Workpiece slipping

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yukon_rose

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I'm trying to cut a thread on a 1/4" 304 stainless pipe. The pipe is an odd length at about 2-3/4" but quite short.

I've chucked it up in a 3-jaw chuck and set the back gear to the slowest speed.

I've managed to cut a thread that is about 1/4" long. But at this point the pipe is now spinning in the chuck and becoming quite galled. I've tightened the chuck heavily without success. I need to get the thread to at least 1/2" long.

What would you do to keep the pipe clamped in the chuck?

Gary
 
if you were so inclined, you could put a wrap of sandpaper ,with the grit to the pipe.
you could bore a bushing from a piece of soft material, plastic ,aluminum, brass, copper to the od of the pipe and slit the bushing.
the bushing would compress against the pipe
if appearance is not critical, you could knurl the pipe in a small section too, to gain friction
 
What Boswell said is probably on the mark. It was unclear whether you were doing "single point threading" with a 60 degree tool, or using a Die to thread the pipe. If it is a tapered pipe thread, I suspect you are using a die. For operations using a die, I leave the motor off, and stick the chuck key in the chuck, and hand turn the spindle, as a lot of back and forth passes are needed to thread the pipe. Also look at your jaws to make sure they aren't slightly tapered and having limited contact with your part.

Most importantly, most chucks have trouble gripping small items firmly. This is especially true if your workpiece is only 3/8ths (or smaller) inch thick. For that application I would likely use a full length of 1/4 pipe, pass it through a spider then the spindle and through to a "collet". Then I would cut it to length and then turn the part around and face the freshly cut end.

If you can only get a short piece of your 1/4inch pipe (I.D. measurement?), then I would just go with a collet for the maximum grip surface. Even a basic ER-32 six sided collet holder held in your 3 jaw chuck would work well for this application.

The last two parts of the equation is the tool itself, and tool height. If the tool is not properly ground, or is at the wrong height, it takes more force to cut threads, which can encourage slipping. If it is a die, is it a quality die, or one that came in a "value priced" set. Not all dies are created equal. If it is a tapered pipe thread, you can step cut the end of your Part, to reduce the amount of "work" your die must do as it threads more deeply on your part.

What type/size lathe are you doing this operation on?
 
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