How tight to clamp glue-ups?

[Chasgroh]

...I use "rub blocks" frequently and have had opportunity to demo same for whatever reason...damn, they are VERY strong.

cg

[dadiOH]

I *like* squeeze out. A little bit, proves I used enough glue.

[Larry Jaques]

On Wed, 10 Mar 2010 09:35:11 -0800 (PST), the infamous "SonomaProducts.com" scrawled the following:

topposting repaired

I've used masking tape as a clamp to glue two pieces of pine together and the joint was perfectly strong. Oak to MDF is the same: tape for alignment and the weight of the project (reject countertop) for clamping pressure. I agree that you need some pressure, and that's especially true if the boards you have aren't perfectly flat, but all of us tend to overclamp things. I believe that 100psi is optimum, and out little HF bar clamps are capable of 1kpsi, so don't worry about wimpy clamps not being enough.

If you disagree, grab a pair of your favorite clamps and your household scale. Use boards (1" square if you're a purist or anal engineering type ;) to protect the metal as you gingerly tighten them on the scale. Note how softly you can twist before it tops out, remembering how snugly you crank them suckahs down on a project. Amazing, isn't it?

-- There is no such thing as limits to growth, because there are no limits to the human capacity for intelligence, imagination, and wonder. -- Ronald Reagan

[jev]

I have lost the identity of the original poster of the text below but a rep of Ttitebond addresses the 'pressure' issue .

Response from Frnklain/Titebond rep re:Calmping Pressure

After our discussion here, I wrote to technical support at Titebond regarding our discussion of clamp pressure. I got a next day reply from a very knowledgeable and helpful gentleman, Mr. Zimmerman. I'm posting it here.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I am writing in response to your question about clamp pressure. First, your calculation and understanding is correct. If you wanted to produce 200 psi over an area 12" x 12", you would need 28,800 pounds of force. On the other hand, it is not clear whether you often, or ever, fell short of the actual, required clamp pressure.

The actual required clamp pressure for any bond involving a wood glue is a combination of the small amount of pressure required to squeeze the glue into a thin, consistent layer, and the pressure necessary the compensate for any distortion or lack of fit in the wood stock being used. That means when the surfaces of the pieces being joined are true, and there is no gap between the pieces when they are dry fit, very little pressure is required. If, however, the same assembly is being made using pieces which are bowed, twisted or ill-fitted, the required pressure is much greater, and is largely the pressure required to straighten the wood and pull it into position. Thus, the actual required pressure for a bond also reflects the thickness, or fight, of the wood involved, with much more pressure obviously required to straighten a very thick piece of maple or oak than to straighten a thinner piece of the same species.

In many applications, then, pressure, serves to compensate for some lack of diligence in wood preparation. That being the case, good wood preparation lessens the need for, or dependence on, pressure. In the case of our literature, the high suggested pressures reflect the fact that those individuals being addressed include those who, at least on occasion, are trying to bond thick, poorly fitted pieces of wood, and for those readers, the high, suggested values are, indeed, necessary.

Finally, because the bond strength produced in a joint is the result of the entanglement of the glue particles which have been drawn into the pores and anchored to the wood on the two sides of the joint, there is rarely any concern for applying so much pressure that the glue is all squeezed out.

In fact, the bond strength achieved increases as the bondline or layer of glue becomes thinner. Given that fact, there are only two situations in which high pressures may be counterproductive. First, there is always a concern that the pores of wood at the bonding surfaces not be crushed, and that is the reason that our listed pressures are lower for the softer woods. The second situation deals with bonds involving end grain or other open grain. There the concern is that the open grain is prone to suck up a large amount of glue and, if that thirst has not been quenched before clamping, that excessive absorption of glue may result in a starved, and weak joint. Because most bonds involve face or edge grain which is relatively straight, that particular risk is rarely a concern. I hope this response is helpful, and ask that you feel free to write again or to call me at 1-800-###-#### if I can be of any further assistance.

[SonomaProducts.com]

[Larry Blanchard]

More or less. I move the wood just 1/4" or so till I feel it start to grab. Then press it into the desired location - no clamps needed.

[Larry Blanchard]

Amen, brother!

