"Gonadicus" wrote in message
. au...
On 6/08/2013 10:09 AM, Terry Coombs wrote:
"BQ340" wrote in message
...
On 8/5/2013 9:02 PM, wrote:
I was trying to make two rings from copper pipes, 1.625" OD (wall
thickness 0.061") and 2.125" OD (wall thickness 0.035") respectively.
The length of the ring was to be 0.75".

I cut the pipe oversize on my bandsaw. As expected the cut was not
exactly square. I clamped the first ring initially on the inside in my
3-jaw chuck trying to make sure that the sides of the piece were at
right angles to the body of the chuck as determined by a square.

I faced off the first side and deburred it. I reversed the ring and
clamped it the same way with the faced-off part flat against the jaws
base. I faced off the other end to the required dimension. Everything
looked good.

However, when I checked the final product with a square it was obvious
that neither plane of the ring was square to the sides. I did an
additional check with a height gauge with the ring on a flat (well, as
flat as I could get) surface. There was a difference of 0.017" in
height of the ring around the circumference.

I tried a second ring of the same diameter, this time clamping on the
outside. I checked the position with the square in three different
spots. The interesting part was that the fit was square in two of them
but not in the third.

Anyway, the result was almost identical and I got the same result with
the larger ring.

Although the result is adequate for my purposes I would like to know
why I was unable to make the rings properly. Was I not following the
correct procedure? Where in the procedure could the error have
occurred? I have never before had a problem with facing-off being out
of square with the sides.

Thanks.

Michael Koblic,
Campbell River, BC


Copper as you know, is very soft. You can't clamp it very hard before it
distorts & also it will move in the chuck unless you take very light
cuts.
You did not say but I assume you deburred it before chucking in the
lathe?

Maybe check it with the square after you faced one end but before you
unclamp it to see if it had moved in the chuck during facing?

You would probably need to put it on a mandrel for perfect accuracy.

MikeB

--
Email is valid

Yup , mandrel . Bolt in the end with a rubber washer under a regular
steel
washer I'd machine the steel with the mandrel , trim the rubber with a
knife/sandpaper . Tighten the bolt , rubber expands and grips the
sleeve .
Take light cuts ...



I tried a similar thing but to make the mandrel I used a bar just under
the ID of the tube I wanted to machine. I then put pieces of thin wall
tube over the mandrel with O rings spaced along in between. When it was
tightened down the O rings expanded outward holding the tube in place.

===============TUBING TO BE MACHINED==================
|tube O tube O tube O tube O tube O tube O|
================================================== ====|
mandrel BOLT
================================================== ====|
|tube O tube O tube O tube O tube O tube O|
===============TUBING TO BE MACHINED==================



How much runout did you get ? Were the pieces of thinwall between the
o-rings a tight slip fit ?
--
Snag