Experiment: Manual Gear Cutting

One of the final touches in completing my school clock will be making its key.

The supplied design for our clock includes a cutout at the 6 o'clock mark in the dial for the winding key to pass through to the barrel arbor. Since my dial is solid (and I hope to keep it that way), my winding mechanism will have to be more complicated.

The idea is to make a ratcheting key that will slide on the arbor behind the dial, but that will require the creation of a ratchet wheel. Ideally, this would be formed through milling... But we haven't learned that yet. Instead, I'm doing it by hand.

Our lathes have 12 dividing holes in the pulley, so I used those to lay out 12 teeth. Using the cross slide, I marked a 7.00 mm circle inside the 12.00 mm diameter of the gear blank, and used a spring divider to form the arcs of the Breguet teeth required for the ratcheting mechanism.

Here's the wheel roughly sawn out, and you can see the remains of the tracing lines. You can also see where my sawing went awry (5 o'clock).

With some filing and facing passes done, the wheel is starting to look more like a working part. The interior recesses between the teeth need to be evened out, since they're what keep the clicks anchored during use, and should be fairly uniform.

I'm using a double click design to compensate for manufacturing defects in the wheel itself. If one recess isn't properly formed and kicks the click out, the other will hopefully catch it. I still have to make springs to push on the "tails" of each click to give them tension, and I haven't even come close to beginning work on the key housing itself. The ratchet wheel will eventually be drilled and given a square hole to match the barrel arbor, as well.

Since this wheel will be hidden in the key, it just has to work—complete precision isn't required. I'm hoping that the safeguards built in with the double click system will keep my clock from unwinding uncontrollably, but there's only one way to find out. Hopefully that will be soon!