what is a caul?

A project I'm working on suggests using cauls for gluing up the carcase of a shelf. Is there anything special about cauls or are they only something like a 2x2 to even up the pressure of the clamps?
Thanks Tom
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a
A caul can be made of anything. I use scraps because I usually make it specific to joint being clamped. Depending on the situation, you might want to put wax paper between the caul and the project being clamped. It is pretty sad when you realize that you have attached the caul to the project.
Bob
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They are something like a 2x2 as you say. They are used to distribute clamp pressure over areas that otherwise probably wouldn't see any pressure. The only thing special about them is that they are usually tapered or curved somewhat to be slightly thicker (in the clamping direction) at the center than at the ends where the clamps are applied. The reason for that is to insure that as the caul flexes under load, the center still exerts roughly as much pressure as the ends.
Tom Veatch Wichita, KS USA
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Yes, a caul is used to clamp and spread the clampingvforce across the entire span as well as lining the board edges/ Typically they are a bit thicker in the middle. When clamped on the ends, the thicker part, (actually a bit of bow) squeezes the middle. Hardwood with a 1/4 " bow is usually enough/ The piece may be two inches thick, with the middle three inches and the ends 2 3/4 thick. This span would be good for a two to three foot span.
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Ditto what everyone else said and also,
Judge the pressure of a clamp head radiating (fanning out) on a line of 45 degrees each side of the clamp head. This 90 degree (2 X 45) arc is the area the clamp head is exerting pressure to/on. By pulling the clamp head away from what's being clamped the width of this "pressure area" is widened. With a caul a wider area can be clamped using fewer clamps.
The above is also true when gluing up boards into panels. Let's say you are gluing up two boards 5" wide. The first clamp can be positioned 5" (the width of your board) in from the end. The remaining clamps can be position 10" (twice the width of your boards) on center with the last one 5" from the end. Any more than that is overkill and of course it never works out to be exact but the idea is to be as close as possible without beating yourself up over it.
This is not to say that if your stock is good and true you cannot get by with less, just that if you are ever wondering, this is a good rule to go by.
Your Mileage May Vary and Bingo Was His Name-O.
Oh, and before I forget, cauls are tools. It doesn't hurt to have a few on hand. They can be made from left over scraps and stored away until needed.
UA100
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Thanks for the info. Wouldn't you know, I just cleaned the shop and burned the scrap pile? I was wondering how it would work to take a 2x2 and instead of spending a lot of time tapering, put a couple of door shims under the middle to make the bow.
Tom

a
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Your clamping pressure would be focussed on the shims, methinks.
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Wouldn't the shims go on the outer portion so as to flex in the center? Ed
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says...

Takes me about 2 minutes per 4' caul with a hand plane.
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Ok two minutes and a hand plane. What could be easier.
Thanks
tom

burned
instead
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Tom Young wrote:

Nail the caul bowed to a piece of scrap ply. Run it through the table saw.
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