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?
RE: Subject
When stuff stays stuck together where you want it after the clamps are taken off, the clamps were tight enough in the first place.
Lew
Leon wrote: ...
Again, that's the right answer to a different question...
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Again, right answer; different question...
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"dpb" wrote:
----------------------------------- No, right answer for the SUBJECT.
Different question requires different SUBJECT.
Lew
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.
Actually I don't recall "you" asking a question. Did I miss your question?
snipped-for-privacy@att.bizzzzzzzzzzzz wrote: ...
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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.
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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.
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).
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Practice... :)
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No, but there was lots of response to the OP's that I think wasn't quite right...addressing the difference between ultimate and "adequate" strength and effect of clamping pressure thereon.
I'm not disagreeing it's possible to get "adequate for most circumstances" glue strength w/ moderate to even minimal clamping force but otoh there is a relationship between clamping force and ultimate strength obtainable that seems worthy of being recognized...
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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. ;-)
On Sat, 13 Mar 2010 21:44:53 -0600, the infamous dpb scrawled the following:
Isn't glue a pass/fail kind of situ? You either do it right and it survives punishment or you get it wrong and it fails catastrophically. Adequate = ultimate in this instance, wot?
-- No matter how cynical you are, it is impossible to keep up. --Lily Tomlin
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