[Larry Blanchard]

That ought to settle the issue once and for all, but we all know it won't :-).

Wonder what those high pressure advocates would say about yellow glue, veneer, and a household iron?

[Phisherman]

How the joint fits is of most importance with regard to strength. I apply just enough glue so that just a small amount oozes out. My clamps are "hand tight," but not "muscled tight." I dont believe a joint is starved of glue, some is absorbed into the wood fibers. Apply a thin coat of glue to both mating surfaces if using carpenter's yellow glue.

[Leon]

Ahhh! My comments are backed up by Franklin!

[Leon]

Precicely, and for those that like the "glue starvation" cautionary statement, it helps to indicate that there should be no glue starvation.

[Leon]

Actually I can not remember the last time I actually put glue on both sides of the joint, my practice is one side only. Except when end grain is being glued.

[dpb]

Of course, since you quoted _very_ selectively... :(

The actual quoted message goes on to say

That last sentence is the key to the argument; that while perhaps an adequately strong joint is achieved w/ minimal pressure (given the caveats above) if one wants or needs the full strength achievable higher clamping pressures will be required.

This is what has been unequivocally demonstrated in test after test after test and can be found in US Forest Products research reports for (afaik) virtually all single material glues and most others such a resorcinol, etc.

Same thing...while not normally required for the application, a vacuum press and cauling would yield a stronger bond.

Whether it's necessary or not for the application is a different question than whether it does or doesn't have an effect.

--

[SonomaProducts.com]

Unsolicited end-grain gluing info.

End-gra> > If you have tight fitting joints, hardly any pressure is needed. =A0If = not,

[Leon]

Ahh but you leave out the key comments. The thinner the glue layer the better the stronger the bond. The comment was made and as I have pointed out, that extra clamping pressure is only needed if the surfaaces do not touch evenly along the joint line. If you have flat mating surfaces, squeezing the glue tighter is not going to strengthen the glue.

[Leon]

Yes especially when the joint is visible before the glue is applied. If you have an ill fitting joint more pressure is required to close the joint.

Just holding the joint together with out any pressure can be a perfecectly strong joint if the glue is applied properly and not in excess. Too much glue may in fact keep the joint from actually closing properly unless clamped tightly enough to squeese the excess glue out. Ironically glue squeeze out is an indicator the the joint is probably not going to be starved of glue but it is also an indicator that more glue was used than necessary.

But you sure can warp the boards.

Yup.

[Leon]

End-grain to any other grain, face, edge, end does not have any of the same great qualities of a typical glue joint. The fibers being perpendicular to the joint just don't allow for any long chain attachment. One technique is to "size" the joint (not sure if that is the right spelling for "size", I am speaking about the same concept as used with wallpaper). Anyway, add glue to the end grain and then let it mostly dry before putting the joint together. Then glue it again so the joint is gluing the glue on the end grain to the other piece. Haven't seen any science on this but have seen it recomended many times. Could be old wives tail.

That is exactly what I do. Coat the end grain and let it soak in and then reapply additional. Now if I am using pocket hole screws on the joint I simply apply one coat to the endgrain. Kreg claims glue is not necessary but IMHO it will not hurt.

[Father Haskell]

For the same reason, 2x4s or even 4x4s make better cauls for general glueups than thin scrap, because they widen the Vs at the glueline.

Even pressure is better than crushing pressure applied at few points. The glue is stronger than needed in most cases, but the finished joint will look neater.

[dpb]

Leon wrote: ...

...

No, the effect of the extra gluing pressure contributes by helping in creating the thinner bonding layer as well as inducing (as the above alluded to as well) more integrated into the wood pores.

As noted above, these results have been verified over and over and over w/ identically prepared samples taking out the variable of the quality of the jointing surfaces, etc.

--

[Leon]

If you put the proper amount of glue on in the first place there is no need to squeeze excess glue out. If you are working with flat mating surfaces and don't use excess glue there is no need to clamp except to keep the pieces properly aligned.

And for the last 30 years I have verified by my own practice that the joint is fine if properly set up to start with. Clamps are not necessary. Try this, put some glue on a "flat" scrap, and "lay" another flat piece of scrap on top, let set a few hours, see if you can break apart with out the wood actually failing.