus 20%. I don't believe you'd have much success convincing an event organizer that your homebrew parts have an industrial certification.

---

Q: I've heard that paintball CO₂ regulators from Palmer-Pursuit work in combat robots, but which one should I choose? I also need a suggestion for a solenoid valve -- it doesn't need to be anything wonderful since I'm using a buffer tank.

A: Mark J. here: Suppose you were working in a pizza parlor and a customer came in and asked, "Which pizza do I need to feed my family?" They haven't given you enough information to answer their question. How many people are in the familly? How old / large / hungry are each of them? How do they feel about anchovy?

Same thing here. I don't know what weightclass your robot is in, the size of the actuator, the geometry of your weapon linkage, the flow rating for the other components in the system, or your expectations on performance. I cannot answer your question.

Note: a buffer tank will not make up for a poor-flowing actuator valve. A buffer tank allows the accumulation of a volume of gas at full actuation pressure between the pressure regulator and the actuator valve. This allows use of a regulator that may not have the flow capacity to keep up with your actuator clyinder, but the critical importance of the actuator valve in flowing enough gas fast enough to the actuator is unchanged. A pneumatic system will perform no better than the poorest performing component in the system.

---

Q: The wire section at Robot Marketplace they doesn't specify the insulation material used on most of the wire. Any idea what it is?

A: Some of their wire choices specifically say they have silicone insulation. If it doesn't say, it's a good bet that the insulation is standard PVC.

---

Q: I can't find an 'A size' v-belt the length I need (14"-15") from McMaster-Carr or the Robot Marketplace. Can I cut a longer one and solder it back together?

A: Mark J. here: no!

V-belts have internal cords that prevent stretching and provide structural strength. Cutting a belt and heat welding it back together would give you a belt that would fail immediately under load.

The smallest conventional A-size belt I can find is 16" at www.mainsupplies.com, but it wouldn't hurt to check your local auto parts store or lawnmower repair shop.

Your best bet may be a resizable PowerTwist belt. These are made up of individual linked sections and can be assembled by hand to any reasonable size. I have never used them, but they are reported to be as strong as conventional belts.

---

Q: Where can I buy large sheets of S7 Tool Steel? The widest sheets McMaster-Carr has are only 4 inches.

A: Mark J. here: I've never seen S7 flat stock wider than 6". onlinemetals.com has 6" widths up to 1/2" thick. Both suppliers offer other grades of tool steels in widths to 12" and thicknesses to 4".

Q: In a previous question regarding a spinner-killer's scoop, Mark J. said that S7 tool steel comes hardened. In another question about ramming spikes, he said that it doesn't. Which is true?

A: Raw tool steel stock from a metal dealer is shipped in a relatively soft annealed form so that it may be machined, shaped, and formed. It will require hardening before use. If purchased in a finished tool form, the steel is hardened. If in doubt, ask your supplier.

If you buy from a metal dealer you'll know what you're getting, but I have access to an exotic scrap metal yard where the material may be either annealed or hardened. Sorry if you found my comments regarding the spinner-killer scoop misleading -- I've edited that answer.

Q: Which other grades of tool steel would you recommend for a featherweight spinner with a 8in disk?

A: I wouldn't recommend making your whole disk out of tool steel. Take a look at disk weapons from other builders; they tend to be aluminum or titanium with hardened steel teeth. You use tool steel for impact surfaces where hardness counts in delivering damage, not for the main body of the weapon where toughness is much more important. Also, a piece of tool steel that large may warp significantly when hardened.

If you decide to ignore my advice, go with an oil quenched tool steel like 'O1' that forms a relatively thin hard surface and still retains interior toughness, or a warp-resistant air quenched alloy like 'A2'.

---

Q: What do you think would be the best way to mount a Colson wheel to a 42mm Banebots motor? Could I drill out the bore of the Banebots hub larger and rebroach the keyway?

A: The press-fit BaneBots hub for 3" to 5" Colson wheels is only 0.59" in diameter. If you drill out the bore to 0.50" there won't be enough material left for a keyway.

The best solution is to order your Colson wheels fitted with with custom hubs from Team Delta. They will modify the wheel and mount a bolt-on hub that will fit perfectly and give no trouble.

Q: Hm... Are there any Colson Hub solutions adaptable to either the 36mm or 42mm Banebots motors for the 2" wheels? Could I Loctite in the keyway included from either the Robot Marketplace or Banebots and broach a keyway?

A: The Colson wheels are easy to modify. The 13mm bore of the 2" wheel can be enlarged to accept the 15mm BaneBots hub for the 36mm gearmotor. Alternately, you can broach a keyway into the wheel and mate it directly to the 1/2" shaft of the 42mm gearmotor.

Don't bodge the hub with Loctite, tape, or glue. Hold the wheel in place with proper mechanical methods: the banebots gearbox shaft is tapped on the end for a bolt/washer combo, and the inner limit can be set with a locking collar or spacer. I've seen more robots break down from bodged hubs than any other single cause. Do the hubs right or you'll wish you had.

---

Q: Could I use the 2 inch aluminum antweight wheels sold by the Robot Marketplace in a 30-pounder if they're mounted properly?

A: The Du-Bro wheels are made for R/C airplanes -- your local hobby shop probably stocks these. The wheels are not solid aluminum chunks, so the 1/8" axle hole can only be drilled out a little. They are entirely not suitable for a featherweight combat robot.

---

Q: What bearings should I use to mount my beetle's spinning weapon?

A: Mark J. here: you haven't given me enough information about your weapon for me to make a specific recommendation. In general, ABEC grade 3 steel ball bearings are adequate. Higher grades are are more expensive and are made to higher precision, but precision isn't critical in this application. A 'shielded' bearing design will help keep grit and debris out of the bearing.

