3lb Plastic Combat Robot - Build Log
So you want to know how to make a Battlebot out of plastic! Here you go: Milling the chassis and weapon, 3d printing the internal structure, and the finished product.
I bought a 12x6x1.5 piece of UHMW so that I could make two chassis if necessary. For cutting UHMW a 4-10 tooth per inch wood bandsaw works great, whereas a fine toothed metal bandsaw causes the plastic to melt and gum up.
I underestimated how much time it would take me to go from a solid model to a program for the CNC mill to run. "Computer-Aided Manufacturing" (CAM) software seems like a misnomer to me. I was fighting Autodesk Invenotor + MasterCam2015 at every step. I had problems importing my solid model, I had to hack around in the back end of the program to fix some settings that were wrong in its definition of a the Tormach 1100 CNC Mill I was using, and then the program was terrible at generating good tool paths. After much fiddling we began:
I could not use liquid coolant because coolant + plastic generally makes sludge that is hard to clean out of the machine. The airgun worked well to clear the chips out but because they were so light and statically charged they went EVERYWHERE.
Machining the frame from a single piece of solid material went well, but hey! If we are already over-doing it we might as well go crazy! Apparently polycarbonate can be bent at room temperature. If you haven't bent sheet metal before there is a little bit of math to it. If you want to make a single right angle bend to form a bracket that is 2 inches on each side, you actually start with a piece of material, called your flat pattern, that is less than 4 inches long. A more detailed explanation of this can be found here. My armor plates were effectively very shallow boxes that fit the body of the robot very snugly. To get that tight fit the flat pattern had to be just right. After a few test bends I had everything dialed in.
Since there are bends on all four sides of the armor-boxes I had to use a break with removable fingers. Without that feature I would crush two sides of the box when trying to make the final bend. As you can see below I avoid that problem by using a set of fingers that are slightly more narrow than the box.
And the final result! The near side of the robot has a snug fit polycarbonate shell.
Additionally a number of holes had to be drilled to attach the polycarbonate armor, and internal components. It was largely uneventful UNTIL THE MACHINE CRASHED ON THE FINAL OPERATION. I noticed that something was wrong and hit the emergency stop, but the inertia of the machine carried the chuck of the drill into the side of the robot. I took a chunk out of one of the weapon supports and bent it but the UHMW saved me. I was able to bend everything back into place. Sorry I dont have a picture!
The weapon was one of the more challenging things to make. Not because the shape was very complicated, but because the tolerances had to be kept or the whole thing would be out of balance and vibrate itself apart. Oh yeah, and because cutting fiberglass emits dust that you need to control lest you enjoy breathing glass or destroying your mill by adding abrasive particles to the coolant. With help from Techshop's professional machinist I was able to calculate feeds and speeds that made big beefy chips that were easy to clean up. A shop vac was able to suck up any of the smaller dust that was created.
The weapon actually came out great, everything was within 2 thousands of the nominal dimensions. Unfortunately I made a pretty bad decision in the very first machining operation. The rod I received was not exactly 1.5 inches on each side. In order to make the weapon slightly bigger I put the teeth on the sides that had the greatest distance between them (1.56 inches if I recall). That negligible increase in size came at great expense. Rather than having the teeth impact the enemy with the surface of a fiberglass layer, they strike with the laminated side! That makes it very easy to shear layers off. Doh!
The internals consisted of 2x top motor mounts, 2x bottom motor mounts, a battery holder, a weapon cable guide and an electronics tray (to keep the wires from being able to enter the wheel wells). They were 3d printed out of PLA and heat-set threaded inserts were pressed into them. I printed them with 3 shell layers and 30% octagonal infill. Printing went smoothly except that the Makerbot printed screw holes smaller than they appeared in the model.
It was not until the night before the event that all of the parts were complete and ready for assembly. I knew this would be the case, so I had 3D printed mock-ups of the major components and test fit them earlier in the week. Unfortunately upon assembly I noticed a few things that were off. The wheels stuck out from the top much more than they did from the bottom and the battery was being compressed in one dimension. I quickly redesigned those parts and began them printing.
While those parts printed, I turned my attention to the weapon motor. I removed the rotor (a.k.a. the bell) from the outrunner stator. I have heard that the rotor magnets are sometimes not secured well. I carefully filled the gaps between them with JB Weld Epoxy. This turned out to be a mistake. I did not realize the JB Weld literally contains iron. The magnets pulled the iron filled epoxy into mounds as the part dried, creating interference between the stator and the rotor. Coincidentally, the stator of that motor was dead on arrival from Hobbyking because a stray winding stuck outside of the stator and its insulation had been rubbed off. That motor was a complete bust, thankfully I ordered two! Below you can see the one I ruined with JB Weld.
I disassembled the second motor and secured the magnets with standard clear epoxy. I then epoxied the rotor into the weapon. I would have preferred a press fit, but I had machined the hole a little big. The assembly process was clearly becoming an all nighter. At some time around 3AM the new motor mounts and battery holder finished 3D printing. I assembled everything and began to unceremoniously tape all the electronics in a 9mm gap between the internal components and the top cover.
Even after changing the height of the motor mounts so that the wheels were centered I was still having ground clearance issues. My polycarbonate armor, which I had not included in the CAD model, was the culprit. I openly wept as I removed some of the offending sections from my pristine armor with a dremel. Lastly I created a top cover to hold the electronics in and glued neodymium magnets to the robot. It is very difficult to glue things to polycarbonate or UHMW. Luckily I had done my research and purchased some special Loctite two part plastics glue. Unfortunately the glue created a very weak bond and most of the magnets fell off before the robot even began it's first fight.
In my next post I will give a play by play account of the fights once the footage is edited and posted.
EDIT: February and the fights are not up yet... startups are a busy place.