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

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.

bp.skp
Download

Step 2: Materials and Tools

Materials

(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) ServoCity.com 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

Tools

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

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

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

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

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

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!

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

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

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

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.