After sitting and pondering and sitting and thinking and sitting and staring off into the distance for far too long I think I have finally cracked the center ankle problem. This whole time I’ve been trying to figure out how to make this shape:
from either a solid billet of aluminum (expensive and hard with many chances to break bits), or by forming/welding a rectangular bit of channel and then welding onto a solid hunk of aluminum for the curvy bits. The problem with the second approach is that aluminum is such a good conductor of heat that the thin walled channel will overheat and melt before the solid hunk even gets up to melting temp. A true welding pro can do it, but me? Not even close.
So what to do, what to do? More sitting and staring off into the distance, and then a long, slow walk around Techshop. I do my best 3D thinking while moving.
Then I looked at the layered horseshoe assembly I’ve been working on and thought OF COURSE! Could I adapt that idea to the center ankle?
So I used Fusion 360 to design the two different layer shapes, exported them as .dxf files into Illustrator, checked the geometries, did a little bit of redesign on the interior of the foot, and then dropped them onto the laser cutter with some 1/4″ plywood for a proof of concept. The plywood was being stubborn and didn’t cut too well (and there was a guy waiting to use the laser anyway), so unfortunately I had to finish the cut on the bandsaw. It turned out very rough but I was more worried about seeing it prototyped than seeing it neat.
Here’s the Illustrator file of the layers:
With the layers (badly) cut, I quickly glued them together, trying my best to keep the layers aligned. For the aluminum version I might cut a couple of alignment holes and then tap threads so that I can run some permanent alignment screws. That way they’ll be nice and tight and aligned before welding.
Once the stack dried I took them into the wood shop and hit them with the big stationary belt sander to clean up the edges and sides.
So if you’re keeping track, I had a brainstorm and was able to use the following process to go from idea to physical prototype:
Fusion360 CAD software
Trotec Laser Cutter
All in the course of about two hours (a little more if you count drying time). The end result?
You can see the inside lip that the corresponding mounting block will rest against. This means that the weight isn’t being borne by the screws (as in the blueprints), but by the structure itself. That was my little idea. Yay me.
And here it is on the upside-down frame. It works! It works GREAT! I’ll double check the measurements tomorrow but it looks great! I’ll just have to make sure that the layers are perfectly aligned before I weld them together since there is literally maybe .001 of slop in that inside joint. If I mess up the alignment it’ll be a whole lot of filing to get it to fit together. Lining up the screw holes isn’t going to be fun either. But hey, if there’s one thing I can do, it’s the slow and steady thing. I might not be the most talented guy around, but I’m stubborner and mule-headeder than anybody.
My prototype isn’t pretty, but it does the job and I think it’s exactly the right way to go about this. I’ll make the curved outer part out of 16ga (.063″) 5052 and bend it on the slip roller, then cut and weld the curved part with the slot in it last (and probably file and sand for hours).
I’ll still have to TIG weld the edges of the aluminum once they’re cut on the Water Jet and ground down to make the little “V” shaped edges. But since I’m working in consistent 1/4″ layers, uneven heat build up shouldn’t be a problem. That’s the beauty of this design and I’m actually rather proud of my solution. I’m fairly certain that I’ll be able to use the same procedure to make the outer feet with just a few alterations for a slightly different geometry. The best news? If I bought these feet online from a builder they’d run around $900 (!!!!).
Doing them this way will be about a quarter of that.
I feel really good about my progress lately. I had been feeling stuck lately what with the horseshoes. But all the thinking and staring off into space finally got things unjammed. I feel like I have some momentum now.
After the ankles will come the feet (a huge multi-month job) and then the motor mounts but those are both on the far side of 40 hours of TIG welding practice. But that’s basically Year 2: Legs in a nutshell. I can’t see the end of this particular tunnel but I feel like I finally have a map that makes sense.
This morning (really early… insomnia) I also bought some aluminum prep cleaner (called, naturally enough, “Alumiprep 33”) that should make the TIG learning a lot less painful. Dirty aluminum is a pain to weld and this stuff cleans it right up. Wasn’t too expensive either.
So yeah…. Center ankle!