In MEW 291, I was interested in Brian Wood's account of coping with the curse of the wrong size draw bar thread. Having previously standardised on the once predominant 3/8 Whitworth threaded arbour, I too looked for an easy way to cope with the larger threads. There is no special merit in an M12 drawbar – 3/8 Whitworth can over tighten MT3 more than sufficiently! At one time, there were 14mm spark thread repair inserts that used only the outer half of a 5/8 ANF thread, and the same principle can be used here.

The modification is outlined for a M 12 threaded arbor, but will work similarly for ½ inch. First screw in an M 12 bolt to measure the length of thread. Next, bore out the inner portion of the M12 thread by drilling with 11 mm. A HSS bit usually works even for a hardened thread, or use a carbide milling cutter. Now screw cut an adapter that fits this truncated thread as follows.

Making the Insert

Grind up a lathe tool with a 60 degree V thread profile, then carefully form a flat on the end slightly less wide than one half the thread pitch – say 0.75 mm for the 1.75 mm of the M12 thread – it need not be exact. With a suitable steel bar in the chuck, reduce the diameter to about 11.9 mm over the required length with an additional short section at the beginning to gauge thread depthing, turned to be a close fit to the root of the truncated M12 thread (ie circa 11 mm).

Do not angle the top slide. Screw cut until there is a slight witness on the 11 mm gauging section. At this point there will still be a flat on the crests. Now, while keeping the same diameter setting, take cuts advancing the top slide (deburring as necessary), until the arbour can be screwed on, but without slack.

Finish the adapter by making the internal 3/8 Whitworth thread (ensure deep enough), and facing off the 11 mm stub. Screw on the arbor with thread lock, and once set (apply hot air gun to speed up), part off if feasible safely, or go part way through and hacksaw.

One made much earlier.

Perhaps I should have spelled it out, but the object of using the truncated M12 thread is to achieve a reasonable thickness of material between the threads. For 11mm, it is about 0.7mm, which is quite adequate to take the 3/8 Whitworth tapping forces. It is also fully strong enough in service, after all a wire thread insert works.

Manufacture is easy as outlined provided you can screw-cut in a lathe.