5508 Questions and Answers about Combat Robotics
from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for a question and answer page to document our responses.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
In Memorium: Aaron Joerger, 1991 - 2013
The 'Ask Aaron' project was important to Aaron, and I continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me. Aaron's obituary
- Mark Joerger   
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

Q: Is using four wheel drive for enhanced drive control good? Yes, more friction but less swing-around like I've seen. And would I use a long or short wheelbase? [Dearborn, Michigan]

A: [Mark J.] Worried about control versus traction? Start by reading the Run Amok Guide to Combat Robot Gyros.

With four-wheel drive a short wheelbase gives greater maneuverability and a longer wheelbase gives greater stability. I think an ideal balance comes with a wheelbase close to the wheel track -- a 'square' wheel arrangement.

Don't overlook a six-wheel configuration. If the center set of wheels are set just a fraction of an inch lower than the fronts and rears you get great maneuverability without sacrificing stability. Search the Ask Aaron Design Archive" for "six wheel" to find several discussions of this design option.

Q: Is making a counter-rotating saw blade a good idea at all? I did a little test on some aluminum with my dad's saw (counter-rotating). It didn't have any throwback, and it made a clean cut just touching it. Using a single blade, it had a LOT of throwback, made not very deep but nasty looking cuts, but wasn't good at cutting from the flat side. Any thoughts? [Dearborn, Michigan]

A: [Mark J.] I know that the new batch of 'BattleBots' are all complex examples of 'machine porn', but that's only because the audition process for the show demands that type of design. If you want to build a robot to win matches in open competition you need to keep the design simple and robust.

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

So, no counter-rotating saws. In fact, no saws at all. Effective use of a saw requires that you immobilize your opponent, and your opponent has other ideas about that. Keep it simple.

Q: I can't think of one, and there may not be one just because I haven't seen many bots with it; but is there any pros to having a non circular kinetic weapon? Like Tillah from Team Juggerbots' square drum? [Spring, Texas]

A: [Mark J.] Can't think of one, you say?
  • Bar spinners like 'Tombstone'?
  • Asymetric mini-discs like 'Witch Doctor'?
  • Cutaway discs like the 2015 version of 'Nightmare'?
  • Snail drums like 'Touro Maximus' (image right)?
  • Monotooth drum hybrids like 'HyperShock'?
Toro Maximus' snail drum

Plenty of examples, some of them requiring a great deal of engineering and construction effort to produce. As covered in the Ask Aaron Spinner FAQ the best energy storage does not come from these designs, but builders don't generally waste effort so it's a safe bet that there is an advantage to be gained. The advantage varies with the style of weapon:
  • Bar spinners are simple to construct, robust, and durable;
  • Asymetric mini-discs are single-pieced, durable, and can spin at high RPM while retaining 'bite';
  • Cutaway discs... Well, they're pretty;
  • Snail drums are durable and show you have design and serious machine skills; and
  • Monotooth drum hybrids have the advantages of asymetric mini-disks but can store more energy.

Q: Hello Mark, it's the Deadblow to beetleweight guy again. This time I was wondering if it would be better to use Loctite to hold the side panels to the top panel or, if it would be better to invest in a soldering iron to do it instead? Thanks-Luke. [De Funiak Springs, Florida? I thought you were in Alabama?]

A: [Mark J.] The 'Deadblow' Metal Mechanics model has four very small machine screws and nuts holding the top panel down to the side panels. A drop of Loctite on the threads should do very nicely. Loctite recommends their Purple #222 threadlocker for small screws like these, but I've used the more commonly available Blue #242 threadlocker on small screws with good results.

Q: I remember there was a recent question regarding brushless motors on 60lb+ robots and I found some info regarding that. In a recent video by Team Velocity it's shown that 'K2' uses brushless inrunner motors for its drive motors. [Chicago, Illinois]

A: [Mark J.] 'Recent' is right -- it was uploaded just three days ago. Kevin Barker has gone a different direction than other teams with his robot and weapon drivetrains. Most builders are comfortable using brushless motors for weapon drive and stick to brushed motors for the robot drivetrain. Keven sticks to a proven brushed AmpFlow motor for the spinner weapon but has patched together a brushless robot drive from new inrunner brushless motors and old gearboxes from his parts bin.

