Versatile Clamps

[Van Huskey]

+1 on cauls, this is a perfect use for them!

[Steve Friedman]

Just to be clear, I am edge gluing two slabs that are each 48" x 24" x 2" to form a 48" x 48" x 2" table top. I like the idea of using cauls, especially if it means buying less clamps.

Sorry to be thick, but I'm confused because two clamps (one on each end) are only going to produce around 2,000 pound of pressure (according the FWW test). The glue line is almost 100 sq ft., so that means I need 25,000 (assuming 250 pounds per sq inch) without the cauls. How is it possible for the cauls to make up the 23,000 pound difference? Seems too good to be true.

[Eiji Fuller]

Just to be clear, I am edge gluing two slabs that are each 48" x 24" x 2" to form a 48" x 48" x 2" table top. I like the idea of using cauls, especially if it means buying less clamps.

Sorry to be thick, but I'm confused because two clamps (one on each end) are only going to produce around 2,000 pound of pressure (according the FWW test). The glue line is almost 100 sq ft., so that means I need 25,000 (assuming 250 pounds per sq inch) without the cauls. How is it possible for the cauls to make up the 23,000 pound difference? Seems too good to be true.

I was taught that your clamping force radiates out from the head of the clamps at about a 45 degree angle. In practice that has worked out for me in determining the number of clamps needed for a particular glue up. Since your boards are 24" wide and 48" long you only need 2 clamps to get evenly distributed clamping pressure. Thats with no cauls. If you had 16" wide boards than you would need 3 clamps. 12" boards than 4 clamps would be needed. I like pipe clamps but if you want the jorgy i-beams seem to fit you task. If your boards are properly jointed 2 clamps will do it for sure.

I think that there is a misconception that if you have clamp that exerts 1000 lbs of force and can than say somehow distribute the force over the entire glue up than you have to divide the total force by the surface area. IME that just is not the case. If you had but one clamp plus 2 cauls, imagine reverse bow clamp, that required 1000 psi to close them and clamped. I think you would measure 1000 psi at any given point along the joint. Or maybe I'm the one that's screwed up
I still get invisible glue lines though.
Another way to look at is that I believe that 2 - 1000lb clamps don't equal 2000 lbs of pressure but rather 2 areas of 1000lbs of pressure. Its not cumulative unless you can crank one down and then add the other at the exact same point. It then points to distribution of pressure which in the case of your glue up 2 clamps will suffice.

[Steve Friedman]

I was taught that your clamping force radiates out from the head of the clamps at about a 45 degree angle. In practice that has worked out for me in determining the number of clamps needed for a particular glue up. Since your boards are 24" wide and 48" long you only need 2 clamps to get evenly distributed clamping pressure. Thats with no cauls. If you had 16" wide boards than you would need 3 clamps. 12" boards than 4 clamps would be needed. I like pipe clamps but if you want the jorgy i-beams seem to fit you task. If your boards are properly jointed 2 clamps will do it for sure.

I think that there is a misconception that if you have clamp that exerts 1000 lbs of force and can than say somehow distribute the force over the entire glue up than you have to divide the total force by the surface area. IME that just is not the case. If you had but one clamp plus 2 cauls, imagine reverse bow clamp, that required 1000 psi to close them and clamped. I think you would measure 1000 psi at any given point along the joint. Or maybe I'm the one that's screwed up
I still get invisible glue lines though.

Another way to look at is that I believe that 2 - 1000lb clamps don't equal 2000 lbs of pressure but rather 2 areas of 1000lbs of pressure. Its not cumulative unless you can crank one down and then add the other at the exact same point. It then points to distribution of pressure which in the case of your glue up 2 clamps will suffice.

Eiji,

Thanks. I've seen pictures of your amazing work and if you say it's so, it must be!

Steve

[Van Huskey]

With the wide boards and the radiation pattern that Eiji mentions (which I have seen referenced countless times in literature) you don't need a lot of clamps. To be honest in practice I would probably use 4 parallel clamps (jet and Jorgensen are my favs) and more likely 6 since they are just hanging there and I don't have to buy them. In any event unless you are using more than 4 clamps don't set the outside clamps right at the edge, move them inboard to take advantage of the full "cone of pressure". If budget allows get good parallel clamps since you will sue them tons down the road.