How to hold this part?

If you are getting scratches from the soft jaws you are not keeping them clean and probably clamping with much more force than needed. If you can't manage to not get marks after the blind hole is done, maybe you can make a little false center to fit between the .25 hole and tailstock center. You can then pressure drive the part against a spud and finish turn or polish the o.d. There may be some other options if you have any steps or other holes in the part, but the description, as I see it, is a slug with a blind hole on 1 side.

michael

.(clip) Any other ideas would be appreciated ^^^^^^^^^^^^^^ I am a woodturner, and not a machinist--this is how I would do it. Turn a recess in a piece of hardwood that fits the piece. Turn the outside of the wood to about 1/2" larger OD. Hold the piece in the wood, using a 4-jaw chuck and drill the hole. You will not get any marks. The compressive force of the jaws will squeeze the wood against the outside of the metal disk, so it should not slip.

The woodworking scroll chucks I am familiar with have curved jaws which grip all the way around. I'm not sure whether machinists chucks would work the same way.

Just wrap a bit of masking tape round the o/d before you hold it.

Having said that you shouldn't get scratches with a properly bored set of soft jaws if they are clean and bored to the right i/d. Try boring them just a tad oversize then you won't get nicks at the edges of each jaw. Deburr and polish the jaws and blow or brush them out after each op.

Alternatively, machine the last few thou off the o/d as the last op. Once the counterbore is in you can hold the part by pressing back against a wood or nylon pad held in the chuck using another pad against the tailstock into the counterbore. Then turn the o/d and you have perfect concentricity to your counterbore plus a newly machined finish.

How about simply holding it in a "pot" or "step" collet?

This is what they look like:

Ive seen a number of them with a large flat flange, you can bolt soft jaws to, then turn to the proper diameter, though a properly fitted step collet will not leave much if any marks.

Gunner

"At the core of liberalism is the spoiled child - miserable, as all spoiled children are, unsatisfied, demanding, ill-disciplined, despotic and useless. Liberalism is a philosphy of sniveling brats." -- P.J. O'Rourke

Ok, here's another idea. Don't polish the part until you have drilled the hole. You're going to have to deburr it anyway.

Gary

How about cutting a slit in the length of a 1.2" ID brass or plastic bushing to act as a collet. Cheap and fast!

Don't use soft jaws. Use SOFTER jaws. Keep them clean. Nylon or plastic should be fine. You don't need either any real accuracy or gripping force.

You do realize that the soft jaws can be machined in place to fit the part very closely, don't you?

John Martin

Line the jaws with leather.

Ron Thompson On the Beautiful Florida Space Coast, right beside the Kennedy Space Center, USA

The ultimate result of shielding men from the effects of folly is to fill the world with fools.

--Herbert Spencer, English Philosopher (1820-1903)

Use low temp metal. Mount it flat and pour it around it as a clamp. Make the form that you can grip on. Then grip on the low temp metal and drill the work. When done - boiling water or like heat if needed dry and the metal will once again become liquid.

Martin

I made up a special purpose soft chuck from a chunk of Brazilian walnut a couple of years ago to hold some antique doorknobs I was modifying to fit onto the modern Schlage locksets in our home:

You should be able to do something similar, though you'd probably want to come up with a faster closing means than the couple of screw type hose clamps I used.

If you can stand it, more about that insane doorknob project may be found at:

Jeff