Introduction: Abrasive Personality -- 12 Pound Combat Robot

Picture of Abrasive Personality -- 12 Pound Combat Robot

Maxim 70: Failure is not an option. It is mandatory. The option is whether or not to let failure be the last thing you do.

This was my first attempt at a 12-pound (aka, "hobbyweight") combat robot. It was built to compete at Robot Battles events. As of this writing, it has competed at the Dragon Con event in 2015 and 2016, the 2016 Momocon event, and the 2015 Freeside Street Fight.

To my knowledge, at the time I built this it was the only combat robot to use a belt sander as a weapon. In the time since I built it, AlexHrn built a clone of Yeti with a drum-sander weapon (maybe if you poke him enough he'll post a build report). Hopefully his was more effective than mine...

The idea here was to use the wedge to get under my opponent and then grind down their wheels with the sander. Real combat has shown that...didn't happen so much. I think I one time kinda sorta nicked an opponent's wheel. So consider this a build report to bring up my Instructable count to give ideas for others' bots. It worked pretty well as a standard wedge-bot, so there's that.

Step 1: Design

Picture of Design

Pretty much everything I build these days, whether robots or furniture, gets modeled in Sketchup before I make the first cut. Attached to this step is the Sketchup model file.

Of course, "No battle plan survives contact with the enemy"; as I built some things had to be adjusted on-the-fly. If you're actually building this -- and not treating it as a dire warning to others -- don't fool yourself into thinking you can trust the plans 100%.

The pictures include labels to certain parts I'll reference throughout the build

Step 2: The Rollers

Picture of The Rollers

Tools this step

  1. Bandsaw
  2. File
  3. Large pipe cutter (or hacksaw)
  4. soft mallet

Materials this step

  1. (appr. 2 feet) 1-1/2" PVC pipe
  2. (2x-3x) 1-1/2" PVC coupling
  3. PVC cleaner and PVC glue
  4. (4x) conveyor roller bearing for 1-1/2" PVC pipe
  5. can of Plasti-Dip spray
  6. masking tape

Using a pipe cutter or hacksaw, cut two lengths of pipe, about 8-5/16" each. Then, using a bandsaw, cut seven 1/4" wide rings from the PVC couplings and glue at the following positions, as measured from the left end of the pipe:

  • left edge
  • at about 3-3/4"
  • at about 7-7/16"
  • at about 8-1/16" (only need to do this for one roller)

The actual measurements aren't as important as getting the rings at the same position on both rollers -- you need two channels approximately 3-1/2" wide (this are the sanding belt channels) and one channel at the edge about 7/16" wide (this will accommodate a belt to drive the weapon).

Drive the roller bearings into the ends of the pipes using a soft mallet. It should be a tight fit; no need to glue them.

For the drive roller, rough up the belt-contact areas with sandpaper. Mask off the roller bearings and rings with masking tape, then spray the belt-contact areas with Plasti-Dip. I gave it 3 light coats, allowing an hour or so to dry between coats. The Plasti-Dip will "rubberize" the roller and help the drive and sanding belts grip it better.

Step 3: Frame: Side Rails

Picture of Frame: Side Rails

Tools used

  1. miter saw
  2. clamps
  3. drill press
  4. scratch awl
  5. center punch
  6. mallet
  7. jigsaw with metal-cutting blade
  8. rotary tool with a grinding bit (the smaller the better -- it'll be a tight fit)
  9. hand file

Materials used

  1. 2"x1/4" Aluminum bar
  2. electrical tape
  3. cutting fluid

The side rails are the most critical part measurements-wise, so I took my time to get it right. Everything else will follow from this. I started by cutting two identical lengths of 2"x1/4" Al bar, each about 16-1/2" long, on the miter saw.

There are several holes in the side rails that need to be at the same location on both, so I wanted to tape the rails together and drill straight through both rails. I stacked the two pieces, lined them up, clamped them, and then wound lots of electrical tape around them to keep them aligned. One picture shows where I used the fence on the miter saw, along with a stop block, to help align them before clamping.

