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Congratulations on breaking through 6000 questions, the community warmly thanks you from here in western Pennsylvania. [New Castle, PA]
Q: Why do saws have less bite than discs with less teeth? And why do hyperspeed weapons not get bite? Yes I did read the spinner weapon FAQ. It said the reason was longer insertion time. Over speed weapons should do the same thing as mid-speed weapons except faster. Bots like Saifu, Weta, Algos, and DDT all use high speed weapons and they have no problems. [Arlington, Virginia]
A: [Mark J.] I believe you misunderstand what is ment by weapon 'bite'. A faster weapon, or one with more teeth, allows less time for the opponent to be inserted into the arc of the spinning weapon before the next tooth rotates around to strike. The Spinner Weapon FAQ points readers to Section 6.3 in the RioBotz Combat Tutorial for an explanation of the relationship between weapon speed and bite, as well as the formulas for calculating bite depth. If the explanation in the FAQ is unclear to you, I suggest reading the RioBotz tutorial.
The robots you list do have effective weapons - it is possible to have an effective spinner that has poor bite if you compensate with other design elements.
Examples:
Q: This is the bite guy and I have a question. Does bite apply to hammers? A: 'Bite' as decribed here applies only to continuously rotating 'spinner' weapons. The concept of 'bite' does not apply to 'strike on demand' axe or hammer weapons. Q: Hello, Mark! Back with some questions: this time, these questions are regarding a heavyweight build. The robot would be heavily based off of Electric Boogaloo:
A: [Mark J.] Having conquered the lower weight classes you have decided to move up to the heavyweight class?
Your questions are well thought out and nicely presented. I'm not taking this position to be cruel, I'm attempting to do you a favor by giving you the best advice I can provide.
A: [Mark J.] The group owner/admin didn't like my comment to a post from a builder who was looking for info on his idea to build an updated 'Buddy Lee' robot: a manequin in a ride-on toy -- now improved with a flamethrower in its mouth. I believe you were one of the commentors on that post.
I made three points [from memory -- I cannot access the actual post]:
The moderator and I have had similar run-ins previously, and this was the second personally derisive reply he had made to me that day. I will not request reinstatement. Combat robot builders who want my opinion or advice know where to find me.
Q: I have a custom pulley i wanted to press fit to a 5mm shaft, the bore came back from the cnc shop at 5.3mm. How do i fit it without getting another cut? i've heard of loctite and shimming? [Ile-de-France]
A: [Mark J.] First, don't use that machine shop again. A just-passable machine shop will hit a critical bore to ± 0.03mm, and your guys are off by ten times that much. I'd reject it and ask them to try again, but that may not be an option for you. Solutions:
Q: I would like to build a beetleweight version of Razer, and I was wondering if I could use a high torque motor for the crusher? Or do I have to use hydraulics? If I have to use hydraulics could you please tell me how to use hydraulics? [Livonia, MI]
A: [Mark J.] There are several posts about insect crushers in the Ants, Beetles and Fairies archive. Search there for 'crusher'.
A quick summary of these posts:
Q: My antweight is a direct drive drum. When I tested it,
A: [Mark J.] The more info you can provide about your 'bot, the better chance I have to give a direct and useful answer. Given the symptoms, I'm gonna guess that the motor is a brushless outrunner, since brushless is 'in' and an 'inrunner' style motor would be a horrible mistake for this purpose.
Possible cause #1
Brushless motors rely on the brushless ESC to provide power to the correct motor windings at precisely the correct time. With the non-sensored brushless motors commonly used in insect-class combat robots, the ESC is not given data on the position of the rotor relative to the motor coils -- it has to take a guess based on small changes to the electrical properties of the motor as it rotates. At slow speeds and heavy loading, the ESC may guess incorrectly and send power to the motor windings at the wrong times. What you call 'sputtering' is in this case more correctly called 'cogging'.