---

Date marker: March 2008

---

Q: Can I use Aluminum for a bot frame?

A: Aluminum is a popular material for bot frames, but you need to know that there are various grades of aluminum: 6061-T6, 2024-T3, and 7075-T6 are strong and durable alloys well suited to bot construction. Other alloys may be very soft and relatively weak. Browse thru this archive for information on different aluminum alloys.

---

Q: Can I use Laminite for bot armor?

A: Yes -- but Laminite is very heavy and somewhat brittle in thin sheets. There are better armor materials.

---

Q: What's the best way to cut Garolite?

A: An abrasive cutting disk works very well. Wear eye protection and a dust mask. A fine-toothed hacksaw will also do an acceptable job. Square up the cuts with a sanding block.

---

Q: I just went to a bunch of thrift stores and I bought a bunch of metal plates and pans and things like that. i just thought this was a cheap way of getting ant armor for a first time 'bot builder. Might want to post this idea. Thanks a ton for all the help you have given me!

A: Yes, I've seen many antweights with recycled kitchen components for armor. I'm always thinking about robot applications when I wander thru a dollar store or thriftshop.

---

Q: Is there any way to increase the traction on standard Colson wheels?

A: This is the first complaint I've ever heard about traction with Colson wheels. The rubber is pretty grippy on smooth, clean surfaces. Clean the tires with rubbing alcohol between matches and let dry. Move heavy components around on your chassis to concentrate weight on the drive axle. If you're still getting excessive wheelspin it's just because you have more torque than weight.

---

Q: What exactly is a linear actuator? What can they be used for in combat robots?

A: In combat robotics a linear actuator is a device which converts the rotational motion of an electric motor into the extension/retraction of a rod to provide a pushing and/or pulling force. Their speed is not nearly as great as pneumatic actuators with the same force, but they require only electric power and simple controls to operate.

Typical uses in combat robotics include lifter weapons (BioHazard), clamping jaws (Jawbreaker), and self-righting mechanisms.

Q: What linear actuator would be best for a Lightweight clamper? How much power would it need, and what speed controller should be used?

A: Mark J. here: I'm not sure that I can recommend any electric linear actuator for a lightweight clamper. There are dozens of manufacturers offering hundreds of models, but it's still unlikely that you'll find an off-the-shelf unit that will meet your needs. Tell me about your design and I'll see if I can match an actuator to it, but effective combat clampers larger than insect class use custom actuators or pneumatic power.

A linear actuator is typically controlled by an R/C Dual Ended Switch, not a speed controller.

---

Q: I want to add some steel spikes to the front of my lightweight ramming 'bot to help absorb impact and deal some damage. What should I make them out of, and how?

A: Mark J. here: I'll take the metallurgy question. The alloy of choice for impact weapons is 'S7 tool steel' for a good balance of hardness and impact resistance. The best material source I have is McMaster Carr: search for 'S7 tool steel'. The 'unpolished mill finish' rods are fine for your application and are much less expensive than other options.

Tool steel is supplied in an annealed form -- soft enough to be machined to a point or chisel profile. After you have cut and shaped the spikes the steel must be heat treated to produce the hardness needed to penetrate armor. Check your local phone book for companies that heat treat metal. Tell them what the spikes are made of and how they will be used.

Too much trouble? Your local tool store has heat-treated chisels and punches that would be a reasonable substitute.

Now you have some spikes that will take enormous punishment and punch thru armor like cardboard. The next problem is that spikes don't 'absorb' impact, they just transfer it back to their mounting points. Make those mounts strong and well braced. The judges won't be impressed if your spikes fall off or your chassis buckles under impact.

Q: Okay then, what would be the best way to mount the spikes? Could they be welded after heat treatment, or could the chisels be welded out of the package?

A: Yes, you can weld heat treated steel, as long as you're careful not to heat up the whole mass and ruin the treatment. Chisels are generally heat treated only on the pointed end, so they retain ductility at the 'hammering' end. I'd rather make a mount for the spikes from which they could be removed and replaced quickly if needed. Maybe a steel tube welded to the chassis that the spike could slide into? Run abolt thru both the tube and spike to hold it in place. Drill the hole in the spike before you heat treat it, and weld a plug at the back end of the tube to take the force -- the bolt only holds the spike 'in'.

---

Q: Can I use the two battery packs that came with my Harbor Freight drills to power my 'bot?

A: Mark J. here: the HF NiCad battery packs are not 'combat quality': they are heavy and bulky for their capacity. You can use them if you're on a really tight budget, but don't try to use the battery chargers that came with the drills. They are far too slow.

Each pack has a capacity of about 1.2 amp-hours, so a single pack should be plenty to run two HF drill motors in a light 'bot for a 3 to 5 minute match. Check the Team Tentacle Torque & Amp-Hour Calculator for an estimate of the battery capacity your 'bot will need.

---

Q: Where can I get a low-voltage shut-down circuit for my Li-poly battery? I need it to be shut down when voltage drops below the recommended minimum cell voltage. I saw one at Baldwin Controls, but I want to have some more options.

A: Really bad idea! You're gonna be in combat and suddenly have your 'bot shut down to prevent possible battery damage? How about the damage your opponent is gonna do to you while you're sitting there immobile?

No fuses! No circuit breakers! No auto shut-down in combat!

Best solution is to make sure your battery pack has enough capacity to last the entire match. If you're still concerned, there are devices available that flash a light and/or beep loudly when the Li-poly pack voltage drops too low (example). That gives YOU the option of shutting down.

---

Q: I'm making my own 4 inch wheels for my hobbyweight with a hole saw out of 3/4 inch Lexan for the main body of the wheel. What should I use around the Lexan for grip, and where can I buy it?

