How tight to clamp glue-ups?

Page 1 of 3  
I've seen completely opposite opinions. On the one hand I have seen "scientific" perspectives saying that even a dense set of the best clamps can barely supply the "optimal" clamping pressure.
On the other hand, I see the woodworking shows warning about not clamping too tight so as to prevent glue squeezout and starving the joint of glue.
So who is right the "scientists" or the "practitioners" or both?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

If I am not mistaken, titebond recommends 100 to 150 psi, this makes good tight joints that won't creep, with a lot of clamps this is not too hard to get close to on edge gluing, but for laminating larger items it would be almost impossible without spending a lot on equipment.
Get your fitting as close as possible and apply the most amount of pressure you can, without damaging your work or clamps, works for me.
basilisk
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 3/10/2010 7:53 AM, blueman wrote:

The proof is in the pudding ...
--
www.e-woodshop.net
Last update: 10/22/08
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 10 Mar 2010 08:17:24 -0600, the infamous Swingman

You clamp pudding? Um, are we talkin' shaky puddin' here, Swingy? I clamp onto that when it's around, too. (Go, Gator!)
-- 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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Things to consider. The best joint contains 2 mating surfaces that are perfectly flat.
Clamps sole purpose are to hold the pieces in alignment until the glue cures.
Clamps are often used to squeeze an ill fitting joint into submission. See above comment about the best joints.
You can use masking tape to clamp a properly fitting joint.
You will never starve a joint of glue by clamping too tightly. Glue starvation is a condition that is caused by not applying glue properly in the first place. Running a bead of glue down the edge or surface of a board and not spreading the glue over the entire surface is the first step to glue starvation. A thin glue line is the best. Tightly clamping an ill fitting joint to close the gap will aid in a better bond at the joint, it creates a thinner glue line, unfortunately an ill fitting joint may appear to fail as the wood splits "next to" the actual glue line.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 03/10/2010 09:57 AM, Leon wrote:

I disagree. Tests have shown that PVA glue joints are stronger when clamped tightly than when just "held in alignment". Whether that added strength is actually necessary is a different question.

Agreed.
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Is that because it takes pressure to minimize the glue (joint) width? Just holding the boards in alignment may cause there to be too much glue in the joint.

But you sure can warp the boards.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

My point was that glue has a non-zero surface tension. To get the minimum glue line width it takes some pressure, even if the surfaces are "perfectly" flat. Is this the only pressure that matters, or is there something else going on, like there is with pressure sensitive adhesives?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Yes some pressure is required if you apply too much glue so that you can get the minimum thin layer of glue. Done properly the pressure that "tape" exerts is adequate. With a normal PVA glue extra pressure on two properly prepaired mating surfaces only insures that the union does not slip out of alignment, maybe. With too much glue clamping pressure can be troublesome with maintaining alignment, the parts want to shift. If there are glues that work better under pressure I have not aware of them. That is not to say that added pressure with PVA glues is a bad thing, added pressure can close an ill mated joint where the two surfaces so not make contact on the entire mating surfaces. BUT as we have both pointed out this can also make the glued up panel warped or bowed if the clamping force was considerable to bring both mating surfaces together. Basically, as you are asking, you only need enough pressure to insure a thing layer of glue. You can help insure a thin layer of glue by simply applying a complete thin coat of glue to one surface. Less glue = less clamping pressure.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@att.bizzzzzzzzzzzz wrote: ...

...
Yes, there is something else going on.
In testing it's been shown that glue line thickness much thinner than are possible by manually distributing the glue and light pressure _does_, in fact, create a stonger bond (up to the point at which actual damage by compression of surfaces, etc., happens, of course).
This is owing to to effects according to the analyses I've read -- first, the glue itself crosslinks and is less material relying on it's on strength and the increased pressure also forces more into the wood pores where it adds strength as well.
Again, granted, one can w/ well-prepared jointing surfaces that mate well get glue joints that are as strong as most applications need w/ fairly minimal pressure but the research is clear that ultimate strength for identically-prepared and well-fitting joints is correlated w/ increased clamping force/pressure.
So, again, there are two questions here -- one is "how good is good enough for practical applications?" and the other is "what affects maximum glue joint strength achievable?"
Most of this research is, of course, oriented to applications for manufacturing facilities, not the casual (or even not so casual :) ) woodworker w/ handcraft applications and clamping.
--
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks, but of course a little understanding raises even more questions... ;-)
Thinking about panel gluing here - seemingly the hardest for me. Does the "cross linking" of the glue depend on pressure - a non-Newtonian fluid sort of thing? Since the pressure forces the glue into the pores a high clamping force is better (up to failure, of course). Excessive force can also warp the material during the clamping time. To avoid this warping, and assuming perfectly fitting components (yeah, right), the best strategy, might be to clamp the hell out of it to force a thin glue line and into the pores, then back off so the chance of warp is minimized? Will this release of pressure cause a weaken glue joint? How long should the clamps remain on? I've always tried to leave them on for 24 hours, but perhaps this isn't the best idea.

Well, there is a third issue. I'm certainly not a great woodworker and "well fitting joints" are a relative thing - what is the best strategy for the home woodworker?

Indeed.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@att.bizzzzzzzzzzzz wrote: ...

A) For PVA, etc., I don't think the pressure is affecting the glue itself but is more the increase into the pores and the effect of less glue itself to be the weak point if you will...
B) I don't think you want to clamp so strongly as to actually cause warping of the components to begin with. Again, the actual strength under relatively moderate pressures is generally all you'll really ever need.
C) Clamp time is important -- it's dependent on the glue and the temperature/humidity so can't say unequivocally. In hot weather, I routinely continue working glue ups after a few hours; cooler weather wouldn't risk it. Overnight is generally pretty conservative unless really pushing the limits on temperatures (low).
...

...
Practice... :)
--
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It doesn't take a huge pressure to warp wood, given time.

I read that backwards, at first (continued working glue ups = were still futzing with joints).

That's certainly my intention. ;-) I've always been interested in woodworking. I now have the money, if not time, to play like I always wanted. I retired once, but figure I'll try it permanently in another few years. I want to be ready. ;-)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Well you are certainly welcome to disagree but I have on many many occasions not used clamps on glue ups and the joints have never failed. Including the panel glue up that was clamped, but not to my melamine TS extension table. Squeeze out glued the panel to the laminate and the laminate broke after jerking the panel up to free it from the TS. Squeezing glue does not add to its strength.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Waaaa?
No, you need to clamp.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 10 Mar 2010 09:35:11 -0800 (PST), the infamous
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 11 Mar 2010 10:26:01 -0600, jev wrote:

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?
--
Intelligence is an experiment that failed - G. B. Shaw

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Larry Blanchard wrote:

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.
--
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.