Introduction: Doom Service: Multi-saw Combat Robot (Hobbyweight Class)

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

Introducing Doom Service-- a drum-spinner-style hobbyweight (12-pound) combat robot. Except its drum weapon has a bunch of circular saws instead of, well, a drum. Because saws.

This is my second attempt at a hobbyweight, built largely from meat picked from the bones of my first attempt.

As with most of my bot builds, this can be made without the need for fancy schmancy machinist's thingies like lathes, milling machines, tight tolerances, or skill. All the needed tools are things you can get at your friendly local multinational orange-flavored bigbox hardware store. (I did cheat and use a 3d printer for some parts, but it isn't required...)

Doom Service conforms to the slow-spinner rules for Robot Battles and will debut at Robot Battles for Dragon Con 2017.

Step 1: The Game Plan

Doom Service was designed in Sketchup (Free version because I'm a tightwad. Also I have adult responsibilities like prechool tuition and mortgages.). All dimensions for the build can be found in the attached skp file. (Warning: the wedge and its brackets are best eyeballed to fit -- don't rely on the dimensions for it).

Step 2: General Guidelines for Working Aluminum

This bot, like many before it and many yet to come, is basically an angry pizza box made of Aluminum. (Being a hobbyweight, maybe it's more of an angry breadsticks box?) If you've never worked in Aluminum bar and plate stock before, here are some general techniques that have served me well.

Personal Protective Equipment (that's "PPE" for those of us down with the slang)

Wear safety goggles, a dust mask, and earplugs for any kind of cutting, drilling, hammering, etc., operation. Aluminum shavings can be brutal.

Drilling Aluminum

  1. A red Sharpie makes a good poor man's alternative to marking fluid. Fill in an area of your metal with red before marking and it'll make your mark a lot easier to see.
  2. Mark your location with a scratch awl. If you use a transfer punch to mark your location, got back and poke it with the awl, too.
  3. Use a center punch and mallet to make the divot a little bigger. This will help center your drill bit. It's also a very satisfying way to channel your animal rage.
  4. Drill with a twist bit and use a drop or two of cutting/tapping fluid.
  5. For small holes, clean the edges with a countersink bit (gets rid of the tear-out burrs and gives a lovely chamfered edge). Just a few turns should do it. I do most of the actual drilling with my drill press and keep the countersink bit in my hand drill so I don't have to keep switching bits.

For larger holes (like the 5/8" hole for the weapon shaft bushings) start with a smaller bit (e.g., 1/8") and work your way up with a couple of different sizes. You may also want to use a ream if things are a little too tight.

Cutting aluminum

Aluminum bar stock can be cut to length on a woodworking miter saw. Just go slow and use cutting fluid.

For long cuts in Aluminum plate, use a jigsaw with a wood cutting blade. Don't use a fine-tooth metal-cutting blade; Aluminum will just gum it up. If you don't have a miter saw you can use the jigsaw (or hacksaw) for cutting bar stock to length, too.

De-bur the edges of any cuts with a metal file.

Step 3: The Saw Weapon

Materials used this step

  • (6x) 4-inch mini table saw blades (purchased from Harbor Freight, about $6 each)
  • (6x) Actobotics 1/2" clamping hubs (purchased from, about $6 each)
  • #6 cap screws
  • 1/2" shaft with an 1/8" keyway

Tools used this step

  • drill press
  • hand drill
  • #4 center drill bit

Since I'm building this bot by hand, most of the tolerances and fit are...not tight. This is not one of those steps -- go slow and get this one right.

Each of the blades of the weapon is a 4-inch mini table saw blade with a 1/2-inch arbor hole, bolted to a 1/2-inch clamp hub from Actobotics that clamps it to the long shaft. Getting the holes properly drilled to attach those hubs takes some care.

I was going for repeatability and as-much-precision-as-possible-with-my-tools here (the screw holes for the hubs are approximately 1/8" inch from the arbor hole, so there isn't much wiggle room). The following helped a lot:

  1. As shown in the third and fourth pictures I used a spare piece of 1/2" shaft to center a hub on a blade so I could mark the hole locations with a transfer punch.
  2. As shown in the fifth and sixth pictures, I built a simple jig for my drill press. The jig had another piece of 1/2" shaft positioned so that all four mounting holes were the same distance from the arbor hole. Using that setup I could rotate the blade from hole to hole as I drilled. I left that jig set up until I finished drilling all six blades.

