How much pressure needed to glue up drop top on solid body?

[Satnam Singh]

I'm trying to decide if I can use weights instead of clamps to glue up a 1/4" drop top on a solid body blank. If I know the needed pressure, couldn't I multiply that times the gluing surface area to get the needed weight? If I chamber the body blank, that would reduce the gluing surface area for this calculation, reducing the amount of weight needed, no?

I have a few 24" by 36" slate paver tiles that I believe are 75-80 lbs. each, that I could use. Is this a bad idea?
Thanks.

[Brian Evans]

The point of using clamps is you can distribute the pressure over many separate points somewhat equally. Using a single weight will usually put most of the pressure in a single high spot. You can distribute the weight using cauls or foam so the weight is evenly distributed

[Pat Foster]

Over on the OLF there's a member who used to design and build propellers. He knows more about gluing than anybody I know. His name is Stuart Gort. If you search on pressure or glue, you'll get more info than you can use.

Pat

[Barry Daniels]

I once saw on Franklin's website that they specified somewhere around 200 psi for clamping pressure. So, if you were compliant with the specs, you would be placing several thousand pounds of pressure on something the size of a solid body drop top.

We discussed this issue here once before and it made for some strong opinions. My belief is that Franklin is grossly overstating the pressure needed for a well fitting joint due to liability issues. After all, most of us have at least heard of luthiers successfully using a rubbed joint which in effect has zero clamping force.

So one answer to Satnam's question is how well does your joint fit and how much force does it take to completely close up the entire joint. Then add some more weight for a safety margin.

[Bryan Bear]

Another thing to consider is that clamping pressure radiates out at around 45 degrees in all directions. If you plop a bunch of weights down on a body and they only actually touch the surface in a few points (due to irregular surfaces) the bulk of the clamping force will be placed in a 45 degree cone from those spots. If you do use weights, tall flat cauls that raise the weights high above the actual glue line will spread that force out much better because the cones at the glue line will noww be much wider. . .

[Clay Schaeffer]

If you use epoxy ( a non waterbased glue that won't make the wood swell or cup) and a piece of styrofoam that will compress under the load of the paver tiles and help distribute the weight more uniformly, it will probably work O.K.

[Barry Daniels]

What Clay said.

[Jason Rodgers]

All good advice here. Mechanically, I think you'll be able to get the top secured sufficiently. Cosmetically, getting a tight, closed joint all around the perimeter is going to be your challenge. Even with a crap-load of clamps, I've ended up with gaps between the drop top and body core. Those bodies are in the burn pile now. I do my drop top glue ups in a vacuum bag to avoid this problem.

However, if you have a plan for hiding that joint, you could get away with minor gaps. If you were to bind the edge, with the ledge extending over the top and into the body, nobody would be the wiser. Of course, binding a guitar presents its own challenges.

[Dan Smith]

I glued a 1/4" Maple top once and clamped it very tight to minimize joint lines.
I used tite bond. When dried, the Maple cracked due to glue moisture expanding and contracting the wood.
I now just use firm pressure or I use slow-set epoxy on thin pieces.
Something to consider.
Dan

[Satnam Singh]

Lots of things I had considered. Much appreciated!

I had planned to use Titebond; I'll have to give epoxy some consideration. Caul to spread out/even out the pressure-not a difficult adjustment. Maybe a sandbag layer under the weight to even out the pressure.

The first project is gluing up 2 layers of 7/8 inch bubinga, which will need to be chambered anyway (sorta like the later rosewood tele bodies). I thinking less surface area will reduce the effort to make sure I have no high spots, good mating surface. Not sure if the harder exotics (especially at 7/8 inch) are less prone to swelling/moving due to moisture from the glue. Thanks for that warning, I hadn't thought about that. Less gluing area should reduce the weight needed (greater PSI with same weight).

My next project does have a 1/4 inch bubinga drop top, so I'll have to consider if I need to make adjustments there.

Thanks again.

[Daryl Kosinski]

Make a sandwich 3/4 plywood, foam board, parts to be glued, foam board, 3/4 plywood. Take the whole thing out in the driveway and park the front wheel of your car on it overnight.

[Eric Baack]

Consider that vacuum bagging a top only applies about 14 psi then I would say 200 psi is more then a bit of overkill

[David King]

Franklin's figures actually range from 60Psi to 220 Psi depending on the wood's hardness. The ideal clamping system involves a hydraulic bladder and a very rugged frame to contain it. Vacuum bagging works for a very thin top or one that is precisely shaped to fit the top but it's really better suited to a glue like epoxy where an thicker glueline is expected and desirable.

[Mark J Tierney]

15 psi doesn't sound like much, but it's more than a ton per square foot.

[Brian Evans]

15 psi doesn't sound like much, but it's more than a ton per square foot.

If you think Franklin (whoever he is) is right, then a guitar top needs over 30,000 lbs, or 15 tons, of clamp force. Just a reality check. The only thing I own that weighs over 30,000 lbs is my house.

[David King]

When I visited the Warmoth factory years ago they were using vacuum bags and placing them in a pressure chamber and pumping that up to 60psi for a total of 75psi.
You do the best you can. You live with a little glue line if that's the best you can do or you buy a bunch of clamps and clamp around the edges so at least no one will see the glue line. The last time I did a drop top I lined the clamping caul with some heavy duty neoprene rubber to spread out the clamping pressure and that worked out well.

If you want to use a vacuum bag then use a glue that's designed for that application like epoxy but bear in mind that the epoxy will get sucked up through every pore and crevice and probably get all over the top surface forcing you to use that same epoxy to fill the grain come finishing time.