expansion chucking with tommy bars?

[Stephen Hibbs]

I have a grizzly chuck (the cheap $50 one) and I am having trouble keeping the grip on the piece from coming undone when I am expansion chucking (pushing from inside out). I realize it's becuse the way it is tightened with the tommy bars, when expansion chucking, the chuck undone by the force of the lathe spinning. Is there any remedy for this, or is it just not possible to expansion chuck with a tommy-bar style chuck.

[Lee DeRaud]

To some extent it's an issue with any scroll chuck: they are constructed so that the rotation of the lathe tends to tighten it...which of course actually loosens it when used in expansion mode. It's not really the tommy-bar mechanism that's the culprit: the rather loose tolerances of the Grizzly chucks make them more vulnerable to the effect than some others.

You might try mounting the piece, getting it as snug as possible with the bars, and then apply some duct tape to the chuck to "lock" it (inhibit rotation between the two halves). In a really tough case, driving a folded "wedge" of sheet brass into the slot between the halves is even better.

Or buy a better chuck. :p

[Don Baer]

What Lee said, I have the oneway with tommey bars and have no problems with it in the expansion mode.

[Jim Becker]

Ah...a third hand would help, but... ;) I don't use this method, so I'll be of little help with the technicals.

Seriously, be sure that any time you are doing expansion chucking that you have enough wood to support the jaws/pressure. Platters...OK. Deep Bowls...Not!

[Bill Boehme]

It is nearly impossible to back drive a scroll gear (unless tolerances are really sloppy, it might back off). I suspect that maybe it is the wood moving and not the chuck. You could use a Sharpie to make a line across the two halves of the chuck and then check if there was any movement when the turning becomes loose in the chuck. When compression chucking, there will be a certain amount of "spring force" in the wood that will usually keep it from slipping, but when expansion chucking the forces applied by the jaws can exceed the cross-grain tension failure point. The wood will then yield and create a very small crack that you may not be able to see partially because when you remove the chuck, the crack will close back up. The problem with tightening it even further is that it will just exacerbate the problem. It may be that you created the problem initially by applying too much force when tightening the jaws. Next time, use only moderate pressure or even better, make a tenon and use compression chucking.

Bill

[RL Johnson]

"Exacerbate", Bill. I sure hate it when you use those big words around me, especially when I can't find my dictionary.:D

[Stephen Hibbs]

Thanks guys, duct tape had occured to me, but I'm not looking forward to that residue getting all over my chuck. Using a wedge might work too, I'll look into that. Getting a better chuck probably isn't oging to happen any time soon though. I wonder if a very stiff rubber band or some string could hold it still.... Thanks for the input.

[Lee DeRaud]

Thanks guys, duct tape had occured to me, but I'm not looking forward to that residue getting all over my chuck.Actually it's not that bad. I use quite a bit of it to hold bowls to the chuck for finishing up the bottoms...sort of a poor man's vacuum chuck. Good duct tape doesn't leave that much residue unless you leave it on for extended lengths of time, which won't be happening here. And you only need enough to keep the halves of the chuck from turning relative to each other, not to wrap a mummy.

In any event, you're applying it to smooth surfaces that are easy to clean with some mineral spirits or Goo-Gone.

[Bernie Weishapl]

With mine Stephen I found if I made my recess slightly deeper it helped hold better. Now that I have bought a Vicmarc chuck I cannot believe the difference. I can cut a recess 1/8" deep and it will hold no problem.

[Bill Boehme]

Duct tape = solution to all of the worlds problems and even some that are out of this world (remember Apollo 13).

exacerbate = screw things up even worse. Sorry about that -- I let the engineer inside creep out a bit.

If Bernie can use technological, then I ought to be allowed to exacerbate things.

Speaking of exacerbating things, I was given a honey-do to make a wall mounted magazine rack and it has been to long since I have done any flat woodworking. I forgot the correct way to adjust the height of a rail and stile bit set on my router. To make a short story long -- I made a mess of things when I did the cope cut and when I tried to a flying-by-the-seat-of-my-pants fix, I managed to exacerbate things.

Bill