You can see the 'bot in action at the 2016 RoboGames in this Team Velocity playlist. This was the third event for the new brushless drive and Kevin says the brushless drive is working great but that driving feels different and "takes some getting used to."

Team Velocity lightweight combat robot 'K2'

Q: How can u self right using a 4 bar lifter? i saw this video on youtube and even after watching it i just cant figure it out..

P.S. i really love ur site, its the best out there and u gave me the inspiration the build bot thank u :) [Quebec, Canada]

A: [Mark J.] Thanks for the props, Quebec.

The video of antweight 'Pad Thai Doodle Ninja' self-righting is taken from an awful angle to actually see what's happening. I think you'll get a much better idea of the process by watching this video of 'BioHazard' self-righting. Getting a 4-bar lifter to flop back upright requires estensive pre-planning and a fair amount of tinkering. You'll notice small extension 'claws' on the back of PTDN's lifter that I'm sure were added to get the self-righting to work.

Charles Guan's 'Equals Zero' website has an archive for PTDN that includes the design requirements for getting a 4-bar to self-right:

Antweight combat robot 'Pad Thai Doodle Ninja'
"[Self-righting] is kind of tricky with 4-bar lifters. You really have to take into account the center of gravity of the bot, and the length and extension of the arm, in order to facilitate this. Generally, 4-bar lifter bots flop onto their backs and come to rest on the arm whenever it is then deployed, as the CG is too far forward, and no self-righting is possible. [The classic video of former Battlebots heavyweight Biohazard shows how a 4-bar can self right.]

Notice how [BioHazard's] center of gravity is far enough back that the bot hinges on its rear edge and does not come to rest on the arm. The arm’s retraction then keeps the CG within the line drawn between the arm’s contact point and the bot’s rear edge, and it gathers enough momentum to push back over. Making it able to do this meant making the arm extend all the way back across the bot. Notice also how Biohazard had a ‘tang’ at the very back of the arm, a part that sticks up – this aids in the maneuver by making the contact point with the ground further forward, so the ‘line’ is longer.

This goal meant that I was continually watching the bot’s center of gravity in autodesk Inventor, and also continually modifying the linkage to suit. The arm had to have a certain amount of extension to make sure the CG was in the right place, and that extension had to jive with everything else’s placement."

The full text, with some pictures, is way down at the bottom of the Equals Zero archive for Pad Thai Doodle Noodle. When you scroll down to the part of the archive where Charles is doing a brake job on his van you're about half-way there. Keep going. WAY DOWN. Seriously.
Q: can the same robot armed with a 4 bar lifter enter in the featherweight and the sportsman class as well? [Quebec, Canada]

A: [Mark J.] Technically yes -- but be aware that the sportsman and featherweight class fight schedules may overlap. That will have you scrambling in the pits to charge batteries and repair damage. Some event organizers don't allow dual-class enteries over conccern for delays in the fight schedule and/or the (very real) possibility that the 'bot won't survive the first class and take up a spot it can't use in the other class tournament tree. Check with the event organizer.

Q: Hi Mark,

First of all, I know. You heavily advise against ring-spinners. However, there is one that catches my eye due to what seems to be a pretty solid design. This upcoming season of ABC's Battlebots will feature The Ringmaster. A Single-tooth ring spinner with the weapon system being driven via gears rather than friction wheel. You can see all how it all works on their facebook page.

I'm heavily considering using a similar drive design in a lightweight ring-spinner that i'd like to build (not a ripoff, I have standards.)

Now, you've almost completely crushed my dreams of building a ring-spinner. But if I would be wasting my time by building one with this type of drive system and ring support, I would indeed like to know.

For knowledge-sake, lets assume that I built a lightweight clone of The Ringmaster. Good RPM, Quick Spin up, "perfectly" Counter-balanced weapon, and so on. What deal-breaking issues or concerns do you think I'd most likely run into?

Thank you sir, David R. (Livermore, CA)

Quick addendum

I know that I am speculating that The Ringmaster works well even though we haven't even seen it in action, and we probably won't get a good idea of that until the new season airs. In fact, just about ALL of my question involved speculation. So I understand that nothing from the scenario I gave you is completely concrete. So upon your response, I'll be sure to have a pinch of salt ready. Thanks :)

A: [Mark J.] 'Crusher of Dreams' -- maybe I should put that on my rèsumè...

