Converting an MT3 Arbor to 3/8th Whitworth Drawbar Thread

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.

Have done something similar, several times, and found that the thin wall between the crest of the 3/8 and the root of the M12 is not always strong enough to drill and and tap, outside of the arbor.

The alternative is, having pilot drilled for the 3/8 BSW thread, to screw the M12 into the arbor, and secure with an anaerobic adhesive. When cured, the bore can be opened up to tapping size, or very slightly over, for the 3/8 BSW and then to tap the 3/8 BSW thread.

Ideally the M12 "blank" should be screwed into the arbor until it is just below flush.

Howard

I’m not sure that “There’s no special merit in a M12 drawbar” is true. You can buy metric threaded rod almost anywhere, 3/8” Whitworth on the other hand ...

This is a problem which needs to be addressed from time to time.

Some suppliers now only sell M12 threaded arbors/tooling. This is a nuisance for those with lots of existing 3/8 BSW tooling. Some Moll/Drills have a very small bore for the drawbar, so that M12 could be too large to pass through,, so making a new drawbar may not be a solution.

A less satisfactory soultion would be to make up a double threaded bush with a female thread that isd smaller than 3/*BSW, (5/16 BSW or M6 spring to mind ) so that the wall thickness gives the required thread, and then to make up a drawbar to suit.

But some might consider that to be a PITA

A variety of choices are available; take a pick!

Howard

I've come across this before and simply had two 12mm drawbars made from studding, one with the end turned down and threaded 3/8" Now with the Tom Senior R8 x 7/16 UNF captive drawbar, all the odd sizes are being relegated to the drill mill. You have to be careful with R8 arbors from China as some have 12mm threads rather than the proper 7/16 UNF.

 Of course, in many mills which have a 3/8 drawbar, a 12mm one would not fit in the first place. I had to ream out the spindle bore from a 3/8" clearance to 7/16" when changing to R8 from MT2.

Edited By old mart on 24/02/2020 18:32:18

Edited By old mart on 24/02/2020 18:37:59

I have a slight variation on the problem.

I'm running a Centec 2B which has a 3/8"BSW drawbar, and also a Dore Westbury, which has a 1/4" BSW one.

To ensure interchangeability of tooling, I have a number of little slugs made up to convert from 3/8" to 1/4" (and also M10 to 1/4" for a couple of more recent purchases.)

I haven't got round to it yet, but my intention is to make a new drawbar for the Centec, out of 3/8" rod, so it fits the bore OK, but drilled and tapped 1/4" BSW. I will then Loctite a length of high tensile studding into the hole, thus giving me 1/4" drawbars on both machines, so I can use 1/4", 3/8" and M10 threaded MT2 tapers in everything without needing to swap drawbars.
Most likely I'll use a sawn down Allen bolt to provide the high tensile stud.

Bill

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.