A: Mark J. here: it's very ambitious of you to build your own wheels. Any particular reason you don't want to use the wheels everybody else is having success with?

Getting a high-grip material to bond to the plastic wheels you're making isn't going to be easy. Industrial plastic wheels have their tires bonded in place in high-pressure molds that aren't practical for a small-scale builder to duplicate. Some sumo robot builders mold a multi-part urethane resin around their wheels for their special high-traction needs -- but this requires a custom mold and their wheels don't have to stand up to spinner attacks. I've also seen sumo builders glue a wide rubber band around the outside of a plastic wheel with contact adhesive.

You could try a layer of RTV silicone sealant applied to the outside of your wheel. You can get silicone sealant at any hardware store. It isn't going to stick very well to the Lexan, and getting it on in an even layer without a mold is going to be tricky.

A reasonable solution might be to machine a groove around the outside edge of the wheel and glue a rubber O-ring into the groove. Industrial suppliers like McMaster-Carr can supply O-rings in a wide range of sizes and specific materials.

My suggestion: don't make it hard on yourself. There are lots of reasons Colson wheels are so popular. Use them and you won't regret it.

---

Q: I need to make the inner diameter of a Lexan tube larger. Is there some kind of simple tool that will do the job?

A: Mark J. here: I wish you'd mentioned how big and long the tube is, how much material you need to remove, and how precise you need to be! If the tube is big enough, you can use a 'flap sander'. It's a bunch of abrasive flaps attached to a shaft that fits in your drill. With a shaft extender, you can work down a pretty long tube with one. They come in several sizes - check your local hardware store.

For a smaller tube, you could use sandpaper glued around a wood dowel a little smaller than the inside of the tube and work it by hand.

If the resulting tube has to be very precise, a machine shop could put it on a lathe and bore or shave the inside diameter. Best luck!

---

Q: What is the weight of a 20:1 Radicon gearbox?

A: Mark J. here: Radicon makes a variety of industrial gearboxes with different designs and capacities. I don't know which one you want, plus I don't have their catalog. Industrial gearboxes are made for durability, not light weight. A typical industrial 20:1 worm gearbox with a 200 inch-pounds output capacity weighs about 10 pounds.

---

Q: I have a few questions about garolite:

1. There are so many different kinds, which one should I choose for a 30lber baseplate?
2. How thick should it be? The only motors are 4 Small Johnsons at 14.4V with T-Boxes, to give you an idea of the torque.
3. How dense is the garolite that I should use from Mcmaster Carr?

A: Mark J. here:

1. See the earlier post on garolite.
2. Thickness will depend on the dimensions of the baseplate, the distance between load points, the location and number of stiffening elements, and many other design factors. The Totalinsanity chassis design tutorial (archived) may provide you with some help. Note that I've never seen a garolite chassis for a 'bot bigger than a beetleweight.
3. Garolite weighs about 1.1 ounce per cubic inch.

Q: My bots footprint is 12x12 inches. The garolite would have to cover the bottom, so a 12x12 piece with it being screwed into .25 titanium at each edge. So the maximum span would be 11.5. The motors would be as far as possible to the corners to get maximum benifit from wheels on the outside. From center of front left motor to center of front right motor is 6.128", and center from front right motor to back right motor is 6.044. Could .125" garolite work for this setup? I've heard that it is less bendy than an equal thickness of aluminum, and people do use garolite in 30's, maybe more but 30's are the largest I've seen. Thanks.

A: My best advice is to ask a builder of one of those featherweights you know about how thick their plates are. As an educated guess, I think you could get away with 1/8" garolite -- if you have it supported on all edges and can add a stiffening vertical center support up to the top cover. Going to 3/16" would add about 9 ounces to the weight. I'd go for that.

Like carbon fiber, when garolite fails it fails big. Aluminum dents, garolite comes all apart. Use metal washers to spread the load at equipment mounting points, and have a spare baseplate in the pits.

---

Q: I don't have access to many powertools at the moment. Can polycarbonate be cut with a woodsaw or hacksaw? Will polycarbonate crack if I attempt to drill holes in it with a normal electric drill?

A: Polycarbonate cuts, drills, and machines very much like other soft plastics. It is a good armor material only because it can absorb a large impact by deforming. A normal sharp drill bit bores through it just fine, a fine-toothed hacksaw cuts it easilly, and sandpaper will smooth the edges.

---

Q: How do I bend carbon fiber composite? What is the thickest carbon fiber that can still be bent to shape? What machine do you use to bend it?

A: Carbon fiber should not be bent - it seriously weakens or breaks. Angled or curved carbon fiber was formed to shape when made, not bent afterward. The whole point of CF is how stiff it is.

---

Q: Where can I buy the parts formerly available in the Robot Marketplace's Micro Pneumatics kit?

A: The antweight-sized micro pneumatic parts were made by Inertia Labs, but they are no longer listed at either thier site or at the Robot Marketplace. There is contact info at the Inertia Labs site -- you could drop them a note and see if they still have a few pieces around, but it looks like they no longer make them. Try a 'parts wanted' post to the RFL Forum to see if anyone has spares in their parts bin.

---

Q: I'm building a hobbyweight and I'm very tight on weight. How would 2.5" Lite Flite wheels compare to 2.5" Colsons for a 12-pounder? Any other insect weight wheels that might work better?

A: Mark J. here:

Advantages:

• The Lite Flites would certainly save weight. A 2.5" Lite Flite wheel / tire weighs less than half an ounce, versus 2.25 ounces for a Colson.

Disadvantages:

• The wheel is molded snap-together hollow plastic that's very flimsy compared to the Colsons.
• Matching the 5/32" wheel center bore to the 3/8" shaft on your gearbox will require machining for the hubs.
• The foam rubber tires don't give as much grip as the Colson rubber.