Where to start...

  • First: run a Google search for Ringmaster's team leader 'Hal Rucker'.
  • Second: examine your budget and technical skillset to see if they match Hal's.
  • Third: no-spoilers, but check in on the historical success of ring spinners.
  • Fourth: consider the wisdom of the Team Run Amok Design Philosophy:
Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

Here's the problem with FBS in general: the impact vector is lateral, parallel to the floor, and uncomfortably close to the center of mass of the robot. You hit your opponent and newtonian physics throws you off in the opposite direction like a high-powered hockey puck. They are nearly as dangerous to themselves as to their opponents. A ring spinner has the advantage of being able to operate when inverted at the cost of much greater mechanical complexity. An extended center pole of the type used by lightweight champion FBS spinner 'Ziggo' is a much simpler solution to this problem.

If you want to build a ring spinner as an example of 'machine porn' go ahead and do it for that reason -- and yes, gear drive it. Just don't expect it to dominate.

Quick addendum

1999: people watch BattleBots, see some fratboys build a robot out of a beer keg, say to themselves 'I can do that!', hundreds of teams do just that and combat robotics as a popular hobby takes off.

2016: people watch BattleBots, see robots that appear to be built by NASA contractors under DARPA funding, say to themselves 'I can't do that!' and combat robotics as a popular hobby dies.

Just a theory.

'Ringmaster' detail

'Ringmaster' -- Not a Simple Tool.

'Ringmaster' detail

Q: Your team has built a machine with a drive system that uses a 36 tooth gear from the motor to a 12 tooth gear on the drive wheels. Why would your team choose this option? What problems could arise from this choice? [Broomfield, Colorado]

A: [Mark J.] Aaron wrote a haiku that covers this:

Robot haiku:

That's obviously
A question from your homework.
Do your own research.

The least you could do is re-word your assignment before you send it to me. Start your research here: Optimum Robot Drivetrain Gearing.

Q: Hello Aaron:
[pneumatic] cylinder has two holes: inlet and vent. As we know the speed of cylinder depend on air pressure and flow, So I got a idea: if we make cylinder have two "Inlet holes" the flow of the cylinder will be larger, so the cylinder speed will be faster? [Yunnan, China]

A: [Mark J.] A pneumatic system has multiple restriction points: the pressure tank valve, the pressure regulator, the solenoid control valves, and the actuator cylinder ports. Typically the speed of the system is limited by the performance of the regulator and valves. Gas flow at the actuator port is typically quite good; improving the flow there will not provide noticeably greater speed.

Q: A question regarding the original Battlebots: Was there ever a rumble where one bot was left standing? I know Toro came close in the Season 2 SHW rumble, failing only to KO War Machine, but were there any bots in any BB weight class who went 'all the way', so to speak?

On that note, do you think Toro could have been able to flip War Machine before the rumble ended? [Portland, Oregon]

'La Machine' combat robot
'La Machine'
A: [Mark J.] The only rumble with 'one bot left standing' I can recall was the middleweight melee at the 1995 US Robot Wars where 'La Machine' was the sole operational competitor remaining. 'La Machine' went on to win the heavyweight melee at that event, and upgraded versions of the 'bot won the 1996 and 1997 heavyweight melees -- a string of rumble victories never equalled.

I suggest you watch the BattleBots S2 super-heavyweight rumble and decide for yourself if 'Toro' had a reasonable shot at flipping 'War Machine'. From where I sit, Dan Danknick pwned 'Toro' the whole match with a wedge that was lower than the lip of the big flipper.

Q: Hello Mark, I was wondering if I bought a turnkey saifu from kit bots, what else do I need to complete it? Also, do I need to program it? Or would you even recommend it for an absolute beginner? I plan to build my own robot eventually but, I don't really have the supplies right now. Thanks-Luke [Huntsville, Alabama]

A: [Mark J.] The 'turnkey' version of the Kitbots 'Saifu' is fully assembled and ready to use. It is complete with battery, charger and 2.4Ghz radio. You'll need to figure out how to properly use the battery charger (the Ask Aaron LiPoly Battery FAQ can help you there) and familiarize yourself with the controls of the robot. You'll want to read the radio manual to learn about the adjustments available on the R/C transmitter to allow you to match the response of the robot to your liking, but no programming of the robot is required.

