Vacuum press for chambered body glue-up?

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Vacuum press for chambered body glue-up?

Postby Steve Sawyer » Mon Apr 16, 2018 8:11 pm

In gluing the front and back plates to the center of a chambered solid-body guitar, would a vacuum press be a good idea? I was looking at the pieces today, and thinking about how I was going to caul and clamp, but then had the thought of using the vacuum press. This would certainly be more convenient, but not sure if the clamping would be adequate. I know that a vacuum press can't develop more than 14psi (atmospheric pressure) but it's over the entire surface, unlike parallel-jaw clamps and the like which can develop a couple of hundred pounds, but only directly at the jaws.

Anyone tried this?
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Re: Vacuum press for chambered body glue-up?

Postby Barry Daniels » Mon Apr 16, 2018 8:48 pm

I would not advise it. Vacuum presses are great for veneer but they have insufficient pressure for larger wood assemblies. Also, on a chambered guitar the veneer would be applying pressure in places where you don't want it; on the unsupported areas of the chambers, however, a thick caul would remedy this issue. But with C-clamps you can apply the pressure where its needed.
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Re: Vacuum press for chambered body glue-up?

Postby Steve Sawyer » Mon Apr 16, 2018 9:34 pm

Barry Daniels wrote:I would not advise it. Vacuum presses are great for veneer but they have insufficient pressure for larger wood assemblies. Also, on a chambered guitar the veneer would be applying pressure in places where you don't want it; on the unsupported areas of the chambers, however, a thick caul would remedy this issue. But with C-clamps you can apply the pressure where its needed.

Ok - thanks Barry!
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Re: Vacuum press for chambered body glue-up?

Postby Bill Raymond » Mon Apr 16, 2018 11:49 pm

I should think that clamping pressure would be sufficient, but I'd sandwich the pieces between 2 sheets of plywood, probably 3/4 inches thick would do, so that the chambers won't collapse. Note, I've not tried this, but I have glued up laminated arched backs in a vacuum press; I believe that if you have well-fitted joints that the pressure should be enough.
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Re: Vacuum press for chambered body glue-up?

Postby Steve Sawyer » Tue Apr 17, 2018 12:29 am

Thanks, Bill. Based on your experience, I might just do an experiment and see how it works.
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Re: Vacuum press for chambered body glue-up?

Postby Bill Raymond » Tue Apr 17, 2018 5:07 pm

That's a really good idea, Steve. It's always best to do a test run on new procedures to make sure they work as you want them to.
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Re: Vacuum press for chambered body glue-up?

Postby Steve Sawyer » Tue Apr 17, 2018 8:11 pm

Bill Raymond wrote:That's a really good idea, Steve. It's always best to do a test run on new procedures to make sure they work as you want them to.

Yeah - especially in the absence of a chorus of "Oh yeah - I do that all the time" :D

My suspicion is that it might work just fine. You figure a dozen clamps, all generating 200lbs per sq inch, but each clamp only exerting that force on one or two square inches distributed over the entire surface of a guitar body...

But a test run will tell the tale.
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Re: Vacuum press for chambered body glue-up?

Postby Christ Kacoyannakis » Tue Apr 17, 2018 11:55 pm

I recently purchased a vacuum press setup to glue the top caps on carved top solid body electrics. I have not used it yet, but will in the next week or two. I did a bunch of research and talked to several vendors, and luthiers who use vacuum setups. My take on it was that the vacuum pressure will be more than enough to glue the top cap onto the body. Basically, you have two large well mated surfaces, and the for the most part, the suction created from the just spreading on the glue and rubbing them together holds them somewhat, and then the vacuum is more than enough (I was told). As mentioned above it is uniform pressure all over the top, not just where the clamps are. I had built some homemade clamps for my last glue up - basically sets of 2" x 2" pieces of hardwood with 1/4 bolts on either end. I slightly cambered the face of each piece of hardwood so that they would first apply pressure to the center and then as the bolts were tightened, apply pressure to the edges. They worked, but it was huge hassle, and every time you got one set of clamps tight the adjacent ones felt loose. Hopefully, the vacuum bag will be simpler.
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Re: Vacuum press for chambered body glue-up?

Postby Steve Sawyer » Wed Apr 18, 2018 12:44 am

I'm thinking you're going to be ok, Christ. I just did a little arithmetic, and learned that the pressure on a 13x18 body blank is 3,276 lbs!! Obviously a fully-profiled body would be much less, but it gives an idea the kind of forces involved.

If I try this I'll definitely need to provide a caul for a chambered body.
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Re: Vacuum press for chambered body glue-up?

Postby David King » Wed Apr 18, 2018 12:53 pm

Get a vacuum gauge to make sure your pump actually gets close to 29"Hg. I tested a friend's set up once and we couldn't get over 10"Hg which is only 5PSI instead of the 14-15 we were hoping for. Really all you care about is glue lines since it's not a structurally important joint. Dry fit and see if you can live with what you see. If you need more clamping pressure around the edge then build a gluing frame and make a bladder (firehose) around the edges that you can fill with compressed air to 30 or 40PSI. Fire hoses are pressure tested with fluid, not air so never take the rating at face value. If it says 210PSI do not go much over 50 with air. Vessels explode very differently with air than with fluid and air is infinitely more dangerous due to the potential velocity of debris.
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Re: Vacuum press for chambered body glue-up?

Postby Steve Sawyer » Thu Apr 19, 2018 5:26 pm

My vacuum press already has a vacuum gauge attached. I built one of Joe Woodworker's EVS rigs. For veneer I set it to 22" Hg, but the adjustment can be a little iffy, and it has gotten "stuck" and I've seen it pull well over 25" Hg before I started running in the adjustment screw! Don't know how much more it would have drawn if I'd let it run, but I wouldn't be surprised if it can pull a full 29"Hg.
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Re: Vacuum press for chambered body glue-up?

Postby Randolph Rhett » Thu Apr 19, 2018 11:22 pm

There is no reason it would need to pull a perfect vacuum just for clamping pressure. 10” is very different from 22” or 24”. In my experience vacuum bagging is very effective for providing clamping pressure. I use a bag for glueing braces, fingerboard, headstock veneers, etc. I would glue tops and backs in there except I’m pretty sure the vacuum would crush an acoustic body.
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