I'm guessing that your hobbyweight doesn't have much in the wheel protection department. I don't see any insect wheel standing up to hobbyweight punishment unprotected. I'd find someplace else to save the weight.

Q: My wheels would extend only a .25" from the top and bottom.

A: The protection sounds great, but there may be another problem. The Lite Flites are pretty 'squishy'. With a few pounds of weight on them, your .25" clearance might squish down to almost nothing. You can always go to the 2.75" size if you need to. Keep a few spares in your toolkit in case things get rough.

Q: What about HPI Geolander R/C wheels? Foam-filled pneuamatic tires, 4"x2.1", only 90 grams each including foam insert, and they appear to have good traction qualities for $17 a pair.

A: Those tires are a fairly common offroad R/C item. You can match them to a variety of R/C wheels. Drawback: 90 grams for the tire plus 30 grams for the wheel ($12.95 a pair) equals 120 grams -- more than 4 ounces. The 2.5" Colsons you rejected as too heavy only weigh 73 grams, have great traction, and are combat proven. Colsons are popular for a reason -- use them.

---

Q: In your gyro article, you say:

Look for a heading hold gyro that has the ability to adjust the sensitivity or 'gain' of the gyro via a spare channel on your radio. If your 'bot gets inverted, the gyro correction signal is reversed - shutting off the gyro will avoid a 'death spin'.

I plan on using 2 IFI victors with the Robot logic IMX-1 mixer, and that mixer has the ability to adjust when inverted. If I use the invert feature on the Robot logic mixer, will that avoid the 'death spin', and what do you mean 'death spin'? Thanks.

A: Mark J. here: when your invertible tank steer 'bot flips over, the forward/backward response to your control inputs is reversed -- a forward command will back the 'bot up and vice-versa. However, the turning response is not reversed -- a left turn command still turns the front of the 'bot to the left because both the direction of the wheel rotation and the side of the robot the wheel is on have reversed and the two reversals cancel each other out.

Unfortunately, an upside-down gyro reads left as right and right as left. When your 'bot drifts a little left, the inverted gyro reads that as drifting right and dials in some left steering. This causes more left drift, to which the gyro responds by dialing in even more left steering. It's a 'positive feedback loop' that sends the 'bot into an uncontrollable 'death spin'.

A mixer with an invert feature will correct the forward/reverse response issue on an inverted robot by reversing the interpretation of the throttle input, but that won't avoid the death spin. The gyro itself must be shut down. Note that this isn't a problem in the aircraft that the gyros were designed to work with because they do reverse directional control when inverted.

I've added an explanation to the gyro article to make it a little clearer.

---

Q: Should I use 7075, 6061, or 2024 aluminum for my armor? It will have an outer layer of Lexan on it for shock absorption, as well as being bolted to my bot using shockmounts. That should stop those spinners!

A: Mark J. here: Multi-layer armor like you suggest is tricky and often less effective than single-layer armor of the same weight. Lexan is effective at energy absorption only if it is allowed to flex. By backing Lexan with rigid aluminum you will greatly reduce the desirable energy absorbing properties and impact will be transmitted directly to your relatively thin underlayer. You can vary the hardness of armor in layers, but the layer stiffness should remain fairly constant to allow the load to be spread evenly. Search the Ask Aaron archive for 'composite armor' for more comments.

I'm not a big fan of aluminum armor, but T6 tempered 7075 is generally the best aluminum alloy for that purpose. It's harder and has higher shear and yield strength than 2024 or 6061. It's price is comparable to 2024, and about twice that of 6061. Note that both 7075 and 2024 are very difficult to weld.

---

Q: Another question about the scoop for my middleweight spinner killer. Steel is to heavy, and I just can't afford titanium -- it would be thousands of dollars for the scoop alone. So what about half-inch thick 6061 aluminum?

A: Yes, titanium is expensive -- but thousands of dollars?? Check with Titanium Joe. Aluminum is not a good match against the tool-steel teeth of a serious spinner. With scoring based so heavily on visual damage, you can't afford big gashes in your scoop. I'd rather see you go with a thinner, hardened or tempered steel scoop reinforced across the back as needed.

---

Q: When they say titanium has an ultimate tensile strength of 150,000, how thick of a piece of titanium is that measured on? Obviously 1/8" thick titanium isn't as strong as 1" thick titanium.

A: Mark J. here: Tensile strength is measured by placing an increasing linear pulling force on a material sample until it breaks. The unit of measure is 'pounds per square inch', and is calculated by dividing the maximum resistance provided by the material sample by the cross-sectional area of the sample in square inches.

Tensile strength is not a very useful measure in evaluating the suitability of a material for most combat robot uses. More interesting for applications such as armor are measures of impact resistance, toughness, elasticity, and hardness.

---

Q: Hi Aaron: I am building a small robot. I plan to purchase the Sabertooth 2x10rc ESC with built-in mixing for the two drive motors. I know the bot will move forward and reverse in a straight line and turn left and right, but will the 'bot turn left and right in reverse?

Thank you for your answer.

A: All variable-speed dual-channel robot motor controllers on a robot with differential steering will allow you to:

• move straight forward and backward;
• rotate in place clockwise and counterclockwise;
• move forward while turning left or right; and
• move backward while turning left or right.

Some dual-channel ESCs (Sabertooth included) have a plug for a third control channel that allows you to reverse the direction of the steering response if the 'bot is flipped. Very handy in an invertible 'bot.

---

Q: How do I attach 1/8" polycarbonate to 1/8" aluminum for armor?

