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Hey guys, I'm thinking of going to vacuum to glue bridges down...got hold of an AC man's vacuum pump, but it'll pull a continuous 29.90" inches, with no relief valve...am I gonna pull the bridge thru the top, or suck all the glue out of the joint like this? I suppose I could introduce a slow leak into the system somewhere, or just turn the thing off every 5 or 10 minutes...what do you with the experience think? TIA.

Larry

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http://www.presnallguitars.com

I think anything over 20" is over doing it. I'd be most concerned about starving the joint, especially on a bridge.

Vacuum is a natural phenomenon that exists in all of nature. Vacuum is merely the absence of matter, a space that contains air or a gas at less than atmospheric pressure. Our high school physics taught us that a 1 sq. in. column of air from sea level to outer space exerts 14.7 pounds of downward force. When we apply this force to a larger area like 1 sq. ft.(144 sq. inches), then we generate more than 1 ton of holding force (14.7 x 144 = 2,116.80 lbs.). This is very useful in guitarmaking.

Vacuum is normally measured in "inches of mercury" and written as “"Hg”. This is the same term the weatherman uses when talking about the barometer reading. The barometer is 29.52 "Hg and falling." The maximum barometer reading on a standard day is 29.92 "Hg and the maximum force or weight of 1 sq. in. of air is 14.7 pounds. From this we can develop an approximate 2:1 ratio between vacuum level and holding force (29.92 Hg = 14.7 lbs. per square inch)
I sincerely doubt that your vacuum pump can pull AND maintain 29.9" of mercury. Wood leaks so your actual amount will be something less. But, even at 29.9" that is only 14.5 pound per square inch. If your bridge is 6" x 1" then the vacuum would exert about 90lbs of pressure. Plenty for gluing but not destruction!

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Sylvan
http://www.wellsguitars.com

if you are concerned you can get vacuum controllers from joewoodworker.com at a reasonable price. he also has a lot of info on their use and installation.

With a perfect vaccuum (which you will not get with an AC pump), you will get 14.7 psi of clamping pressure. The Franklin website recommends 100-250 psi of clamping pressure for Titebond (depending on the species of wood), which is roughly somewhere between five and fifteen times the pressure you could get with a perfect vacuum. http://www.titebond.com/DetermineTB.ASP?UserType=1&ProdSel=P roductIntroTB.asp

Note that both of these numbers are psi, not total clamping pressure. If the surface area of your bridge's glue surface is six inches, then Franklin's recommendation would be 600-1500 pounds of clamping pressure, whereas the vacuum would get you closer to 85 pounds.

Now, nobody really uses 600-1500 pounds of clamping pressure to glue a bridge on. Many of us use vacuums with which we would get 85 pounds of pressure on a good day. I lay these numbers out simply to show that there is absolutely no risk of starving a joint with a vacuum clamp no matter how good your vacuum is. If anything, the only real question is whether a vacuum can give you enough clamping pressure. Fortunately, the answer you will get from any of us here who have tried it is "yes." It turns out that you can get a perfectly good joint with far less clamping pressure than Titebond recommends.

If you have a vaccuum pump and didn't pay anything for it, use it!

Hey Larry,

can you tell us what you're using for vacuum? Curious minds.....

Kelby. those are very informative numbers. I had no idea that you could exert that much force and not starve the joint.
When I was setting up my vacuum, I thought I starved a joint but I was obviously wrong. Must not have gotten enough glue on in the first place.
With that in mind, to answer the original question, go for it. If you can pull that much vacuum, you're in better shape than I am.

Well, it's not pretty Terry...a guy who installed a new central air conditioner for me last summer (turned out to be a guitar customer!!) used this pump on my job, and just dropped by and gave it to me last week. It's a small motor attached to a vacuum pump by way of belt and pulley, and uses lawnmower oil, so it's not a diaphragm type like the one I tried using to glue braces last year...(I went back to go-bars because of the lack of clamping pressure with that one)...so, I can't really say what kind of pump it is, only that it's strong enough to vacuum down a central air conditioning system.

Larry

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http://www.presnallguitars.com

I am pulling 23-24 "Hg, and it seems like plenty. This is using a small Gast diaphram pump. Excavates the brace jig in under 15 seconds.

Not sure what is typical, but it must be in the normal range. The joints are far stronger than the wood, and no starvation - as several destructive tests have shown.