Centering Rotary Table

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I've got a Jen-u-wine Bridgeport rotary table. It has a smoothly ground, 1.000" hole in the center that I figure I could put to good use. I turned a dowel that is a tight sliding fit into the hole, and about .75" on the other end to fit an ER-32. Took me a good three hours and as many attempts to get that one together, but I turned it as one piece to guarantee concentricity.

Set it up on the mill. Indicated the body of the R8 collet adapter, and the needle didn't move. Indicated the inside, and still perfect. Put the dowel in, and get 7 thou runout. Replace the bearing nut with a plain one, and the runout drops to half a thou. Good 'nuff.

Drop the dowel into the rotary table center hole, jiggle the table a bit, and snug the nuts up. Now, you'd think that would be the end of it. But, I can't leave well enough alone. Put the indicator on the table, indicate off the collet body (that I just verified had no measurable runout), and spun the table with the crank. The needle went around the world. Made a full sweep of the dial. At least 30 thou runout.

I spent the next hour getting the table situated so that there is about half a thou runout, with a plan to true up the center hole by dropping a boring bar through it, followed by a grinding stone. I figured I could touch the side and spin the table till it was all clean.

Then I thought to myself, "Self, you're fixin' to destroy this heavy piece of American iron. Go ask those guys on HM if this is a good idea." Self, is a purty smart fellar at times, so here I am. Is this a good plan? Or should a rotary table have some sort of adjustment for concentricity?
 
If the center bore of the RT has a precision taper (I would expect it does), then I'd find a gentle way to clean it up. If the bore is just a plain bore with no features, then heck yeah, a freshly bored surface would be much easier to pick up with an indicator.
 
I would verify where the problem exist. Either on a granite plate and indicators. Or center the RT on mill. Then put a dowel and get it to be center on the RT. Rotating the table can you get zero runout. If so then remove part and indicate center hole and see the difference. If you can’t get the part to zero then it’s in the RT and my guess is your gonna have to take it apart to see where the problem lies. How flat and parrallel is it?
 
I have a 12" Bridgeport rotary table. It's a straight 1" hole in the center, not tapered.
 
Correct, @Winegrower. It is not tapered. It is awfully smooth, though.
@Cadillac, I think I've done what you're suggesting, just in reverse. I verified that the dowel was centered in the quill. Then brought the table up to center the table's hole on the dowel. Then dropped the table down enough to clear the dowel, so that I could rotate the table, indicating of the part that I just determined had no runout.
That said, I've never used this table. Got it months ago off of a machinist that had it sitting in the back of a storage shed. I stripped it down, degreased, de-rusted, oiled and back together it went. No adjustments that I could tell. Base, big taper bearing, then the table. One big nut to hold it all together. The table itself seems to rest on the base around the sides. Hard to imagine where I might the .030" play from.
 
Long less than 1” diameter end mill in the hole, touch off moving x or y on the inside of the hole, and rotate the table until cleaned. Wouldn’t this ensure the hole is concentric with the rotation?
 
No. I didn't indicate the hole off the spindle directly, but it is so far out, I didn't see much point.
I have a new, long, 1/2" carbide end mill that may get some use tonight.
 
And the plot thickens.
I indicated the center hole as I spun the table. Couldn't see any movement in the needle at all.
Frustrated, I even made a new longer dowel. Didn't help.
Now, I'm starting to think that maybe the hole is tapered just slightly, so that I'm hitting it in some wonky way.
 
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