Previously, I described the overall plan for my improved foot. To make that work, I needed to cast a 3d printed part into the squash ball such that it would likely stay attached during operation, be suitable rigid and yet damped, and do so repeatably.
To start with, I used a random single yellow dot squash ball with a hole cut in one side using a pair of side cutters. For the casting foam, I just used Smooth-On Flex Foam-IT 17, which is what Ben Katz originally used at least. Initially I just mixed up a batch, poured it in to a random level, stuck my bracket in and hoped for the best.
Well, something sure happened! But not exactly what I wanted. The foam didn’t fill in the interior cavity, nor make a great connection overall with the bracket. On top of that, the process wouldn’t exactly be described as “repeatable”. Since I just eyeballed the level of foam, there was no way to get the same amount in.
For my next runs, I decided to do everything by weight. I tried a few different foam masses, curing orientations, and venting strategies. Eventually, I got something that seems to look pretty good. We’ll see how well it works on the actual machine shortly!
Here’s a bunch of different intermediate attempts:
And here’s a cutaway of the process I’ve settled on for now. This particular one has a slight bit of overfill on one edge that is more than is typical, but the inside fill is pretty good:
As mentioned long ago in my post on failing more gracefully, it was obvious I wanted to strengthen the lower leg and foot mechanism to remove the point of failure observed there. For now, I’m attempting to basically copy the original Mini-Cheetah foot principle, although with more 3d printing and less machining.
The basic idea is to print the entire lower leg in a single go laying on its side, so that delamination is unlikely. The foot bracket will be cast into a squash ball, then epoxied onto the lower leg.
One of the parts on the original quad A0’s leg that was prone to failure was the “knee stud”, a little cylinder that acted as the mating interface between the upper leg and the lower leg. It directly attaches to the upper leg, and has bearings that ride between it and the lower leg. The entire tension of the leg belt is born in shear by this part.
In the mk1 leg, this part was 3d printed with heat set inserts used to form the threaded holes. This mostly worked, although occasionally the stud could shear along the 3d printed lamination lines. Thus, for the mk2 leg, I’m making this part out of 6061.
The first op takes a 0.875 inch cylinder, and does all the work on one of the sides. That includes roughing it down to length, getting the outer diameter that the bearing rests on accurate, and drilling and threading the holes.
At that point, the part is turned over and bolted into a 3d printed fixture.
Then, all the tool paths are repeated on the other side, as well as the middle being cut away. I didn’t really worry about surface finish on the middle section, since it will never be seen. This of course would be much easier on a CNC lathe with live tooling, but hey, you use what you’ve got!
Since the mk2 moteus servo has slightly different dimensions and a different mounting pattern than my original, I needed up update the full rotation leg design to handle it. The basic concept is the same, except for some in-progress work on the foot design which I’ll write up later. The only significant changes were that because of the mk2 design, access to the power and data connectors is much easier.
First, after doing some analysis, it appeared that the 3mm pitch 6mm wide belt was unlikely to be able to carry the full torque from the motors. So I’ve switched to a 5mm pitch 15mm wide belt, which while still unable to carry the full torque indefinitely is only a factor of 2 or 3 off instead of a factor of 20 off. Secondly, I added a bearing opposite the upper pulley so that it is supported from both sides. The recommended belt tension for this belt works out to something like 120lb, which is a fair amount of cantilevering, even over the 16mm wide pulley. The updated CAD looks like:
And the newly added bearing can be seen in this section view:
I did a first test print of all these parts and put them together. While there were a few tweaks necessary for the second revision, it looks like this leg is probably usable.
Another of the failure modes observed during the 2019 Maker Faire was in my quickly slapped together leg design. The shoulder joint was required to squeeze two motors together against a strongly tensioned belt, using nothing but a relatively thin section of printed plastic. This caused it to deform, leading to belt tooth skipping, and then eventually to fail, leading to delamination of the shoulder joint.
My plan to resolve this is to switch to a leg design where the upper and lower leg are in series rather than opposing one another. This is more like the Mini-Cheetah design from Ben Katz. This has the benefit of getting the leg out to the side, so the upper leg is free to rotate 360 degrees, only limited by cable harnessing. As seems to be my pattern, I’ll try making something out of 3d printed PETG first, optimize it some, and if I fail there, switch to metal. Here’s a render of the current CAD:
Eric from CireRobotics helpfully pointed out that I’m way over the design limit for the 6mm Gates belt I was using, so I’ll also be trying to bump up to a beefier belt in this iteration.