Difficult to understand, but it looks to me that differential threads are involved to get the super fine pitches.

To get 100tpi,you only need 20 and 25 tpi threads working against each other.

No, you have lost me there. Perhaps it would have been better to read "a device for cutting very fine pitch screw threads".

Edited By old mart on 20/01/2023 21:37:18

I have not read the patent beyond what is in the image above but I think the idea is to produce a movement 1/15000" per revolution of a knob or shaft which is equivalent to a very fine pitch screw but is not a physical very fine pitch screw. Easily achieved with differential pitches.

Martin C

This reminded me of a Sci Am article – Vol 232 no 4 April 1975 in their Amateur Scientist section. Described a grating ruling engine built by Brian Manning for ruling interferometer gratings.

The screw pitch problem was addressed by a fine pitch screw of sane tpi driven by a small motor/gearbox with a feedback system controlled by a small interferometer looking at the exact number of wavelengths moved at each step. Used a photomultiplier and a homebrew discharge lamp.

All mounted in a big cast iron box to control temperature etc. This chap even evaporation coated in aluminium his own glass plates and made the diamond cutting tool to make the grooves.