A: Polycarbonate and aluminum can be bonded with a 'Goop' type adhesive, but mixing materials to make a composite armor is tricky. You're trying to get the best properties of both materials, but it's just as easy to end up with the worst properties of each! I don't recommend it.

Q: Would 1/8" polycarbonate plus 1/8" aluminum be adequate heavyweight armor?

A: Heavens, no! You'll need much thicker armor for a heavyweight. Current design favors at least 3/4" high-strength alloy aluminum or 1/2" titanium for protection from heavyweight spinners. Quarter-inch plastic or aluminum is purely sub-light class.

Q: Titanium is expensive! How thick should polycarbonate armor be for a heavyweight robot?

A: Mark J. here: good armor is less expensive than having your opponent punch thru your poor armor, destroy your 'bot, and blow you out of a tournament. Heavyweight 'bots are expensive. If there was a less expensive alternative to exotic metal armor, builders would be using it. The current field of heavyweight spinner 'bots are brutal!

No matter how thick you make the plastic, polycarbonate is not a direct replacement for titanium. They have different strengths and weaknesses. Take a look at the Team Stupid materials page (archived) for a listing of suggested uses for different materials and alloys.

---

Q: How do I trim sintered bronze bushings? When I trim them they don't seem to fit right. What tools should I use to trim bushings?

A: I'm not sure what you mean by 'trim'. If you're reducing the outer diameter of the bushing, it should ideally be done on a small lathe. Small adjustments may be made with a jeweler's file. Bushings should be a snug fit -- they should not rotate in their mounting.

---

Q: What is 'spring steel'?

A: There are a lot of different types of steel. The properties of the metal vary widely according to the elements alloyed into the mixture, the process that is used to form the steel, and various heat-treating processes that may be applied to the metal after it is formed. A 'high carbon' steel that has been specially heat tempered takes on properties that allow it to bend and 'spring back' rather than permanently deform is called 'spring steel'. The process often leaves the surface of the steel darkened with a blue tint. Spring steel is more resilient than structural steel, but it is not as tough -- it will break rather than dent.

---

Q: I want to glue 1/16 CF on to 3/16 UHMW for armor. What glue should I use?

A: Mark J. here: Let me pose another question first -- Is it a good idea to combine Carbon Fiber and UHMW Polyethylene to make composite armor?

Consider the properties of the two materials: carbon fiber is extremely stiff with a fairly hard surface, while UHMW is soft and so pliable that it is considered 'unbreakable'. If the proposed composite was placed under stress, all the force would be placed on the stiff CF with the UHMW simply bending out of the way with little resistance. Only when the CF fails would the properties of the UHMW become useful. Bad combination!

Back to your original question: if you decide to do this anyway, what glue should you use? Polyethylene is almost as slick as Teflon and very difficult to bond. Epoxy, silicone, and cyanoacrylate adhesives won't work. The surface has to be chemically modified to allow an adhesive the chance to get a grip! 3M makes a couple of two-part adhesives that will do this: Scotch-Weld Structural Plastic Adhesive DP8005 and DP8010. They are difficult to find, expensive, and tricky to work with. You can view the product data sheet for DP8005 here: DP8005.pdf.

If you really want to bond CF to something for effective composite armor, try Garolite and an epoxy adhesive. UHMW, like polycarbonate, needs to flex to absorb energy and be useful as armor. Adding a stiff CF layer doesn't make good sense.

Q: My armor is also the chassis side plates. I need to tap holes into the side plates to mount the top and bottom chassis plates. Garolite (like CF) doesn't hold edge taps well. Can I glue CF to Lexan?

A: Lexan is much easier to glue than UHMW polyethylene, but you'll still have the problem of the Lexan flexing a lot more than the CF. You can use a gel cyanoacrylate 'Super Glue' to bond CF to Lexan, but I think I'd use 'Goop' adhesive or silicone sealant for a little 'give' with a countersunk screw thru the CF into the Lexan in each corner for a little mechanical insurance.

---

Next Mystery Post »

Q: There are so many steel alloys that I don't know which one would fit my purpose! Do you have some good links about steel alloys?

A: Mark J. here: there are a HUGE number of specialized steel alloys, and several different standards for referring to them. There are entire college courses devoted to steel alloys and library shelved filled with books on the topic. Add to this the various heat treatments, casting and milling options available and you're quickly in deep water!

To narrow your search I suggest that you first determine which alloys are available from your supply sources and research their properties. It won't do you much good to find an alloy you want that you simply cannot get in the shapes or quantities you need!

• A good introductory article on steel and aluminum alloy designations: ESPRIT Mechanical Design.

• An overview of the different designation types for carbon steel: Key to Steel.

• General descriptions of commonly available stainless steel alloys: eMachineShop - Stainless Steel.

• ASTM, ISO and IEC Test Methods: IDES property descriptions.

• Physical and mechanical properties of specific metal alloys and plastics: MatWeb Material Search.

That should get you started. If you get bogged down, look at the materials used by successful 'bot builders in similar applications.

---

Q: What are tensile, impact, and yield strength? Which of these properties should I look for in robot armor?

A: Mark J. here: there are a lot of different measures of material strength, some of which are overlapping. Each measure may have several different test procedures that can be difficult to directly compare. Some useful material physical properties to examine include:

• Tensile Strength is a measure of how much pulling force a material can withstand. It is commonly measured as the peak pulling force a material can withstand before it begins to fail (ultimate tensile strength).

• Yield Strength is a measure of a specific attribute of tensile strength: the amount of pulling force required to permanently deform the material. Materials with low yield strengths tend to `smear' when hit with a high energy weapon.

• Impact Strength is a measure of the amount of energy a material absorbs when breaking by impact. This is also known as 'toughness'. Some materials that are very strong are also brittle and can shatter under sharp impact. These make poor armor!

