assembly

After 3D printing a few more plastic parts and cutting all the aluminium plates, the custom chassis is finally complete! Below are some quick notes on the progress over the last weeks.

More 3D printed parts and painting

I printed off some of the remaining parts of the chassis. The battery compartment was best printed upright with minimal support structure needed. The rear bumper was trickier than the front bumper, because of the additional hole space for the battery, so I found it was best to print upright, with a raft and curved supports at the bottom.

Once all parts were printed, and some more sanding, I spray-painted all the parts with plastic primer, then blue paint, and finally clear sealer.

Metal parts

I was initially thinking of finding an online service to cut out the aluminium chassis parts, but then decided it would be faster and cheaper to just get some sheets 1.5 mm thick aluminium sheets from eBay and cut them on a jigsaw table.
I used Fusion 360’s drawing tool to export to PDF the parts I needed to cut out: four chassis plates and four foot plates. I then printed them in actual scale and glued them on the aluminium to uses as traces.

I threaded the holes on all the 3D parts, which were either 3 mm wide where the aluminium plates attach, or 2 mm at the leg and spine bracket attachment points.
Using a tap for the 3 mm holes worked pretty well, but the 2 mm holes were more prone to being stripped or too loose, so manually threading the holes with the bolts worked better. Another issue was the infill surrounding the internal thread cylinder sometimes being a bit too thin. In retrospect, I’d try designing the 3D parts to use heat-set or expandable inserts, especially for the smaller threads.

The servo brackets attaching to the chassis have a large number of holes (16 for each leg and one of the spine brackets, and 12 for the other spine bracket) so the screws so far seem to secure the brackets well enough. The spine section is under a lot of stress from the wight of the whole chassis and legs, so it will not be able to withstand much twisting force, and the servos may not be strong enough at this area, but I will have to test this in practice with new walking gaits.

Conclusions

The custom chassis has finally made it from a 3D design to a reality, with relative success so far. Some of the threaded holes on the 3D parts are not as strong as I’d like, the AX-12 may be under-powered for the spine connection, and the brackets anchoring the spine may be the first to give way due to twisting forces. Also the chassis as a whole would benefit form weight-saving exercise and perhaps being thinned down. But this has only been the first iteration of the main chassis, and the robot design has now become a reality and seems to stand up well. The next step will see how it performs some of the basic walking gaits!

With the hardware for all four legs gathered, I have assembled the first standalone version of the quadruped. The MakerBeam XL aluminium profiles were adopted as before, to create a temporary chassis.

The fact that the robot can now stand on its four feet meant I could quickly give the walking gaits a test on the real hardware: The Python test software reads the up/down and forward/back position of each leg for a number of frames that make up a walking gait, the IK is solved, and the resulting joint values are streamed via serial over to the Arbotix-M, which simply updates the servo goal positions. No balancing or tweaking has been done whatsoever yet, which is why in the video I have to hold on to the back of the robot to prevent it from tipping backwards.

I took some time to make a video of the progress so far on this robot project:

A chance for some new photos:

Finally, here is an older video showing the Xbox One controller controlling the IK’s foot target position, and a simple serial program on the ArbotiX-M which receives the resulting position commands for each motor (try to overlook the bad video quality):

In the next stage I will start building the robot’s body, as per the CAD design, which is for the most part 3D printed parts and aluminium sheets, combined with the 2 DoF “spine” joints.