wrote:
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.

Thanks,

Michael Koblic,
Campbell River, BC

I think a faster and easier way to make this part would be to start
with .187 plate, use a hole saw or tram cutter in the drill press to
cut the ID and then trim the corners with a shear or saw and then
finish the OD by mounting the piece on a mandrel.

You could also make a fly cutter / tram that would cut the ID and the
OD in one op. Here it would probably be best to have the ID cut first
and then have the od cut as you might need to have a speed chang

If for some reason you need to use bar stock rather than plate, Then
I would face one end, drill/boar the ID part or cut off the blank and
then use the mandrel to face and size the OD.

As I see it the disadvantage top the way you are doing it now is that
it requires a whole lot of set up changes and having to reestablish
center each time.

Roger Shoaf