Introduction: Unskinny Bot: 3-lb Horizontal Spinner Combat Bot

Picture of Unskinny Bot: 3-lb Horizontal Spinner Combat Bot

This is my fourth Beetleweight (3-pound) combat robot and, hopefully, the first with a usable weapon.  Let's just say those earlier attempts at weaponized 'bots were "learning experiences"...the kind where you go home and hope you can salvage it for parts.  On the other hand, my bot #3 has held up well in combat over the last year, but it's a wedge, so this is my attempt to level up and throw some non-boring into the mix.

"Unskinny Bot" is my take on the tried-and-true horizontal bar spinner, just a rectangular chunk of metal that spins really fast and (hopefully) does a lot of damage to the other bot.

It will debut at Robot Battles 47 Microbattles at Dragon*Con in September 2013.

Step 1: Design

Picture of Design
I spent a lot of time modeling it in Sketchup before even ordering parts.  Attached is the CAD file (in Sketchup) for the 'bot.  Some notes about the design:
  • dead shaft for the weapon: This means the weapon shaft doesn't rotate; instead, the weapon blade and weapon pulley (which are bolted together) spin around the fixed shaft.  There were a couple of reasons I chose this arrangement.  First, that having the dead shaft clamped to the armor at the top and bottom might make it easier to change a weapon belt during competition.  And second, it would have required some machining beyond my abilities and equipment to build a keyed weapon shaft with high-speed bearings at the ends.
  • it had to fit within the milling limits: which are about 11"x5"  for the machine to which I have access
  • battery cover for quick battery access: very important during competition!
  • power switch: the rules require this if a bot has a powered weapon.  I always forget this until it's almost too late...
My original design called for belt-driven wheels (as shown in the CAD file) which are better at taking combat abuse.  I found out late in the build that those didn't allow enough internal space for the electronic guts, so I had to rebuild using direct-driven wheels.  The upside is I had enough weight left over to put armor around the wheels.

Step 2: Materials and Tools

  • (1x) 16T Fingertech timing pulley
  • (1x) 42T Fingertech timing pulley
  • (1x) 300mm Fingertech timing belt
  • (1x) Fingertech power switch
  • (1x) Turnigy Park300 1600kv brushless outrunner motor
  • (1x) 30A brushless ESC
  • (1 pack of 2) Kitbots B16 motor mounts
  • (1x) 3/8" ID needle roller bearings, McMaster-Carr p/n 5905K22
  • (2x) Beetle B16 gearmotors
  • (1x) Futaba R2004GF radio receiver
  • (2x) 3/8" bore set-screw hub clamps
  • (2x) Banebots bb-3-9 brushed ESC
  • (1 package of 2) LiteFlite 2-1/2" wheels
  • (1 package of 2) Dave's Hubs, 4mm bore
  • 12"x12"x3/16" Al-6061 plate
  • 2"x36"x3/8" Al-6061 bar (McMaster-Carr p/n 8975K593)
  • (3 ft) 3/8"x1-1/4" UHMW polyethylene bar (McMaster-Carr p/n 8702K731)
  • 3/8"OD x 12" hardened steel shaft (McMaster-Carr p/n 6061K32)
  • (1 box of 50) 18-8 stainless steel plastite screws, 6-19 thread, 3/4" long (McMaster-Carr p/n: 96001A268)
  • (1 box of 50) grade-8 6-32 5/8" cap screws (McMaster-Carr p/n 91251A150)
  • (1x) Turnigy 1000mAh 3S 30C Lipo battery pack
  • CNC milling machine
  • drill press
  • miter saw
  • hand files
  • transfer punch set
  • center punch
  • scratch awl
  • Sharpie
  • Torx screwdrivers
  • tap & die set
  • utility knife
  • try-square
  • hex keys
  • soldering iron
  • rotary tool
  • pipe cutter
  • angle grinder

Step 3: Milling the Armor

Picture of Milling the Armor
Tools This Step
  • CNC milling machine
  • hand files
Materials This Step
  • 3/16" aluminum plate

I have access to a small CNC milling machine at the university where I work.  I used it to cut out the top and bottom armor plates from 3/16" 6061-Al plate.  The last picture shows the top armor and the (old) battery cover after milling.  All the milled pieces required some hand filing to remove burrs, soften edges, etc.

