What you are attempting is called swaging. A three roller tool can be made
to do this, but if you are looking for an extended length of swage on the
tubng, relative to it's length, you may have to resort to a commercial
swager. Torrington is one of the manufacturers. Unless you have a screw
machine or similar lathe-type tool, I suggest the swaging machine for
thousands. Standard cam rollers with the leading edge rounded off on a belt
sander and buffed smooth work well in short swages. They are mounted in a
triangular pattern in slotted adjust ment holes on a holder and forced over
the rotating tubing or vice-versa. Good location control is going to be
needed, and expect crinkled tubing until you get the learning curve downpat.
Use lube on it in the rollers.
http://www.torrington-machinery.com/
No affiliation, just a regular user.

RJ

--
"You're just jealous because the voices are talking to me, instead of you."


"Robert Haston" wrote in message
link.net...
I am prototyping an invention that has a thinwall aluminum inner core of
5/16" on one half and 1/4" on the other.

My attempt to neck it down failed, as it buckled and bent.

I guess there are dies for such a thing, but I'm pretty clueless.

Having noticed how well my tubing cutter necked the tube before it cut it,
I
wondered if there are any simple roller necking devices, preferably
something to be used with a drill press. I know that roller necking
devices
are used in mass production, and I am looking at potentially making
thousands of these.

--
Robert Haston
Satellite Beach, FL