Next, I made a drilling template from my CAD model and taped it to one of the rails. I've included the template below -- the left edge of the template should be located 5" from the left edge of the rails. Using the template, I marked the hole locations with a scratch awl, then dimpled them with a center punch.

Now to the drill press! My press works best if I step up to large holes, so I began by drilling all holes with a 1/8" bit. Next, I switched to a 13/64 bit for the tensioner hole. Then a 1/4" bit for the remaining three holes, which I next stepped up to a 3/8" bit, and then finally finished with a 1/2" bit. Stepping it up this way reduced chatter and gave me very clean, very straight holes. I find if I start with a large bit like 1/2" that the bit wanders a lot.

I separated the two rails, then used my jigsaw to rough-cut the adjustment slots. I finished them up with a rotary tool and hand file.

Step 4: Tensioner Disks

Picture of Tensioner Disks

Tools used

  • jigsaw with metal-cutting blade
  • drill press
  • hand file
  • bench grinder

Materials used

  • scrap 3/16" Aluminum plate
  • (2x) 1/4"x1/2 machine screws

These disks are used as cams to apply tension to the rear roller shaft, which pulls the sanding belts taught. 1/4" machine screws will hold them to the side rails and keep them locked into position during a fight.

Each disk is 1-1/4" in diameter, with a 1/4" hole located approximately 3/8" from the outside edge. The offset-from-center hole turns the disk into a cam. I rough-cut the disks with a jigsaw (not an easy process!) and smoothed them with a hand file and bench grinder. They ain't pretty, but they work quite well (at least until that well-placed hit on my flanks disabled my weapon...)

The disks are attached (using the 1/4" machine screws) to the tapped holes just forward of the rear roller slot.

Step 5: Frame: Cut the Cross-rails

Picture of Frame: Cut the Cross-rails

Tools used

  • miter saw
  • jigsaw with metal-cutting blade
  • hand file

Materials used

  • 2"x1/4" Aluminum bar
  • cutting fluid

I cut three lengths of Al bar on the miter saw, each 10-1/4" long. I set one aside. The other two must be narrowed by 1/4" (unfortunately I couldn't find bar stock in 1-3/4" widths!). I used a jigsaw and lots of cutting fluid to shave the extra 1/4" off each bar. Clamping was a nightmare here, so I used a jig I'd made for cutting small bits of stock with a jigsaw (see the pictures). Instead of actually clamping, I drilled holes in the bar stock and screwed them directly to the jig. (The picture shows pan-head screws, but I had to switch to counter-sunk flat-heads to get the saw past them.)

Step 6: Frame: Attach the Back Cross-rail

Picture of Frame: Attach the Back Cross-rail

Tools used this step

  • miter saw
  • drill press
  • torx screwdriver
  • transfer punch
  • center punch
  • mallet

Materials used this step

  • nutstrip
  • 10-24 pan-head torx screws

I used nutstrip (tapped for 10-24 screws) to attach the back cross-rail to the side rails. One of the most important tools in this step is a transfer punch, used to mark the location of holes on the rails.

I wanted to get everything as square as possible. So after marking, drilling, and attaching one side-rail, I laid it on the flattest surface I could find -- my table saw. The fence and miter sled of the saw helped me dry-fit and mark the other rail so the whole assembly would be both flat and square -- one of the pictures shows this better than I can explain here. I temporarily installed one of the weapon shafts to help line everything up.

Step 7: Frame: Weapon Motor Box

Picture of Frame: Weapon Motor Box

Tools used this step

  • Miter saw
  • Jigsaw (with metal-cutting blade)
  • center punch
  • mallet
  • transfer punches
  • torx screwdrivers

Materials used this step

  • 2"x1/4" Aluminum bar
  • 1/8" Aluminum plate
  • 10-24 machine screws (both flat and pan head)
  • 6-32 machine screws
  • nutstrip

This is the box that sits inside the sanding belts, between the rollers. It contains the weapon motor and a speed controller for the motor.