I have attached DXF files for the armor pieces to this step.

Step 4: Frame Assembly

Picture of Frame Assembly
Tools This Step
  • drill press
  • utility knife
  • cutting board
  • try square or other metal straightedge
  • torx screwdrivers
  • miter saw
Material This Step
  • milled armor plates
  • UHMW bar
  • motors
  • motor mounts
  • plastite screws
For lack of a better term I call the vertical supports (which sandwich between the top and bottom armor pieces) bulkheads.  These are made from 3/8"-thick-by-1-1/4"-wide UHMW polyethylene bar.  I cut each to 6-5/8" long on a miter saw using a stop to ensure both pieces are exactly the same length.

I laid out the electronics and motors for fit, and used that to choose the location of and drill the holes for the motors.

The motor and motor mounts are assembled and attached as shown in the pictures.

With the wheels and hubs assembled and in place, I  sized the rear bulkhead so that it sticks about a wheel's thickness beyond each wheel.  This gives space to easily remove and replace wheels during competition as well as provide more buffer between the wheels and the side armor.  More buffer means that opponent's spinning Blade O' Death has to push farther to take out my wheel...

Step 5: Wheel Guards

Picture of Wheel Guards
Tools This Step
  • drill
  • red Sharpie
  • utility knife
  • bandsaw
  • cutting board
  • try square or other metal straightedge
  • torx screwdrivers
  • miter saw
Material This Step
  • UHMW bar
  • 1/8" thick UHMW plate
  • plastite screws
Since I couldn't do belt-driven wheels as I originally intended I want to use the weight savings to protect my wheels from opponents' weapons.  The first pic shows how this should look.

First, I cut the front wheel guard supports from short scraps of UHMW bar, drilled pilot holes, and attached them to the side bulkheads with plastite screws. (If I had to do this over I would have designed it such that those guards were a single piece going across the bot, boxing in the electronics and firewalling them away from the weapon motor.)

Then I took some scraps of 1/8" thick UHMW plate and traced a wheel on it as shown in the pictures and then screwed the plate to the front and rear wheel guard supports so I could finish tracing the wheel guard profile.  Then I cut it with my bandsaw.  Repeat for the other wheel.

Step 6: Attach Weapon Shaft Collars

Picture of Attach Weapon Shaft Collars
Tools This Step
  • drill press
  • tap & die set (with a 6-32 tap)
  • hex keys
Material This Step
  • shaft collars
  • 3/8" shaft stock
  • 6-32 cap screws
  • tapping fluid
Many of the measurements on this bot can be fudged -- for example, the bulkheads don't have to be exact.  The weapon shaft should be aligned as accurately as possible, however, so we pause frame assembly to get it squared away.  We will actually let the shaft dictate proper placement of the top armor (relative to the bulkheads) in the next step.

The shaft collars are pre-tapped, but my design calls for the armor to be tapped and not the collars, so I drilled them out just enough to slide a 6-32 bolt through.

My CAD file has a too-small weapon shaft hole in the top and bottom armor plates, so I bore it out to 3/8".  Tighten one of the shaft collars onto the shaft and slide the other end of the shaft into the bottom armor plate.  Use a transfer punch to mark the corresponding hole positions on the armor plate; drill and tap these to accommodate a 6-32 bolt.  Repeat for the top armor.

Fasten the collars to the armor with grade-8 6-32 cap screws.  Also, replace the stock clamping screw in each collar with one of the grade-8 cap screws.  Can't hurt to make things a little stronger...

It's really annoying to have to deal with that long piece of weapon shaft...maybe I'll get around to cutting it one of these days...

