Vacuum press for chambered body glue-up?

[Steve Sawyer]

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?

[Barry Daniels]

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.

[Steve Sawyer]

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!

[Bill Raymond]

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.

[Steve Sawyer]

Thanks, Bill. Based on your experience, I might just do an experiment and see how it works.

[Bill Raymond]

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.

[Steve Sawyer]

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"

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.

[Christ Kacoyannakis]

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.

[Steve Sawyer]

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.

[David King]

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.

[Steve Sawyer]

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.

[Randolph Rhett]

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.

[Steve Sawyer]

To "close the loop" on this topic, I wanted to pass along my experiences.

Bottom line for the impatient is that a vacuum press works GREAT for assembling a chambered and/or capped solid-body guitar.

For the technically inclined, I'm pulling about 22 inches Hg with my setup. This translates into a pressure of 10.805 lbs per square inch. For the 14" x 18" caul I had sitting atop the "sandwich", this comes out to 2,723 lbs! That's like having a very small car balanced on top of this glue-up!

The first experience was correcting a screw-up. My initial re-sawing left me with stock for a nice cap for the top, a nicely book-matched plate for the back, a thick slab in the center to bandsaw for the chambering. The only trouble was, the entire stack was coming up too thin. I had to joint, glue and re-attach the sections that were going to be the veneered top cap, and then thickness, joint and glue-up another plate to be veneered for the top cap.

More screw-ups ensued. I jointed the two center slabs before gluing using a glue-line rip blade in my table saw. Unfortunately the blade was just a bit off of 90* so the center slab ended up ever so slightly "V" shaped. Not obviously so, but a straight-edge revealed about a 3/32" hump on one face, and a corresponding 3/32" valley on the other. Because I wanted to ensure that the top cap was less than 1/4" thick after veneering the front and back, running the center section through the planer to get the two faces flat and parallel would have put me under the minimum 1 3/4" thickness I'd targeted.

Grrrrr.....

Also, when I jointed and glued up the plate that I wanted to re-attach to the center slab prior to band-sawing the chamber, I book-matched it. Don't ask me why, as it was going to end up being veneered, but as it turns out, when this got re-glued to the center section the grain matches very closely as the pieces ended up right back where they started before being re-sawed! Anyway, as so often happens with thin re-sawed stock, both pieces warped considerably so what I had was decidedly S-shaped.

So - I had a slightly V-shaped body blank and wanted to glue onto one face an S-shaped 1/4" thick plate...

I coated both pieces with Titebond, slipped the stack into the vacuum bag with a beefy 1" particleboard-and-melamine caul underneath and breather mesh on top and let the pump pull a good vacuum for about an hour. The pressure in the vacuum bag flattened everything beautifully. The result still had a tiny bit of a V-shape, but was down to less than .040", so about half what I'd started with.

I made up the top plate from some additional stock I had that I jointed, edge-glued then planed down to about 3/16" using an MDF sled in the planer, then veneered front & back in the vacuum bag. Came out to about .225" total, so it will be hidden nicely behind the binding.

I band-sawed the center section to create the chamber, glued a piece of contrasting wood (some birch veneer) into the saw-kerf at the butt end of the body blank, and used the result as a pattern to mark out, cut and glue-up a caul to allow me to use the vacuum press to glue up the back plate, the chambered center section and the veneered cap without creating "dimples" in the cap where the chamber is, or even worse, crushing the cap into the chamber. The pattern I made for the body has two 1/8" holes drilled smack in the center of the positions of the pickup routs, and I used these to drill corresponding alignment holes in the back plate, center section, top cap and the caul so everything would be easily aligned with some 1/8" dowels.

This time I used some Titebond Extend glue so I wouldn't have to worry about anything setting up while I assembled this "sandwich", and inserted the entire stack into the vacuum bag and let it sit in there for about three hours. This is much longer than Titebond recommends, but after 90 minutes the squeeze-out was still gooey, so I stuck it back into the bag just to make sure.

When it was all done, I couldn't detect ANY deviation from flatness across the face of the body blank, and only had a little valley of .026" to deal with on the back. Ran that puppy through the planer with one very light pass so the back is perfectly flat so I could go on and band-saw the outline of the body and run a router around the pattern.

So...onwards!!

Oh - by the way, I weighed the pieces that I removed in the chambering. This will reduce the weight of the body by about 2 3/4 lbs!