Hi again jephw:
I'd say if you're going to go to the trouble of making and using the plug, you will have better success with it if it's metal...at least aluminum and for preference, steel.
Remember, you want to keep the stainless tube from collapsing under the force of turning, and if you use a soft material I'm not certain it will prevent that deflection.
The problem with the tailstock is that on some makes and models of turning center, you need a minimum tailstock pressure to allow the program to run...I'd find that out before investing in making the plug.
If it turns out you can't use the tailstock, you have to use a piece of threaded rod through the tube with some kind of cap at the headstock end and your plug snugged up with a hex nut at the tailstock end.
If you have to do that for several hundred parts you will grow to hate the job because you'll be tied to it, forever screwing around with that stupid plug.
If it was me, I'd first try the custom ground turning tool.
If you can find a triangular or rhomboid insert with a super small nose radius and if you have a grinder with a 150 grit or finer diamond wheel, you can put some top rake on the insert and make it dead sharp at the same time.
Next step is to bore the cone with your normal boring bar.
Last step is to turn an OD taper say 10 degrees or so and get the tip of it dead nuts on size.
Then take another pass making an 8 degree taper, then on down until your OD is cylindrical.
You don't take anything further off the tip after the first pass.
With a 4 degree internal cone, the part wall is still awfully skinny, but with a really sharp tool you just might get away with it if you baby the cuts.
You can also try to take it all in one big pass after your first OD taper cut, using a super slow feed (like 0.0005" per rev) and not trying to break the chip.
You want the chip to come off like unrolling toilet paper...violating every rule you ever learned about speeds and feeds.
If the tip radius is small enough and if the tool has positive rake and is fed slowly enough you can take an enormous cut with it before it starts to chatter, and with a tiny tip radius the forces going radially into the part stay minimal so with luck the tube doesn't collapse.
Cheers
Marcus
www.implant-mechanix.com
www.vancouverwireedm.com