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It has been a bit of saga but worth it. Finally cut a screw for the 8 days French alarm clock. One of the four to fasten the pillars to the base, three were missing. I am almost sure that all its sizes are in French lignes (symbol: ‴), to begin with the pitch of 4 threads per ligne (0.564mm). The head is 2‴ x 1 exactly.

The pitch is easy to get if your lathe has a bunch of gears. But the profile is another story, antique threads have rounded crests and valleys. I can only hypotize on how that could be reproduced in an home shop - not easily. 

Anyway, just like my Swiss-exiled counselor has confirmed, with a bit of care in tool grinding and cutting the result is working and acceptable. Original on the right. 

DSC_0405_copy_458x396.jpg

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