I don't know what instructions come with the 'Saifu', but if you get stuck you can always call on 'Ask Aaron' for help.

The price of the turnkey Saifu robot is $740 plus shipping (as of April, 2016). Less complete versions of the Saifu are available if you have the skills to complete the 'bot yourself. See the Kitbots website for options.

Have a look at the post right at the top of the Toy Hacks and Kits archive for a rundown of all the current antweight and beetleweight combat kits.

'Saifu' antewight robot by Kitbots

Q: Hello Mark, this time I have a question about paint. Is spray paint going to affect my internals if I use it without taking anything apart? If I accidentaly get paint on the LiPo battery will it ruin it? Also, will it affect my axle/wheel or weapon connection if that gets sprayed? The bot that I will have is the Turnkey Saifu antweight from Kitbots. Thanks-Luke

P.S I am sorry for your loss, I am also sorry for not saying this sooner. I know what it is like to lose someone you love and that it is typically better not to mention it.

A: Thank you for your note of condolence. I miss Aaron greatly, but I find comfort in continuing our project here at 'Ask Aaron'.

The Kitbots 'Saifu' is constructed largely of Ultra High Molectular Weight (UHMW) Polyethylene plastic which is remarkably tough, slick, and resillient. One thing it does NOT do well is hold paint. Standard spray paint 'puddles' on the surface and simply flakes off when dry.


There is special spray paint for plastic that will stick to UHMW fairly well, but wherever your opponent hits you the paint will scratch away and leave a brite mark that looks like real damage for the judges to see. I advise that you do not paint the polyethylene parts of your robot.

The weapon drum will also take a lot of very hard impacts that will quickly rip the paint from the surface. I'd suggest leaving it unpainted as well.

You'll need to learn how to take the robot apart to perform routine maintenance and to replace parts damaged in combat. If you want to add some paint to your Saifu you can remove the upper and lower aluminum plates by taking out a few screws. Spray paint the plates, let them dry, and re-attach with the screws.

About spray paint and robot internals: most parts won't be bothered by a little stray spray, but show your 'bot some respect and take the time to mask off things you don't want paint-spattered. In particular you don't want to get paint on/in bearings, electrical connections, and surfaces that get hot (motors, speed controllers).

Sumo lifter robot Q: Dear Aaron, I'm trying to build an 30lb, middle weight, or any weight that is not a heavy weight combat robot which looks very close to the robot in this image (not mine) and I would add a Biohazard type CO2 type flipper. Can you tell me the complete parts list with controllers and all that please? [Team Combat Robotics Malaysia - Selangor, Malaysia]

A: [Mark J.] I'm happy to answer specific questions about robot design, construction, and materials, but I'm not going to design your robot and spec the parts for you. See Frequently Asked Questions #4, #17, and #21.

Q: do u have a diagram on how is actuated an axe like on shunt? can u explain how the linkage works? thx [Quebec, Canada]

A: [Mark J.] 'Shunt' uses a complex pneumatic powered multi-bar linkage to provide a full 180 degrees of axe motion. You can see Shunt's designer explaining and demonstrating that linkage in this video.

The diagram at top right is from Chris Hannold's book "Combat Robot Weapons". It shows a much simpler overhead axe/hammer weapon linkage.

Additional linkage options and a full explanation of pneumatic systems can be found at Team Da Vinci: Understanding Pneumatics.

Q: hi axe guy again in the picture u put, is the hammer able to do a 180 degree rotation?

A: No -- see the small diagram at bottom right. When the axe retracts the axe bracket runs into the pneumatic actuator, and when the axe extends the actuator shaft runs into the axe pivot axle. It's only good for about 90 degrees.

You can play with the design and get a little bit more, but a simple single-pivot design like this is limited to an efficient swing of about 120 degrees. That's why Shunt's designer went to the complex linkage to get 180 degrees.

Q: axe guy again, do u have any diagram or image of the multi-bar linkage u were talking about?

A: The only images I have of Shunt's axe linkage are in the video link I provided above. I've sketched up an approximation of the linkage (far right) taken from the video. A couple of the links are fused, so this is functionally a 4-bar linkage.