Step 7: Frame Assembly, Part 2

Picture of Frame Assembly, Part 2
Tools This Step
  • Torx screwdrivers
  • hex keys
  • drill press
Material This Step
  • plastite screws
Ignore that this step shows inner bulkheads from the earlier design.  Otherwise the steps are the same. 

Begin by threading the weapon shaft through both armor plates (and their shaft collars).  This forces both the top and bottom armor plates to be aligned on the weapon shaft.

Now mark the bulkheads and drill pilot holes based on the corresponding top armor holes.  Fasten with the plastite screws.

Finally, drill pilot holes for the battery cover and attach with plastite screws.  I also added 6-32 washers.

Still need to cut that weapon shaft...

Step 8: Annoying Shaft: BE GONE!

Picture of Annoying Shaft: BE GONE!
Tools this step
  • clamps
  • angle grinder with metal-cutting blade
Material this step
  • 3/8" shaft
I assembled the armor and frame and used that to measure the needed shaft length.  Then I cut it with the angle grinder.  IT'S ABOUT TIME.

Step 9: Weapon Blade

Picture of Weapon Blade
Tools this step
  • drill press
  • files
  • try square
  • miter saw
  • scratch awl
  • center punch
  • hack saw

Materials this step
  • 2" wide by 1/4" thick Al bar
  • Fingertech 42T timing pulley
  • 6-32 cap screws
  • roller bearing
  • 3/8" inside diameter bronze bushings (with flange on one end)
In this step, I attach a timing pulley to the blade using 6-32 cap screws.  Note that the pulley is tapped to receive the screws.

I used a hacksaw and file to remove the set-screw nub from the timing pulley.  Then I drilled a single hole (with a bit intended for 6-32 tapping) as shown in the second picture.

Then I measured and cut a 6" piece of the Aluminum bar using the miter saw and drilled a 1/8" hole dead center.  I used the drill bit as a temporary axle to mate the bar and pulley along their common axis.  The third picture shows this from the back side of the blade.  While these were together, I put the 6-32 drill bit in the pulley's off-center hole and rotated the pulley: this scratched a nice circle on the blade, a circle that is where all my pulley-to-blade fastening holes will be.  Once done, I tapped the single pulley hole to 6-32 thread.

Next, I drilled four holes in the blade, big enough to accommodate 6-32 screws a little loosely.  I used the inscribed circle to guide them (as in the third and fourth pictures).   Attached the pulley to one of the holes (again, using the 1/8" drill bit to keep the center holes aligned) with a cap screw.  Used a transfer punch to locate the remaining three holes on the pulley, which were then drilled and tapped for 6-32 thread.

Then I attached the pulley to the blade with all four screws.  It may be necessary to make the holes in the blade a little bigger so as to adjust for alignment.  Finally, I drilled the center hole of both the blade and pulley -- at the same time! -- to 9/16" so as to accommodate the roller bearing.  (Since I wanted this hole to be as precise as I could get it, I enlarged it a little at a time, in increments of about 1/8" diameter each time.) 

Finally, I assembled the blade as shown in the last pictures.  I cut down some bronze bushings to keep the blade at the proper distance from the top and bottom armor pieces.

Step 10: Weapon Motor Mount

Picture of Weapon Motor Mount
Tools used this step
  • Miter saw
  • metal files
  • drill
  • countersink bit
  • hacksaw
Materials used this step
  • brushless motor
  • UHMW bar
  • angle aluminum (just a small scrap is enough, should be about 1.5" wide by 1.5" tall by 1/8" thick)
  • random bolts (probably 6-32) to hold the bracket
Sorry about the lack of detail this step; it was another victim of the redesign.  Hopefully the pictures tell the story well enough.

Basically I took a scrap of angle Aluminum and drilled the motor mount pattern on one leg and drilled holes for attaching it to the UHMW riser block on the other leg.  Then I hacksawed and filed to get the bracket to fit in the space required.

