On 2008-11-09, Harold and Susan Vordos wrote:
Wes offered excellent advice, but not for the circumstance at hand. The
use of a center will not disclose the condition of the machine, due to the
influence of the location of the tailstock as it relates to the spindle. If
it has any offset, be it by wear or by having been moved for the purpose of
cutting a taper, you introduce error that may have nothing to do with the
condition of the machine.
In order to determine if your lathe is cutting straight, or not, after it
has been leveled properly, the material you use for the test should be such
that the amount sticking out of the chuck is greater in diameter than 1/3 of
the length of extension from the jaws. That will keep deflection out of
the equation.
You mentioned that the finish you're achieving is uneven. That's a dead
giveaway that your tool is wrong. Leaded steel will cut without skipping or
tearing when you have the geometry right----a statement that can't be made
when machining mild steel. Do not use carbide for this test. Use HSS
with a honed edge, one that has a slight chip breaker ground, which will
increase rake angle. Carbide tends to cut with greater tool pressure, so
it isn't conclusive.
What I'd suggest for the test is a piece of aluminum. 6061-T6 or 7075 T-6
is good, as is 2024-T3 or greater. I strongly suggest the diameter be
fairly large, no less than 1-1/4", to minimize bending of the material under
tool pressure.
Check your jaws to see if the chuck is sprung, which allows the material to
deflect easily. When you chuck a known straight pin, there should be full
contact of the jaws, front to rear. If you find clearance at the outside
edge of the jaws, you can expect material deflection, even with large
diameter material.
Small chucks are often sprung, a function of having been over tightened on
short pieces of material.
How about a report once you've changed a few things?
I will try again, with a four jaw, bigger bar, and HSS. I will post my
results.
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