Hello Colin,

In my experience 3C collets are a bit of a mystery. With my Boxford AUD 5″ lathe (1978 vintage) came a drawbar and eight 3C imperial collets. I had trouble in getting them all to hold properly, eventually discovering that four of the collets had a 26tpi thread c. 0.626 diam, while the other four had a 26tpi thread c. 0.644″ diam. No wonder some were sloppy in the drawbar and others tight! I got into serious trouble when one of the collets jammed through over-tightening, the pin in the collet chuck shearing. Took a lot of headache to discover how to release the spinning collet – but that’s another story!

I have therefore made two new drawbars, using a 0.625 x 26 tap to cut the thread in the smaller one, and screwcutting the 0.650 x 26 one (not easy, but satisfying to do!).

Boxford describe the correct thread as 0.647/650 x 26

Rotagrip supplies new ‘3C’ collets threaded 0.640″ x 26

In all cases we’re talking about the 26tpi Brass Gas Thread.

Not sure you should try to reduce the diameter of your collet threads. You could try using a 26tpi chaser (as available from, say, Tracy Tools).

Very interested to hear how you get on.

Best wishes,

Patrick Carnegy