Note: I cannot recommend that a novice builder attempt to duplicate the 'Shunt' linkage or the other full-range linkage diagrammed on the Da Vinci Pneumatics page. Overhead axe weaponry is not effective in modern robot combat.

Hammer/axe mechanism from 'Combat Robot Weapons'
Limits of pnumatic axe travel Shunt's 4-bar axe linkage Limits of pnumatic axe travel

Q: if i have 2 18v dewalt motor, does it mean i have to get a 36v battery to power them? [Quebec, Canada]

A: [Mark J.] No. Drive motors in combat robots are wired in parallel to the battery via the motor controller(s). A single 18 volt battery will supply 18 volts to each motor. See Frequently Asked Questions #19 for a diagram of a typical circuit diagram.

Note: The 18 volt DeWalt drill motors are often run at up to 24 volts for greater speed and power in combat robot drivetrains.

Li-Poly FAQ
The 'Ask Aaron' Lithium Polymer Battery FAQ
'Ask Aaron' gets a lot of questions about Lithium Polymer (Li-Poly) batteries -- enough to merit a special 'Frequently Asked Questions' page on the topic. I consider this to be a 'beta release' for the moment. What did I leave out? Comments welcomed.

Q: In your LiPoly Battery FAQ, I'd recommend adding a "how to charge" paragraph, highlighting the importance of charging via balance-charge, and discharging only to storage voltage (the latter's particularly important, because lithium ion batteries by contrast SHOULD be fully discharged from time to time) I would never have figured those things out if someone didn't tell me. Also, this video really helped explain how to use a standard charger. [Killeen, Texas]

A: [Mark J.] I appreciate your feedback on the LiPoly FAQ. Adding that charging video is a fine addition. I'll take this opportunity to clear up some misconceptions about LiPolys and Li-Ions:

  • LiPoly batteries sold on the hobby market are lithium ion batteries. They're packaged in a lighter, flexible polymer casing rather than the rigid case used in conventional lithium ion batteries.
  • Discharging any lithium ion cell below its minimum safe voltage will cause permanent and irreparable damage.
  • Lithium ion batteries in commercial products have protection circuitry built-in. The Li-Ion battery in your computer or phone that says it's 'completely discharged' has been shut down by the protection circuitry while the cells are still at a safe voltage.
  • LiPoly batteries sold in the hobby market are raw Li-Ion cells directly wired together without protection circuits. They are succeptable to being over-discharged if the circuit they are powering does not incorporate undervoltage protection.
  • Most speed controllers used in combat robots have a 'LiPoly protection' mode that shuts down the controller when the battery voltage drops to some specified level. Most combat robot builders will (correctly) turn this protection off to prevent the controller from shutting the 'bot down in combat.
Next time 'somebody tells you' about lithium batteries, check their sources.

Q:Thanks for clearing that up for me. I think you've still highlighted an important point worth mentioning: that "raw" batteries for robots need to be treated differently than that of your phone or laptop - it strikes me as an easy (and dangerous) mistake for new builders to make.

A: I think you're right. I'll add a note to the LiPoly FAQ. Thanks!

Q: is there a way for a featherweight lifter to be actuate by a motor with gear and chain, without any sprocket hanging out of the bot and without having it be a 4 bar lifter?
how could i make it to be like a rear hinged flipper? i dont want to have something like sewer snake... more like dantomkia.. but not gas operated
thx :) [Quebec, Canada]

A: [Mark J.] A long-armed rear-hinged featherweight lifter requires enormous torque to lift an opponent out at the far end of the arm. A short-armed featherweight lifter like 'Nyx' (pictured) can get away with a small sprocket on the lifter axle, but each time the length of the lifter arm doubles so also does the torque required to operate the lifter double. By the time the arm reaches all the way back to the rear of the robot the torque needed raises to gearbox-shattering levels unless a much larger sprocket is used on the lifter axle.

You can run the torque calculations to see for yourself how much torque is needed and shop around for a gearbox that can survive the load and provide the needed reduction ratio -- but you'll find that a suitable unit is heavy, bulky, and expensive. Four-bar mechanisms are popular for electric lifters because the torque requirement for a long lifter arm is greatly reduced. Perhaps you should reconsider your design.

