In article ,
Greybeard wrote:
On Sun, 13 Feb 2005 08:36:36 -0800, Eric R Snow
wrote:
SNIP
Plugs won't show out of round condition, three point won't show out of
round, you need two point, and there are some out of round conditions
that even two point won't show. (Your chances of seeing that are
somewhere between slim and none.)
SNIP
It is actually pretty common to turn thin walled parts in a three jaw
that will show round when the part is actually triangular. A plug
gauge and a two point gauge will show this. If it gauges larger with
the two point gauge than it does with a plug gauge then there's a good
chance it's a three sided bore. These shapes are called REULEAUX
traingles. You can draw one by first drawing an equilateral triangle.
Then draw three intersecting arcs using as the radius the length of
one side. This will give you a a rounded triangle with a constant
width.
ERS
We always just called it trilobe runout. And yes, it can be pretty
common from chucking.
And this condition, at least, would show up with one of the
three-legged tri-mikes or similar. If it measures differently when you
rotate the mic from pointing to where the chuck jaws contacted to
half-way between them, you can be pretty sure that this is what you
have. (Mark where the chuck jaws contact before removing it, so you know
what orientations to try. And this would *not* be caught by the
telescoping gauge or the inside micrometer with extension tubes.
But if you are working something this thin, that is an argument
for a 6-jaw chuck to minimize the springing. Or even better, turn a ring
with about a 1" radial thickness and a slit to put between the jaws and
the OD of the workpiece, so the force of the jaws is better spread.
Make the ring at least as thick as the length of jaw engagement, and
ideally somewhat longer.
I did not consider ellipticality from wear in my suggestions
before, simply because he was apparently making this *new*. The spring
from thin walls, however, is another matter.
Enjoy,
DoN.
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