What is this small hobbing cutter for?

What is this small hobbing cutter for?

This topic has 4 replies, 4 voices, and was last updated 19 April 2020 at 10:23 by Phil P.

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18 April 2020 at 23:29 #465593
John Paton 1
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@johnpaton1
I found this in my box of cutters. As you see the tooth form suggests it s for cutting a ratchet gear but the helix suggests it may be to form a buttress thread.

It has a Bore of 5/16", O.D. 23mm and Width 9mm. Pitch 0.0457 and Helix 0.47

Does anyone recognise what it is for?
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18 April 2020 at 23:29 #19716
John Paton 1
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@johnpaton1
18 April 2020 at 23:38 #465595
Pete Rimmer
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@peterimmer30576
Clock escapement/ ratchet wheel. I've got a bunch of them. That one is in pretty sad condition.
19 April 2020 at 10:02 #465651
Anonymous
The helix value is a red herring. When hobbing a spur gear, or ratchet wheel, the hob is set over at the helix angle of the teeth on the hob so that the tooth forms cut on the work are parallel to the axis

Andrew
19 April 2020 at 10:23 #465660
Phil P
Participant
@philp
Yes it is a ratchet hob, they only cut a very small range of tooth counts to the correct profile.

You will see that the face of the cutting profile is slightly convex, but when the hob is in action the ratchet tooth is produced by a rolling action and so ends up flat when used for the correct tooth count.

I have some of these type of hobs for my Mikron 112 hobber, they are not always marked with the tooth count and a bit of trial and error is need to establish what you can use them for.

To be honest for most of the ratchets I have made for my musical boxes and clocks, I still do them by the good old one tooth at a time method, but if you have a need for a large quantity of one particular size then hobbing is the way to go.

Phil
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