Material pulling out of 3 jaw chuck during cutoff operation.....Why?

[nc5a]

For the most part I've had very few problems doing parting or grooving operations on a lathe. But lately I've had more than my share, or so it seems. Take today for instance, I'm parting off 12 each 1/2" washers, 0.150" thick from 1.75" cold roll stock. I'm chucked up full depth in a new Bison 6" 3 jaw chuck and the material extends 4" from the jaws and the parting tool is perpendicular to the work. The facing and turning operation go smoothly as does drilling the 3" deep 1/2" center hole.

So I'm turning at 85 rpm and feeding the 0.120" insert cutoff tool at 0.0024" ipr. It's clear in short order that something is wrong. Then 3/8" into the parting operation the insert breaks. I install a new insert and finish the 1st washer but I notice the finish is very poor. So I face the stock and start to part another one off. Once again it's clear that something is wrong. Then I noticed that the stock is being pulled out of the 3 jaw chuck during the parting operation, no problem I'll just tighten the jaws.

Well as you probably guessed that didn't help either so I put the 4 jaw on and finished the job without issues.

So, assuming the 3 jaw chuck is tight and everything else is ok what could cause the material to be pulled out of the chuck when parting? The only thing I can think of that might cause a problem is having the stock extending 4" from the jaws. But the material is 1.75" in diameter and unlikely to flex. I don't think my feed rate was too aggressive and my spindle speed was about right. Even if I was a little off on my feed and speed it shouldn't have caused the stock to be pulled out of the 3 jaw.

Any ideas?

[dalee100]

Hi,

Carbide insert? Kick your rpms up to 900 - 1000rpms and feed .004" to .006" min. Your finish will be better and the insert much happier with you. And the pull out will probably go away.

Dalee

[J Tiers]

Bell-mouthed jaws is one possible reason.

The stock flops around in the jaws, moving outward slightly every revolution. It may be only 0.001" of "flop", but the movement it creates adds up over a number of revs

[Mcgyver]

4" is way to much, even with the large dia. Part of close to the jaws or saw/face. You're putting a high radial load way out from the chuck - its the same way work in a steady will walk out of the chuck. and Dalee, is also right, if using a carbide us it at its proper speed.

What Jerry say as well, if the grip is at the back of the jaws, the the how/why of it walking out is exacerbated

[JoeLee]

Out of round stock would be my first guess. CR isn't perfectly round to begin with and you may have some really out of tolerance stuff like some of the "sub prim" crap I got from Discount Steel years ago. If it's out of round enough your 3 jaw may not be biting down equally on all three jaws no matter how much you tighten up the chuck. If this is what is happening it's not doing the chuck any good either.
What I would try is turn one end of it to make sure it's round and then see if it still pulls out.
Measure the stock and / or indicate it and see how out of round it is.

JL..................

[CalM]

Support the work with a tail stock center ........ until it parts away.

When the holding portion is less than 2mm dia. there is not much force left to rip the work from the jaws.

[nc5a]

Hi,

Carbide insert? Kick your rpms up to 900 - 1000rpms and feed .004" to .006" min. Your finish will be better and the insert much happier with you. And the pull out will probably go away.

Dalee

Damn, I thought I was closer to the right speed and feed but after checking the chart you are way closer than I was.

Bell-mouthed jaws is one possible reason.

The stock flops around in the jaws, moving outward slightly every revolution. It may be only 0.001" of "flop", but the movement it creates adds up over a number of revs

I think J Tier may have nailed the major cause. I checked the imprint the jaws left and they were clearly deeper inboard than at the nose of the jaws.

4" is way to much, even with the large dia. Part of close to the jaws or saw/face. You're putting a high radial load way out from the chuck - its the same way work in a steady will walk out of the chuck. and Dalee, is also right, if using a carbide us it at its proper speed.

What Jerry say as well, if the grip is at the back of the jaws, the the how/why of it walking out is exacerbated

I knew 4" stick out was pushing it but I figured if I didn't push the feed I'd be ok.

Out of round stock would be my first guess. CR isn't perfectly round to begin with and you may have some really out of tolerance stuff like some of the "sub prim" crap I got from Discount Steel years ago. If it's out of round enough your 3 jaw may not be biting down equally on all three jaws no matter how much you tighten up the chuck. If this is what is happening it's not doing the chuck any good either.
What I would try is turn one end of it to make sure it's round and then see if it still pulls out.
Measure the stock and / or indicate it and see how out of round it is.



JL..................

Joe, I did clean it up before I chucked it up.

Thanks to all for your suggestions.

Ron

[BCRider]

You said it was a NEW Bison chuck? If so I'm amazed that it would have any amount of bell mouth issue. That just doesn't seem right.

With that light a feed I would not have expected too high a load. Or perhaps it's one of these insert issues where it NEEDS some minimum feed or the edge tends to rub instead of cut. If so that could be the reason behind the high force that caused the jaws to let the piece walk in the grip.

Besides, out of round on a three jaw should not matter. A three jaw will provide equal pressure on any shape. It just won't center it correctly if not round or hexagonal.

I'd tend to check for bell mouthing of the chuck with some sort of method If it IS gripping far more inside than at the tips then it would appear to be time for warranty replacement or repair.

[oldtiffie]

There is a fair chance that due to wear in and under the tool post that the tool "dives" under load and the job tries to climb over the tool.

A similar effect and result can arise from excessive clearance in the spindle/head-stock.

[Arcane]

It wouldn't hurt to make sure your cut off tool is 90^o to the spindle axis.

[PStechPaul]

I know a 4-jaw grips a bit better but I would not expect that much difference, where the 4-jaw worked perfectly and the 3-jaw failed with all else the same. I think the 0.120" cut-off blade is a bit wide for cutting 0.150" thick washers, but other than wasting material I don't see a problem.

[Rosco-P]

I knew 4" stick out was pushing it but I figured if I didn't push the feed I'd be ok.

Ron

3x to 5x diameter is a typical stickout guide for unsupported work.

[Carm]

"Besides, out of round on a three jaw should not matter. A three jaw will provide equal pressure on any shape."

Not so. Gently snug a three jaw on some cold rolled and spin the stock in the chuck, you'll feel grabby spots.
Even on perfectly round stock, one of the jaws will dominate.

[dian]

if you suspect the chuck is bell mouthed, do the following: take two pieces of round stock of same diameter with through holes, preferably ground/hardened, put an all thread in one of them and slide the other one over the allthread. chuck both up, one in the back one in front so you can grab it. tighten the chuck slowly and check if you can move both in the same maner. i like to turn my jaws so they are tighter by about 0.01 mm in front btw.

[projectnut]

You haven't told us what type of machine you are using. If it's an industrial grade machine In good condition I doubt the amount of stock protruding from the chuck jaws is the problem. If the chuck and the headstock bearings are in good shape the length of the stock protruding shouldn't be a factor. The rule of thumb (on a commercial/industrial grade machine) is that stock should be able to protrude 5 times its diameter from the jaws without support. In this case if all is well with the chuck the stock should be able to extend 8.75" without support.

If you have a hobby grade machine it is possible the machine isn't stiff enough to support that much material extending from the chuck jaws.