The critical measurement here is the distance between the motor and the blade.  My design does not have a belt tensioner, so the motor must be placed so that the belt has enough enough tension to not slip off either pulley.

(If I were to do this over I'd mount the weapon motor to one of the side bulkheads instead of in the center of the bot.)

Step 11: Electronics

Picture of Electronics
Tools this step
  • soldering iron
  • screwdriver
  • 4-40 tap
Materials this step
  • 30A brushless ESC (for the weapon motor)
  • battery
  • battery connector
  • (2x) BB-3-9 ESC (for the drive motors)
  • Radio receiver
  • power switch
The wiring is pretty standard for an R/C bot:
  • connect the weapon ESC output wires to the weapon motor
  • connect the left and right drive ESC output wires to their respective drive motors.
  • connect together the positive power wires of all three ESCs and route to one leg of the power switch
  • connect the other leg of the power switch to the positive of the battery connector
  • connect together the ground wires of all three ESCs and route to the ground of the battery connector
  • plug the servo-style cables from the ESCs into the radio receiver.  For the receiver I used, the drive motors are on channels 1 and 2 and the weapon is on channel 3.
Power switch placement always merits some thought -- it needs to be accessible to the operator, but not to hits from other bots.  As shown in the pictures, I placed it on the inside of the rear bulkhead just behind the left wheel.  Per Fingertech's suggestion, I tapped the mounting holes for 4-40 thread, which meant I could mount it without nuts.

Step 12: Battery Cover

Picture of Battery Cover

This is pretty quick and dirty simple.  Just a scrap of 1/8" thick UHMW screwed to the armor to cover the battery access hole.  It works.  :)

Step 13: Finished!

Picture of Finished!

Step 14: Appendix: the Original Drive Train

Picture of Appendix: the Original Drive Train

I left the pictures for this step in case anyone wanted to see how the belt-driven version of the bot would (should) have looked...


pkottwit78 made it! (author)2016-09-14

Great Instructable! A few of the parts they no longer make, the beetle-weight motors have been back-ordered for years, and I also couldn't find anywhere that had 4mm Daves hubs. I substituted them for Robotmarketplace's Hexhub's and generic 16:1 motors and mounting brackets.I also used a Sabertooth 2x12 ESC so I could make the bot tank steer

I think it came out great! the CAD file was a huge help and my local water jetting place was able to cut it out no problem. Thanks so much!

lewisb42 (author)pkottwit782016-09-16

Fantastic build!

Yeah, the B16's have been "on backorder" for some time. I have learned since there are plenty of drop-in replacements available with some ebay diligence.

aaronanger3000 (author)2016-07-30

im having trouble downloading the files..could someone tell me whats the final length and width of the top and bottom plates please

lewisb42 (author)aaronanger30002016-08-01

It's about 10-7/8 inches long by 4-1/4" wide.

If you like, email me at lewisb42 AT yahoo DOT com and I'll send the files (if I still have them...)

gladamirflint made it! (author)2016-04-14

Made one with some modifications. Won 1st place at a competition with 11 bots competing. I'm making an Instructable for my design modifications, should be out soon.

Pictures are before and after the competition.

Jamie Toeg (author)gladamirflint2016-07-08

Could i ask what competition you entered and how you found it ( as i am interested in building a combot but want a competition that i can compete in and a set of rule guidelines to follow.


gladamirflint (author)Jamie Toeg2016-07-08

Check out for the competitions, and for general information and rules on Combat Robotics. Hope this helps!

lewisb42 (author)gladamirflint2016-04-15

You showed me this on Facebook, but it bears repeating here: this makes me insanely happy!