• Modulus of Elasticity is a measure of material stiffness -- now much the material flexes in response to a specific amount of force. Materials with a higher elastic modulus flex less than materials with lower values.

• Hardness is a measure of the resistance of a material to surface penetration. Hard materials do not scratch easily, but may be more brittle than softer material.

• Density is a measure of the weight per unit volume of the material. Tests of material strength are conducted on samples of a specific size, not of a specific weight. A one inch diameter rod of 4130 steel may have twice the tensile strength of a one inch diameter rod of 6061 T6 aluminum, but it weighs three times as much! With a combat robot, you're generally looking for strength per unit weight.

Which of these properties are important depends on your armor design and what you expect your armor to do. Does your design allow the armor to flex and deform, or must the armor remain rigid and retain its shape? Is it important that your armor resist scratching and any sign of damage, or are you willing to sacrifice the armor to protect the internal components of the `bot?

Expense also enters into the equation. Polycarbonate (Lexan) and UHMW polyethylene plastics are incredibly tough and light materials that are easy to work with and quite inexpensive. However, they are soft, flexible, and show surface damage after a tough match. Top of the line 6AL-4V titanium alloy is very hard, impact resistant, and has a high yield strength. It rarely shows even a scratch after a match, but does give off bright sparks when hit with a rotary weapon. It's also expensive: about $80 for a square foot of 1/8" material. Be sure to have a look at our Armor Guide for more ideas.

The www.matweb.com site has information on the physical properties of a wide range of metals, plastics, and related materials.

---

Q: Can old VCR parts be used in a 'bot?

A: I really like recycling useable parts to make something new. You can also learn a few things about mechanical engineering by taking apart an old VCR. You probably won't find anything very useful in the way of drivetrain or electronic components but there are plenty of gears, belts, pulleys, and maybe even motors that could be used for a weapon system on an insect-class 'bot. Be aware that some of the motors in a VCR are likely brushless motors that require special electronic driver circuits to operate.

One problem with recycled parts is replacement spares. Unless you have another identical scrap VCR, it may be difficult and expensive to replace a recycled component damaged in combat.

---

Q: My lipoly seems to be bulging with air under the shrink wrap. Will that affect the performance of the battery? What should I do to maintain my Li-Poly battery?

A: Mark J. here: Your Li-Poly battery is toast! The usual cause of this 'outgassing' is overheating brought on by too high a discharge rate -- you're asking for more amps from the LiPoly than it can provide. Replace it with a larger capacity battery. It's also possible that your charger is providing the wrong charge rate. Never charge Lithium cells with a charger not designed for the purpose.

Whatever the cause, *** IT IS NOT SAFE TO USE THE BATTERY ***. Do not attempt to charge or discharge the battery. Drop the battery into a plastic bucket with 1/2 cup of salt dissolved in a gallon of water, cover the bucket, and let it set for at least two weeks. It will then be safe to toss out with the garbage.

Li-Polys are not nearly as robust as NiCad or NiMHd cells, and can be dangerous if abused. Electrifly.com has a good article on Li-Poly care.

---

Q: I've heard that silicone increases the traction of Lite-Flite tires. Where can I get silicone and how is it applied?

A: Mark J. here: A thin layer of silicone rubber does improve the traction of many types of tires and treads. Pure silicone rubber can be found at auto supply stores. It's used for making automotive gaskets ('RTV silicone gasket maker'). Pure silicone kitchen and bathroom caulking from a hardware store is also OK, but do not try to substitute similar products like 'Goop' or 'siliconized latex caulking'. Silicone rubber comes in assorted colors, but they are all about the same for grip.

The trick to application is in getting a good bond so the silicone rubber doesn't peel off. Clean the surface of the tire very thoroughly with rubbing alcohol and let dry. Uncured silicone rubber is sticky and messy to work with, so spread out some newspaper and have paper towels handy. Use a popsicle stick or old knife to spread a very thin layer of the gooey silicone evenly onto the tire surface. Apply enough pressure to get the rubber down into the tire pores, Wipe excess off the edges with a towel. Let 'cure' for at least a full day before use. Curing RTV silicone smells like vinegar.

Clean silicone grips better than dirty silicone, so clean your tires before each match. I use a little lighter fluid on a towel, but be very careful with any flammable liquid!

Remember: better traction means less wheel spin, and less wheel spin means higher amperage consumption by the motors. Make sure your speed controllers can take the extra load.

---

Q: I heard that magnesium is twice as strong as aluminum and only a quarter the weight. Have people used Mg in combat robots? If so, where is the best place to get some?

A: Mark J. here: Judging the relative strengths of metals is difficult and sometimes misleading. The tensile and yield strengths of magnesium are more than twice as great as common aluminum alloys, but the shear modulus is four times less, which means that magnesium is brittle; it tends to break rather than bend. This restricts its use in combat robots to internal braces and mounts that are not exposed to high impact. It would be really embarrassing to have your armor shatter and fall off!

Also, magnesium is not that light! Common alloys are about 67% the density of aluminum. You'll save a little weight, but it may not be worth the trouble for small parts.

One last thing - magnesium is flammable! If exposed to a high-energy weapon impact it could ignite, and water or CO2 fire extinguishers won't put it out! Your event organizer wouldn't like that.

I know that Biohazard uses some magnesium in its internal components, but in general people use more common materials. My metal suppliers don't stock magnesium, so you're on your own for a source.

---

Q: Can I hacksaw 1/16 in thick carbon fiber?. Right now, I don't have access to any other equipment. I just need small rectangular parts for my antweight. I know its bad to breathe in the CF dust, so should I wear some sort of mask?

Thank you.

