Odd lathe issue

I needed to shorten the tailpiece for a bathroom sink drain: chrome-plated brass tube 1.250 OD with 0.030" wall. Obviously the tool of choice would be a tubing cutter, right? Except that
my tubing cutter has a maximum opening of 1.220", despite being labeled as "1-1/4". Grrrr.
OK, no problem, I'll chuck it up in the metal lathe and part it off. Should be easy, right?
No sooner does the parting tool start removing stock, than the tube begins slowly but inexorably sliding out of the chuck. Not crooked or anything like that, still concentric, just creeping slowly in the direction of the tailstock. OK, maybe I didn't tighten the chuck enough. Stop the lathe, reseat the tube in the chuck, make sure it's darn good and tight, try again.
Same thing. I get maybe 0.0005" (that's right, half a thou) removed, before the tube begins sliding again.
I tried low rpm, high rpm, slow feed, rapid feed, lots of oil, no oil -- no difference that I could see.
After 45 minutes, I finally got it turned down far enough to slip it into the tubing cutter, after which it took about 45 seconds to complete the cut.
Why is this happening? And, more importantly, how can I prevent it? This was number 1 of two pieces that I have to shorten by the same amount.
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Which way was it creeping? If out, then just set the lip on the tube up against the back of the chuck jaws, and use a bullnose center in the end. If slipping in, put a block of wood inside the chuck behind it, and tighten down the screws of your spider to keep the block from moving. If you don't have spider bolts on the back of your lathe spindle it might be time to add them. Never know when you might need to do some turning on a rifle barrel next.
I would suspect it was slipping in because the end with the lip would be more rigid effectively making your tube into a cone with the large end at the lip when you clamp down.
Personally I would have probably cut it on the bandsaw or with a hacksaw, and then debuirred it with a file, or deburring tool depending on what was handy. There is a huge amount of overlap in the joints of that type of fitting, and it doesn't have to be very precise at all.
Lots of tools could have done the job. Tail pipe cutter. Pipe cutter. Tubing cutter. Hacksaw. Bandsaw. Abbrassive saw. Dremel tool. Jigsaw. Sawzall.
That style fitting and assembley is designed with ease of use in mind. Hence the huge overlap. As long as it reaches past the geasket, and doesn't have any burrs to catch hair and clog its fine.
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Creeping out. I don't have a bullnose center that large.

It can't. This is a small lathe (7x14 mini), and the chuck is too small to allow anything beyond about 5/8" to pass through it.

:-) That's probably not in my future, not with this lathe anyway.

I had to chuck it with the lip outward, away from the chuck: the lip apparently isn't perfectly square to the axis of the tube, and it wobbled pretty badly.
But that may explain why it was creeping outward -- in the direction of the lip.

I would have, too, except that those tools are currently about fifteen miles away. We're in the process of rehabbing another house, to downsize into, and almost all of my hand tools and portable power tools are over there.

Don't have.

Don't have one big enough.

At the other house.

Don't have.

Didn't think about that. Thanks! We do have cutoff wheels for that.

At the other house.

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http://www.autozone.com/loan-a-tools/tail-pipe-cutter
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That is an option I never think of. Its a good one too. I've used my tail pipe cutter maybe a half dozen times in 25 years, and most of those times I could have used something else. The one or two times it was really the right tool for the job I could have just borrowed one from Autozone. Doh!
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Tubing is difficult to part with a larger lathe too, because it flexes too easily. I sometimes score it with the lathe bit and then saw along the groove with a 24 TPI or 32 TPI hacksaw.
Whenever you are turning wood on the lathe you could make a pair of cones with holes through the center for threaded rod, to hold hollow tubes. Just don't try parting work held at both ends because when it becomes flexible enough to deflect away from the cutting pressure, the near side will close and grab the bit.
--jsw
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I've gotten spoiled in a very short time. Last month I took delivery of a new 3HP PM1440. It does things.
For the last decade or so all I've had is a small lathe (8x18) and a mini lathe (7x14), and I have found quite often a hacksaw is the right tool for the job. For a lot of materials the mini lathe just doesn't have the horsepower or the rigidty to use a parting tool or a parting blade. I've often scored a groove and used the hacksaw to make the cut. Just have to remember to move the saw back and forth to get the chips out from between the teeth.
Now on the new 1440 I can actually use a parting tool. First thing I did was waste a foot or so of 1018 I had laying around being amazed at what the new machine would actually do. I put knurls on impact sockets just to see if it would do it. Threaded things in one pass to see how hard it would push. LOL.
My very first actual part on it worked the very first time when I took it off the machine. Almost never had that happen with the mini lathe. LOL.
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Nice! http://precisionmatthews.com/PM1440BLathe.html
My 10" lathe is fine for a hobbyist but the 15" lathes I used in company shops were better for commercial jobs. I tend to make one-off prototype parts whose design I refine while machining them so I never used the CNC capability.
The optional accessory I like most is 5C collets. Right now there's a chuck like this in the collet holder: http://www.ebay.com/itm/like/391006105565?lpid &chn=ps&ul_noapp=true
I can move the 5C-mount chuck and work to the milling machine to drill holes, mill slots or add wrench flats and then return it to the lathe. When making repair parts I can test the fit on the machine without losing registration in the chuck. For most jobs this is enough to hold and index it on the mill: http://www.ebay.com/bhp/5c-collet-block
I have a nearly complete set of second-hand collets but only the sizes that accept my supply of ground drill rod and shafting stock get much use.
Machined brass pipe fittings run much truer than iron ones and are very handy to chuck and modify other fittings.
I still use my small lathe at its higher speed to drill deep holes such as grease passages in axles and for polishing.
--jsw
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On Thu, 18 Aug 2016 17:27:26 -0000 (UTC), Doug Miller