Near Chaos Robotics' featherweight robot 'Nyx'

Q: Hello Mark, I have recently purchased a Deadblow Metal Mechanics chassis and I would like to turn it into a 4 wheeled axe robot. (Like the Judge) but without using CO2, what are my best options for electronics and wheels? I intend to compete in the antweight class but I have no equipment whatsoever and a low budget so could you help me? -Luke from Alabama. [from a server in Tennessee]

Deadblow Toy A: [Mark J.] You're not making it easy -- a toy chassis not intended for combat, no tools, a low budget, an ineffective overhead axe weapon, and I'm guessing that this is your first robot. You're going to need at least some tools to drill and mount the gearmotors and carve clearances for the weapon in the steel top cover. Realistically it isn't going to be competitive against high budget, custom built, spinner-weaponed antweights.

I pulled my 'Deadblow' out of the robot collection for a look. It weighs 10 ounces without any drive motors, radio gear, or functional weapon. It's 4.5 inches square and a scant inch thick. That doesn't leave much weight or space for the conversion. If I were doing it I'd raid FingerTech Robotics for the parts: a pair of FingerTech 11:1 'Silver Spark' gearmotors driving their 1.5" Snap Wheels and Hubs, a pair of TinyESCs, and perhaps an HXT Metal Gear Servo to power the axe weapon.

Add a 7.4 volt battery and an R/C setup of your choice and that's about as good as you're going to do with a small budget and off-the-shelf parts. Still, it's unlikely to last more than 30 seconds in combat. You might want to rethink this.

Q: Hello Mark, it's the Deadblow Metal Mechanics guy again. I have taken your response into consideration and would like to know what is my best option for a chassis? With the same setup and conditions. Thank you sincerely -Luke from Alabama.

A: I don't know enough about your building skills and actual budget to suggest a specific chassis design -- see Frequently Asked Questions #4.

Go take a look at current successful antweight robots: click on a name; click on "Search for this bot at the", then click on the photo to enlarge. There may be a link to the team webpage as well.

Put together a design you like and that matches your construction skills. Keep it simple. Tell me about your design and I may have some tips and adjustments, but I can't design it for you.

If your budget is really low, you may want to consider an R/C toy conversion -- see the Ask Aaron - Toy Hacks and Kits archive for some ideas.

Q: Hello Mark. This time I was wondering if it would be better to use motor mounts rather than bearing blocks for mounting silver spark motors on the chassis. Oh I forgot to mention that I intend to participate in the Beetleweight class, my bad. Thanks, Luke from Alabama.

P.S - I realize using this toy chassis is a terrible idea but, I have a "secret defense" (that will probably break) to solve the issue. Also I realize that axes and hammers are inefficient weapons but, I don't want to build what everyone else is building (wedges, flippers, spinners etc.) Thanks, Luke.

A: You don't have room in your chassis for either the FingerTech Bearing Blocks or the Flat Motor Mounts, Luke. They're too tall and likely too wide to fit. I think you'll be fine with the compact L-Bracket Motor Mounts.

For a beetleweight with two motors and 1.5" wheels you'll want to switch to the 22:1 gear ratio Silver Spark motors. The 11:1 Silver Sparks could stall with that much weight on the drive wheels. You have enough weight allowance to go to four motors if you have enough budget.

This may not be a 'terrible idea'. With a 3-pound weight limit you should have enough allowance for some decent protective armor and a thick front wedge that may keep you from turning into instant road kill. Best luck with the 'secret defense'.

Q: Hi Aaron:
 I build a flipper combot (220lb) , now i got a problem. The material of flipper weapon ,As we know the titanium is widly use in flipper weapon, but it`s so exp , I want use the thick aluminium alloy Instead of titanium , give me some advice plz :) [Yunnan, China]

A: [Mark J.] You don't build a robot out of a single material. Each component should be evaluated for the stresses placed upon it, the constraints of weight, and the processes needed in manufacturing. The goal is to build the lightest possible part that will withstand the expected forces by considering the need for surface hardness, toughness, and strength.

The builder of a well-known heavyweight lifter robot added up the materials used in the chassis by weight:

  • Titanium: 49%
  • Magnesium: 18%
  • Aluminum: 22%
  • Steel: 11%
Each part is made from the lightest material with properties suited to withstand the specific forces to be placed upon it.