A: Mark J. here: The preferred method of cutting carbon fiber is an abrasive cutting disk, but you can use a fine-toothed hacksaw.

Safety precautions when cutting or sanding carbon fiber include a dust mask (always!), safety goggles (always!), and gloves (always!). CF dust is very irritating, and jamming a loose carbon fiber into your flesh is painful, easy to do, and difficult to remove. Don't take shortcuts!

---

Q: What is 'unobtainium'?

A: Unobtainium:

1. an item so expensive, rare, or otherwise difficult to get that it is effectively unobtainable.

2. a reference to a material with physical properties that do not exist, as in "I broke the titanium hub, so I'll build the next one out of unobtainium to make it stronger and lighter."

---

Next Mystery Post »

Q: How can I make custom gears to fit a tiny motor so that I can create a 6-wheeler tread tiny robot?

A: Making custom gears requires machining skills and special equipment. It's much better to find gears that will work, but that can be difficult as well. Try a local hobby shop, or sift thru a surplus supply store. Old toys can yield valuable parts, too. Try a web search for 'slot car gears'.

---

Q: I've seen a white powder spill out of damaged drum weapons. What is it? What does it do?

A: Mark J. here: I did some research. A free-flowing powder placed inside a hollow cylinder can improve rotating balance. There's a product available for balancing automotive tires called 'Equal' that might be what you saw. Drum weapons are difficult to build precisely enough to be well balanced and can use all the help they can get!

---

Q: Where do builders buy flamethrowers?

A: They don't buy them, they build them. See the post on flamethrowers. They're ineffective and dangerous -- not for novice builders!

---

Q: What is the least expensive brand of drill? Is there a place that sells cheap 2 channel speed controllers for a 12 pound bot?

A: Mark J. here: One last time: 'Cheap' and 'Robot Combat' don't mix! If you're going to spend your time and effort to design and construct a robot and take it to to a competition, you don't want some crappy component to fail and flush the whole project.

---

Q: I saw a ant made with 'Delrin'. What is Delrin? Compared to Lexan, which is better?

A: Mark J. here: Delrin is an Acetal plastic developed more than 50 years ago by Dupont. It is a little heavier and stiffer than Lexan polycarbonate, but it is not nearly as impact resistant. Delrin makes good machined components (gears, bolts, actuators), but polycarbonate makes better armor.

---

Q: Are LiPoly batteries a good choice for a hobbyweight? What mAH capacity range do I need? Is it a good idea to have separate batteries for each ESC?

A: LiPoly batteries are a good choice for any weight class -- if you can afford them and can maintain them properly. They provide a lot of power for their weight, but they do have limits on their safe output amperage. Be sure to allow for that in your calculations.

Take a look at my dad's previous post about calculating battery capacity. For a starting point, I'd try about 2000 mAH for a hobbyweight with an active weapon.

Most builders use a single large capacity battery rather than multiple smaller batteries for each ESC. Multiple dedicated batteries could leave you with no drive power but plenty of weapon power still available. The single battery will also be better able to survive large amperage surges.

---

Q: Have you heard of a 'bot named 'Cobra'? It's a small (12 or 30 pound) wedge bot that's really fast. Where do I get motors like they use for under $30 apiece?

A: I don't know 'Cobra'. The official name database at BotRank.com does not show a 'bot named 'Cobra' in any weight class. Sorry.

Most hobbyweight (12 pound) and featherweight (30 pound) 'bots use cordless drill motors and gearboxes for their drive train. Top of the line is the DeWalt 18 volt drill motor and transmission, but a DeWalt with all drive components will run more than $150. Many builders in these classes use motors and gearboxes from less expensive, lower voltage drills. The 'Handiworks' 7.2 volt drill was once a favorite for this purpose, but they are no longer widely available. Look around your local discount tool store and you should be able to find something for $30.

You might want to reconsider your desire for 'really fast'. Robot combat is not a race and trying to drive a fast 'bot can be a real handful.

Q: Aaron, this is more of an answer than a question: in an earlier post someone asked for information about a 'bot named 'Cobra'. I have been privileged to see Cobra in action. It's a fast wedge bot from Sydney, Australia -- powered by modified DeWalt drills.

A: Thanks for the info! DeWalt has powered more top flight 'bots than I could possibly name here. Robot Marketplace offers a wide range of DeWalt motors and gearboxes, and Team Delta has the 18 volt DeWalt motors and gearboxes plus special combat mounts and driveshafts.

---

Q: Does scrap metal work well for light armor? [Chris Marnati]

A: 'Scrap metal' could be almost any type of material, Chris. Some could be great, some would be useless. I live in an area where I can get exotic scrap titanium from the aerospace industry that makes tremendous armor, but you may not be as lucky. Check my dad's Armor Guide to get some ideas.

---

Q: I don't know how to solder. Can I build a robot without soldering?

A: Soldering is a very good skill to develop for making all sorts of things. A soldered electrical connection is strong and unlikely to fail in combat. Some speed controllers do come with either pre-soldered or screw-type connectors, so you can build a 'bot without soldering. It would limit your choices of components.

Maybe you could find a team-mate who can solder?

---

Next Mystery Post »

Q: After shopping for Garolite at McMaster-Carr I realize there are many different kinds. Can you give me a general idea of which types are stronger or lighter?

A: Garolite is a branded composite fiber/resin material similar in properties but much less expensive than carbon fiber composite. Stick with the woven glass cloth varieties like G10 or G11 for general robotics construction for their high impact resistance. There's a good discussion of Garolite at the Spambutcher Robotics site.

---

Last - Back to Top »

Written in 2006

Q: What is the best material to use to armor my `bot? How thick should it be? Where can I get it cheap?

