Hi Michael G ,

Its a DIY regrind of an ordinary gear cutter for some special purpose . The only use I can think of is to make Meccano style gear wheels which will mesh with any 20 tpi worm .

Its too big in diameter to make curved profile gear teeth for a proper worm wheel so I would guess that its purpose was to make straight across or angled straight teeth which , to a reasonable approximation , would work for light instrument drives or models ,

Very unlikely but cutter could also be used to thread mill the worms themselves .

Regards ,

Michael W

Its not a hob of any sort .

In use to cut worm wheels ordinary ' one tooth at a time ' cutting and indexing would be used .

Really need a shot of the tooth form to get a better idea what the reground has come up with.

— and 20 tpi would be exactly right for Imperial gauges .

It has a demonic undertone…send it back from where it came!

Given that the tooth form appears to be trapezoidal it wouldn't be much use for cutting a worm wheel one tooth at a time. Unlikely as it seems, I think it was probably used for thread milling the worm itself. Like here:

**LINK**

Quite why an involute cutter is being used I'm not sure, as it doesn't produce the normal trapezoidal form. I guess it might be because the cutter was to hand, and traction engine steering is hardly the most precise of mechanisms?

Regards,

Andrew

PS: It is well worth reading all the pages of the above link, fascinating stuff