Clamping Techniques for Flat Panels

Tips and tricks for gluing wood door panels up nice and flat. September 27, 2009

I'm having trouble keeping raised panels flat after glue up. I first face joint one side of each board being glued up and plane close to thickness. I then joint one edge and reverse the board and joint edge again (a technique I recently learned from you guys - thank God for WOODWEB). By this time the boards are flat. I then glue them up on a shop made glue clamp rack which is made of pipe clamps that slide on a pipe. I let them sit for a couple of hours and when I release them from the clamps, I notice they kind of cupped. I run them through the planer to clean both sides, cut to size and raise the panel. Problem is that the cupping did not go away, resulting in the panel not fitting in the stile and rail grooves as expected, and if they do, the panel makes the door wobble. I then plane the back of the panel some so it can fit a little loose in the grooves, and problem solved, but by doing this, it leaves gaps in certain areas around the panel when the door is put together. Can anyone help?

Forum Responses
(Cabinetmaking Forum)
From contributor H:
Rip your glue joints on the table saw. They will be straighter than jointing. Don't apply so much pressure when clamping.

From contributor L:
You can't face on a planer! (Joint a face.) Well, maybe you can with a carrier board and lots of shimming. Check out how you are clamping. Is the clamp pressure being applied in the center of the edge of the panel? Pipe clamps have sloppy fitting clamping faces made all the worse if you over-tighten.

From contributor R:
Get rid of the pipe clamps. They are not rigid enough and tend to bow glue-ups, especially if you are using a lot of pressure. Pipe clamps and pipe wrenches have no place in a woodworking shop.

From contributor D:
Try alternating the clamps. One on top, one on bottom, etc. Can't argue about the pipe wrench, but I use pipe clamps all the time.

From contributor E:
If you glue them up, they need to be flat and true in the clamps. If you glue them up and they are curved in the clamps, they will retain that shape when you release them.

From contributor J:
You could try lifting the panel a bit before you tighten the clamp. This will center the screw in the middle of the edge. We've always used pipe clamps, and edged boards on a jointer, and it works for us.

From contributor F:
Three elements: flat boards, square edges, and proper clamping. You can use any clamp under the sun as long as you take the time to ensure that the panels are flat at the moment of glue up. I use a straight edge for this purpose.

From contributor W:
I also use pipe clamps all the time! Different clamps have different uses and it's good to have a variety, especially when I need a 10' clamp for some odd job. I also have pipe wrenches in my shop. How else would I tighten my cast iron airlines? Anything else I shouldn't have in my shop?

I use jointed edges for glue-ups also; just make sure your fence is set accurately. It's usually easily visible when you dry fit the pieces if your jointer is off.

I use Bessey bar clamps with the parallel jaws for gluing up smaller panels. Put your straight edge on as soon as you have clamped up to see if you're causing the problem with too much clamp pressure. I'm guessing this is your problem, so start here and see what happens.

From contributor F:
There is a knack to everything about working wood. I mean, sure... it would be great if all tools just worked perfect with no thought or special effort from the user. I have a pile of pipe clamps myself and they work great if you learn how to use them. You have to adjust the panel edges and their relationship to the clamp jaws and also use the correct clamping pressure along with a straight edge to check, clamp and glue a flat panel.

I also can and do clamp a flat panel together with long "F clamps" all the time. I do not depend on or need a rigid clamp surface to index a flat panel glue up. It's all about finesse and using a gauge (straightedge).

Many woodworking books teach that if the panel faces are flat and the joining edges are straight and square, a panel can be "put in glue" and simply stood on its edge against a wall or vertical surface and left to dry, resulting in a sound joint by gravity and the weight of the boards alone. This is true; I have tried it.

From the original questioner:
Thanks. I think I do apply a little too much pressure when gluing up. What is the least amount of time I should leave the panels clamped up before I release them? I use Titebond original wood glue.

From contributor W:
I usually try to leave them for half an hour before I take the clamps off. I have enough clamps that it takes almost that long to go through them all anyway. Of course you'll need to let them set longer before machining; that's just the time for being in clamps.

As far as pressure goes, I use just enough to bring the boards together and get a line of little beads along the edge. If you keep squeezing you'll actually do more harm than good as the joint will get starved for glue.

From contributor M:
The "too much pressure squeezed all the glue out of the joint" myth has been busted by the fellas at Fine Woodworking. They found that more clamping pressure doesn't affect yellow/white glues. I believe it was in the same article that found Gorilla Glue type poly glues are poor for woodworking.

Pipe clamps work fine.

Running a cupped board through a planer will accomplish nothing in terms of flattening. The feed rollers will flatten the cup. You just end up with a thinner cupped board.

From contributor L:
Several years ago we tried to produce "starved joints" by clamping different kinds of wood on our Taylor clamp rack. Serious clamps! Put as close together as we could get them, put as much twist on the big handles as we could muster. The result, no different than just a nice snug pull up. If you have lots of joints in the panel you need a little extra effort to make sure the pressure doesn't slacken due to the excess glue slowly oozing out. Just go back over the clamps a second time to be sure they are still snug. Few jointers produce perfectly straight edges, so with several boards being glued into one panel the errors add up. Going real slow on a jointer also tends to glaze the edge (compress the fibers), making for a poorer glue joint.

For slightly larger shops going to a straight line rip saw produces an excellent gluing surface much faster. A 3 man door shop near here went to a straight line rip a few years ago and wouldn't go back. We bought an Extrema (XR-12C), 15hp, laser, blade, digital readout on fence, 3 sets of anti-kickbacks, feeds 4, 5 or some 6/4 @ 99'/min. Variable speed for thick material. Produces beautiful, ready to glue edges. We rip 1/8" strips for laminating curved casing, no problem. The Extrema (Taiwan) was a little more money than the cheapest Chinese saws, but has been very good. There are lots of bigger saws made for those of you that need them. Sorry for getting off track here, but producing the best quality you can as efficiently as possible is my justification.

From contributor B:
I have been using pipe clamps for far too long. This looks like a good place to ask who's using what clamps and why?

From contributor E:
Pipe clamps, inexpensive.

From contributor F:
Pipe clamps, F-clamps, C-clamps and spring clamps in my shop. You can make a pipe clamp as long as you need if you have enough pipe. I glue up small panels with F-clamps all the time. As born out in this thread, clamp pressure does not need to be intense to get a good glue joint... you just need enough.

From contributor Y:
Pipe clamps for over 40 years. And while we're at it, I'm surprised no one has mentioned using cauls. Even simple, 3/4" plywood strips 2" to 3" tall clamped to each end of the panel, top and bottom, will keep the glue-up flat enough to dramatically reduce thickness sanding. Put the panel on T-stands to gain top and bottom access. Oh, yes - be sure to use waxed paper, Formica strips or some other barrier between the cauls and the work piece to prevent the cauls from becoming part of the panel!

From contributor N:
As the old saying goes, you can never have enough clamps. I have bar clamps, c clamps, etc. I now almost always use Jet bar clamps. I was at a woodworking show about 6 months ago and a local dealer was selling them 50 to 70% off. I bought over 40 clamps that day and wish I had bought more.

I have found the bar clamps have worked the best for me. They sit upright on the assembly table and the Jet style clamps tighten very easily. As long as the joint is prepped right I have not had any issues with the panel being flat. The other nice thing about these clamps is that you can sight from the side to see if the panel is in full contact with the bar. If so, the panel will come out flat and true.

I prefer the Jet clamps over the Bessey clamps, but both work fine.