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Patrick Carnegy 1
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    @patrickcarnegy1

    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

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