Quote Originally Posted by Brice Rogers View Post
... I think that I recall people using bleeder valves to bleed off excess pressure of very thin or delicate pieces.
Every vacuum chucking system should have a bleeder valve. This picture, again, is from the excellent Joe Woodturner vacuum chucking page:

vacuum_manifold.jpg

The vacuum pump is connected at the bottom. The elbow at the top connects to the rotary adapter on the lathe. The valve has a filter on the left side to keep sawdust and the like out of the vacuum pump.

For those new to vacuum chucking, this is a typical way to use the bleeder valve:
- the valve is opened to the atmosphere so almost no vacuum is on the chuck,
- the vacuum pump is started,
- the piece positioned on the chuck in the approximate center (with help from the tailstock, if possible),
- the bleeder valve is closed enough to hold the piece lightly on the chuck,
- the work piece is adjusted on the chuck until it is exactly centered when the lathe is rotated by hand (I like to use a dial indicator on a magnetic base for this),
- the bleeder valve is closed enough to hold the piece firmly and (hopefully) without cracking.
- the lathe speed is turned up slowly.

Joe says this about the amount of vacuum needed: "You'll find that most projects will have no trouble with the pressure from the vacuum. It is generally accepted that 15" of Hg is the minimum safe vacuum for most small to medium sized projects. As always, exercise caution and good judgment. Wear appropriate protection and don't crank the lathe up to 8 million RPM. And for goodness sake, make sure the project is centered on the jig before you get the lathe up to speed."
http://www.joewoodworker.com/veneeri...umchucking.htm

JKJ