I used a #4 center-drilling bit to make the holes on the drill press. The saw blade metal is fairly hard and the short stubby bit held up well. I did have to go back and enlarge the holes slightly with a conventional bit.

The first picture shows what the completed weapon will look like. Note that I'll be swapping the hollow shaft in the picture for a solid keyed shaft later (after a trip to McMaster-Carr...)

Step 4: Building the Frame

Materials used this step

  • 2"x1/4" Aluminum bar (appr. 24")
  • 1/8" Aluminum plate (two 12"x12" pieces)
  • Medium Nutstrip (enough for 9 pieces with each having at least 2 holes per side)
  • #10x1/2" machine screws
  • 1/2" diameter shaft with 1/8" keyway (appr. 12" long)
  • (2x) bronze bushings (1/2" inner diameter, 5/8" outer diameter, 1/4" long, flanged)

Tools used this step

  • drill press
  • jigsaw
  • miter saw
  • 5/8" twist bit (I'm assuming you have a drill index with smaller sizes, but they won't have one this big.)
  • screwdriver appropriate for your fasteners
  • a very small C-clamp (I think mine is a 2" clamp?)

This is the other place where you need to be careful. As you build the frame it is important to keep the weapon shaft holes aligned, or else the shaft will bind and not turn.

After cutting the side rails to length I taped them together and drilled the weapon shaft hole through both rails at the same time to keep them aligned. The holes aren't quite centered top-to-bottom, which is ok, but I make sure to mark which end is up on both rails so I don't mis-align by flipping one over by accident.

The top and bottom armor pieces are made from 1/8" thick Aluminum plate. I deliberately designed the bot so that each plate could be cut from a standard-size 12"x12" piece (or save a little money and buy a 12"x24" piece). (You could save a little weight and money by making the top armor out of 1/8" UHMW plastic -- use the extra weight for alternate weapon configurations, blinky lights, googly eyes, etc...)

Most of the frame is assembled using medium nutstrip. Work methodically: drill and fasten nutstrip, then dry-fit to the next part to mark the next set of holes. You'll disassemble and reassemble a lot as you go, making sure everything fits (and that the weapon shaft spins freely) before moving on to the next set of holes-to-drill.

To keep the shaft in alignment, I built the frame in the following order:

  1. Right-side rail to back armor. Make sure the L made by fastening these stays level with the bottom armor.
  2. Using the L from the previous step, drill and attach the right-side rail to the bottom armor, then the back armor to the bottom armor.
  3. Left-side rail to bottom armor, making sure to not bind the weapon shaft. Leave the shaft in place (and rotating freely) while marking this rail's holes, and drill the holes in the bottom armor a little oversized so adjustments can be made.
  4. Left-side rail to back armor. Basically force the back armor onto the rail. :)
  5. UHMW firewall, leaving a gap for the weapon belt
  6. Top armor as a solid piece (we'll cut a quadrant out for the battery cover in another step)

Step 5: Interlude of Reuse

You're probably building this from scratch, making this step irrelevant. But on the 0.00004% chance you were nuts enough to build Abrasive Personality and then rip it apart and build this bot from the loot drop, then this step is for you.

Pictured is the drive electronics and motors from Abrasive Personality. I pulled them out without disassembling because I wasn't thinking ahead and I was in a hurry. Then I realized everything was still tuned properly -- "tuned" meaning the leads on the drive motors were already in the proper orientation to make it drive properly (Explanation: one-stick driving requires some trial-and-error to get everything working properly: soldering ESCs to motors, driving, seeing that wheels aren't turning in the right direction, then opening the bot up and swapping the leads on the motors -- it's a process that's something of a hassle).

Before disconnecting all this rats nest I made sure to label the ESCs, motors, and motor leads so that I could reassemble it the same way. Maybe -- just maybe -- I'll not have to tune it when I stuff all this back in its new shell. (Spoiler: it worked!)

Step 6: Drive Motors

Materials used this step

Tools used this step

  • drill press
  • hand drill
  • jigsaw
  • files

The first picture shows a dry-fit of the brackets on a drill motor. The brackets are designed with cutouts so that the screws only hold them to the bottom armor plate. I didn't want to go all the way through the bottom and top plates because that's harder to fabricate *and* harder to disassemble for maintenance (learned that one the hard way...).