I suspect that the ESC you have chosen for your weapon is a poor match to the motor. There are so many different motors and ESCs that it is very difficult to track their compatability. I suggest that you join the Combat Robotics group on Facebook and tell them:
Possible cause #2 It's also entirely possible that your battery is not fully charged and/or does not have enough capacity to deliver full voltage under the heavy load of your unspecified weapon motor starting up from a full stop. The drop in voltage can 'brown out' your electronics and cause a sputtering of the type you describe. LiPoly batteries are shipped with only a partial charge, so before you go off and ask the Facebook group for help give your battery a FULL CHARGE and try another test. Five Days Later... I haven't heard back from Arlington, nor have I seen any posts on this topic in the forums. I'm guessing that a battery charge did the trick. Q: For a heavyweight horizontal spinner weapon system, is the perm PMG 132 a better choice than the E-tek Motoenergy ME0909 PMDC motor? [Portsmouth, Ohio] A: [Mark J.] The preferred brushed heavyweight weapon motor - for a balance of power and durability - is the Motoenergy ME0708.
Q: Here is a hypothetical question. If you used a 40 volt battery to drive a 20 volt maximum motor how would you get the motor to run without it breaking? I hope this does not lead to the hamburger is bad. [Portsmouth, Ohio]
A: [Mark J.] A couple of more-or-less practical methods:
Disadvantage - some of the individual cells will be drained more than others which may unbalance charging. The whole pack must have excess capacity for this to work.
Disadvantage - it is VERY hard on the ESC to spend equal time flowing and restricting current flow under all motor load conditions. You may blow the ESC, but it will save the motor. Make sure the ESC can handle the full 40 volts.
Q: Hello! I was wondering if you could help me out. I'm a bit new to the whole Robot Combat thing, with hopes of making some amazing machines (hopefully soon!). First, I would like to know what some of the cheapest, but EFFICIENT, motors, batteries, power supplies, tires, etc, are for an antweight. I'm hoping to have my first ant to be a drum spinner, but that'll have to depend on how much money I have. [Champaign, Illinois]
I'm not willing to provide a shopping list -- see Frequently Asked Questions #4 and #16. Q: Second, I'd like to know if a particular metal would work or not for an antweight: it was a thin panel from the school's old fountain, but I managed to save it from being scrapped. I don't know exactly what it is, but I can provide you with a pic if need be. A: I don't do psychic metal identification. There are hundreds of metal alloys and useful/useless ones look the same in photos. Common metal identification methods. Q: Third, what kinds of printers can be used to make 3D printed robots? I know a lot of people in the antweight category 3D print, but I didn't know if my school's 3D printer would be able to print parts off: I'm assuming the 3D printers used by roboteers either prints things off in HDPE or UHMW, which is why I was asking if you knew any printers that would be best for this.
A: [Mark J.] Robot parts are rarely printed in HDPE or UHMW. I suggest that you join the Facebook 'Combat Robotics' group.
Q: Also, while on the topic of plastic, if one were to make a 250-pound robot with 100% plastic armor, how thick must said person make it so that it can be as strong as the armor of, say, Minotaur's, and is it really worth the time, money, and weight? A: By the time you are ready to build a heavyweight robot you will know the answers to those questions. See Frequently Asked Questions #17. Q: ALSO also, and this is off topic, do you happen to know where I can find a picture of two of 'The Bishop' from the Robot Wars that was unaired in 1997?
A: No photos. A web search for -- "Robot Wars" "The Bishop" -- will turn up a very poor video of the only match Stephen Gaudio ever won with the 'bot. I believe the robot was taken from the event and thrown off the Oakland Bay bridge. See Frequently Asked Questions #32.
Note - none of the US Robot Wars (1994 to 1997) were 'aired'.
Q: Alright, I have a pic of the metal (and also a pic of another part of the fountain I found as well!) I asked about in my previous question. The one on the left was the one originally in question. I was unable to do any of the main tests in the helpful link provided (due to the lack of tools with my only tools being a soldering iron and some Allen Wrenches), but I was able to find out the following:
A: I understand how enthusiasm leads new builders to plunge past multiple prompts in the 'submit a question' process to 'Read the FAQ'. Don't worry -- if I didn't want to answer your questions I simply wouldn't do it. Nobody has a gun to my head.
