Taping a centering hole in MT3 Rotary table sleeve

[TomB]

I have always found that the fastest and most accurate method is to put a big dead center in the mill spindle then gradually lower the spindle until the dead center hits up against the hole in the rotary table(RT). By leaving the rotary to table hold down screws a bit lose I can then jog the RT around until the dead center drops fully into the
RT hole at which point I tighten the RT to the mill table and then use an edge finder and the DRO system to zero the table at that point. My machine is a Shopmaster 3-in-1 with an R8 mill spindle and to make the alignment process work I bought a R8 type dead center with a big cone point. I read all table move distances from the DRO so alignment of crank handles is irrelevant.

[larry_g]

Iam not sure if I can ask this succinctly enuf... but here goes: is it reasonable to expect to be able to achieve .001-.002" accuracy when drilling and taping a 3/4" deep 3/8 x16 center hole in a shop made MT3 sleeve? The MT sleeve fits very well in the center hole of my rotary table. BUt, a 3/8" shoulder bolt screwed into the hole is out of true by .010" in one direction, and .015" in the other direction. TIR is .025" over 180* of arc.

I want to use a 3/8" bolt screwed into the tapped hole to quickly center the RT with the quill of my mill. ALSO, use the bolt to center and hold several parts that need a radius cut on the outer edges. This bolt would be the center pivot for the radius for many parts.

Method: I turned down the sleeve by center drilling both ends, then mounting on centers in the lathe and tapered the raw stock by Offsetting the tailstock the required amount. Finally mounted the piece in a four jaw chuck and drilled and tapped the big end for 3/8" threads. I tapped the hole by hand in the lathe- holding the tap on the center line with the tailstock, and turning by hand.

However when mounted in the sleeve, and the sleeve mounted in the RT, the bolt has a total runout of .025" out of true.

If I make another sleve with this method, is it reasonable to expect less than .002" deviation from true center for the threaded hole. If not, what would be reasonable runout, given a hand tapped hole.

(BTW. My alternate plan is to mount the existing sleeve and dial in the RT on the mill. Then plunge mill a new slightly larger center hole over the top of the existing threaded hole. Then either use a 3" drift pin or larger threaded bolt as a centering stud/work center.)

Thanks much for any comments on feasibility, or improvements to my method.
Glenn

A lot of good stuff has been advised above but I'll offer another. If your using a shoulder bolt to screw in the taper then bore the first 1/2" or so of the hole to fit the shoulder size and then the threads below that. Then your centering should be on the ground shoulder of the bolt and not rely on the threaded portion. Leave the threads a bit loose to allow centering to take place if you happen to get the threaded hole a bit out of concentric with the reamed/bored part.

lg
no neat sig line

[Magicniner]

A lot of good stuff has been advised above but I'll offer another. If your using a shoulder bolt to screw in the taper then bore the first 1/2" or so of the hole to fit the shoulder size and then the threads below that. Then your centering should be on the ground shoulder of the bolt and not rely on the threaded portion. Leave the threads a bit loose to allow centering to take place if you happen to get the threaded hole a bit out of concentric with the reamed/bored part.

lg
no neat sig line

You just described "A Register"

- Nick