On 6/15/2011 10:15 AM, Tim Wescott wrote:

While we're on the subject -- how do you spin a piece that's rounded on
the end? Make a dimple in your sheet, then spin down from there? You've
got to have some surface area on the tail stock, don't you?

Years ago I watched a metal spinner making teardrop headlight shells for
high quality reproduction pedal cars. It was formed almost completely in
one setup. Now I wasn't close enough to see any real detail, but I did
see a small dimple in the shell when he was done, the pressure pad and
form being shaped to form this dimple when the disk was clamped. The
clamping pad was fairly well relieved to allow tool access. He spent
what seemed like a long time getting the metal started forming, then all
of a sudden, he zipped along and got 80+% of the shaping done in
seconds! Some back and forth work to smooth out the ridges, then the
pressure pad was pulled back. At this point, there was sufficient
friction between the shell and the form to hold it, and he finished off
the shaping in the region near the end that he couldn't really reach at
the beginning. The owner of the shop explained the dimple was pressed
out in secondary operation.

Be careful about going too thin on material, it often tends to thin out
as you form it.

Another trick I just recalled, he used the main roller tool and a back
tool to form a small and shallow upturned edge on the blanks as a first
step after clamping. Upturned being away from the spindle, toward the
tailstock. What this does is strengthen the rim and helps prevent the
edge of the disk from buckling.

I have no idea if the disks were annealed or not. But I suspect getting
it done as fast as possible helps avoid work hardening the material.

Hope this helps some.


Jon