The first feet I built for the quad A0 lasted for maybe an hour of walking before snapping off. The current design, has been much more robust – completing a lot of intensive walking and jumping. However, all things must fail:
Looking at the failure, I was surprised I used so little material in the region in question. For now, I just made it 4x thicker and we’ll see how long that lasts, although ultimately it may need to be a different design or machined instead of 3d printed.
The quad A1 was the first robot I built with foam cast feet. When I did the first feet, I jury rigged a fixture from some old toilet paper rolls to hold things in place while they were curing. When I went to rebuild with my most recent leg geometry, I figured it was time to get at least a little more serious. Thus, my new leg casting fixture:
When an insert is cast into place, it is set on one of the trays, the tray is inserted into a slot, and then a weight can be placed on top and constrained by the fixture.
This makes the casting process more repeatable and faster as I scale up production. As a bonus, it can also be used as a fixture to epoxy the lower leg to the insert:
After casting the feet, the final step was to join the lower leg with the 3d printed foot bracket. This I just did with some slow cure epoxy.
It seems strong enough for now, I was able to manually apply 10kg of load to a single leg while perfectly horizontal with no signs of stress, which should be good enough for a 4g 4 legged jump.
All the legs (and a spare) are now assembled with belts and a lower pulley ready to go on a robot!
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.
Next up, we’ll see my experiments in casting!
A Modicum of Fun 