About those metals: the link to the metal identification tests was there largely to convince you of the difficulty of the process. I guess you didn't beleive me when I said that I really can't identify metal from a picture. Although that is generally true, given the history of the pieces and their apparent use I can take a guess.
Summary - there are lighter, tougher, harder, and stronger materials available for combat robots. Your opponents will be built from them.
Q: What is your least favorite bot design? Okay, got it? Now the real question: how would you go about building something similar to it anyway without feeling bad the entire time, and trying to win some fights? [Buffalo, New York]
A: [Mark J.] I can't see all your input fields in the image you sent from the
Team Tentacle Torque Calculator
, but it's clear that you have not entered the correct values for the RS-395 motor. Let's start over with the correct values:
The 20:1 ratio P60 gearboxes with the 2.375" wheels give the best performance from the RS-395 motors in a moderate size arena: around 5.5 MPH, reaching that speed in about 6.5 feet. That's ample speed and power to push around a big spinner weapon. The 26:1 gearboxes would be a little easier on the motors, but the 20:1 should be fine.
The RS-395 motors run well between 12 and 15 volts -- you'll need four LiFe cells wired in series to provide 12.8 volts. The drivetrain will use about half of the capacity of the 1100 mah cells in a 5 minute match.
Your weapon motor is a powerful choice for a hobbyweight. It operates in the 22 to 30 volt range, so you will want a separate battery to operate your weapon. That battery will require between 7 and 9 LiFe cells wired in series. You have not given me enough information about your weapon to calculate the battery capacity (mah) needed. Heavier, longer blades will use more battery power to spin up, and the reduction ratio between your weapon motor and blade will also impact current usage. Example, courtesy of the Team Run Amok Excel Spinner Spreadsheet:
So, you'd need one four-cell LiFe battery to run your drivetrain and one 7 to 9 cell LiFe battery to power your weapon. Combined, they should run your 'bot in combat for about 10 minutes on a full charge.
Are you sure you wouldn't like to build a nice wedge robot instead?
Q: Thanks for the quick response. My weapon's current plan is a 5 pound 12 inch diameter 1/2 inch thick S7 steel asymmetrical "disc", shaped somewhat like a teardrop. The thickness is to avoid vertical spinners from breaking the blade. None of this is set in stone but the 12 inch diameter, and somewhat the weight.
My fights will only be 3 minutes long. The bot does strategically not need to be fast - in fact it should be as slow as reasonable to save weight and power consumption. The 26:1 gearboxes look best to me to that effect.
I hope to have as little power left over after a fight as reasonable, perhaps only enough to last 4 minutes in total. That being said, I still think I can squeeze 12 LiFe A123-18650 batteries into my bot, in 2 groups of 4 and (with slight design alterations) 2 groups of 2. But of course the fewer needed, the better.
Funny you should mention wedgebots, as I eventually hope to build a bot to take down Original Sin! Anyway, do you have any blade alteration or battery number suggestions? Thank you.
A: A 12" diameter, 1/2" thick steel disk weighs more than 16 pounds, so your 'teardrop' design must have a LOT of material cut away from the full disk. A simple steel blade 12" long, 1/2" thick, and 2.875" wide weighs 5 pounds. A 5-pound weapon is heavy for a hobbyweight, so be careful with your weight calculations.
The number of cells I'm suggesting for your batteries are not there for current capacity, but simply to obtain the voltage needed for your two motor systems running at differing voltages. You can't run your drive motors at 30 volts, and running your weapon motor at 15 volts would drop it's power output from 1000 watts to 250 watts. 'Tombstone' has this same problem and solves it the same way that I suggest you do: two battery packs with differing voltages.
I'm a bit concerned about the ability of the cells you have selected to provide the start-up amperage needed by your monster weapon motor. The motor is rated 40 amps continuous, but can briefly draw as much as 140 amps under starting load and will draw over 70 amps for as much as two seconds while staining to pull that heavy weapon up to speed. Given that your cells are rated for 30 amps continuous draw, pulling more than twice that current may damage the weapon battery. You may want to reconsider your LiFe cell choice. Q: Thanks for the advice. I'm happy to lock in 4 1100 mAh batteries (the yellow ones) for the drive power with 26:1 gearboxes, if you think that's not too many.
