New three-part elbow joints

In the last update I described work on a three-part hinge that was to start with the use of the drill press to drill a hole through a block of styrene, perpendicular to the bottom face of the block, and use this as the core segment of the hinge. This was the basis of my most recent prototype for the hinge. What I found early on, however, was that the two faces of the block were not parallel due to variances in how I stacked the styrene sheets to create the block, and I had various difficulties in creating the remainder of the hinge. This frustrated me for a while, and I began to consider other ways of making the hinge part, including recasting a 14mm ball bearing and attempting to drill through its center, then cut it up to produce the hinge. I did a couple trials, and found that the process was far too error-prone with the equipment I have, and all the approach’s merits depend on being able to get it right the first time. In contrast, my other techniques have all allowed for initial imprecision, by emphasizing stepwise refinement.

I recently returned to the prototype I’d made with the help of my drill press, and refined my method to improve the work. Earlier work had all been done with a styrene rod as the joint’s axis, when testing or spin-sanding the joint. I found a supplier for a 3mm steel rod to do those jobs: in comparison the styrene rod was a bit undersized (meaning it wouldn’t totally fill a 3mm hole, meaning the parts would wobble) and it was flexible – meaning my precision was limited and my ability to detect precision problems was compromised. The steel rod solved those problems. I also began using the drill press to spin the parts both for sanding the sphere to spherical shape, and for sanding the planes flat and perpendicular to the axis. This combination made it easy to make a lot of progress quickly, and now I feel I have a prototype that is nearly ready for recasting.

I had debated on whether or not to create the counter-sunk areas where the outer hinge parts overlap the inner hinge parts before recasting what I’d built so far – I was concerned about the risk of destroying the work I’d done so far in the attempt to improve upon it. Ultimately, I decided to go for it, and so today I added the overlap between the hinge parts.

As for the joint’s current state: I think it may have a slight wobble relative to the axis but if so it’s minor enough that I’m willing to accept it. There are also minor problems with the planes where the hinge segments meet: as the hinge is turned the distance between the segments changes a little. But at all times there’s a gap there, so the variation isn’t as bad as it’d be as if the gap closed and opened as the joint was turned. The joint end cap was originally made with a hole all the way through it, and I’ve just started capping that hole so the entire outer surface will be spherical in form – that’s a bit rough still. The whole joint is a bit rough in places, in fact, as a result of various poly putty work I’ve been doing, little bits of poly putty have wound up on the joint surface. That’ll be relatively easy to correct. Also note that the pictures show an outer hinge segment on the end of the rod, an inner hinge segment next to it, and another outer hinge segment strung onto the rod: the second hinge segment is relatively crude because I stopped development on it. I had been working on all three parts but at a certain point I decided I had to choose which of the two would be recast (as they’re not identical, and the outer hinge parts should be identical.) It’s in the photos mostly just to show how the final joint will look. Obviously there’s still a lot of cleanup to do – the joint is going to be painted in Alclad Chrome, so all these little pits and bumps have got to go.
The result of recent modificationsAnother viewThe assembled hingeAssembled, another view

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