4-Jaw Chuck - Indicating?

After observing the advice from the board, I have pretty much ditched my 3-jaw chuck for my 4-jaw that also came with the lathe. I have found that I am getting much quicker at centering pieces in the chuck, so I really have not great desire to go back to the 3-jaw. I am only having one little problem with the 4-jaw, though. Even on small pieces, if I indicate and center near the tailstock end of the piece, the headstock end with gradually become off as I slide the indicator toward that end. If I then center the headstock end, I will gradually become more and more off as I approach the tailstock end? Are my pieces a bit out of true, or what would cause this? Thanks in advance.

Benny

Benny,
It's caused by the chuck not holding a part in line with the spindle axis, but rather "wobbling".

Swap the indicator for a lead, brass or plastic hammer and coax the tailstock end into better centering before final tightening.

I anxiously await Forrest's and other 4 jaw gurus answers to your question as well. I've seen more slop in 4 jaw chucks than 3 jaw and still have questions about their "precision".
Den

benny try this indicate a short section close to the chuck face and center drill. loosen jaws 1 and 2 and move our part out to the length you want lightly engage the tailstock center into the center drilled hole and re indicate close to the chuck this should put you very close to centerline thru your part...jim

It sounds to me like your tailstock center is not aligned with the center of the head.

If the center of the tailstock is pushed towards the tool or away from the tool, it will cut a taper. Put a piece in the machine and take a cut over 12" and see if it cuts a taper. If it is, then you have to re-align the centers. It will take some time to align them. Like Den said, the other guru's will explain how to do this. I do not have the time at the moment. But before you do anything else, run a test piece in there over 12 inches just to see if it cuts a taper. If its smaller at the tailstock then the center is pushed towards the tool. It's a bitch to align, but you will figure it out.

There could be several reasons why your work is not concentric at one end when it has been dialed in at the other. You didn’t mention the amount of error that you observed or the length, diameter and composition of the workpiece,

It may be hasty to assume that there is a problem. As-milled material is not particularly straight and may not even be concentric from end to end.

Obtain a short, stiff, precision bar and repeat your exercise to see if the problem still exists. Ideally, your test piece would be something like a 1 inch diameter heavy-wall steel tube, precision ground and no more than 12 inches long (tube is better than solid rod because the sag, due to it’s own weight, will be less).

If you still find that you have a problem, other tests can be devised to isolate the cause and determine a correction. (Could be something as simple as re-leveling the machine or grinding the chuck jaws …)

But how good are "good" 4 jaw chucks in this regards.

A quality, set-tru 3 jaw, adjusted to zero runout at the chuck may only have 0.001" 8 to 10 inches out. Will a good 4 jaw compare? The ones I've seen look a little crude for that.

i frequently use a .0001" indicator on my 15" clausing. since you are clamping in opposite directions the accuracy is up to you...jim

Glad your having success with the 4 jaw chuck. From your post I gather only little more practice is needed to bring you economy of motion.

4 jaw chucks may have jaws that are splayed to some degree. It's very seldom you can "swallow" the work with the jaws, dial in the chuck end, and have the far end run anywhere near true. You have to be able to move the work in the jaws if you want both ends of the work to run true.

If you're having trouble dialing in long finished work, pad the jaws at or near the tips with short pieces of copper of equal length maybe 1/2" long by the width of the jaw. (I prefer to use heavy gage copper wire rolled into a circle snapped into the first jaw serration.) Dial in the chuck end then tap in the tail stock end. A couple of times back and forth and you'll have the work within 0.001".

Needless to say you will need to support the end of long work with a steady or the tailstock.

Dialing in rough work works the same way but you grip only a short length of it with the bare jaws.

My next article in The Home Shop Machinist magazine (soon in the mail to subscribers, on the stands Nov) discusses 4 jaw chucks (shameless plug disguised as a helpful tip).

For tight tolerace work I do in the Monarch EE I use 4jaw chucks. I have ground and tested the jaws for even contact on the workpiece. This will give the most ridgid work holding, this will also make the workpiece run pretty true far out from the jaws. A light tap with a piece of lead will bring a shaft dead on at a distance from the jaws.
I generaly try to use the smallest body 4jaw I can because when Iam doing this Iam running at very high speeds to take advantage of the machines capability. I should add that Iam gripping ground tool steels.

Don

Sorry for the late response (was out over the weekend), but thanks for all of the suggestions. Forest - I will try your copper trick while truing my piece. Sounds like a neat trick of the trade. Also, I have checked my tailstock center, and I am turning a couple of thousandths off over the length of a 1' piece, so I will be readjusting the tail tonight and seeing if that cures the problem. Thanks again all.

Benny