Clamping Techniques for Consistently Flat Panel Glue-Ups

Jigs and simple methods for maintaing flatness in solid wood panel glue-ups. October 19, 2014

Question (WOODWEB Member) :
A technique for clamping flat panel glue-ups I use is to take four pieces of square tubing, 1" or larger. Stainless would be the best, but it can be done with black pipe. If black pipe, laminate one face with vinyl edge banding so that the glue does not react with the metal. Stack the tubing two high and drill corresponding holes through both pieces about 30" apart. I drilled to allow a 3/8" carriage bolt. Run a 4" carriage bolt through the hole, and lay out two pieces of tubing on the table with the bolts pointed up. Lay up as much panel, drawer front, table top, etc. in between the bolts as possible. Drop the top tubing over the bolts, and snug down with a wing nut. Clamp the panel together as tight as you want. With the panel sandwiched between the tubes, they cannot cup when pressure is applied by the clamps. Using 60" tubes, and regular pipe clamps I can lay up about 14 square foot of door panel at a time, and they will come out perfectly flat.

Forum Responses
(Cabinetmaking Forum)
From contributor C:
We do something similar to this except that our cauls are spring loaded at both ends. One end pivots and the other is buckled with a destaco latch clamp. The cauls themselves, in our case, have a UHMW liner on the bottom to keep glue from sticking and iron from staining the wood.

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From Contributor O:
Why not just run clamps alternately under and over the panels? This way they counteract each other and act as cauls. I think this is so basic as to not need to be mentioned.

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From contributor C:
You have obviously clamped a lot of boards in your life. I am sure you have done some oak. Ever wonder what those deep black stains are that magically show up wherever the iron and moisture made contact with the oak? Alternating pipe clamps under and over a board will mitigate bowing to some extent but not do anything to keep the boards coplaner with each other. Try the clamping caul method sometime. You'll never go back.

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From Contributor O:
Our work is all face and edge jointed, planed to thickness and then glued up, so it is pretty flat and easy to level boards. We do 2 to 20 glue-ups a week from 10" to 6' in width, from 20" to 12' long. It is impossible to keep pairs of cauls on hand for the wide variety we do. We place the boards on the (level, flat) bench on top of clean S4S pine bumpers that hold the boards flat edge to edge. The bumpers are sized to allow the clamps (either pipe or bar) to pass underneath without touching the boards. We glue the precisely jointed edges of the boards and level the joints with one hand or a mallet while snugging up the clamps.

Once all the boards are level and held in place, we then add clamps above the boards to balance the compression. These can be set above the glue lines also to prevent ferrous stains. The bumpers are waxed to help repel glue, but the clamped assembly is removed from the bumpers so they don't all set up. This makes for flat and true panels every time. Our glued for width stock is also planed after glue up at times, so then the stains are not a problem, but the use of bumpers (we have quite a collection) makes any glue up easier.