If it's an insert shell mill you are looking at then it's more than likely to be 16, 22 or 27mm bore so the metric arbors these fit come with metric drawbar threads, If you find a 3/8W drawbar one it is more than likely to be 5/8 or 7/8 so won't be much use anyway

As M10 is reasonably coarse and the MT taper will do the griping consider making a drawbar from 3/8 round bar with an M10 thread cut on the end

Edited By JasonB on 17/09/2022 16:37:10

I have converted M12 tappings in blank arbors to 3/8 BSW by Loctiting in a piece of M12 studding and then drill and tap to 3/8 BSW.

M10 (0.393" )is 8.5 mm ( 0.3362) Tapping size (Core dia 8.159 mm 0.321" ) 1.5 mm (p 0.059" ) pitch

3/8 BSW is 0.3125" Tapping Size, (Core diameter 0.295" ) 16 tpi (0.06252 ) pitch

The sizes and pitches seem all a little too close for plugging and retapping to be feasible.

My method would be to make up a drawbar with a M10 thread on the end, if that will pass through the spindle.

Howard

Loctite in a short bit of M10 studding, drill and tap M6 and use M6 studding for drawbar. For MT tapers you don't need a strong pull, M6 plenty strong enough.

It would help if the exact bore size of the spindle was posted.

I have two drawbars for my Amadeal mill. The milling R8 arbor is M10, the drill chuck and boring head are 3/8th BSW. I don't find it a problem, I just use the appropriate one.

I have a DW mk1 mill that I made from the kit circa 1980. The spindle blank was supplied pre-drilled through at 11/32" for a 5/16" drawbar. Neither 3/8" nor M10 will fit. The tail of the spindle is 5/8" diameter with two opposed keyways cut into it 3/32" deep. There isn't enough metal left to drill it out for the larger drawbars. The OP might have the mk2 version of course and that might have a larger spindle – I can't help if so.

Andrew