A: Mark J. here: "Best" depends on lots of factors. Various metals, plastics, and composites are all useful for different designs, applications, budgets, and builder skill levels. Commonly used materials include:

• Aluminum: Different alloys vary widely in strength. Don't use the stuff you can buy off the rack at the local home center - it is soft and weak. 6061-T6 alloy is nearly as strong as mild steel and is widely available thru metal dealers. Aircraft alloys like 2024-T3 and 7075-T6 are harder and nearly twice as strong, but more expensive and difficult to find. Aluminum armor has a tendency to `smear' and show damage.

• Steel: There is a huge array of steel types: hot rolled, cold rolled, tool, stainless, high carbon, low carbon, plain carbon, mild, nickel, invar, chromium, tempered, annealed, quenched, ferritic, pearlitic, martensitic, killed, capped, spring, and chrome moly - to name a few. If you want to use steel, do your homework and pick out something suitable. Mild steel (called `1018') and Chrome Moly (`4130') are commonly available. Other alloys offer greater strength for the same weight, but mild steel is cheap and you can find it in sheets, angles, and rods at your local hardware store or home center. See also other posts about steel alloys and terminology.

• Titanium: Top of the line `bot armor, titanium is nearly as light as aluminum and nearly as strong as alloy steel. It's also very expensive, tricky to work with, and makes pretty white sparks when hit hard with a rotary weapon. A common alloy used for `bots is 6AL-4V.

• Lexan: Also known as polycarbonate, Lexan is a clear, lightweight plastic with an enormous capacity to absorb impact and pop back to its original shape. Leave room for the material to flex when designing - fastening points are common failure sites. Lexan can also be chemically welded for very strong joints. You can find Lexan at your local glass shop or home center, but make sure you're getting polycarbonate and not clear acrylic plastic - acrylic is useless for `bot armor.

• Polyethylene: Ultra-high molecular weight (UHMW) polyethylene plastic is lightweight, cut resistant, unbreakable, and very easy to machine. It takes huge abuse and bounces back for more. Thin sheets are flexible, but thick plates can be used for motor mounts and main chassis components.

• Carbon Fiber: Amazingly stiff, strong and light -- but when it fails it comes all apart! Great for small `bot chassis plates and top armor. Expensive!

• Fiberglass: A blend of woven glass fiber and a resin media, there are lots of fiberglass formulations. A high performance type called Garolite is popular with some insect-class builders for chassis plates and armor. It is heavier, less strong, and less stiff than carbon fiber composite but much less expensive.

Thickness: The simple answer to "how thick" is, "as thick as you can make it and still make weight." The strength of your armor will depend on the material you're using, the support the armor has, the location of the armor, the angle of the armor, and the size and shape of the piece -- as well as thickness. In general, top armor can be fairly thin, chassis plates and side armor thicker, weapon/drive mounts and wedges thicker still.

Look at successful 'bots with designs similar to your own and use their experience as a guide to your own project. If calculations like this were easy, it wouldn't take four years to get an engineering degree. To quote Team Juggerbot, "Damage is weakness leaving the robot." If it fails, make it stronger next time.

Sources: Locally, I have specialty aerospace manufacturers with scrap outlets from which I can buy titanium at low prices. Check your local resources for unusual opportunities. For things you can't find locally, The Robot Marketplace offers a wide variety of metals, plastics, and composites in small quantities for `bot builders.

Remember, you don't have to use the most exotic materials to make a winning `bot. I've built two champion robots with wood armor!

NOTE: This guide was written in 2006 -- today's combat robot weaponry is much more destructive. Pre-hardened abrasion-resistant steel alloys (AR-400, AR-500, Hardox...) are now in common use in all weight classes. As it says above, look at 'bots with designs similar to your own and use their experience as a guide.

---

Date marker: February 2006

---

Q: Where can I find an inexpensive servo? [Garret B.]

A: OK, everyone say it at once: EBay! You should be able to buy standard servos for $10 or less. Your local hobby shop may cut you a deal on some and save you shipping costs.

---

Q: Where can I find strong tape that sticks to plastic? I need something better than duct tape.

A: Clear plastic 'Package Tape' is available pretty much anywhere. It might be good enough for your purpose.

If you really want the good stuff, 3M makes a specialty tape that's super strong, very flexible, and so sticky it will even bond to polyethylene. I use it to hinge the side skirts on my beetleweight. It's called 3M #396 Super Bond Film Tape. It's hard to find and it isn't cheap.

---

Q: Where can I buy polycarbonate plastic (Lexan) for 'bot armor?

A: Check with local sources that sell window glass. They often sell polycarbonate as burglar-proof window material. Your local home center may also have polycarbonate, but make sure it's the real stuff -- they may also sell clear acrylic plastic that is not nearly as strong. Check the yellow pages under 'Plastic' as well.

If you can't find what you need locally, www.robotmarketplace.com sells a variety of materials suitable for 'bot armor (Lexan. carbon fiber, titanium). Check their Metals & Materials section.

---

Q: My bot design has tank threads, but I don't know where to get them. Can you help me?

A: You don't mention how large your robot is going to be. For smaller 'bots, take a look at the 'Rubber Treads' section at www.classictintoy.com. For a medium sized 'bot automotive V-belts or timing belts might do. Big 'bots can use treads from light equipment like snow blowers.

Remember that treads are heavier than wheels, often fail in battle, and are easy to attack. They're also expensive.

---

Q: Were do you get your parts? (Anon)

A: We get parts from many sources: our local hobby shop, pieces scavenged from toys, stuff we make ourselves, and web stores. Places you might try:

Team Delta
Microbotparts.com (no longer in business)
Sozbots (no longer in business)
Robot Marketplace

---

Copyright 2006, 2007, 2008, 2009, 2010 by Mark Joerger -- all rights reserved.