I done this sort of thing on larger stainless tube, 20mm and 25mm about
1.6mm wall. I turned up a thick collar with the tube OD bored in it and a lead in taper then cut it in half and mounted one in the ram of my fly press and the other on the base. The tube was then repeatedly squeezed and rotated and it could be quickly reduced in size. You might try something similar and if you don't have a press then maybe a vice would do, likely somewhat slower though.
Otherwise I've reduced the size of short lengths of tube by pushing them through a sizing die, the length being short enough not to buckle under the load. A good lubricant helps a lot. If the tube is long and likely to buckle you might need an internal and/or external support.
No, I don't have a press, but thanks for the idea: make something=20 similar to what you suggest, but only pierce the length only on one=20 side. If necessary, an hydraulic jack could be pressed into service.=20 (Sorry...)
Keeping the alignment wouldn't present a problem then.
Or, take two blocks of gauge-plate and peg them together; place =
in drill-vice and drill, cut and ream the die at the junction of the=20 faces.
Yes, I've resized, necked-down and otherwise manipulated brass cases for =
obsolete arms in the past, so I have a good idea of the culpabilities of =
sizing dies. Doing so with 3" lengths of open-ended tube (@ =A33.15/foot)= =20 makes me reluctant to go down that route.
The split die seems favourite to try. I do have a big vice (but don't=20 talk about it online...), and there's a general auction next Saturday in =
a nearby country market-town, and I'd be surprised if there isn't an=20 hydraulic jack amongst the deadstock lots.
Otherwise, they're not expensive to buy new.
I've been postponing this job now for several years=20 : Now I shall lie awake for several nights=20 fretting^h^h^planning...
I have never had to do anything like this but ever the optimist how about--- Replace the knurls in a caliper type knurler with plain discs with maybe a very slightly rounded outside face. Make up a steel tapered mandrel to the required length and taper with a parallel section a push fit in the tube. chuck tube and mandrel in a 3 jaw chuck. Pinch the rollers onto the tube at the chuck end and whilst rotating traverse slowly to the other end, traverse back apply more pinch traverse etc.
Three rollers would be better but I think this explains the concept
I can't remember where they came from, or even when they came, hey've been around so long, but amongst my 'may cumminandhi even if I never use it' box I have four one inch bearing balls, each with a three sixteenths axial hole through it, and, surprisingly, I have laid my hands on them.
The knurling tool and tapered inner steel core suggestion is coming back into play.
Three of three-sixteenths inch silver steel bar, hardened, and immersed in molten lead which is gathering a straw-brown film over it will temper the steel to spring quality.
The spare Super Adept bed, head and tailstock (courtesy of Jim Guthrie) might acquire a special chuck, just for tapering and/or moulding brass tubing.
Watch this space: I expect you can see how (or if?) my mind is working...