The process here is:

  1. Attach the front brackets to the bottom armor. Use the measurements in the design file. (3rd picture)
  2. Mark the "flattened oval" profile of the front backets onto the side rails, and square off the sides as shown in the 4th picture. This is where the motor will poke through the side rail. (Note: It's fine to have a rectangular hole here as long as it is a close fit. A rectangle hole is much easier and cleaner to fabricate than an oval one. Ask me how I know -- there's a reason I'm showing you the left side hole...)
  3. Drill out the corners of the motor hole (5th picture). The holes should be big enough to get the jigsaw blade in.
  4. Use a jigsaw to remove the rest of the material (6th picture). Clean up the hole with a file (a circular file works well here).
  5. Reassemble the side rails and front motor brackets.
  6. Dry fit the motors and rear brackets to the side rails/front brackets.
  7. Mark the location of the rear brackets, disassemble everything, and drill the holes to mount the rear brackets. You can cheat here and drill these holes with a hand drill. It avoids too much disassembly.

Step 7: Weapon Motor and Pulleys

Materials used this step

Tools used this step

  • screwdriver
  • hand drill
  • files
  • reamer
  • drill press
  • arbor press
  • broach set
  • tap set

Overview: The weapon system is pretty simple -- just a drill motor driving the weapon shaft via timing pulleys and a timing belt. Since this is for the open-stage Robot Battles event the spinning weapon must obey the 20 feet/second tip-speed limit; if you're building for another event make sure you check their rules. In my case, the 900 rpm drill motor driving a 4" circular saw blade at a 1:1 gear ratio gives just south of 16 feet/second so I'm good.

My drive motors and weapon motor are all the same. So, in a pinch, the weapon motor can be used as a spare drive motor. It would just be a pushy-bot at that point, but I'd still be in the competition!

Prepare the motor pulley. The motor pulley has to be bored out to, um, whatever size the threaded end of the motor shaft is. Probably 3/8", but your motor might be different. ( Yep, 3/8", but technically a "Q" sized bit to get the tapping sized correctly). Then it gets tapped to match the motor shaft threads (3/8" with 24 threads per inch in my case). Attaching to the motor is now as simple as screwing it on and using the left-hand screw that came with the motor to hold the pulley in place.

Prepare the weapon pulley. The weapon pulley gets bored to 1/2". I used a reamer to bore it a little larger so that it has a sliding fit on the weapon shaft. Now find a buddy (or your local makerspace) with an arbor press and a broach set and have them broach a 1/8" keyway in your pulley.

Prepare a piece of key steel. Cut a piece of key steel as long as the weapon pulley is wide, then round the edges with a file (or grinder) so they don't bind. Now go back and cut and grind a second one. Now you'll have a spare when you inevitably lose one during competition...

Prepare to move out."What are you preparing? You're always preparing! Just go!"

The rest of this step is getting the weapon motor mounted in the correct spot. The pictures tell you pretty much everything you need to know. Use shaft collars to hold the weapon pulley (and its key) in place, then position the weapon motor accordingly. The belt doesn't have to be tight, but you shouldn't be able to stretch it off once the motor is in place and the belt is around both pulleys.

Step 8: Install the Switch

Materials used this step

  • (2x) 4-40 screws, about 1-1/4" long (I cut down some long ones, so not entirely certain what the length should be...), with nuts and washers
  • (1x) very tiny setscrew (again, not sure the size -- the bag I had was unlabeled, probably #3, maybe #4)
  • Deans connector (both male and female)
  • heavy-gauge wire (at least #16, thicker if you can manage it)

Tools this step

  • 3d printer
  • screwdriver
  • allen (hex) wrench
  • soldering iron
  • drill

In Robot Battles rules, a switch is required if your bot has an active weapon. As it turns out, finding an inexpensive off-the-shelf switch that handles the current needs of a bot like this is pretty hard. Most switches you can buy in electronics shops only handle a couple of amps -- this bot could potentially pull 100 amps for a short period of time.

This style of switch is called a removable link and is acceptable for Robot Battles (and other events, so I'm told). The idea is you take a connector with a high current rating (like the Deans connector used here) and short the leads of the male to each other with a loop of wire, creating a handy pull-tab as you do so. One lead of the female is connected to battery hot; the other lead is connected to the load hot (i.e., all the ESC hots). That way, plugging in the male turns the bot on and removing the male turns the bot off.

Protip: make a spare removable link.

The female is mounted to one of the inner rails with a 3d-printed bracket (Sketchup and STL files are attached). Mount it so that the loop of wire is contained within the bot; later we'll cut an access hole in the top armor so the operator can reach in to activate/deactive the bot. Don't let the wire protrude as that gives an opponent a weak point to exploit.

