Wilding egg timer - construction question.

Youra -

I suspect that you are about an order of magnitude too big there - I would go for something nearer 2 thou, possibly less.The point of leaving a witness is just to ensure that you don't end up with a wheel that is undersized - what you do is to cut the wheel blank to the correct OD, then coat the cut edge with a marker pen so that the witness is obvious when you cut a pair of adjacent tooth spaces. You adjust the depth of cut so that the witness is only just visible; at that point the depth of cut is correct. If you leave a witness that is too wide, the depth of cut will be too small. This probably isn't really a big deal on a ratchet wheel, other than that the tooth shape won't look quite right if the depth of cut is too small; on a gear wheel if you get the depth wrong it will change the pitch diameter of the wheel, so the centre to centre distance between it and whatever it meshes with will be affected.

Regards, Tony

Tony - many thanks for the reply.

I think I see what you say - however the depth of cut is clearly specified (1/4"), so I had been planning to set up the wheel in a dividing head on the milling machine, and cut each space in one go - that's why the tip thickness is so important - adjusting it will adjust exactly how I make the ratchet wheel cutter (I think) as the vertical part of the tooth is not on a radius of the circle segment that makes up the curved bit.

I'll do some more sketches and see if I can simplify it any further in my own head...

Cheers,

Youra.

Youra -

I think you are talking about the escape wheel (not a ratchet!), in which case, if the tip of the tooth is too wide it may interfere with the proper operation of the escapement.

What cutter does John recommend in the article? It looks like a pretty conventional recoil escape wheel from the pics I can see on the web. Making an escape wheel cutter of the right shape is going to be interesting...

Regards, Tony

Hi

Yes - it is the same shape as an escape wheel - John refers to it as the ratchet wheel in the article though - the escapement proper is made up of the single pin escape wheel crank and the pallet arm slightly higher up the mechanism - in most of the pics on the web, is the thing that's just above the egg marked '6' at the top of the dial ring.

He doesn't recommend a cutter - the article originally had the radial cuts done with a slitting saw on the mill/dividing head, all the curved edges of the teeth marked up by hand and then cut with a hand saw and filed to shape - I thought I'd give making a cutter to suit the teeth a go, based on the instructions in "Making Clocks - workshop practice vol33" - it looks relatively straight forward .

I may rough them out by hand and use the cutter to finish them off, mostly so the cutter lasts for all 48 teeth !

Your point about interfering with the action of the suspended lantern pinion that mates with it is very well made - I'll try and keep below

0.005"....

I'll post some pictures with links when I'm done - unfortunately a specific radius cutter I've ordered to make the form tool with hasn't arrived in time to be used over the BH weekend, so it won't be for a couple of weeks yet.

Thanks !

Youra.

I don't know of this clock, or any special gears in this clock, however all clock train gears (those over 20 teeth made of brass, cyclodial tooth form, and that are driven) have this ''witness'' mark. It does not matter how small, one or two thousands of an inch, the thickness of a human hair, or...... none at all.

It is not necessarily formed by the design of the cutter, but by how deep you bury the cutter. If you have left too much of a mark, make another pass and leave less.

As you have stated that you are making a ratchet gear(click gear in horo speak) will it not have a pawl (click in horo speak) to engage the teeth? Will not this click have a spring tension against it? The surface fit of these two parts is large.

Holding the O.D. on a gear blank(leaving a witness mark) is important to have train teeth mesh properly(at the pitch circle) especially if you have pre-drilled the plate holes. If you fit the gears first(use a depthing tool)then you can plant the gears and drill the holes in the plates to ''fit the fit''.

1/4" inch depth of cut is very unusual unless you are working on turret/tower clocks, an easy way to work out a given ratchet cutter is to multiply the diameter of the wheel with Pi and divide by the number of teeth required, this will give you the pitch, you can then shape your fly cutter to suit, Dave