See if there's anything here that helps:
Roy
--- In 7x12minilathe@..., "John" <moran03@e...> wrote:
I recently attempted to make a 3C collet chuck to fit the 3MT taper
in my lathe. This seemed to go OK but the first try was not a good
fit (rear of the collet fit sloppy) so I made another which fit
better. The chuck is based on an article by Exactus, 2984 found
here:
As a newbie, I've never seen a collet chuck in action so I don't
know
exactly what to expect from it. What I hoped was that it would
provide simple, non-marking gripping of work with zero runout. My
chuck may be poorly made or perhaps I'm overly optimistic on how it
should work.
I bought a 3C South Bend 3/8 brass collet on eBay as a model to
judge
fit; my goal was to eventually make steel collets to fit the chuck.
When I put this collet into my chuck I find that the runout varies
each time I tighten the drawbar, typically 2-4 tenths, sometimes up
to 1 mil. When the runout is large it gets larger as I move
farther
from the chuck so apparently it is angled somewhere (the work or
the
collet itself); if I tap the test piece on the high side with a
block
of wood it generally will settle to the more typical 2-4 tenths
anywhere along the length of the 1 inch long test piece (a
commercial
3/8 D bit shaft.
Ths narrow part of the SB collet tapers about 6 mils, getting
larger
as one goes from the threads toward the steep taper. I made the
chuck
so it is an easy fit for the first part of the collet but I have to
press firmly to fully insert it due to the collet's taper.
Similarly,
it requires a push with the drawbar to get enough to protrude so it
can be pulled out.
With the above as background, I have a number of newbie questions
on
collets and collet chucks:
1. What is the expected runout when using a collet?
2. Is the taper on the narrow part of the collet machined in or
does
this happen when they add the slits? Will this also be present on
steel collets or just on brass?
3. What surface(s) are critical in providing the collet centering.
That is, is the fit of the chuck onto the rear of the collet
critical? Does the chuck need to be tapered in the narrow bore to
match the collet? Is there a specification for the steep angle on a
3C?
Any help here would be appreciated. I've put quite a bit of time
into making this chuck and now don't know whether it is useful or
just a fancy doorstop.
John
