Margery, any engineering shop should be able to do this for you,
cutting threads is so fundamental in lathe work. They'll want to
know what threadform and pitch you want..... well, you'd best discuss
it with them, but for something soft like silver you'd want a coarse
thread rather than something fine, and perhaps the coarse threads
normally used for cast-iron would be appropriate. Might be a pricey
operation, especially if they have to make up a soft faced fixture to
hold the silver without marking it. Since you're asking for help I
assume there's no home machinist that you know of?
But wait, there's another way, which may be more appropriate for a
hand crafted piece. In his "Jewellery Techniques" I'm sure that Oppi
Untrach describes handmade threads. You wind round wire around the
male part and solder in place, to make the thread on that piece.
Then wind a separate length on top, lying in the grooves, carefully
remove it by unscrewing, and solder it into the female portion.
Never tried it myself though.
Is this to be a travelling inkwell? Seems to me that desktop models
usually have a hinged lid.
Kevin (NW England, UK)