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I didn't want to hijack Mario's thread for information on vacuum bags. But he led me to another subject need help with.
When worked in shop in Maine, we used vacuum for the top and back braces and was a great way for doing whole top and back pretty much one shot and for me worth doing.

I have a hinged jig that is round and use a radius dish (just bought the same radius dishes without abrasive)There is a notch in the dish so the vacuum isn't blocked.

My vacuum pump is electric (can be converted for air, but no compressor right now). All of my previous experience was with air and could read the pounds in regulator (tired and been setting up new shop all day so may not be right term)

I guess A/C dial works different. It shows HG and barametric numbers. When checking out the vacuum jig, it sucked down over an old back with braces on it already. I could pull on the top and it did have enough suction so that it didn't lift without some effort.

The thing only went to little less than .5 HG and about .1 bar. The dish is 24". Thing is nothing shows pounds that it may be at for clamping pressure. Does anyone have experience in this and what is math to determine where I am at.

Thanks

Stan, the gauge is reading inches of mercury. A complete vacuum will pull 29.something ( don't remember the exact number off the top of my head right now), which translates out to 14.7 psi at sea level. 29 inches of mercury is a huge amount of pressure. On a 10" x 10" area (100 sq inches)this would be the equivalent of 1,470 (100 X 14.7) pounds setting on it.
While you probably wont get 14.7 psi, you should get more than .1 bar, or 1.47 psi. I would like to see at least 15 inches of mercury or so.
You probably have a large leak somewhere. Take your finger and plug the tube and you should see the gage jump up close to what the pump is capable of pulling. If it's more than the reading when in actual use, you have a leak. Most pumps will pull at least 20 inches of mercury.
Hope this helps some.

_________________
Jim Watts
http://jameswattsguitars.com

Stan, .5 hg is not enough. A good vacuum should pull between .8 and .95 bar or about 25" - 27" hg. As to how much less than that is ok for gluing, more info here  http://www.ukuleles.com/Technology/bridgeglue.html

Conversions here   http://www.industrial-vacuum.net/cinhg.htm

 

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http://www.chassonguitars.com

I was thinking about my quick answer here and it could be a little deceptive. There is no mis-information in it, but to really understand how much clamping pressure you have you really need a good grasp of psi and how pressure is distributed.
For the clamping pressure your basically getting the area of the top of the brace times the pressure of the vacuum.
Sorry for any confusion or misunderstanding.

_________________
Jim Watts
http://jameswattsguitars.com