Three-part Elbow Joints

Since the last update I’ve abandoned the idea of creating a two-part spherical hinge for the elbow in favor of a three-part hinge. I could explain the design but I find pictures communicate these ideas better, so there’s a mockup photo to see. Just picture the middle part attached to the model’s shoulder, and the two outer parts pinned to the forearm. The middle part rotates about the axis and that’s the hinge action.

I had a lot of false-starts on this effort. My first attempts involved creating the outer parts first – I trimmed down one of my plastic hemispheres (from the full 7mm radius thickness to 4mm) and attempted to mount that, perfectly centered, onto a strut. When that didn’t work out, I tried spin-sanding it on the dremel (and then building the part back up again with poly-putty, and repeating) in the hopes of making the part more symmetrical about the axis. I think the (styrene) axis I’d mounted it on was a problem, though, subject to flexion. I got as far as mastering the part this way and recasting it, but now I’m not sure if I’ll be able to use those castings.

From there I tried to use two of the castings to create the middle segment – I had all kinds of problems trying to do different poly putty casting maneuvers (many of which wound up stuck to the pattern rather than the part) and ultimately I wasn’t happy with the results. This work went through the same process of spin-sanding and re-puttying as well, but I ultimately gave up on that approach.

My current approach is to build the middle segment first. I started by cutting twelve circles of .5mm styrene and stacking them into a cylinder. I used my new drill press (best Christmas present ever!) to drill a hole down the middle of the cylinder, and I created a turning template with which I’m attempting to form the spherical surface of the part. Once that is done I’ll either re-use the end part I’ve already cast, or I’ll make a new one.

The work so far is promising – more successful, I’d say, than my earlier attempts at using similar template-turning to create spheres. I think my template this time around is more precise, thanks to my circle-cutter and a slight bit more patience. However, there are still other problems. Mainly, the flat surfaces of the middle hinge part still aren’t quite completely planar and perpendicular to the axis. However, I have a plan to correct this. I’ll start with a thick plate – something I’m confident has parallel and planar upper and lower surfaces, and I’ll drill a hole in it with the drill press, and install a 3mm rod in it. Then I’ll apply putty to the hinge part and slide it down the rod until it comes in contact with the plate. The result should be a planar surface of poly putty perpendicular to the axis applied to the hinge part. This depends on the precision of my drill press and the straightness of the rod I use – both of which I’m reasonably confident in.