My bet is that the tool is distorting the tube enough that the pressure is reduced bewteen the tube and each jaw in turn as it's approaching the tool. As that that jaw leaves the tool the tube springs back. The result is a squirming of the tube in the jaws. The cure is a plug inside the tube, a collet, a chuck with more jaws, a bullnose center, a piece of sacrificial wood between the part and the tailstock, or a hacksaw.
--
Ned Simmons

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On Thu, 18 Aug 2016 17:27:26 -0000 (UTC), Doug Miller

The tube is moving because the wall is so thin and the tool pressure is enough that the tube distorts and pushes away from one jaw at a time and works its way out. It could be that your parting tool is dull too. Next time put a spider inside the tube to support it so that the tube material is clamped between the jaws and the spider. An easy to make spider is a nut with three tapped holes, one through every other flat on the nut. Eric
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Doug Miller wrote:

Machining thinwall tubes is very difficult. One way to do it is to have a mandrel just a couple thousandths smaller than the ID, the chuck jaws squeeze the tube onto the mandrel and then it will hold. Six jaw chucks help, but not too many home shops have these. Or, make soft jaws for the OD of the tube, a grip around the entire OD of the tube will also be more secure.
But, holding a 1.25" tube with .030" wall in a 3-jaw chuck is not going to work well.
I'd just cut it with a hacksaw and then clean up the edges.
Jon
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    [ ... ]

    The tubing is rather thin -- and is flexing as you try to cut it.
    First thing is to turn a plug to fit inside the tubing to support it before tightening the jaws on the OD of it. A collet (if you had one large enough) might work, but support the ID and you can get a much better grip on it. Hardwood or aluminum depending on what you have available.
    Depending on the length of the tube, perhaps also a plug with a shoulder in the other end, and support that with a live (ball bearing) center.
    Enjoy,         DoN.
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Many thanks to all who responded -- I now have multiple solutions for the next time I'm in this situation. I'm a very experienced woodworker, but I'm sure you all realize that I'm just a beginner in the metal shop, and I learned a lot from reading your responses.
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Doug Miller wrote:

Well, we pretty much guessed that, as this is actually a VERY common problem in turning operations. Any time you are turning a tube, unless the wall is REALLY thick, it will happen to some extent.
There are all sorts of ways to deal with it, up to even wire EDM.
Jon
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And turning a tube isn't a real common operation on a wood lathe, so this was completely outside my experience.

Where SWMBO works, they have a plasma cutter, but I figured that was overkill.
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On Fri, 19 Aug 2016 21:42:00 -0000 (UTC), Doug Miller

FWIW, when I have to cut off thin tubes, I pad the tube in my four-jaw with some pieces of wood (I have even bored a piece of wood to the approximate size, and then bandsawed it to make a "collet") and turn the tube slowly against a hacksaw.
Crude, but it works, without crushing the tube.
--
Ed Huntress

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I made a collet-like hub for a 10" solar tracker pulley by boring the center of a piece of 2x4 to slip over the water pipe support, drilling stress relief holes a few inches out on the centerline on either side and slitting between them with a handsaw. A screw on either side of the center hole closes it tight on the pipe. Other than the center hole that I bored on a lathe face plate it was bench vise and battery drill work.
I could have bored the hole to size after cutting the slots to hold a large piece of thin-walled tubing in the lathe.
If you have a metal ring, perhaps cut from leftover scrap at the Home Depot pipe threader, one of my old-time lathe books mentions using it to close a custom wooden collet like Ed mentioned, by tapping it onto a taper on the OD.
Usually I hacksaw with the lathe off and the drive engaged, which keeps the work from spinning too easily unless I bear down hard on the saw to advance the cutting area.
It's a leather-belt-drive lathe, which has both advantages and disadvantages. An advantage is that the spindle stops almost instantly (if the tool is cutting) when I raise the tension lever, so I can thread right up to a shoulder or stop in a drilled hole.
--jsw
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One easy way is an abrasive cutoff disk in a table saw. I set the disk as low as possible and the drive belt loose to reduce problems with jamming.
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wrote:

The simple way to do that, since the tubing ID is difficult to measure exactly enough and it may be distorted, is to turn a plug close to size, file the end tapered until the (deburred) tubing will jam onto it, then touch the lathe bit to the mark the tubing left and turn the larger end to that size. The grooves and ridges from turning or coarse filing let the plug compress and adapt to the tube somewhat. You don't want the plug so tight that you damage the tube pounding it out.
On my old lathe the plug may need to be supported by a tailstock center to keep the diameter constant enough for a light press fit..
--jsw
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