Picture the end of a trombone that the music comes out of. That's a curve very
similar to one I want to make one of. It doesn't have to be perfect, but
shouldn't have any abrupt discontinuities.

Here's my first thought: cut out a bunch of plywood circles and glue them up to
form a rough blank a bit larger than the desired shape ID profile. After the
glue dries, screw the largest flat face to a faceplate and use wood turning
techniques to generate a form. Screw the form down to a table securely, measure
its diameters at several locations, then using projection techniques lay out six
strips on flat sheet metal. The strips would be drawn and cut such that they
could be stitched together with vertical seams to make the part. Then I was
thinking of rolling a flat curve in the strips such that they would touch the
form in the middle of each strip. Then, using several helpers probably, arrange
the strips around the form and use a big radiator hose clamp to pull them all
into the form about half an inch below the form's top lip. If I could succeed in
getting the clamp tight and the pieces all aligned, then it should be possible
to tack weld the pieces at the top, and then work my way down the form tacking
along the seams like zipping up a zipper. I figure that would take a fair amount
of peening with a hammer to stretch the metal, but it seems like it could be
done. Then pop it off the form and weld it up solidly using a MIG welder.

Sure sounds like a bitch though. I wish there were a nice shape I could just cut
out and use, like the top of an old welding cylinder or something.

I'm working on a design for a shopbuilt vibratory tumbler, like the one Bob
Engelhardt made which is pictured:
http://www.metalworking.com/dropbox/vibepolisher.jpg

Bob used a turned wood form. I put his picture in a place where it won't go away
for a bit: http://www.tinyisland.com/images/temp/conemandrel.jpg

Ideas?

Grant Erwin
Kirkland, Washington