MIMF

Yes, briefly. The problem is that the back/top joint on an acoustic body is only about 1/4" to 5/16" wide. You would not want the compressed air to apply ANY pressure inside the line at the inner edge of the kerfing. This would cause the back to be pressed down, opening the outer edge of the joint and possibly breaking the back. I just don't see how you could get that fine of control unless you built a very accurate frame and used small diameter tubing which would have to be semi flexible. Too many hurdles. I am starting construction of my 50 cam clamps so my course is set. Thanks anyway, David.

Barry, I use HHG to attach my tops and backs. I set up my clamping dry, using a flashlight to check that all of the seam is tight. I usually put a rubber-backed piece of carpet face down in the dish to make up for any imperfections between the radius dish and the rim (which is sanded in that dish). I use bar clamps on the blocks and cam clamps, only as necessary, around the rim to get a tight fit—you only have to clamp where needed to draw the joint together. If the seam is closed, the hide glue takes care of the rest. I warm the gluing surfaces on the rim with a heat gun. Mario squirts his glue on the rim; I use a brush because it makes less of a mess for me. And then, if there are any defects the next day, I fix them just as Chris describes above. I really think it’s worth it to use the HHG for this joint.

Glad I got it right describing fixing gaps in HHG joints! I am a rank amateur by comparison with many on here.

I've no experience using fish glue, but I've read a lot about how it softens with heat and moisture. So the same principle ought to work here. But it's not my own instrument, maybe I'd do a test piece with a less than perfect joint just to be sure.

I haven’t ever used more than 8 cam clamps gluing a top or back to the rim. Three to five is usual. There’s just enough flexibility in my rims that clamping the blocks isn’t enough.

Thanks, Bob and Chris. Great information.

I'm a bit late to the party here, but let me explain what i do and see if anyone cares.
I also use HHG for gluing tops/backs via go-bars, and for a long time I glued backs to rims with the back down in the dish. Even with a complaint layer in the dish I would always get gaps in the joint that bugged the crap out of me. Viewed from the inside, there was never much of a real gap, but... you know.
So I do it the other way round now. I put the top rim down (flat - i use a flat top rim). I heat the rim and the back, and squeeze a bead of glue around the back rim. Then I place the back on and make sure it is centered. Then put a caul, ass-gasket looking thing, over the outer edge of the back. Then I place go-bars quickly around to clamp the joint.
The gasket caul and back are a compliant sandwich, and the gasket protects the back from go-bar dimples. I find this gives nice uniform closed joint. I can even close gaps where gelled glue holds a gap by just hitting it with a heat gun while it's under the go-bars.

Where could I get one of those ass-gaskets?

I think I had the same issue whereby using the dish for the caul is not sufficient for a tight joint, and your "compliant sandwich" is the remedy. Thanks Chuck.

Chuck, do you have issues with glue squeeze out dripping down the inside of the rims with your method?

Squeeze out - good question, i've really not had many problems with drips going down the inside of the box. I usually use reverse kerfed lining, and that stuff invisibly soaks up glue into the hidden kerf tunnels. The place where it does come out is where the brace ends fit into pockets in the lining. The glue has a tendency to pool there, and the tight-fitting brace displaces it out all over the place. So i try to make sure there is only enough glue in the pocket before putting the back on (or top - same procedure). Even if it does drip in one or two place, its an easy cleanup.

I make ass-gasket's from cheep-o hardboard (see pic). This one was the only one i could find right now (my shop is in total disarray due to an ongoing patio-cover re-top).
The pictured one is solid, but I have them cut-out as well. The cutout is only needed if you have a bridge or other feature to avoid.

I wonder how a bicycle inner tube would work for compressed air clamping ? A simple wooden frame could hold it in place around the perimeter of the guitar and it wouldn't take much air to inflate it.
Something I have used for light duty clamping is bubble wrap with a board on top of it. The pieces to be glued are placed in position, the bubble wrap is placed on top , a board on top of the bubble wrap and some weight on top of the board. The bubble wrap conforms (somewhat) to the irregular surface and applies enough pressure for gluing things that don't require a lot of clamping pressure.

i was thinking a similar thing but with closed cell foam around the perimiter of the caul board. 100 lb weight on the center, around 5PSI on the rim joint equally distributed.

I think any method needs to be cautious of placing any force inside of the kerfing edge because that will flex the top into a reverse arch which will tend to open the outer edge of the joint. It is a very narrow edge that you are trying to glue to.

I was just playing with an animal shaped twisted ballon art thingy and un-inflated it looks like thin walled surgical tubing, about 1/4" dia. I think that a compliant layer could be made from silicone tubing as used for fuel lines in model airplanes. It's pretty stiff and could be fluid filled to maintain a higher pressure over a narrow clamping area. Glue won't stick to it which will be handy. Comes in lots of sizes too.
https://www.towerhobbies.com/cgi-bin/wt ... fuel+lines
Also check out the Tygon tubing recommended for peristaltic pumps.

I just closed a body, top yesterday, back today. Note how few clamps it takes. I have learned the hard way to rub my fingers around the edge after gluing before I rout it flush. The change in pitch signals a loose spot. The router grabs loose spots in a bad way.

Bob, that is pretty amazing.

One of the tricks is to get it to fit perfectly when it’s dry clamped. The back, with a tighter radius, likes more clamping pressure for the angle on the heel block, hence the screw clamp. And, the carpet piece in the dish helps push against any small irregularities. I’m thinking that I’ve done somewhere between 80 and 100 guitars with this method with small tweaks and improvements all along the way. I have always used reverse kerfed linings, so that does help some with rim rigidity.

When there is a bad spot in the joint, a drop of water, a hot iron, and a clamp takes care of it.

2nd Barry - that is amazing.
I get it tho - must take a bit of fussing to get everything laying down, and to figure out where the few clamps need to go.

I am knee deep in clamps. Making them out of 1/4" baltic birch ply stacked 3 layers deep. These seem to work much better than the ones I made back in the '70s with the design in Sloane's book.

The flexible tongue is made from a piece of 1/8" baltic birch which is flexible enough that it can go into a slight "S" curve that allows the pad some swivel action, keeping it flat against the workpiece. I didn't really plan that but will take it as a bonus feature.

I added a small brass bushing on the cam.

During the construction of the clamps I am also working on tightening up the back to rims joint using Chris' method. It is working good. I have a new infrared heat gun which I use to judge the heat that I apply to the back directly over the kerfing. What seems to work is getting the top surface of the back up to about 220 degrees Fahrenheit. Then apply the clamps. I am dampening the joint with a bit of water twice during the heating phase. Takes only about 5 minutes per section.

I tested my new cam clamps on a bathroom scale and they easily can apply 50 pounds of force. I think that should be sufficient.