A: My earlier explanation was not sufficiently clear. A battery pack for a specific application must supply three things:
We have also calculated that the peak current draw of the drivetrain is about 10 amps, so your 30 amp continuous output cells also have that well covered.
That leaves voltage. To get adequate performance from your chosen drive motors you need a battery pack that will supply 12 to 15 volts. Each of your cells produces 3.2 volts:
Q: Now for the weapon power. Apparently the bigger A123s (green) are back in stock. They have a very slightly lower voltage (3.2V) but a higher amperage(50A), plus a maximum impulse discharge of 120A.
However, as I understand it, the same number (7-9) of these green batteries would be needed for the weapon motor's voltage as of the yellow batteries. Since the greens weigh more and are slightly bigger, this is less ideal. LiPos are not allowed at my upcoming event. Is there another battery you recommend instead?
Not using 100% of my motor's capability seems ok to me, as it might help prevent breakage (I think). Obviously I want to use as much as possible, but I'm okay with using less. Perhaps not filling up the voltage requirement completely but satisfying the corresponding Amp requirement would do the trick. The green A123s might be better at that. I currently have 1-1.5 pounds to spare for batteries, and the fewer cells I can reasonably use the better. What do you think?
A: Your problem is that your weapon motor is, as I mentioned before, a powerful choice for a hobbyweight. It requires both high voltage and a high peak current capacity. Having a big weapon motor and not running it at full voltage is a serious waste of power. Power varies with the square of voltage, so dropping a couple of cells cuts your power by almost half:
If you ask around the on-line forums, somone is going to suggest that you build a 9-cell battery pack to power the weapon and 'tap' the pack at 4-cells to power the drivetrain. This is possible, but I cannot recommend it. You'll be drawing more mower from some of the cells than others, and the draw from that big weapon motor may 'brown out' the drive and electronics. Proceed on that path at your own risk.
I think I'd go find a more reasonably sized weapon motor that would run well at four or five cells, downsize the weapon rotor, and run the whole bot off a single battery pack.
Q: what is the scale factor in combat robot? and what is its importance? [Al-Jizah, Egypt]
A: [Mark J.] There have been several questions about 'scale factor' and the 'square-cube law' recently. Take a look at this post from 'Urbana' in our Weapons archive and this post from Buzzards Bay in our Design archive.
Given this interest, I've summarized the earlier posts and added it to Frequently Asked Questions #17.
Q: Can you make a permanent page for those series of questions about crazy weapon designs? I think you'd be doing a great service to dissuade new builders from doing something too crazy. [Waltham, Massachusetts]
A: [Mark J.] I generally assume that builders writing in to 'Ask Aaron' are interested in combat robots that will win matches, but an increasing segment of builders are interested in impressing audiences and other builders with 'show-off' designs. While our focus remains on supporting simple and effective designs, I also recognize that one man's crazy is another man's awesome.
The recent long series' of questions about... unusual... weaponry and design has found a place in the Ask Aaron Design and Construction archive and I'll keep trying to set reasonable expectations for any design thrown at me.
Q: In the weapon FAQ or something it says discs are better than bars. Why do bots like tombstone or last rites use bars if discs with carvings such as in Nightmare or DDT are better?
A: [Mark J.] A spinning disk stores more energy at a given RPM than does a spinning bar of the same mass and swept diameter. This is because the disk has more of its mass located farther from the axis of rotation. The Ask Aaron Spinner Weapon FAQ features a comparison of the energy storage capacity of four spinning weapon designs with the same mass and swept diameter: bar, disk, eggbeater, and hollow drum.
Better energy storage is a good thing, but it is not the only factor to consider when designing a spinning weapon.