Also, mount so that you have enough space under the bracket to solder wires to the female. I used a piece of 1/8" plastic to space it properly. DO NOT let the leads touch the aluminum armor or they'll short out.

The last pic shows switch mounted, with the battery connector and other leads soldered on. I used Liquid Tape to insulate the switch since space was dicey.

Step 9: Wiring the ESCs

Materials used this step

  • ring terminals
  • wire (I used #10 stranded; anything #16 and up should be fine)
  • (3x) Botbitz 85a ESC (this is a TZ85A brushless ESC that has been firmware-flashed to be a brushed motor controller; buy pre-flashed from Botbitz or you can go cheap like me and flash it yourself!)
  • a couple of random 1/2" bolts and their nuts, sized to go through the ring terminals (probably #6 but who knows?)

Tools used this step

  • wire strippers
  • crimping tool
  • soldering iron

The guiding rule of this step: I suck at wiring. Every time I build a bot I end up making a mess of things at this stage.

The best I can tell you is to very carefully, very deliberately, very professionally shove everything in. The pictures can give you a general idea, but your bot's reality will differ from my bot's reality. As Tim Gunn says: MAKE IT WORK.

The fifth pic shows how everything is wired together. All of the following needs to happen:

  1. Prep the power leads of the ESCs by crimping on ring terminals. Then go back and reinforce the crimp with solder. You'll have to remove the plastic sheath from each terminal beforehand -- it should pull off easily with a small pair of pliers.
  2. Find places for your ESCs, receiver, and battery. Dry fit them in.
  3. Try to connect all ESC hot wires (red) to one bolt (see how I do this in the pics), then do the same for all the ESC ground wires (black). Securing the ring terminals together with a short bolt gives us a compact screw terminal (that can be easily disconnected). This will help with layout and making sure all the leads reach where you need them to.
  4. If you're satisfied with the placement of all the components, go ahead and solder the ESC control leads to their respective motor tabs. Secure with electrical tape or heatshrink.
  5. If everything seems to fit, attach the battery ground to the ESCs' ground screw-terminal. Connect the hot wire coming out of the switch to the ESCs' hot screw-terminal. Wrap both really well with electrical tape.

The last picture (and video) show all the electronics connected for testing. Do this before strapping everything down, because you might have to tweak some things (usually reversing ESC-to-motor wires, swapping channels on the receiver, or reversing channels on the transmitter). I prefer to wait until the very last to install wheels, so I used tape "flags" on the drive motor shafts to figure out which way things were turning.

Step 10: Securing the Battery (and Other Electronic Jibba Jabba)

Materials used this step

  • battery (5S LiPo)
  • short scrap of 2" wide by 3/8" thick UHMW bar
  • 3x #6 plastite screws
  • sticky-back Velcro
  • strap-style Velcro (like a ribbon with the hooks on one side and the loops on the other)
  • cable ties

Tools used this step

  • drill
  • miter saw
  • scissors

The first picture shows the end game here -- get all the electronics secured down. Use a short piece of UHMW to make a firewall between the weapon motor and the battery. It has four holes (as shown in later pictures) used to route some strap-style Velcro and secure the batter to the firewall. The firewall is fastened to the left side rail using 3 plastite screws.

The ESCs and receiver unit are secured to the bottom armor using sticky-backed Velcro. Use cable ties as needed to tidy up the wiring.

Step 11: Cut an Access Hole for the Switch

Materials used this step

  • (4x) #6 plastite screws

Tools used this step

  • drill press
  • jigsaw

Cut a notch out of the top armor big enough to reach your fingers in and insert/remove the removable link switch. Do this by drilling holes for the corners and cutting the straight edges with a jigsaw. Make the notch as small as your fingers will allow (I goofed and made it about 1/2" too long...)

Also, while you're at it, drill holes along the front of the top armor and fasten it to the UHMW at the front of the bot.

Step 12: Power LED

Materials used this step

  • servo cable
  • resistor (anything from 330-ohms to 1K should be fine)
  • panel-mount LED

Tools used this step

  • drill
  • soldering iron

The purpose of this step is to have a power indicator on the outside of the bot. Red lighty mean worky, no lighty mean no worky. It's a bit of a hack, but it works: thanks to the magic of BEC (Battery Elimination Circuit), the receiver gets its power from an ESC and passes that along to anything connected to it via a servo cable. So the LED is wired to the power and ground of a servo cable and plugged into an unused channel of the receiver.