Because this is a tight fit, the overall height of the box must be 1-7/8". This is why I previously shaved 1/4" off the cross rails with the jigsaw. I also drilled and tapped a couple of 6-32 holes to temporarily attach the top plate. With that assembled, you can center the box top-to-bottom such that the top of the box is even with the top of the rollers (should be about 1/16" from the top of the side rails). Attach the box' cross-rails to the side rails with nutstrip and 10-24 machine screws. Some screws will need to be flat-head and countersunk to clear the rollers.

I waited to permanently attach the top plate until after I worked out the placement of the box' innards. That way I had a better idea where my screw-holes would be.

Step 8: Weapon Motor

Picture of Weapon Motor

Tools used this step

  • drill press
  • Size "Q" drill bit
  • adjustable hole cutter bit
  • 3/8"-24 tap and tap wrench
  • miter saw
  • dremel with grinding and milling bits
  • torx screwdriver

Material used this step

  • timing pulley, XL, 20 teeth ( p/n A 6A 3-20H3708)
  • 3/8" timing belt, XL, 110 teeth, 11" outer circle
  • cordless drill motor
  • 2"x3/8" UHMW bar
  • #6 plastite screws

The weapon is powered by a gear motor salvaged from a cordless drill (of the $20 Harbor Freight variety). A timing pulley is mounted directly to the motor's output shaft and drives a belt wrapped around the front roller.

First, I drilled the timing pulley's bore with a size "Q" bit, then tapped it for 3/8"-24tpi (the size of the thread on the end of the drill motor shaft). The pulley screwed on perfectly, then I replaced the left-handed screw...which was too long to go completely in, so I shimmed it was a random hex nut. The left-hand screw keeps the pulley from backing off the shaft while the weapon is spinning.

Next, I built two motor mounts from the UHMW bar. The front one is matched to the square(ish) cross section of the front of the motor; I pretty much free-handed it with some well-placed drilling and dremeling. It may take a couple of tries to get it right, but make sure you do because this is what keeps tension on the weapon's drive belt. I cut the rear mount using an adjustable hole-cutter bit. Both mounts are secured to the weapon box with #6 plastite screws.

The last picture on this page shows my impromptu rig for testing the weapon motor.

Step 9: Frame: Firewall

Picture of Frame: Firewall

Tools used this step

  • miter saw
  • torx screwdriver
  • cordless drill
  • drill press

Material used this step

  • 2"x3/8" UHMW
  • #6 plastite screws

The "firewall" is just a piece of UHMW that protects the soft squishy (well, not really squishy) electronic innards from the sander weapon. It isn't really a structural piece like the metal frame parts.

Simple step here: cut a piece of UHMW to the proper length, drill holes in the side rail for the screws to pass through, and attach the UHMW with the plastite screws.

Place the firewall so that it maximizes internal space for the electronics. Do so by first pushing the rear roller as far back in the adjustment slots as possible, then place the firewall about 1/8" behind it -- i.e., just enough space to clear the rollers.

Step 10: Preparing the Wheels

Picture of Preparing the Wheels


  • phillips screwdriver
  • punch
  • hammer


  • (2x) 3"x7/8" Colson wheels
  • (2x) Colson wheel-to-drillmotor adapter hubs from

Most of the information on mounting the wheels to the hubs is available from Team Rolling Thunder. You might have to remove a plastic bushing from the wheel first, however. I used an appropriately-sized center punch and a hammer to gently tap the bushing out, as shown in the pictures.

Assemble two motors this way -- one for each side of the bot.

Hint: have at least one extra motor and 2 extra wheels as combat spares. They're cheap!

