flat glue-ups

[Curtis Niedermier]

Let's say you're gluing up a panel with two 3/4-inch thick boards that are each 9 inches wide. For argument's sake, the boards are perfect flat, edges square, etc. Is it still possible to end up with a cupped glue-up do to offset clamping pressure? What about twist? Can you get twist in a glueup if everything is square? I can't figure out if I'm imagining things or if some of my boards aren't true prior to clamping.

I have been using cheap bar clamps, and it seems as if I get some cupping if I seat the panels too far down into the jaws of the clamps (too close to the bar). This even happens if I match the panels and joint the edges together prior to gluing so that they're at complimentary angles.

I thought about locking in the panel by clamping straight-edge boards on the faces and sort of sandwiching the panel flat.

I'm probably making too much of this, but I'm curious what you all think.

[Jim Koepke]

I thought about locking in the panel by clamping straight-edge boards on the faces and sort of sandwiching the panel flat.

That is the only way I have found to prevent the problem.

jtk

[Sam Murdoch]

Using Bessey K clams I find that I can very often simply stand the assembly on the clamps - in other words the glue up is vertical rather than horizontal. I alternate the clamps on either side of the glue up equally. Considering your "for argument's sake" qualifiers - you only need light pressure. I clean up the glue before I set the panel aside to dry. I do also check that the assembly is flat and will change the pressure and/or move the clamp in or out as needed to correct any shape I might have induced. I will add a caul if needed but that is often unnecessary.

With cheap bar clamps - you will do best to use top and bottom cauls as you describe. I add a film of packing tape to the cauls to eliminate the need to worry about them sticking to the panel. There are other ways to achieve the same result. Packing tape seems easy to use and to clean up or replace.

[Judson Green]

Its a good practice to alternate your clamps. I also try to get the screw of the clamp centered though the panel. Regarding the twisting you may want to have a better look at surface (sawhorses, work bench) that your doing the glueing up on. Need to confirm that its flat.

Oh) Don't know what cheep clamps you have but on some pipe clamps I've noticed that the faces are somewhat crowned.

[Ryan Baker]

It's not hard at all to get those sorts of problems when clamping ... especially with cheaper clamps. Check with a straightedge across the surface to see if you are getting any cupping. Adjusting clamps on alternating sides and avoiding too much pressure helps a lot, as the others have said.

[Tony Wilkins]

As an interested observer / hijacker I have a couple of questions:

1) what are the right clamps?

2) how would using hot hide glue change the answer to the original question?

[Jim Matthews]

If you're getting a curved panel after glue up,
your joint isn't actually square, or the wood isn't completely dry.

I match plane all boards I want to lay flat, that cancels planing errors.

[Judson Green]

As an interested observer / hijacker I have a couple of questions:

1) what are the right clamps?

2) how would using hot hide glue change the answer to the original question?

In the commercial shops I've worked in pipe clamps were typically used for panels. Guess a ideal clamp for panels would be stout but not have long arms on it. If you think of a clamp as an (F) than short horizontal bits would be good. I have Bessy K-bodyes and I turn them ~20° to the face of the panel glue up.

I haven't any experience with hot hide glue but can't imagine there would be much if any difference.

[Tom McMahon]

With hot hide glue, I use a rub joint, no clamps at all to distort the panel.

[Josh Doran]

I've been pondering the same issue for a while too. What I had tried was using cauls across the joint to flatten the boards. As it turned out, the cauls were actually causing the cupping since both sides were not bending equally to get to flat. I've come up with an alternative but haven't tried it yet. Use the flat surface of my bench and clamp the boards to it. Then use bowclamps (not necessary but quick and useful) to clamp the joint. Of coarse laying a protective layer between the joint and the bench. As far as hide glue goes, its viscosity would cause less sliding than regular glue.

[Brian Holcombe]

I try to save the pipe clamps for a last resort (overly large panel, ect). Bessey clamps apply pressure more consistently and less often bow out a panel.

[Judson Green]

I've been pondering the same issue for a while too. What I had tried was using cauls across the joint to flatten the boards. As it turned out, the cauls were actually causing the cupping since both sides were not bending equally to get to flat. I've come up with an alternative but haven't tried it yet. Use the flat surface of my bench and clamp the boards to it. Then use bowclamps (not necessary but quick and useful) to clamp the joint. Of coarse laying a protective layer between the joint and the bench. As far as hide glue goes, its viscosity would cause less sliding than regular glue.

Clamping the glue up boards to you bench can create another problem... your somewhat freshly planned wood not able to breath equally on both sides. Could induce cupping.

To help prevent sliding one of the guys I worked with would use little finishing nails, toe nailed at ends of the board, though the end grain, let just about 1/16 protrude out of the edge to be glued. I've heard of other things to like a few grains of kosher salt or sugar in the raw at the ends. I've never bothered always glued up over sized and just be mindful.

[Judson Green]

I try to save the pipe clamps for a last resort (overly large panel, ect). Bessey clamps apply pressure more consistently and less often bow out a panel.

Hear ya on that! I hate pipe clamps. Don't/won't own any. Seemed to always give me metal slivers. K-bodies are way better. Unless you get them all shnozzed up with glue.. those little serrations.

[Chad Bender]

My only large clamps are pipe clamps, so alternating top/bottom to balance out the clamping load is required. I've also gotten in the habit of using a couple pair of cauls with every panel glue up. The extra 30 seconds it takes to set them up more than compensates for the time saved in cleaning up the glue joint later.

[Sam Stephens]

i'd offer 3 comments: 1) the joints need to be well matched and should require virtually no pressure to close. I vertically stack the panel to check joints for large panels -very easy to tell the good joints from the bad. Sprung joints work well too. 2) minimal clamping pressure -basically till it's hand tight and then another 1/4 -1/2 turn. Definitely alternate clamps between top and bottom. It's easy to induce twist and cupping w/ uneven and too much pressure. Cauls can be really helpful, but can also cause problems as others have mentioned. 3) ensure after glueup, adequate airflow around both faces of the panel i.e. don't just lie it flat on the benchtop b/c it will warp. Sticker it and keep it that way. And sometimes, it just happens despite our best efforts so I try and leave some extra thickness just in case.