Originally Posted by BillTodd  ...He used disc of brass of the correct diameter to give a integer number of gooves, then fixed the disc on a lathe horzontally, so it was free to revolve around its centre. He cut the gooves by pressing the edge of the disc against a long tap held between centres.... |
This can work like a charm.
I took it a few steps further as I show in post #6 in this thread:
http://www.cnczone.com/forums/showthread.php?t=20781
Doing it with the disc spinning freely you have to get the OD of the disc correct so that the initial cut is 'in phase' after a full revolution, but it is possible to do this with a bit of experimentation. With my setup the disc and tap were geared together so the OD did not need to be as precise.
Using an acme tap actually generates a correct gear tooth form that meshes perfectly with an acme screw with very little backlash. A limitation is that you cannot put very much torque through it because the drive is only taken on one tooth at a time.