RE: Lathe Chuck

It's just plastic...why not just chuck up a piece in your chuck, turn a center onto it and use the pressure from your tailstock to hold the work in place. Then again...with it being "just" plastic, you might bend it thinking you need to crank on it real hard to keep it from spinning. Its been my experience that when turning light tubing...this is a easy, fast way to go.

RW:

3/16" wall, minus .125/2 = .125 - .025 (approx 1/2 thread depth) = .100 That's pretty flimsy. Especially if you're trying to push a live center into it. Long expanding mandrels sound like they'd be a high $$$ item.

BUT, like Z. suggested, you could hold your parts in a chuck. I'd add this one idea, turn up some rods that would slip fit (or light tap fit) into your tubes all the way to the front end. Even maybe center drill the ends for your live center. That way you could tighten down on the chuck without distorting the tubes "too" much.

Your understanding is wrong. We're not reducing the diameter and then cutting threads, we cutting threads at the outside diameter. The outside of the tube is full size with thread cut from there FX...4.00 dia will have a 4-20 thread. 20 threads per inch is only .043 per side/.086 overall reduction. we are reducing the outside diameter by .125 overall/.062 per side. .1875-.0625=.125 wall thickness. I may not have explained what we're doing correctly.

We' ve already tried the "just chuck it" route. If you want oblong, egg shaped parts, that's the route. The tubing is poly and not strong but is slick. A three, four or six jaw will distort the hell out of it and the threads are, at best, hit and miss.

What I think I remember is someone had a "stob" chucked up with some "O" rings on it. They slid the part over the "O" rings and then pulled a vacuum to hold it on or maybe it was an air bag on a stob that part was slid over and the bag expanded.

The "just chuck it" just doesn't work, if it's tight enough to prevent slippage, it distorts the tube. R. Wink

Wink, I would machine a stepped plug for the each size tube. The plug would have a large diameter slightly smaller than the OD of the tube, the smaller dia. would just slip in the tube and be long enough to cover either longer of the threaded area of the tube or the chuck jaws. Leave a thin flange on the plugs, and center drill them concentric with the diameter that fits into the tube.

Slide them in the tube chuck it up and snug up the live center.

Hope I've helped.

Best, Steve

RW:

I'm sorry, I must have misinterpreted your following comment:

================================================================ "I would machine the live center end to reduce the outside diameter by

1/8" for approximately 2" then create an undercut (thread runout undercut) approximately 2 1/2" further toward the center of the part. The area between the reduced outside diameter and the runout undercut would be threaded with 20 threads per inch (about 2 1/2" inches)." ================================================================

But you're absolutely correct on the thread depth. .050 pitch X .866 (sine of 60 degrees) is .0433. I was rounding stuff off in my head and screwed that up.

That's why I suggested you turn some internal support bars, so there would be less distortion of the plastic tubes when clamping.

That sounds like a slick item. Does the vacuum line go through the headstock hole and out the back of the lathe, or is it some sort of rotating connection mounted in front of the chuck?

Sorry about my explanation being less than clear. In thinking about this, I've come to the conclusion that I'll end up making a bung for the inside of the tubing. I don't have it engineered yet but what I think will work is to machne a tube outside diameter, add a series of spacer rings with the inside diameter of the main tube and and outside diameter near the plastic tube inside diamter. Between the rings would be rings of a softer material (orings?? urethane?? Die rubber??) that when the rings are pressed/pulled would expand, clamping the tube from the inside. Add a "cap" on the tailstock end. Prime mover could be a cylinder working a drawbar arrangement that would pull the cap towards the chuck end and clamp the tubing stack. Sequence would be load the plastic tubing, introduct air to the cylinder and cap the line (valve??). Turn on the lathe and turn the outside of the tube. Stop the lathe, release the air pressure and remove the plastic tube. Thoughts?? R. Wink

rwwink wrote in news: snipped-for-privacy@4ax.com:

My experiences with plastic tubing/pipe has been that IDs are not round nor are they concentric with the ODs. Be that as it may, Garlic's solution is what I have used successfully for 40+ years.