fixing problems with subtraction

Solving the interference problem describe in the last blog post was fun and gratifying, but ultimately pointless because there was a more fundamental design problem that was revealed by hand-operating the mechanism for subtraction. 

Using the carry sector lifters for four different functions was clever, but one of them failed: forcing a "warn" on carry warning arms that were raised by movable wires (so that the raised sector lifters could be forced down) allowed the wire to fall before it should if it had been in the "detect 0" position for subtraction.

So there needed to be a different way to return the sector lifters. I thought that would require an additional articulated mechanism: another stepper motor in the prototype, or another cam follower in the eventual all-mechanical version. But it turns out that all it takes is a scythe-like keeper above the carry warning arm that moves vertically with the arm but is fixed in rotational position. 

This is a great simplification, and now the carry warning arms don't have any of the pesky overhangs that are hard to 3D print.

An additional problem with subtraction was interference between the fixed wire and the residual disk of the carry sector where the teeth were removed. No problem: just make the disc smaller, since it's not carrying teeth and the only purpose is the provide a surface for lifting and keeping.

Then I realized that the lifter and keeper axles could be closer, which reduces the cantilever, and the pushing surfaces would be better as ovals that are closer to the axle to reduce the chance of jams caused by sleeves that tilt slightly on the axle. It's all starting to look better.