Carbon copy

A project log for Tardygrade

3D printed quadruped with a tiny BOM

Dejan RisticDejan Ristic 08/02/2021 at 14:180 Comments

While waiting for the new Tardygrade PCB to get shipped from Germany I passed the time by trying out some more exotic filaments for the chassis.

I had wanted to try using nylon CF (carbon fiber) filament. This makes for very strong parts. And they also come out considerably lighter than ones printed in PLA or PETG. Both of these properties are of course desirable. The drawback with nylon however is its tendency to warp.

Luckily Nylon CF doesn't seem to warp as much as regular nylon, but still it does warp noticeably even after getting printed perfectly flat on a glass bed.

Because of this I decided to alternate nylon and PETG parts in a layer cake fashion. Once assembled, the nylon parts gets more or less bent back into shape due to the rigidity of the PETG parts. I used nylon CF for the floor-facing parts with thin walls. This should make the robot tougher and more resistant to fall damage. Also the material looks very nice with its matte finish and almost invisible layer lines.

Initial prints had quite a bit of stringing. With other materials that's pretty easy to clean up. But with nylon CF I found the stringing tends to form shaggy carpets that are very tough and resistant to knife blades, especially inside corners. Since I rely on tight tolerances this stuff would seriously interfere with the mechanism.

After a few more tries I managed to get rid of almost all of the stringing by enabling the "Avoid crossing perimeters" option in PrusaSlicer and bumping up the retraction to 4 mm. I wouldn't generally recommend that much retraction since it can mess up the extruder on some printers, but my Prusa I3 seemed to be able to handle it.

I also lowered the extrusion rate to 95% which resulted in a smoother surface finish in my case.

The other problem was the abrasiveness of the material. To avoid friction I shaved 0.5 mm off the main hull floor to make sure neither the main gear nor the drive gear would come in contact with it. 

I then added a little washer for the drive gear to rest on. It's made to have circular layer lines and can be printed separately with a low friction filament (in this case nylon FX256).

The main gear only makes contact with the eight little protrusions around the well leading down to the bottom actuator. 

The steering gear doesn't move nearly as much as the two other ones, so I think a little bit of friction is ok in that case.

The leg parts were printed in black nylon FX256. Nylon is of course especially suitable for the snap fit parts. They were much easier to unsnap once inserted and should withstand many more cycles than ones made with PLA.

Next I want to try and print the legs with flexible filament. I'm waiting for two spools of TPU that should arrive next week. Let's see how that goes.