Chuck won’t keep workpiece centered.

MinnesotaGunsmith · Sep 11, 2022

Hello all,

I have been having trouble with my Jet lathe and it’s 4 jaw chuck not keeping my workpiece centered. What I mean by that is after I dial indicate off of the bore using a range rod I’ll turn the lathe and make a few passing cuts. I’ll then check the workpiece using the range rod again and it is no longer centered. In fact, it’s off by up to a hundredth sometimes.

I cannot figure out what I’m doing wrong here. Will someone please give some insight?

Thank you all!

mhajicek · Sep 12, 2022

Is the chuck mounted tightly and securely to the spindle?

john.k · Sep 12, 2022

The Chinese chucks are not hardened on the slots and sliding surfaces in the cast iron body......they wear very rapidly and the jaws become loose in the body....gripping surfaces tilt as the jaws are tightened,and the workpiece is able to move about under cutting load...........If you want a decent chuck ,you must spend many dollars on a Pratt Burnerd."Steel Body" chuck.

MilGunsmith · Sep 12, 2022

Also ensure that the lathe spindle and bearings are warmed up before trying to indicate in the work. Just turn lathe on an let run for 10-15 minutes to get warm and have the oil evenly distributed.

RC Mech · Sep 12, 2022

Not a gun guy, but it sounds like you’re stuffing something in the bore and indicating off that? When you find the range rod is turning eccentric, indicate first the body of the chuck and then the OD of the jaws.

If the OD of the jaws are square or close to it you can get a solid idea of how concentric they are even if the indicator is touching the points/high spots as it rotates. All this is to say narrowing down exactly where the eccentricity is is required.

0.010” is a ludicrous displacement for a tight chuck to be out of round. My bet is on this range rod not repeating if stuffed into the bore.

john.k · Sep 12, 2022

Those not familiar with the $100 Chinese chucks should buy one,just to see how crappy they are..........in some Ive seen the machining hasnt removed the rough cast surface from supposed precision areas..........nothing is hardened except a small area on the gripping surface of the jaws......even then ,the reversed gripping surface is not hardened.

hvnlymachining · Sep 12, 2022

Those Chinese chucks are indeed terrible, we've had one three jaw self centering, and one four jaw independent, the four jaw didn't even last a month before becoming totally useless. And the three jaw was babied enough to last about a year.

michiganbuck · Sep 12, 2022

qt OP ( In fact, it’s off by up to a hundredth sometimes.)

Do you really mean a hundredth, or is it a thousandth, or a ten-thousandth?
* I agree that a good-quality chuck makes a lathe. At least a name brand, and a good name brand is best.
But take a 2x2 and try to lift and push your spindle to see if the bearings and take-up loading bearing are good. Don't pry hard only a one-hand pull should be plenty.

If the part is sticking out far, that can be a problem.
Oh, and be sure all 4 jaws are at contact/holding..it can be possible to make zero and be off one jaw.
One can straight arm wrist turn the T wrench to .001, and then get serious to torque all the jaws to zero and tight.

akajun · Sep 12, 2022

Are you by chance holding a tapered barrel? may be slipping in the jaws some.
I use aluminum flashing between my chuck jaws and the barrel , one to protect the finish, and two to give a better grip on the barrel, you can fold them over too if you need to like shims if the tapers is severe and you cant grab onto a non tapered part of the barrel. this is in a 6 jaw chuck in a tru bore alignment system.
Otherwise I use a "viper fixture" with copper tipped screws or a 4 jaw chuck and a heavy piece of copper wire and an outboard spider to make sure I have a no slip grip.

You should never grip the barrel directly with the jaws.

john.k · Sep 12, 2022

back in the day,after 50 million cycles ,the jaws of quality chucks would come loose .....usually just one jaw would be very loose........the cure was to regrind all the body slots ,and hardchrome th slots in the jaws and grind them to a suitable fit.........another fix was to turn the whole face off the body ,and make up pieces of steel to replace the cut away metal.....the jaws could be reground in the slots to clean up,then the steel segments fitted and secured to the chuck body with countersunk socket screws....although this was generally done after a massive crash had broken one jaw mortice out of the chuck.

john.k · Sep 12, 2022

In really expensive chucks ,the jaw mortice in the body was a separate piece,and could be removed and replaced .....but this was generally larger diametr,3ft and bigger,chucks.

Mechanola · Sep 12, 2022

If you can, use a collet chuck. Jaw chucks are for single pieces and need to be of high quality. No compromise

john.k · Sep 13, 2022

A common way of dealing with wobblu work in a 4 jaw is to tear some little squares of paper and stick them the the outer parts of the jaws with spit......when the chuck is tightened ,the paper will take up the wear and keep the work centred.......if one thickness wont do ,then double the paper up and tighten again.

300sniper · Sep 14, 2022

Post a picture of you setup. Are you holding a tapered barrel using a copper ring, or some other soft material between the chuck jaws?

wesg · Sep 19, 2022

Maybe you've tightened it up too much. And then tweaked it to 'true'. It was bent inside the spindle and making a cut on it made it shift.

It takes a lot less than you might think. Been there, done that.

michiganbuck · Sep 20, 2022

You might hand whack and and then small hammer handle but whack a held part held under an indicator to try to see what is giving way...you might hammer handle but the chuck in a few places to see it is tight/solid.
Blue up a part and tighten to inspect the marking on the part to see if there is a bug, ding or bump on one chuck jaw.

Is the part very long and high-speed RPM causing a whip/balance issue so stressing the chuck's holding ability.?

*You might tell all the details of the part and how it fixtured other than just holding/trued in a 4jaw.

For one example a gun barrel sticking out 2" highly trued in at the out end, but its length whipping around unsupported in the lathe tube bore would be a chancy setup.
A quick fix for this might be a few long taper paint stir sick wedges, or popsicle tapered sticks lightly tapped in at a few places to hold the but-end of the part steady, with a rubber band around if thought to be needed to keep part from wobbling.

dalmatiangirl61 · Sep 20, 2022

OP has not been back since posting this thread.

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