Woodworkers… Clamping pressure?

[Paul Simmel]

Up until about 15 years ago, I over tightened everything… but not anymore.

On well machined pieces.

Since then, here’s what I do. On panel glue ups for example, I have used moderate pressure and watched my squeeze out as a gage of how tight to tighten the clamps. With a moderate amount of glue, I’m using finger pressure while turning my wrist and when I get a nice, even squeeze out I stop. With ¾” pipe clamps, say every 12” or so (3” boards), clamps on bottom and top (tops used to bring opposing force to keep the panel level only). When everything is nice and flat after finessing with a straight edge, I let it dry.

I have never had a glue failure with this method. I use the clamps more for equaling pressures and pulling things into level or square than consciously thinking about how tight they “must” be.

On another forum, I was reading how they do it. There, they crank ‘em to max and “engineers” and “physicists” supported that… stating that 1800 psi per clamp was justified in order to comply with the following info:

From the Titebond website:

>>> Required clamping pressure Enough to bring joints tightly together (generally, 100-150 psi for softwoods, 125-175 psi for medium woods and 175-250 psi for hardwoods)

I would really like to hear what experienced woodworks have to say about their clamping techniques.

Edited-out image HTLM.

[Paul Simmel]

You know how it is when your sanity is challenged! (You turn toward a source of sanity)

Thank you folks who are responding.

Please comment if you so feel.

By far there is little said on clamping pressure. If you Google it, there’s nothing, really.

It’s not that I can’t take counter arguments. I welcome them. But I prefer to “argue” with woodworkers and not scientists.

Just vote if you don’t want to comment. I thank you!

[Dennis Peacock]

Paul,

I try to do good machining so I don't have to "force" the wood joints to look good by applying more pressure. One thing I learned a while back, you can squeeze most all the glue right out of a joint.....so I do crank'em to the max no-mo.

[Paul Simmel]

Dennis,

I agree 100%.

On the other forum, the counter-argument against starving a glue joint was… no one complains about it.

In all honesty, I can’t remember the exact project on which I had my first failed glue joint, but I do remember it was back before I learned to not tighten so hard.

That was also back before I had machinery to make nice, bow-free joints.

-=-=-=-

There are a lot of experienced and far-beyond-me woodworking talents posting on this board. I’m not putting myself down, I have talent and experience too, but there are some VERY talented people here.

Thank you, Dennis.

[Don Bullock]

I never recall a failed glue joint, but I used to crank my clamps to the max and then some. One time I cranked a table top to the point that it had a bow in it. Fortunately in those days the glue had a longer set time so I was able to discover the problem in time to get the top flat. Part of the problem I realize now, in addition to clamping it too much, was that I did not have enough clamps. Now, that I'm older, and hopefully wiser, I crank them for "enough for good squeeze out."

[Mike Cutler]

I primarily use epoxies for glueups, so low pressure is all that is required.

[Chris Barton]

The purpose of the clamp is to hold the piece in place with good surface contact until the glue dries. However, there are those folks out there that think that the tighter you clamp the work, the stronger the joint (and this used to include me). I have learned what Dennis has, clamp pressure is not a solution for a poorly machined fit. A well machined glue surface can be held together by tape. And, that's often exactly what I use. In some instances I use no clamps at all, just a "rub" fit.

[Greg Deakins]

to me it seems that if you squeeze tighter, it will force it into endgrain, but that doesn't apply for other glue ups. I broke a wood clamp trying to get max pressure once, since then I have never cranked too hard, but good and solid no less.

[Mark Singer]

I give good pressure....a lot of pressure will not make up for a poor fit. If the wood fits well a moderate amount of pressure and applying glue to both surfaces should be fine...

[Byron Trantham]

It took me a while to realize that "tighter is not better". If I have to really tighten a joint, there is probably something wrong with the joint. Dry fitting really helps in making this determination.

[Howard Acheson]

According to the Titebond webpage and the Wood Products Handbook from the US Forest Services, PVA adhesives (white and yellow) require 200-250 psi of pressure for maximimum strength and minimal glueline when gluing hard woods. Soft woods require about 100 psi.

3/4" pipe clamps produce about 1,000 pounds of force. Parallel face clamps (ie: Bessey, etc) produce about half of that.

So, if you are glueing up a panel of 3/4" thickness that is 12" long the pressure needed is 12" x 3/4" x 200 psi which is 1,800 pounds of force total spread equally along the glueline. As a 3/4" pipe clamp, fully tightened, can exert 1,000 pounds, you will need two clamps fully tightened.

In reality, you can not overtighten most joints. The wood itself will deform before there is any damage to the joint. Also, once the adhesive has wetted the joint you will get a good joint regardless of the pressure as long as the minimum pressure is applied. There is no such thing as "starving the joint" in spite of some TV woodworkers opinion.

Now, it's true that you can get a strong joint with less pressure if the surfaces of the joint are mating perfectly. With PVA adhesives, it's the bringing closely together of the wetted wood cells that provides the strength (there is no strength in the adhesive itself unless epoxy is used).

[glenn bradley]

I learned my lesson on a cutting board I over tightened. Squeezed out too much glue and had joint failure before the project was finished. f the joint fits properly and I've got good squeeze-out, I'm done.

[Cliff Rohrabacher]

Tighten 'em up get clean squeeze out and stop there.

I prefer more clamps over more force to distribute the energy more widely.

Epoxies can be goofy. West Systems glues are somewhat runny so they can experience starvation from squeeze out more readily.

[josh bjork]

Byron, I am learning the fruit of dry fitting and it helps a lot. I think it helps you determine the right glue for the job as well. There are instances where you just can't apply pressure in a direction. If you just choose the right glue, all can be well. I ain't a CNC machine. Tolerances aren't near .002 and it is ok because there are glues for such situations.

I think there is a serious lack of education about glues. Many of the threads get quite heated but all the arguement is usually opinion. There is a big difference between what "will get by" and what is optimal.

[Howard Acheson]

>> West Systems glues are somewhat runny so they can experience starvation from squeeze out more readily.

Epoxy adhesives get their strength from the film of epoxy itself--unlike PVA, Polyurethane, Urea Formaldehyde, etc, which get their strength from a chemical reaction between wood fibers and cells. Therefore, epoxy should only be clamped enough to hold the joint together or very lightly. With epoxy you want a layer of glue between the joints.