Make an expanding mandrel to fit the ID. Use method 1 , but use the mandrel held in the chuck to turn the OD. That will let you turn the full length in one go.

I’d be inclined to use a collet rather than a chuck, to spread the gripping load more evenly around the circumference. A 3- or 4- jaw chuck risks some crushing on this thin-walled stock. It’s likely to be more concentric as well.

Other than that I think cutting full-length with a steady would be more likely to give a consistent result. How critical is concentricity? With only 1mm wall left after turning it would not take much deflection or a slight bend in the stock, to give slightly eccentric tube even using a travelling-steady.

A steady with a bush rather than fingers might be an advantage here.

I forget the proper name but there is a capstan-lathe tool that combines a cutting-tool and two ball-race fingers as steady, in one holder. Something like that to clamp in the lathe’s normal tool-holder, would be fairly easy to make and might be ideal for this kind of operation as it holds the material very close to the tool.

I’m guessing this if for someone else so you’ve little choice, but if I were working to my own design I’d not use non-standard tube!

On 17 January 2024 at 21:09 Nigel Graham 2 Said:

I’d be inclined to use a collet rather than a chuck, to spread the gripping load more evenly around the circumference. A 3- or 4- jaw chuck risks some crushing on this thin-walled stock. It’s likely to be more concentric as well.

Other than that I think cutting full-length with a steady would be more likely to give a consistent result. How critical is concentricity? With only 1mm wall left after turning it would not take much deflection or a slight bend in the stock, to give slightly eccentric tube even using a travelling-steady.

A steady with a bush rather than fingers might be an advantage here.

I forget the proper name but there is a capstan-lathe tool that combines a cutting-tool and two ball-race fingers as steady, in one holder. Something like that to clamp in the lathe’s normal tool-holder, would be fairly easy to make and might be ideal for this kind of operation as it holds the material very close to the tool.

I’m guessing this if for someone else so you’ve little choice, but if I were working to my own design I’d not use non-standard tube!

what you’re looking at is a roller box. Start with an overlength bit to allow chucking in a collet (colleting?), or start with a double length bit, do both ends then cut in 2

I suspect many of the suggested methods might have not taken into account how flexible a workpiece of this length and diamter will be. Anything that does not use a steady of some type will probably end up in a tapered (barrel shaped ) result and perhaps a poor finish?

There is or was a the “Jabus” type small diameter turning tool which combines a tubular steady and cutting tool in one unit. There are images on Google. A home made one is not difficult to make, especially if it only has to deal with one size of material and one final diameter.

Bill’s suggestion of a ready sized tube seems very promising.

Rod

I’d get the tube stress relieved before you start cutting the slot. It could easily distort otherwise

The name of the device which escapes you is a Roller Box.

Usually used in Capstan or Turret lathes.

Since you are turning round material, you would have a short length protruding from the chuck and turn it to size. Once done, the rollers would be set to bear on the new surface, to guide and steady the tool when cutting the material over the full length.

Since the rollers steady the material as it is being cut, it will be supported and the diameter should be constant along its length.

Effectively, it isa travelling steady, but set closer to the cutting tool.

Have never yet used it, but made one where the rollers could be reversed, to lead or trail the cutting tool

(Trailing would normally be used when turning down square or hexagon stock, bearing on a short length that had been turned down to size)

Howard

To do the whole length I would Loctite the tube onto a piece of Ø5 rod to stiffen it and use a travelling steady. However looking at the suggested link above it seems only the ends are turned down which makes it a lot easier to do.

Martin C