As it says in Frequently Asked Questions #17, Ask Aaron is not a free engineering service. I'm not able to provide a stress analysis of your chassis design and recommend a specific material and thickness for every part. In general, a change from titanium to aluminum alloy for a specific component will require that the new component be heavier and bulkier, and its ability to absorb shock loading and resist surface deformation may be decreased.

My advice: look to see what other builders with similar designs are using and learn from their experience. If it breaks, make it stronger.

Ask Aaron is not a free engineering service!

Q: Hey, I would like to consolidate some of my understanding on lithium-polymer batteries. If I want to run two separate batteries in together in a single robot, then the milliamp hours are not cumulative, but the voltage is -- am I correct? So, if for example, I run two 6,000 mah 11.1 volt batteries together, the result would be a 6,000 mah 22.2 volt power source? Do I also need to be wary of running the batteries in series or parallel? Thanks a lot! [Bellevue, Washington]

A: [Mark J.] With two batteries you have a choice:

  • Two 11.1 volt 6000 mAh batteries in series will give 22.2 volts with a 6000 mAh capacity.
  • Two 11.1 volt 6000 mAh batteries in parallel will give 11.4 volts with a 12,000 mAh capacity.
You can get either double the voltage or double the capacity, not both.

As long as the batteries are the same voltage and capacity you'll be fine running them either in series or parallel.

Two battery packs in series.

Two battery packs in parallel.

Q: I just had a thought about how I could make the drum for my 60lb robot. My original plan was to machine the drum and drum tooth as one single piece of tool steel, but I've recently been considering forging a quarter-inch square bar of folded tamahagane (Japanese "Jewel" Steel, the material used in Japanese knives and swords) and welding it to a tool steel tube. Do you think this would be worth trying or should I just stick to my original plan? Or is the hamburger bad?

To note, the drum is going to be a simple counterbalanced single-tooth design either way (basically, two teeth but one is on the inside of the drum rather than the outside). [Arden, North Carolina]

Robot drum weapons - end view A: [Mark J.] There are good reasons why most drum weapons have bolt-in steel impactors setting in pockets machined into a thick aluminum tube:

  • Impactor teeth take a beating. No matter what material is used the critical leading edge gets rounded off, they crack, and they deform. Throwing the whole drum away when this happens is expensive. Bolting in a new impactor makes better sense.
  • Correctly welding tool steel is a huge pain.
  • Welds break.
Tamahagane is a high carbon 'bloom' steel. Its performance in knives and swords comes from forging higher and lower carbon layers of tamahagane together to provide both resilience and strength. Blades made in this way were state of the art... 400 years ago. Even if you were able to obtain the different grades of tamahagane and correctly forge the multi-layered block via the intensive and laborious process, the performance of modern 'shock resistant' tool steel is much superior for your purpose.

I'd suggest building a conventional drum for your first attempt and saving the fancy stuff 'til after you've seen the challenges first-hand.

About balancing a single-tooth drum: I'm a little worried by your decription of "two teeth but one is on the inside of the drum rather than the outside". That's like having two kids on a teeter-totter and moving one of them in toward the center -- they won't balance unless the kid moved toward the center is heavier than the kid that stays out on the end. I'm sure you had this figured out, but the next reader might be confused.

Q: What would be the best way to mount a flamethrower for that crowd-pleasing factor? On a wedge shooting upwards so that the flames engulf the other robot when you're already under them, or at the front to be able to gently warm the other robot as you charge forward like a bat out of hell? Or maybe a bit of both like with Ragin' Scotsman? I'm not actually considering building a robot with a flamethrower (well, not to any serious degree, anyway. I know my limits and I still need to finish my 60lb drum), I'm just curious as to what you think looks cooler. [Arden, North Carolina]

A: [Mark J.] So you want to play to the crowd, do you?

Top Five Ways to Mount a Flamethrower

5) Out the rear of the robot: Rocket Power!
4) Out the ends of your drum: Hot Tamale!!
3) Straight up out of the robot: Fire Fountain!!!
2) Straight up out of the transmitter: Flame Antenna!!!!
1) Straight out the rear of the driver: Burrito Night!!!!!

Remembering Aaron... 

Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

Robot haiku:

That's obviously
A question from your homework.
Do your own research.
Killer Robot drawing by Garrett Shikuma

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