The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had it
four years or so. Plus my much older Clausing 12 x 24 never does this. I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

Sure would appreciate any insight. Thanks.

Roger in Vegas
Worlds Greatest Impulse Buyer

Subject: Lathe chuck problem
From: Roger Hull
Date: 19/05/04 04:30 GMT Daylight Time
Message-id: t

The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had it
four years or so. Plus my much older Clausing 12 x 24 never does this. I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

Sure would appreciate any insight. Thanks.

Are you sure the workpiece isn't so small that the jaws are meeting each other
before gripping the part fully?


Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
unsociable and extremely unappealing naked. I'm quite grateful they do though.

"Dave Baker" wrote in message
...
Subject: Lathe chuck problem
From: Roger Hull
Date: 19/05/04 04:30 GMT Daylight Time
Message-id: t

The workpiece slides in (or out, depending on which way I'm cutting) on
my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had
it
four years or so. Plus my much older Clausing 12 x 24 never does this.
I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

Sure would appreciate any insight. Thanks.

Are you sure the workpiece isn't so small that the jaws are meeting each
other
before gripping the part fully?


I'd sure be inclined to consider what Dave said. If not, your jaws might
benefit by some fine teeth, enough to provide a decent grip. Often 3 jaw
chucks have relatively smooth surfaces on the jaws in order to not mar the
items being held. Usually doesn't make much of a difference in that unless
the item matches the profile perfectly (like soft jaws might) the jaws still
tend to indent at the high spots, and often don't grip all that well because
of the smooth surface.

For sure, though, check what Dave said. Makes lots of sense.

Harold

Roger Hull wrote:
The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had it
four years or so. Plus my much older Clausing 12 x 24 never does this. I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

It sounds like you've sprung your jaws. Did you have something
held really tightly just before the slipping started? Or, did you have
a crash?

What it sounds like is the jaw faces are no longer parallel, but
would fit a conical taper. I've had this on larger chucks due
to wear at the front of the jaw faces. Anyway, if the rest of the
chuck is still sound, you need to regrind the jaw faces parallel
to the spindle axis. What you do is mount a Dremel tool or similar
grinder to the toolpost. Open the jaws enough so you can fit a small
grinding wheel between the jaws. Use chewing gum, rubber bands or
whatever you can find to pull the jaws outward, against the face
of the scroll, just like it is gripping a part, but leaving the entire
gripping face of the jaw clear.

Pull the crossslide toward you until the grinding wheel just takes the
slightest bite of the jaw at the back. Run the carriage back and
forth, taking a little off the jaws at a time until the grinder cuts
the full length of the jaws. Repeat on all 3 jaws. Then, set the
lathe spinning at the slowest speed, and use power feed to run the
grinding wheel across the full face of the jaws.

When you are done with this, disassemble the entire chuck, clean
and relube. You will need to run cloth or paper towels through
the jaw grooves to clear all the dust out.

After reassembling the chuck, test it with a smooth, hard rod, like
hardened and ground shafting. If you tried this before, you would
have seen the front of the rod move, and you could have slipped
paper between the jaws and the rod at the jaw tips. now, you
should have no movement and no gap there.

Regrinding jaws is a fairly common procedure in the shop, and it can
make a HUGE difference in chuck performance. When the work starts
walking in the chuck, you know it is time.

Jon

is the chuck at the end of it's chucking travel?
is the part moving or is the chuck moving?
is it tight enough?

"Roger Hull" wrote in message
s.net...
The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had
it
four years or so. Plus my much older Clausing 12 x 24 never does this.
I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

Sure would appreciate any insight. Thanks.

Roger in Vegas
Worlds Greatest Impulse Buyer

In article t,
Roger Hull wrote:
The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had it
four years or so. Plus my much older Clausing 12 x 24 never does this. I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

You don't mention whether this happens at any other size of
workpiece or just this one size. One thing which I would try is to
*remove* the workpiece, and see whether the jaws can be set smaller than
the current workpiece diameter. It might be a chip jammed in the scroll
or in one of the teeth near the outside of the jaws which is preventing
full tightening -- you are tightening against the chip, rather than the
workpiece.

If it fails this test, then it is time to totally disassemble
it, clean the parts individually, inspect under magnification for
embedded chips, and reassemble. If it passes, then the other suggestion
-- that of the jaws being sprung, and only gripping at the inner end
could be the case. If so, then re-grinding the jaws (as already
described in the other followup) might be the best bet.

If it is badly enough sprung, however, a new chuck may be
necessary -- e.g. if the ways in which the jaws move are now warped.

It is generally bad news to try to clamp a short piece at the
tips of the jaws with a lot of force, as this is the kind of thing which
springs the jaws.

One thing which might be worth considering is a 3-jaw for a
Taig. The spindle thread is the same, IIRC, and the Taig has two-piece
jaws, with the top jaws soft, so you can machine them at need to a
precise fit for a project which needs it (e.g. a step behind the
workpiece, to limit the travel into the chuck).

Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

I think Jon is right. Use a spider and hone the jaws.


"Roger Hull" wrote in message
s.net...
The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had
it
four years or so. Plus my much older Clausing 12 x 24 never does this.
I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.

Sure would appreciate any insight. Thanks.

Roger in Vegas
Worlds Greatest Impulse Buyer