A: Who said anything about verticals storing more energy? You didn't find that here. The orientation of the spinner has no effect on energy storage. Q: Hello! I'm here this time on behalf of another builder. He is currently designing a full body, horizontal clampbot. You can check out his design process on his YouTube Channel, AltaPowerDog. I come here asking this question: What are your concerns or opinions regarding what Alta has made thus far? As with every response, your time is greatly appreciated! [Champaign, Illinois]
Q: I would like to chime in the horizontal clamper question. There IS a channel called AltaPowederDog (he won a competition with a cardboard bot).
Anyways,I came across a video of Japanese sumo and those bot are fast! Can you explain how they are so fast? [Perth, Australia]
A: [Mark J.] I've heard of 'Australian Rules Football', so I suppose there may be an 'Australian Rules Baseball' as well. Perhaps three strikes aren't enough in that league?
Here are a few more YouTube channels that do not exist:
If you're asking a favor of someone that requires them to reference a specific internet location, proper 'net etiquette calls for you to include a working link. Failing that, you should at very least make certain that you have spelled the obscure reference correctly so that the person from whom you are requesting said favor does not spend an inordinate amount of time searching the vast internet for something that is not there and who is then forced to back into the YouTube channel by Googling 'horizontal clamper robot video'. Yes, I eventually did find 'AltaPowderDog', but I will make no comment on the design for reasons expressed in 'Strike 1' above. Japanese Sumo: We have discussed the incredible quickness of autonomous sumo bots multiple times here at Ask Aaron. We've posted videos, we've discussed suitable motors, and we've calculated required downforce levels. You ignored the plea appearing directly above the box into which you typed your question:
I'm going to provide you with a correctly spelled and formatted link that will transport you directly to one of several archived Ask Aaron posts which discusses how sumo bots achieve their quickness. By clicking on the link you agree to be nice to some total stranger who asks you for something they could do on their own but who would rather inconvenience you. Here you go: Sumo Magnetic Downforce. P.S. - A cardboard bot? Big deal. Team Run Amok once won a competition with a yam, four nails, and a girl in a mouse suit -- but you'll need to buy me a beer to hear that story. Q: How do I reprogram brushless motor controllers to make them useable for brushless drive motors? Is it easy? [Space Coast, Florida] A: [Mark J.] Builder Robert Cowan has a nice video that covers the entire process of 'flashing' the reversing SimonK firmware onto brushless ESCs. I wouldn't call it either simple or easy: SimonK Firmware Flashing Tutorial. Q: Is there a rule of thumb for how much battery capacity to have for a spinning weapon? [Manassas, Virginia]
A: [Mark J.] Too many variables for a 'rule of thumb' -- how about a calculated solution? The Team Run Amok Spinner Excel Spreadsheet calculates the mass, moment of inertia, stored kinetic energy, tip speed, spin-up time, AND the approximate battery capacity requirement for your spinner weapon. You'll need Microsoft Excel to run the spreadsheet.
In general, the capacity needed for your weapon will be much less than the capacity needed for your drivetrain.
Q: Is there any advantage in hooking 2 brushless motors to a single spinning disk weapon? A: [Mark J.] Two motors = twice the power -- but because each brushless motor relies on it's controller for commutation as well as current, each motor requires its own ESC. It's generally preferable to use a single larger motor with twice the power. Q: Also, can I put a custom aluminium pulley around an outrunner brushless motor retained by a couple of set screws with plenty of loctite in a hobbyweight? [Quebec, Canada]
A: Set screws are best avoided for this purpose: they apply large point-pressure that may deform the rotor and are failure-prone even with a threadlocker. If you're making a custom pulley you should size the bore for an interference fit and press the rotor into place.
A word about threadlockers: some builders recommend applying Loctite to shafts and other sliding assemblies to secure them, but that's well outside the intended use of the standard blue or red Loctite. There are special formulations of green Loctite designed for holding loose-fitting components to shafts, but I can't recommend them for torque carrying applications in combat.
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A: [Mark J.] We all hope to make amazing machines, but 'first combat robot' and 'drum spinner' do not go together. You may be surprised to learn 
Q: And if vertical spinners supposedly generate more kinetic energy then why do people use horizontals? [Baltimore, Maryland]