Again, well done on the 1st place finish. That was all you. :)

gladamirflint (author)lewisb422016-04-15

Yep, just wanted to place it here for the record :)

bharadwaj123 (author)2016-02-10

What is the best motor that to be used for a horizontal spinner

bharadwaj123 (author)2016-02-10

What is the best motor that to be used for a horizontal spinner

bharadwaj123 (author)2016-02-10

What is the best motor that to be used for a horizontal spinner

MicheleG32 (author)2015-12-27

where can I buy one of these

lewisb42 (author)MicheleG322015-12-27

I don't know of any horizontal spinner kits, but there is a vertical spinner kit here:;jsessionid=B3FB80A382A1B4D005A6F578675E2F5C.m1plqscsfapp01?productId=19&categoryId=1

and a wedge kit here:

bumbolbee (author)lewisb422016-01-26

hello can i ask about combat robot cause my father is a technician what can i do to his scrap material

bumbolbee (author)lewisb422016-01-26

hello can i ask about combat robot cause my father is a technician what can i do to his scrap material

bumbolbee (author)2016-01-26

HI I am a NEW her can you help me ALL to make a new combat robots i am a filipino if can you suggest

WarriorBeast (author)2015-08-01

thats a lotta wires

awsome ninja (author)2015-02-17

Why did you change the bot to a direct drive. Where there many issues with the belts?

lewisb42 (author)awsome ninja2015-02-17

I simply ran out of space for the internal components. I thought I'd properly accounted for things in my original CAD design, but the reality was just too tight. In particular, wiring took up more space than I expected.

The upside is that, upon switching to a direct-drive scheme, I had some weight left over for wheel-guards.

The Lightning Stalker (author)2013-08-15

Might I recommend using mild steel for the blade weapon? It may be a little harder to machine but will outperform aluminum in several ways.

It has more mass.
It is less likely to snap and send chunks flying.
It is much cheaper.
It is harder.

Just my 2¢

Most of the bot builders I talk to recommend not using mild steel for a high-energy weapon like this, though to be honest I don't really know the reasons. Typically, the best of these weapons are either completely made of hardened tool steel or are mostly-aluminum (of a strong alloy like 6061 or 7075) with tool steel "teeth" of some sort at the tips (see TitanTechRobotics' comment earlier).

That is interesting. Thanks for sharing that tidbit. With tool steel you could possibly run once again into the shattering problem. Mild steel will bend A LOT before it breaks. I do suppose they have their reasons.

Mild steel has a few problems. The contact surfaces get deformed VERY fast, and can cause dangerous failures if they are spinning at high speed. Failure can happen if they bend and make contact with a frame or support, or sometimes simply by the deformation causing unfavorable harmonics.

Even hardening the work surfaces does not solve the issue of flexing or deformation that can happen elsewhere.

Indeed. Steel has its own share of problems.

Aluminum is however much worse. Lawn mower blades are made of steel for many reasons.

monty324 (author)2013-08-17

May I suggest using 2 counter rotating spinners to do more damage and to cancel out the inertia of the rotation thus making control a bit easier. Also I would love to see what this will do to watermelons.

lewisb42 (author)monty3242013-08-17

From observing more experienced builders, this would actually be counter-productive since the goal isn't to "saw" into them as much as it is to throw them. (Most bots are too well-armored for even a good spinner to cut into.) If you have two blades, with equal energies, coming from opposite directions, the energies could well cancel each other out.

Not to mention, the weight expense of an additional blade, motor, and speed controller would be counter-productive. You'd have to take it out of armor budget, which would be Really Bad. :)

Intertia *is* an issue with these spinners, making them harder to drive. I see it as a check on their power. :)

Now, I don't remember if they went after a watermelon or not, but had an interview recently with the builder of Last Rites, a bot with the same basic design but in the 220-pound (!!!) weight class. They did some awesome videos of Last Rites going after furniture and a bowling ball. It was epic:

Prophes0r (author)lewisb422013-12-06

I never understood the logic behind the single horizontal spinning bar myself. While they do a good job of sending opposing bots flying, Mr. Newton would remind you that your own bot will most likely also move.

I say this with the assumption that you did not design some ridiculous active tracking and compensation system. I suppose you could have a proximity sensor that thrust your bot toward the enemy in the same direction as your bar rotates at maximum motor torque an instant before contact is made. This might reduce the overall instability of the system.

