Re: warning about vac chuck *LINK*
Andrew Hilton
>Oh, ok. I don't have to imagine what you're talking about. I've done it myself plenty of times with vacuum chucking setups. It's just going to happen from time to time. Well, unless you're doing really thick stuff from very stable pieces of wood, that is.
Unfortunately, there's just no way to accurately determine how much force a piece of wood, much less a turned form, is going to be able to handle without extremely considerable calculations being done. Far too many variables working here. Anywhere from the simple (and known) strengths of wood species up through diameters / thicknesses of wood and volumes of suction and into the really complex stuff such as how much that wood is compromised by the design of the turned form, degradation of the wood strength due to spalting, figure, grain direction and so on. This doesn't even address the very real issues of how much force you're putting on that piece of wood as you cut (scraping/cutting/slicing) or how thin you do cut into the bottom or even the porousity of the wood while it's on that vac. system.
Just way too much to consider to give you anything realistic in terms of "use X amount of vac pressure for y type of form and z diameter of piece". At least in my opinion and experience with these vacuum chucking systems.
What I do and what I teach with these chucks is this...
By the way, the valves so that you can adjust the vacuum level is fantastic and can be seen on the included link below. Do try to get one that isn't a "fast opener" so that you can control the pressure better/finer.
Basically, you put your piece (bowl for example) onto your faceplate and turn the vac. on with the pressure regulator valve open fully. Slowly shut the valve until you have just enough pressure to hold the bowl on the chuck but still be able to move it a bit by hand.
BY HAND, spin the lathe and adjust the bowl on the chuck so that it's in the position (usually centered) you want. Once there, slowly close that valve (vac pressure going up) until you can't pull or shift the bowl out of position at all.
If the piece is fairly thin already, go up an additional 2-3 mm Hg. Otherwise, try 5-7 mm Hg.
Again, spin the lathe by hand. If no problems, spin up the lathe under power with yourself out of the way. Stop the lathe and check to see if your piece has shifted. If so, go up a couple of mm Hg. If not, you should be ready to go.
Take light, shearing scrapes or cuts. It's all too easy to go too thin on these and cause a blow-out (literally). Use your tailstock if need be. It's usually there closeby so use it.
For extended vac. chucking sessions, keep an eye on that gauge. You may loose or even gain pressure as you're working for whatever reason. Keep checking to see if things are shifting or appearing to cave in. Usually, things aren't going to cave in much before they just explode instead.
Oh, yes, I highly recommend a battery backup for vacuum chucking systems. It's terrible when the electricity goes out or just wavers for a split second and you have a flying turning all over the place. The battery backup doesn't have to do it's thing for long. Just 5 minutes is enough, usually, to let you shut things down once you notice there's a problem.
Vacuum system & Chuck
