"Andy McArdle" wrote in message
u...
[snip]
It generates sufficient pressure to do light touch ups to the foot of a
bowl, careful removal of the tenon, reshaping, etc.

Even a commercial vac-chuck set up won't allow more than a light
touchup
and
a tin can's a darn sight cheaper...

- Andy


I think the force holding the bowl onto the drum chuck or faceplate is
approximated by 0.5Xvacuum in inches of mercuryXarea of workpiece under
vacuum. For large diameter pieces this can be a very large number and can
implode/crack the workpiece if it is thin. If the diameter of the bowl is
around 6" or larger under a good vacuum you can do a lot more than just
light touchup cuts.
billh

With my setup the smallest dia I can safely hold is around 8-9", but your
formula suggests I should be able to safely hold larger pieces before
needing to worry about cracking. I haven't tried it with anything over
18"
dia so I can't speak from experience.

The only commercial systems I've tried are the Vicmarc & Nova units (newly
set up on friends' lathes) with about 12" bowls and I could still only do
light touch-ups but that could easily be due to incorrect setup. You
know,
the "Oooer! A new toy... let's whack it on the lathe and see what we can
do. We'll RTFM later." type attitude. G

Regardless of the system used, including a pressure gauge & adjustable
by-pass valve would, I think, increase the versatility but this is
something
else I haven't tried. Yet.

- Andy


I have a gauge and bleed valve and I wouldn't be without either, especially
the gauge. Without it you are really flying blind and that might let the
piece go flying too! It is surprising how much leakage you can get through
some open-pore wood if it is thin. Another thing to consider is that the
force is pushing the bowl against the faceplate or drum chuck towards the
rear of the spindle. The only force keeping the bowl from sliding
perpendicular to the spindle is the "friction" of the bowl rim against the
faceplate. If the faceplate is covered with foam then the rim "digs in" due
to the force and is held securely. The drum chuck likely has the curvature
of the bowl bottom against the chuck so it keeps it from moving.
Billh