Hi Roy,
Thanks for the link. I looked at the site and wrote to the owner,
Dave Audette, who provided a considerable amount of information in a
detailed reply. One key bit of data is that his runout is about 4
tenths, similar to mine when the collet settles in the chuck
properly. I also found a CNC site which gave collet runout as 5
tenths max so apparently my collet chuck's performance is in the
ballpark.
However, it is interesting that Dave gets 4 tenths runout with a SB
collet holder which I assume is probably concentric to the headstock
MT3 socket. Makes me think my headstock may be poorer than most
since I find the following when indicating various points on the
backplate:
Outer rim = 5 tenths
Land for chuck registry = 2
Inside of MT3 socket = 6+
Inside of 3C chuck = 2
Piece in collet = 2-4 typical, sometimes 10 (tapping with a block of
wood causes it to pop into alignment yielding 2-4)
Logically, a perfectly concentric collet chuck's runout could be no
better than the socket it is mounted in.I use a witness mark to align
my collet chuck to the backplate/MT3 socket so the socket runout
should be cancelled because the chuck was machined in that socket and
replaced in the same orientation. That is, my collet chuck is
slightly eccentric but in a way which compensates for the MT3
socket's runout.
The above figures indicate (no pun intended) that the runout of the
MT3 socket in my backplate is larger than desirable and that Dave's
machine seems to run truer by quite a bit since he gets the same
runout without having machined his chuck in place.
Based on the above I would guess that during manufacture the
backplate is turned to size on a production machine with the chuck
land left slightly larger than final size. The backplate is then
mounted on the lathe and the chuck land is machined to final size in
place to minimize runout.
I intend to polish the inside of the collet chuck in an attempt to
cause it to register the collet the same way each time so I don't
sometimes have to tap the work with a block of wood.
John
--- In 7x12minilathe@..., "roylowenthal"
<roylowenthal@y...> wrote:
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
