Can Glue Joints Be Clamped Too Tight?

Clamping Clamps Glue

 

Many people are worried about over-clamping. I say that’s absolutely not a concern. Here’s why:

Consider this. I have a cutting board that is 12” x 18” and 2” thick. It consists of many boards glued together to make the whole thing

Let’s take two of those interior boards. They are 2” tall by 18” long.  And I have to glue them together.

2x18 = 36 square inches of glue surface.

It’s a hardwood. So Titebond says I can use 250 pounds per square inch in clamping force.

36 square inches x 250 pounds per square inch = 9000 pounds of clamping force recommended.

Who would have thought that two little boards should be clamped together with 9000 pounds of force??!

Well, the scientists & engineers at Franklin Adhesives (makers of Titebond), who have been studying the chemical & physical properties of PVAs & have been extensively researching how to achieve the very best bond for the last three quarters of a century think that it’s necessary.

So I generally defer to them on this aspect. My take is that if a manufacturer of a product employs research scientists that determine the best way to use their product, then that truly is the best way.

Now let’s get practical. This glue up is only 18” long. How do I get 9000 pounds of force on it? 

It looks like I can get 7-8  pipe clamps on it. But the truth is, that would just barely fit. 

In all likelihood, we aren’t going to try clamping to such extremes. But I did this exercise to prove a point. It’s exceedingly hard to over-clamp. Mostly because we’d have to fill every square inch with clamps.

So my advice is to do this:

Use as many clamps as will comfortably fit. And secure them tightly. Almost certainly you will get a great joint. (Probably not the strongest possible, but that’s ok) And, you will have no fear of over-clamping.

If you are curious as to the science behind why a very tightly clamped joint is so strong, I’ll tell you. PVA adhesives have great adhesive strength (meaning they stick very strongly to other things), but they have a very low cohesive strength (meaning they stick very poorly to themselves).

The reason high pressure is great is that it forces the bond to have a very thin layer of adhesive inside. So that virtually all of the strength of the joint is adhesive strength.  If the joint has a thick layer of glue inside, then we are also relying on cohesive strength. Or the ability of the glue to stick to itself. (Which we know is poor).

This is also why PVA glues are considered poor at gap filling. Inside of a gap is a large amount of glue that not only must stick to the wood on either side of the gap, but it must stick to itself over a considerable thickness. Which it does poorly due to its low cohesive strength.

Epoxy, on the other hand, is a great gap filling glue. It has high adhesive AND high cohesive strength.

 

Titebond official recommendations:

http://www.titebond.com/product/glues/e8d40b45-0ab3-49f7-8a9c-b53970f736af

 

Fine Woodworking article:

“Most woodworkers are underclamping their joints during glue up.”

https://www.finewoodworking.com/2010/05/11/how-to-glue-up-joints-the-right-number-of-clamps


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  • Bradford Penrod on

    Love the answer here! It is hard to explain why a proper, glued joint is actually stronger the wood around it, but I think you did. I love your videos and you sharing your knowledge and skills for the betterment of others. God bless you and your family!


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