Step 11: Motor Mounting Blocks

Picture of Motor Mounting Blocks


  • drill press
  • screwdriver
  • center punch
  • miter saw
  • rotary tool (with grinding bit)
  • scroll saw
  • jigsaw
  • hand files


  • 2"x3/8" UHMW bar
  • #8 machine screws (about 1" long) with nuts

I designed some simple motor mounts in Sketchup and printed cutting templates onto heavy cardstock. I cut out the templates and taped them the UHMW bar stock, traced the pattern, then cut it out on the scroll saw (after breaking a few blades -- spiral blades aren't really made for cutting UHMW!) It took a little cleaning with the rotary tool to get the fit right. Note that I cut the interior holes first, then sliced the mounts off the bar stock with the miter saw. That made things a bit easier to maneuver with the scroll saw.

Then I marked and drilled the mounting holes for the face mount -- this is the one with the "flattened" or "double-D" hole in it. The face mount is attached directly to the side rails with #8 machine screws.

The side rails need holes to match the "double-D" pattern on the face mounts. Once the face mounts were bolted on, I traced the hole pattern to the rails, then removed the face mount from the rails, removed the rails (one at a time) from the bot, and cut the "double-D" holes out of the rails. I cut the hole using a combination of strategically-placed drilled holes, cuts with the jigsaw, and cleanup with files and my rotary tool. Not pretty, but it works.

Step 12: Wedge Scoop

Picture of Wedge Scoop

Tools used this step

  • jigsaw
  • torx screwdriver
  • cordless drill
  • drill press
  • file
  • 10-24 tap

Material used this step

  • 1/8" Aluminum plate
  • nutstrip
  • 10-24 flat head screws

Using the jigsaw I cut the angles on the side rails as shown in the pictures and cut the wedge scoop out of the Aluminum plate. The plate is attached with nustrip and 10-24 screws as shown.

Step 13: Electronic Guts

Picture of Electronic Guts

Tools used this step

  • soldering iron
  • wire strippers

Material used this step

Most of this is standard R/C wire up: solder the ESC motor connections to the appropriate motors, plug the servo connectors into the Rx. I used a Fingertech switch between the battery and the ESCs.

Space is tight -- too tight. Do your best..

The main weirdness here is getting power and control signal to the weapon ESC in the weapon box. As the pictures show, I routed a power cable and servo extension cable out the drive box, over the rear roller axle, then back into the weapon box. I knew that the weapon box had to be removed entirely if I wanted to change the sanding belts, so I planned both cables to have disconnects as shown in the first picture. The second and third pics show how I had to make a jumper with disconnects on each end to properly route the power.

Step 14: And We're Done!

Here are a couple of videos of me losing to other bots. :)


ankesh2002 (author)2017-03-18
mcsmiley (author)2016-12-29

Maybe consider instead of sandpaper, a series of hole saws on drill motors. That'll give you a bit more cutting power, and is more likely to do damage, as the center bit is more likely to be able to grab around axles or possibly catch internal electronics.

lewisb42 (author)mcsmiley2016-12-29

The hole saw idea sounds good. I've thought about similar designs using cheap spade or forstner bits.

H3xx (author)2016-12-28

I see the idea, but it seems that the sanding belts aren't abrasive enough. What you need is something with a lot more bite. Assuming Chainsaw blades are allowed, a row of them would chew up anything. If they're not, then multiple rows of small saw blades would work as well, having each of the rows turn into each other like the blades on a paper shredder would work. They should latch onto any rubber or plastic wheel and tear it up.

lewisb42 (author)H3xx2016-12-29

The grit may have been one issue but the bigger problem ended up being I could never "capture" my opponent long enough for the belts to do much damage. Most of the time they just sailed over me.

As for the "row of small blades" idea: I've already got the Sketchup model for this. :) It'll use six 4" mini tablesaw blades.

And it's funny you mention chainsaw blades -- just a few days ago I stumbled on a "chainsaw blade" for angle grinders: An array of those would be super-sweet, but they're about 10 times the cost of the 4" table saw blades. :)

About This Instructable




Bio: By day, mild-mannered CS prof. By night, husband, father, basement tinkerer, video game player.
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