Vaccum Pressure question

Re: Vaccum Pressure question

John Lucas

>Sean

Sounds like you got it hooked up right. 27 to 28" of mercury is about the maximum you'll get. You don't want to use this much all the time. The bigger the bowl the less vacuum you need. You have 14lbs per square inch of pressure against the bowl when you have 28" of mercury. On a 16" thin bowl you can crush it with full vacuum. On large bowls I use about 10 to 13" on 4 to 6 " bowls I use the full vacuum unless they are thin.

Always use the tailstock as long as possible. Vacuum chucks aren't perfect.

Re: Where do I stand with these numbers?

Al Crandall

>Good shape. I have two systems and the weak one pulls 24". Some people are using the Fein brand of shop vac for a vacuum source and get by with it just "fine". I checked my Fein shop vac with a gage and it pulled almost 5"Hg. This also happens to be the value I use when initially positioning a bowl on a chuck. I find it easy to bump the bowl around with my palm to get it to run true if I keep the gage in the 4 to 5 inch range. With 27" you will never have a problem. I have turned plates from lumber which came straight from the saw mill's kiln using a vacuum chuck as the only method of fixturing from start to finish. You would be surprised how agressive a cut you can take when using this system if it is cranked up to full capacity. Happy turning.

Re: Vaccum Pressure question

Sean Troy

>Thanks for the help. I can't wait to use it. Sean

Re: Vaccum Pressure question

Bill Howatt

>Don't be surprised if you lose a couple of inches or more when you put a bowl on it. It doesn't take much leakage, even through the pores of the wood, to drop your values measurably.

Bill

Re: Vaccum Pressure question

Ric Erkes

>Sean, remember that the inches of pull is multiplied by the size opening of the chuck. If you use a large opening on a thin, wide platter, you can adjust the vacuum pressure down, but still have quite a bit of holding pressure. And the reverse is also true for smaller pieces.

The actual pressure doesn't mean much by itself, until you relate it to the chuck you are using.

Ric

Re: Vaccum Pressure question

Rick R, So Cal

>As John mentioned, 28" Hg works out to about 14psi. I like to approximate full vacuum at 30" Hg = 1 atmosphere ~ 15psi just to make it easy to figure out total force. So if you use a 6" vacuum chuck on a bowl and run 30" Hg, that would be like pushing the bowl with over 420 lbs of force (15psi x pi x r^2). That's too much for most bowls. I normally run about 10" Hg on a 6" vacuum chuck (that's a third of 420lbs, or 140lbs pushing on the bowl), less if the bowl is thin.

On a porous bowl, I feel where the air is coming through and seal the leaks with packaging tape to get adequate vacuum. Just don't forget that applying finish on a porous bowl while being held by vacuum will make the vacuum jump! I almost imploded one as I started applying oil/wax on a leaky bowl- I hit the bleed valve as the gauge was climbing fast.

Rick