Lathe chuck problem

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

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.

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

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?

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.

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