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
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
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.
Actual metal was cut in the preparation of this posting.
13 years ago