How to hold 3 inch alumimun tube in lathe

Have you checked the run out of your chuck just to be sure it is concentric? Perhaps the jaws are placed out of order or in the wrong slots? Just a thought.

The jaws are in the correct slot, but I haven't checked to see if they are all in the same spot. For smaller work, the chucks have worked fine. I think it is very easy to have the tube slightly cocked in the chuck which gets to be a big problem 6 inches later.

Supporting from the tailstock sounds like the right answer. I ordered a bull nose center. While I am waiting on it, I will see if I can use a wood plug with my live center in the tail stock.
 
Don't rely on the saw cut to run the bull nose center if you need good concentricity or roundness. If you push a center into a tube that is not faced square, and turn the OD, it won't be round when you pull the center out. And same general thinking on the chuck end. You can chuck the ID, but it will have 3 lobes where the jaw pressure expanded the tube.

How true must the product run and what is your size tolerance? What is the final wall thickness? Working in an oilfield machine shop, most everything is round, and hollow. I've suffered through learning how to make accurate thin walled cylindrical parts out of many different materials.
 
If real precision is needed saw them close then mount them in your vise on a mill with a V block turned on its side but centered to the part and face off both ends
 
If the inner diameter of aluminum tube is right at 2.5" you might be able to use a section of metal pipe from the hardware store. Cut a few inches from the pipe and slide it in the aluminum tube.

I was also thinking that you may be able to pour a lead/zinc slug in the end of the tube, in theory anyway, as I've never tried it.

-Ron
 
we do parts like this at work all the time. first we face both ends, then we put a piece of solid stock in the chuck and turn a bull nose center.
use a bull nose in the tailstock and take light cuts if you don't have a bull nose for tailstock, turn a tight fitting plug with a shoulder to push against with a small center.


mike
 
Nobody mentioned using a bump centering tool. To straighten up the pipe in the chuck. They work well and are easily made from a piece of bar stock and a small bearing. Just grip the tube semi lightly in the chuck and rotate while running the bearing in to the tube (bump center mounted to tool post) and it will straighten/center the tube right up. Then tighten the chuck. Light cuts could then be made but of course a center at the opposite end would be nice to.
I made a hexagonal "spider" that fits over my live center and set screws to it. It has three sides with 5/16 threads and 3 sides with 3/8" treads. Three appropriatly sized bolts are used and expanded out to touch and center large pipes. Worked well for 8" pipe but probably over kill for 3".
 
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