As a note about the counter-rotating disc idea. You only need the disc to have the same rotational mass, it does not need to be the same diameter. You could have a disc with a greater thickness but smaller diameter mounted so that only the "combat" disc makes contact.

sopzman (author)2013-08-16

Very nice! Good luck at the event!
FYI, FingerTech Robotics now carries needle roller bearings like you've used, and aluminum wheel hubs ("Snap Hubs"). They are new products!

lewisb42 (author)sopzman2013-08-16

Indeed -- they're coming out with a lot of good stuff lately (partly, it seems, by partnering with the excellent site). That said, I think I'd prefer the Dave's Hubs over the new FT hubs because the latter uses a snap ring, not a screw, to hold the halves together.

As for needle bearings, I'm fortunate to live about 20 minutes away from McMaster-Carr's regional distribution facility. I can get those bearings (and all my aluminum, UHMW, etc) at the Will Call desk. :)

sopzman (author)lewisb422013-08-16

Ha, the screw was always the part I hated because you were forced to cut your motor shaft very short. Now the shaft can run through the wheel to be supported on the opposite side. (I realize this isn't a problem with your existing stubby shaft motors!)

Lucky you to live next to the mecca of hardware stores!

lewisb42 (author)sopzman2013-08-16

"Lucky you to live next to the mecca of hardware stores!"

It's not lucky for my wallet... ;)

lewisb42 (author)sopzman2013-08-16

That's a great point about the shaft, one I'd missed. Now it makes sense that FT is carrying them -- the Sparkmotors are overall excellent motors, but the output shaft really needs that double-support.

And of course the FT hubs are lighter and less expensive than Dave's Hubs. Always a plus!

liquidhandwash (author)2013-08-16

nice little ankle biter :-)

JaredsProjects (author)2013-08-14

Nice job, but a park 300 motor is a little undersized for a 3lb bar spinner. You could get faster spin-up times and higher RPMs out of a bigger motor. With the drive train you have you could spin around 10k RPMs and still get good bite. Also, I recommend adding teeth.

lewisb42 (author)JaredsProjects2013-08-14

Teeth are, unfortunately, outside of my machining capabilities right now. Longer-term I've considered a toothed disk, similar to that of Robo-Rooter ( if our little Sherline mill can handle the harder material.

As for the motor, you're probably right -- in theory I'm getting enough stored energy to toss a 3-pounder, but that's all on paper. We'll see what happens in the arena. If the bot survives this competition I'll look into upgrading the motor -- at worst it means hacking another bracket.

Thanks for the suggestions.

JaredsProjects (author)lewisb422013-08-14

You can use a countsink screw but don't actually countersink it to the teeth. It works quite well. I'll post a picture once I'm back on my computer.

That sentence didn't really make sense...I was typing quick on my phone. The picture shows best what I was trying to say.

lewisb42 (author)JaredsProjects2013-08-15

I wondered if that's what you were talking about -- I've seen it done on drum spinners before.

I've actually considered that, but I'd probably have to shorten my blade to do it -- I don't think I have enough clearance for the extra length right now. After the competition (which is only 2 weeks away) I'll look into it as a possibility for the next event.

JaredsProjects (author)lewisb422013-08-15

Good luck at the competition! It should still be an effective weapon, the Weta Kits just use normal socket head screws for teeth, making them completely blunt, and they win competitions all the time.

lewisb42 (author)JaredsProjects2013-08-15

I've gone up against a Weta kit before. Immediately after I was kicking myself for not bringing a hand file to the pits... :)

radmans (author)2013-08-14

I haven't even read through your 'ible yet...I just think the name you gave your bot is awesome.

lewisb42 (author)radmans2013-08-14

Heh, thanks. In this Internet age when every good idea is already out there somewhere I was pleasantly surprised when it didn't come up in a Google search. I went through a LOT of name ideas before that happened...

About This Instructable




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