I recently bought a set of three soft jaws for the 8" Pratt-Burned chuck on the Clausing 5914. I used these jaws to hold the flanged sleeves being machined for the cursed Ryobi bench grinder. What was lacking was accurate boring of the jaw tips, so that a long round piece of stock can be held.
There have been various fixtures proposed and used, but one thing caught my eye about these soft jaws: The wells where the 3/8-16 cap head screws go were 0.760" in diameter, far larger than needed to accommodate a hex cap screw head. I suppose that this could be to allow one to use a hex head bolt, but another possibility occurred to me, to use these holes to mount a fixture.
So I made up a fixture from some scrap 6061 aluminum. It's a ring about
6" OD and 2.625" ID and 0.75" thick, with three 0.75" dia pins equispaced on a 4.25" circle mounted perpendicular to the plane of the ring.The three holes to accept the pins are about 0.725" dia, having been made with a resharpened 0.75" dia end mill used as a drill, the hole having been roughed out with a large twist drill so the endmill had little material to remove. The endmill was used to get a smooth round hole, better than a twist drill can do, without going to the trouble of boring the holes.
The holes were then measured (they are not all the exact same size), and the pins were machined to be about 0.002" larger than their holes, leaving a little shoulder, and were trimmed to not quite go through the ring plate, and to stick out about 0.75" when mounted in the ring.
Press-fitting aluminum into aluminum is a bit dicey, as it tends to gall and jam, so a shrink-fit was instead used. A 0.725" diameter hole in aluminum will grow in diameter by (0.725)(350)(22*10^6)= 0.0056" for a
350 degree F rise in temperature, so the ring was heated in an old toaster oven, and the pins were simply slipped into place by hand, and the assembly allowed to cool.At room temperature, those pins are quite rigidly held. I didn't try to push or pull one loose, but it won't be easy.
The ring mounts on the face of the chuck jaws, with the three pins projecting parallel to the spindle rotation axis into the inner three screw wells. The jaws are tightened against the pins by trying to close the jaws until stopped by the ring. The boring tool enters through the center hole in the ring.
Boring of the jaw tips was uneventful, and the jaws now hold a 0.75" dia bar quite firmly, concentric with the axis of rotation.
Joe Gwinn
Actual metal was cut in the preparation of this posting.