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Doug White

wrote in news:94g3g65m52r78e46mvfiknjsrntsul4khv@
4ax.com:

I am turning a mild steel doughnut: OD= 4.5", ID 2.125", thickness
0.1875". The purpose is to clean it up and make the two surfaces
parallel.

The process is always the same: Hold the piece by expanding 3-jaw,
face off, turn the edge, reverse, apply a spacer (a smaller doughnut
held on by superglue), face off the second side, deburr edges. Remove
from the 3-jaw. Clamp in 4 jaw on the outside, indicate the jaws to
within 0.001". Bore the hole till it "looks right". Deburr the inside
edges.

Today, when I got to the 4-jaw stage, I found that the orthogonal jaws
could not be set to the same number. For all intents and purposes the
doughnut now seemed to be an ellipse with one axis 0.014" longer than
the other.

Never mind, I thought, I centered both axes and proceeded to bore. I
did not encounter any problems but when I re-clamped on the 3-jaw it
was clear that the piece was not rotating concentrically. When I
measured it there was a variation of 0.024" in the width of the
doughnut (average width=0.960"), i.e. the hole is eccentric to the
perimeter.

Not that it matters with this piece but what are the possible causes?

My thoughts:

1) I have not indicated the 4-jaw properly. Possible, but has not
happened before.
2) Crummy 4-jaw chuck that came with the 9x20. It is of the "old"
style and a pain to use.
3) Should not indicate the jaws but the piece itself (difficult to
adjust the jaws then, though).
4) After facing both sides the piece was only 0.154" thick and as such
thin enough to flex in the chuck jaws.

4) is a possibility. Another is that the material had internal stresses
which were released by the machining, resulting in distortion. I've seen
this happen big time with plastics like Delrin, but not so much with
steel.

If it was me, I'd turn the OD, and then use a "Step collet" for the rest.
They can machined in place to fit the part precisely, so are guaranteed
concentric. They also apply very even force to the OD of the part,
unlike the high point pressure of a bunch of chuck jaws.

http://travers.com/skulist.asp?Reque...&q=block%20id%
20114899

Doug White