"Clamp too tight" myth or fact

[newood2]

Anyone ever clamp too tight on a glue-up that caused the joint to fail? In other words, you sqeezed out the glue that suppose to hold the joint together.
I would believe I clamp pretty hard but I never had a joint that failed over the years.
On the other hand maybe my clamping pressure is relative to my wrist strength, so some type of pressure measuring tool would be a better way to figure this out.
Howie

[crokett]

I've never had a joint fail because the clamp is too tight. I have had a joint fail for too loose/not set correctly.

[Tom Slick]

from what I've seen, clamping too tight just wears out clamps.

[Alex Franke]

I clamp tightly, too. Not so tightly that the clamps break, but pretty tightly nonetheless. I don't think I've ever had a tightly clamped joint fail.

[cabinetman]

Clamping too tight means you need no glue for a bond. No, I can't say that I've had joint failure from clamping too tightly.
.

[atgcpaul]

I do what feels right. A recent FWW article said most people don't use
enough clamping force. I thought that was a little overblown. OTOH, I was
at one of the Woodworking Shows and sat in on a talk by Graham Blackburn.
He's an accomplished and published WWer. He says for his students, he
teaches them to joint the boards properly so that no clamps are needed for
glueup. Whatcha' gonna' do?

[LCHIEN]

Typical WWer: Apply glue - clamp hard.

maybe we approach this wrong.
Some days I spread the glue as thin as I can and still wet both surfaces over the entire contact area.
Then I clamp until I see the first hint of glue edging out, then I stop.

Feedback/QC: if you clamp as hard as you can and there's no glue squeezeout, then you didn't put enough glue in. Unclamp and look and see if there's dry-spots where there's no glue, esp. near the edges (maybe that the pieces aren't so flat-jointed as you thought). Reclamp.
If you don't clamp too hard, but start to get squeezeout, then remove clamps, and wipe off excess glue, you put too much. Try again.

[jackellis]

Common sense says all you need is firm enough pressure for there to be no gaps in the glue film. If the joint is set up correctly, that's not a lot of pressure.

I'm still at the point where that's rarely the case, though I am getting better (ruler that measures in 32nds helps a lot). I have been guilty of using clamps to force joints in (slightly) warped plywood parts together. In some cases, the joint without glue is almost strong enough to hold (until the wood dries out), though for the wrong reasons.

Although I'm no expert, I agree with Blackburn's approach. The clamps are really there to make sure the parts stay together until the glue dries, IMHO.

[jking]

I think this is one of those theory vs. reality items. Several years ago, I read through part of book that was put out by the Timber/Wood framing industry. I can't remember the name of the book or the organization that published it; I'm pretty sure they've changed their name by now. Anyway, in this book they had a chapter that dealt with joints that discussed tight joints, glue, clamping force, etc. Too much clamping force will weaken the joint, just as too little/too much glue. This was a technical book & the explanations were rather in depth; they made sense.

From a practical standpoint, though, the forces most of our project encounter aren't that extreme. Many of the joints we use are probably plenty strong anyway. I would think you'd have to have a combination of too little glue & too much clamping force to really run into a problem.

[Gator95]

The argument against clamping too tight is that if your clamp pressure isn't perfectly balanced you can slightly warp the surface- for example on a panel glueup.

I put a thin film of glue on both sides of the joint and then clamp and tighten down until some beads of glue squeezes out over entire joint. Maybe a touch more than that... never had a glue joint fail.

[Alex Franke]

For centuries, quality violin makers have been edge gluing without clamps at all by simply rubbing the pieces together firmly along the grain, which works out any bubbles and excess glue. (It works pretty well, actually, and I do this step whenever I can.)

I think if the pieces are well prepared and the glue is worked into the grain (and not holding the pieces apart) then you'll have a quality joint. Clamping just helps push the pieces together if they're not together already. Clamping more isn't going to push them together more if they're already together -- unless of course you're clamping to overcome defects in the wood or prepared edges.

[dbhost]

Overtension can distort the faces at the joint, I have one of my early laminations where the glue just didn't hold on one edge. Not bad enough to totally separate the joint, but bad enough to be ugly.

I have since learned to apply "sufficient pressure" when clamping, and use cauls where I can.

[sscherin]

I'll have to agree with the warping theory.
I've had joints before that would warp when clamped without a couple culls on either side.

If just excessive pressure squeezing glue out of the joint was an issue nobody would bother with vacuum presses for laminating.

[jonmulzer]

I am in the "warping" category. A long time ago, in a ww'ing magazine far, far away, I read a study on this exact subject. They found that the tighter you clamped the wood, the stronger the joint was. It makes sense really. You want wood to join to wood via glue. If you do not clamp tightly enough you have wood joining to glue, joining to glue, joining to glue, joining to wood. You are NEVER going to squeeze out ALL of the glue and you only need the thinnest of films on a tight fitting joint.

With today's modern glues, it is a lot harder to mess up a joint with clamping pressure. If you were using old school hide glue, perhaps. But not TiteBond. I believe this is one of those sayings that started a long time ago and for some reason stuck around through the evolution of glues.

With all that said though, I do believe that most people use too little clamping pressure. At least the ones I have watched work. For most they barely use any at all. I put a bit of torque on them by hand but never enough to cause warping. I do not tighten them as tight as they can go by any means, but a lot more than anyone else I have personally seen work. I have yet to have a glue joint fail.

[Tom Slick]

At my last job we (me and the engineers) were trying to calc clamping pressure from torque applied to the screw. It torque was applied by an automatic torquing gun. We figured out the parts had been significantly over-clamped for years and never had any issues with the product. This was on 8/4 material with a glue line joint routed into it. What we ended up determining is the clamp manuf. know what they are doing when they put those short handles on the clamps. Look at a bessey k body clamp, its handle is only about 1.5" in diameter and can only be tightened by hand.