In article . com,
wrote:

How about starting with the 1/2"
tube, and plug the ends, then introduce air pressure in the tube and
carefully heat it in the section you want expanded?

While this sounds an interesting idea, I guess I'm looking for a
technique to be able to duplicate with exact precision so I'm able to
produce instruments all look the same. Although, when I get the
equipment I need, I may try this just to see what comes of it!

There is a variation I have read about being used industrially:

Make a two-piece external mold (of metal) that completely surrounds a
piece of 1/2 inch brass tubing, with the annealed part of the tube lined
up with the expanded part of the mold, and force oil into the tube,
hydraulically expanding the tube into the mold.

The key is making the mold assembly strong enough that the hydraulic
pressure won't blow the end plugs out, or spread the mold halves. If
the mold is a split cylinder with two screw-on end caps, it should work.

The end caps would also have a fitting at each end that slides into the
tube, and is sealed with O-rings to the inside of the tube. One fitting
is blank, and the other has a port to admit the oil. These fittings are
loose, and do not turn as the end caps are tightened, but are held
against blowout by the end caps. The external mold restrains the tubing
so it cannot expand enough to bypass the O-rings. Trim tube on lathe
before expansion, to ensure a good fit in the mold assembly.

The slight relaxation of the brass after the pressure is removed should
allow the expanded tube to drop out of the half-molds. If not, split
the mold into three pieces (which will resemble pickles). Or, stick a
steel rod through the tube and nudge it with a hammer.

Joe Gwinn