Wednesday, December 12, 2012

Divide and conquer

OK, I confess. What I am really trying to do is build a clock from the ground up. As with anything else, building a clock is no mean feat. It also rapidly becomes an obsession. So I am obsessed. I do not think this is grounds for a lawsuit. Anyway, the big thing about a clock is gear-cutting. There are several requirements for this. You need two perpendicular axes. On one of them a cutter is revolved, preferably by power. On the other, the would-be gear is held. On this latter you must have an indexer of some kind, i.e. a gizmo that will turn the (proto)gear by a precise amount. In a previous post I chronicled the making of the latter gizmo, the dividing plate. The gizmo allows me to divide a circle into 60 parts. By skipping holes I could also do 30 parts. In fact, any of the factors of 60, and now you know why they made you study factoring in school. Clock gears go from 120 or so teeth all the way down to 6. A clock, you see, is just a big gearbox. But you cannot shift the gears, unlike an automobile.

So in our last episode I had made the dividing plate. Had I about $300 to spare I could have bought a spin indexer to do the same job. But no. We did a dividing plate by hand (previous post).  So I turned up an arbor from a piece of scrap steel. An arbor in machinistspeak is simply an axle. There it is on the four-jaw chuck, a pain to set up but it runs really really true.  This held my victim, a failure from the dividing plate episode. Much too small to be a dividing plate. Next, I  arranged my Dremel (knock-off) device on to the vertical slide on the lathe. Major project. Too many things to adjust. I have blogged on this contraption before, so see previous posts.

This needs some overhauling. Once the DSO (Dremel Shaped Object is bolted in I cannot adjust it except with the vertical slide, and it has extremely limited travel, 50 mm or so. Not enough. This is why most people do this on a mill. But a (small) mill is about $500 plus shipping, almost as much as the mill itself. And again, I have no space for a mill. But lashup though it may be, I have the two required perpendicular axes. And it cost much less than the mill, i.e. zero.

I have no cutter yet. This is work in progress. In its stead (thak you Richard, for pointing out my error. I said "staid" before),  I put an abrasive cut-off wheel on the dremel. I also arranged a spring-loaded detent for the divider plate. So I am really not cutting gears yet. But the whole two-axis lashup arrangement is working! I cut some remarkably gear-like slots in the sacrificial victim! I was a bit worried about wobble, but none was perceptible.

The classical material for clock gears is brass, sometimes steel. But brass is like Unobtainium in Alaska. Small pieces, at a hobby store, maybe. Big pieces, no. So I am doing all this in plastic. Plastic is everywhere. On dismantling an expired (battery) clock I found plastic gears everywhere. So if they can do it so can I. My  plastic is found. I think it is a piece of refrigerator. So I can experiment all day long. Just as well. Winter is here. Lots of time to experiment.

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