Machine the inner diameter first. Boring bars always tend toward chatter, and that can be made worse if machining a thin walled tube etc. So finish machine the inner bore before going on to machine the outer. With 3mm wall thickness you should not have any big problems machining the OD smoothly.

As an aside, it's counter-intuitive but if you were wanting to turn the wall thickness down to a very thin shell, say 0.5mm or something, the way to do that would be to finish machine the inner first, then do the outer diameter in as deep a cut as you could. The strength of the unmachined material supports the point being machined and the thin shell left behind needs no support, having been machined already. But if you do what seems intuitive, ie machine down careflly in small cuts, hoping to take a light finishing cut, the remaining shell gets too thin too soon and starts moving and chattering before you get down to size.

Hello Andy,

I would tackle that slightly differently. Drill out the centre as you have said and then fit a plug at the tailstock end to support the work. I don't see any need for internal boring if the drilled hole up the centre is adequate.

Align the compound slide to the appropriate angle to leave what will become the minor diameter at the tailstock end and machine the outer surface up towards the chuck, you can check your compound angle geometry as well as you go until eventually you have the taper you want over the whole length. I am not a fan of tailstock offsetting for taper turning on a length this short, it is usually a pig to get the proper alignment correct again afterwards

If the motion available on your compound is shorter than 65 mm, you can do this in two bites with a shallow radius shaped step between them in the middle as a 'design' feature, linking the steps, you say the quality of finish is not critical

Cut off the short stub left in the 4J chuck and deburr.

Regards

Brian

Hello Andy,

Now that you have thoughtfully provided a drawing, forget my contribution, I misinterpreted your description completely! For a start I took the wall thickness to apply to the minor diameter end

Retiring to think again, sorry

Brian

I'd just turn as one much like hopper says, done far longer and thinner wall traction engine chimneys like that. Drill to 18mm, then drill part way with 25mm drill followed by boring bar. Turn outside then saw/part off and finally reverse to skim the other end.

If you do it in three how do you propose to join them and keep it all concentric?

Hi Andy

As these don't have to be too accurate, I believe they where made with a form drill. Have you got enough room in the hub to fit a straight stepped spacer?

I use the method John describes above to make tuned pipes for my tether car engines, wall thickness is usually 10 to 12 thou, I machine sockets on the one end of each tapered part the silver solder together.

I use the method used by John for parallel tubes that become displacers and hot caps for hot air engines, that way you can take the thickness to less that ,010", but for the job in hand, use Hopper's method.

Ian S C