By Mel Doran

The clock and watchmaking industry use cycloidal gears which are chosen because they have constant velocity, whereas involute gears don’t. Involute gears are better for transmitting power. Power is not needed in clocks.

In the ‘old days’, all they used was a ‘slitting’ saw and they just cut straight slits. They then used a ‘topping’ tool. When the wheel had been ‘slitted out’ with ‘straight teeth’, it was then put on centres so it was free to rotate. They then used a tool like a scroll, which is a bit like a worm but sharp, to run around the tops of the teeth in order to round the corners off. Very crude, but that was the way in those days. 

Vertical slide supporting a milling spindle. The cross slide centres the cutter and the vertical slide puts on the cut to full depth at one pass. The wheel is indexed by the GHT worm working on the bull wheel.

This wheel is being cut for a ‘Large Wheel Clock’. The wheel itself is approximately 8” in diameter, and it has 260 teeth. The 260 teeth are running into a 10-leaf pinion. This is the ‘Intermediate Wheel’ which is the driving force.

The fusee drive will give me 14 revs of 260 teeth. If that is running into a 10-leaf pinion, then that is driving 100 teeth into an 8-leaf pinion.

The 8-leaf pinion is driving 144 teeth into a 60-leaf pinion which is the escape wheel.

This setup will give me two weeks running in the clock I’ve designed. That’s why I need so many teeth. Although it’s designed for a two week running period, it should have ‘a day and a bit’ over run with the fusee.

This first wheel, with the fusee (the conical pulley with a helical groove around it) on it , is shown above.

It’s quite a simple setup. There is an overhead drive, driving a cutter spindle with a 0.75 module cutter. It is a ‘form relieved’ cutter made by Thorntons.

One pass through with the cutter will cut a tooth. The cutter then comes back through again.

There is a G H Thomas Dividing Head which is directly working on the bull wheel. The headstock, the bull wheel and the Thomas Dividing Head (which has a scroll (worm) which is set into the bull wheel) are being used together.

The bull wheel has 60 teeth (not like the usual Dividing Head which has 40 teeth). In some way it’s a lot easier for cutting multiple teeth. There’s more chance of finding a match with 60 teeth.

This photo shows the cutter in more detail. The teeth on the cutter are ‘form relieved’ which means that when it needs sharpening, all that is necessary is to sharpen down equally on all the teeth. That makes sure the form of the cutter is maintained. In this photo the wheel is mounted on a face plate.
Here is a better view of the Thomas Dividing Head. The worm can be seen engaging in the bull wheel. The ‘hole plate’ and the lever arm can be seen. One revolution moves the bull wheel one tooth so that 60 revs gives 360° rotation of the work piece. How to cut 260 teeth way to do it with the ‘hole plates’ available?

The ‘hole plate’ seen in the slide has a 78 circle ring of holes followed by 66,50,42,32 and 27.

If you take 60 teeth as a full revolution of the bull wheel and 60 is divided by 260 the answer is 0.2307692.

Using this setup means that fractions can’t be used as it could be with a wheel with less than 60 teeth. After looking at all the ‘hole plates’ he had he ended up with the plate shown with the outer 78 holes. This meant that a sector of 18 holes should be available to give the gap for one tooth: 

  1. 60 revs gives 360° rotation.

  2. 60 x78 = 4680 holes to pick from for a full 360° turn.

  3. Divided 4680 by 260 which equals 18 holes.

  4. Set the sectors on the ‘hole plate’ to 18 holes.

All I needed to do then was cut the first slot, move 18 holes and cut the next slot and so on. I set the depth shallow for his first slot (so as not to groove it), got some ‘engineer’s blue’ and smeared it on the outer periphery of the blank. He then cut it through and then moved it 18 holes to the next position and cut that slot. It ended up with a land with bits of ‘blue’ attached, then took the vertical slide down 10 thou and cut again, moved back 18 holes and cut again. And so on until it just cleared the blue.

Once done, all the settings were left alone, and cut around the whole periphery. At the 5th whole turn, the plunger came back to the start position, which confirmed the setup. After another 5 revs around it came back to the start position again. It just worked out that 5 revs and 18 holes worked out right for the 78 hole circle.

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Editor: David Carpenter