making collet adapter from brass (how easy?)

[lynnl]

In the Jan-Feb, 1997 issue of HSM D.E.Johnson published an article 'A 5C Collet Adapter' for the lathe. His design addressed the matter of insufficient spindle bore diameter for a drawbar with internal threads to engage directly the collet threads. He had a separate connector mounted in his assembly that engaged the collet, then the drawbar engaged that connector.
I made the 5C chuck designed by Pat Loop, which is more like the Bison. In this design the collets are pulled in by a closure nut that is sandwiched between the backplate and the collet adapter, with a circular groove on each side containing a row of bearing balls. It functions really well, but I got a little sloppy in a couple of critical places. As a result it has about .003 TIR. I've not been able to determine if that's due to slop in the registry between the backplate and spindle nose, or the hole for the collet itself, or a combination of the two. I've thought of boring the hole out and pressing in an undersize sleeve and then reboring for the collet adapter hole. At any rate this design does permit use of long stock extending back thru the spindle bore, which D.E.Johnson's does not.

P.S. I'd welcome any suggestions as to how best to go about reducing the runout I have in my chuck. It's very useful as is, but it would be awfully nice to have better repeatability.

("... awfully nice.." I guess that's one of those oxymorons isn't it)
[This message has been edited by lynnl (edited 05-10-2002).]

[This message has been edited by lynnl (edited 05-10-2002).]

[Uncle Dunc]

If there was a problem with the spindle threads, I would expect the runout to change every time you mounted the chuck. If it mounts repeatably, it's not a problem, even if the fit is looser than you'd like.

Have you measured the runout at the mouth of the chuck, with no collet installed? If the mouth is on center, then the back of the hole is either off center or too large. It really doesn't matter which, because you can't fix it by boring it out. Your sleeve idea sounds like a very reasonable solution.

If the mouth is off center, and the back of the hole is on center and the right size, you could sleeve the mouth and re-bore it, but you'd have to find a way to measure the runout at the and the diameter at the back. It might be easier to just redo that part.

As a diagnostic, or even as a long term workaround, you might try shim stock between the chuck and the collet, or between the collet and the workpiece, or both.

[This message has been edited by Uncle Dunc (edited 05-10-2002).]