"jim rozen" wrote in message
...
In article , Harold & Susan Vordos says...

Yes, they do, and in the process the shaft is constantly creating more
and
more clearance between the centers as it cold flows to achieve the form.
..
Look closely at centers that have been run offset to see how badly they
are
deformed from the center drilled configuration. Further, if, when
turning
between centers, if the face on either end of a part is not perfectly
square, it has the effect of creating an out-of-round (oval) turn.
This
very concept has been the subject of endless debate, with almost no one
in
agreement, but all it takes is a little experience in precision grinding
to
put it directly into focus. Be certain to maintain right angles on the
ends of offset turned parts unless you don't mind oval turns.

Your memory is incorrect in this regard. Under normal conditions,
the centers don't open up, and in the case I tested, the end of
the part *wasn't* square to the machine axis, and it *did* turn
a round, not oval piece. To remind folks of the tests that were
done:

http://www.metalworking.com/DropBox/_2001_retired_files/offcenters.txt
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters1.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters2.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters3.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters4.jpg

One of the regulars here at that time tested the roundness of the
turned part, it showed no systematic deviation from round to the
limit of the tallyrond tester.

Jim

Chuckle! Or perhaps big belly laugh!!

Yep, I remember, and I commend you for the great pictures, but that's not
what we're talking about. My point is turning a taper with an offset
*tailstock* center, although it's possible I never made that clear in my
original argument. It's not the same thing. The degree of error in
drilling offset centers remains constant and there is no movement of the
part as it relates to the fixed, but *in line* centers in the test you
performed. The machine centers, in your specimen, would pick the high
spots and run there, likely not fully seated, but with enough area of
contact to perform without distorting. When you offset the tailstock,
everything changes. You didn't prove your point originally, I simply quit
talking about it because I had quit following RCM (sort of like not talking
to your family, I discovered).

Try that same test, this time offset the tailstock, and for purpose of
proving whether you're right, or I am, turn a much shorter piece, with a
large offset, so it's exaggerated. Be certain that the faces are not at
right angles to the center, which is a part of my argument. You'll not only
mush the centers, you'll detect an oval. Grinders (the machines, not the
operators) don't lie. By the way, you shouldn't need any special machine
to learn what I'm talking about. Simply measuring the part will disclose
the oval. It will be fairly obvious.

Harold