Step 13: Wheels

Materials used this step

  • (2x) 4x7/8 Colson Performa wheel (buy a 3rd so you'll have a spare!)
  • (2x) drill motor hubs. Available from these sources:

Tools used this step

  • phillips and flat screwdrivers
  • bench or woodworkers vise

Pretty straightforward: press the hubs into the wheels using the vise, then screw the hubs onto the drill motor shafts.

The motors will want to turn instead of accepting the hub, so you can lock them temporarily by shoving a flathead screwdriver into the fan blades at the back of the motor (see pictures).

Step 14: Wedge (not Antilles)

Materials used this step

  • 1"x1/4" aluminum bar, about 12-13" long
  • 1/8" aluminum plate (I used the remainder piece from, I think, the top armor. It was approximately 10-3/4"x4-1/8")
  • 1/4" machine screws (and nuts) with low-profile head. I *think* they're 3/4" long, but I haven't measured.
  • a couple scraps of 2"x3/8" UHMW, approximately 1-1/2" to 2" long.
  • plastite screws

Tools used this step

  • jigsaw
  • drill press
  • clamps

This part is a bit haphazard. I didn't originally design the bot to have a wedge, so this was a late addition.

The wedge is just a piece of aluminum plate held on by angled brackets using the 1/4" screws. The brackets are two pieces of 1"x1/4" aluminum bar, each about 6" long, bent at approximately 145-degrees. I'd recommend you eyeball the angle before bending -- the important thing is to have about 1/4" clearance between the wedge and saws.

Per the pictures, I made a poor-man's press brake out of my woodworker's vise and a piece of 2x4. It did most of the bending, enough that I was able to finish it off with pliers and muscle.

Also per the pictures, dry-fit everything together with clamps before marking and drilling your holes. Measurements are no good here -- just adjust and re-adjust until everything works. The key is to get the bracket holes placed on the bottom armor such that the screw heads don't interfere with the components inside the body. Again, eyeball and adjust until everything looks right.

The pictures show two pieces of UHMW under the front edge of the wedge. These are placed in order to raise the edge about 1/8" off the floor (again, eyeball it until you get it right). The Robot Battles stage at Dragon Con is carpeted, so I didn't want the wedge digging into the floor and catching on the carpet (I learned this lesson with my previous bot).

Finally, you'll notice I cut angled chunks off the front corners of the wedge. The stage at Dragon Con is composed of a bunch of stage risers, each edged with thin aluminum strips. Those strips are notorious for getting wedge-bots stuck underneath them. My *hope* is the angled wedge corners will aid me in getting over those strips as long as I don't hit them head-on.

If you'll be competing on a slick stage (e.g., plywood) you can ignore the corner edges and the UHMW spacers -- both are pretty specific to this competition.

Step 15: Front Legs

Materials used this step

  • 2"x3/8" UHM bar (two pieces, about 3-1/2" each)
  • same 1/4" screws I used for the wedge

Tools used this step

  • miter saw
  • drill press
  • forstner bit large enough to accommodate the screw head (probably 5/8")

The pictures tell most of the story -- just two UHMW legs sticking up in case the bot has to run upside-down (for instance, if it gets "T-boner"-ed).

As originally designed, the legs were used in both the upright and upside-down orientations, but once I added in the wedge they were only needed for running inverted and therefore got shortened. The first few pictures show me building the full-length legs, so don't let that throw you.

Step 16: Storage and Transport Crate

Materials used this step

  • lots of scrap wood, mostly 3/4 plywood and 2x4's
  • drywall screws, mostly 1-1/4"

Tools used this step

  • drill (as a power screwdriver)
  • woodworking saws

This is a little crate to make stacking, storing, and moving the bot easier and safer. There's no real rhyme or reason to it, just use whatever wood scraps you have and build up until you have something that locks the bot into place and lets you stack things on it (and under it).

The top is held on with 1/4" machine screws fastened into insert nuts (the pictures show this better).

Removing the top gives a bonus use of the crate: the floor of the crate becomes a riser block to keep the block's wheels off the table. It is a good idea to do this in the pits during competition -- if something happens and the wheels start spinning the bot won't merrily launch itself off the table. Most competitors have at least a chunk of 2x4 in their toolbox for just this purpose; it's considered a good safety practice.

Step 17: Weapon Lock

This is another safety measure: have something to keep the weapon from accidentally spinning up between matches. A mini vice-grip (as shown in the picture) is quick to attach or remove and lots of competitors use them (translation: this wasn't my idea).

Step 18: Ready to Rumble!!!

Now time to practice driving..