Friday, March 1, 2013

The depth of the wheel (and pinion)

Today I finished the last wheel and pinion set in what clockspeak calls the "going train." This is the gearbox that gives you the proper relationship between the beat of the pendulum and (for example) the second hand. It takes three wheel and pinion (big and little gears) to get away with this.  It might take more if your clock has an oddball pendulum. Traditionally we have the "great wheel" which in turn connects via pinion to the to the "center wheel" and so via pinion again to the "third wheel" and so up to the "escape wheel." These terms date from the 16th century or so. When you start reading clockmaking books, or even looking at the internet, the terms reoccur over and over. By constant reading these terms are second nature to me by now, but it sure wasn't so at the outset.

But one problem at a time. Today's problem is "depthing."  More clockspeak. That is you have to put the axles, which are called arbors, so that the gears mesh correctly. Not exactly CNC. They did this easily in the 16th century! In a wooden clock, the technique is different. The holes are specified the by the paper patterns. The gears, as they come off the bandsaw, are guaranteed not to mesh. We are talking of tolerances of less than 0.5 mm. I defy anyone, Superman excepted, to cut something on a bandsaw to within 0.5 mm. That is the width of a thin pencil lead. So the teeth jam. So you have to sand them down so that they do mesh properly.

There are many reasons gear teeth jam. But a really big one is that the teeth are not the same length.  So today, finally learning from experience, I built a jig to get the same length on all the teeth of my last wheel/pinion combo. I thank the collective Internet for this idea. I am not the first person to build a wooden clock.
Above is the jig. It is a piece of wood with a finishing nail driven through it. It is clamped to my table. It bears against a circular sanding disk that came with my $10 jigsaw. You rotate the wheel (or pinion) and eventually it will sand the wheels down so that the teeth are exactly (well, maybe 0.05mm) the same length. In a brass wheel clock, you turn it on a lathe until the wheels are at the final diameter. Much easier. Well, we have chosen a wooden clock as our first effort. So we must deal with it.

Amazingly when I tried this wheel/pinion combo there was only one tight spot. Another reason your wheel/pinion combo sticks is that the teeth are too fat. A bit of work with a fingernail file and she was done. I am now ready to try the arbor-making business. The arbors, or arbours if you prefer the traditional spelling, or even asses in Italian or ejes in Spanish, are the axles on which the gears all turn.

Clockmaking is an obsessive business. I must be obsessive. I am enjoying  it.  The first combo took me four days, or maybe even five. The next set went in three days. This one went in an hour.

The moral of the story is that if you can see a pen line on the pattern, you have not sanded enough!

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