Bar clamp stresses/follow on to recent HF quality post

[Pete S]

I am just getting into clamping up boards for table tops, etc.. Aren't you supposed to make the jointed edges straight, so the you don't NEED much clamping pressure?

I have a limited variety of bar and pipe clamps. Some are more rigid than others. Sometimes I have to use cauls to keep things flat because some of my clamps do bow somewhat. I assumed that I just have to work harder to get everything straight and to do the gluing and clamping immediately after final jointer work so things don't have a change to deform overnight.

Am I just putting too much effort into making straight boards in the first place? Am I wasting too much wood trying to get things straight? (I only have a jointer with a 4 foot bed.) If I spend the money on heavy duty clamps, then can I get away with wavy boards, or is there a downside to using huge clamping pressures later on?

Pete Stanaitis

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[-MIKE-]

Just my opinion on a few of your thoughts.... I don't think a woodworker worth his salt is buying heavy clamps with the purpose of joining wavy edged boards. Most do their very best to produce a straight board in the first place. The straighter the boards, less clamping pressure you need, obviously. I stopped using the jointer for edge panels once I got a decent table saw (& fence) and glue-line blade. A saw with enough power, properly set up to be perfectly square and parallel, with a good, sharp, rip blade will give results that will allow you to skip the jointer.

Don't looks at cauls as a "have to" tool. Instead, look at them as a "get to." Like any other jig or tool, they make the job easier and improve the results.

I also have a limited supply of clamps. Last time I glued up some wide panels, I did something that made it easier with fewer clamps. I only glued panels wide enough to fit through my planer. I then ran the narrower glued panels through the planer and glued them together to their final, or near final width. I used a card scraper on the remaining glue joint to feather the edge perfect where it wasn't. I found this to be a very efficient and less stressful way to manage the process of making wide panels without dozens of large, heavy cabinet clamps. Also, I found it easier to cut the panels to final dimensions after gluing them up a little larger. Under some circumstances, this also makes things less stressful and more efficient.

[dpb]

...

Test data indicates that glued joint strength increases w/ higher clamping pressures up to very high values (US Forest Products Lab and others; DAGS if you're really into the details. I've posted these links in the past in this newsgroup so the google archives are another place to look; again only if you really care; in general I'd say it's not of significance to worry about).

The thing about the clamping force is that the actual pressure applied along the length of the board between clamps drops off w/ distance so to get a significant force along the entire length requires some way to either distribute that force more uniformly, increase the applied force and/or use more clamps.

And, of course, the quality of a joint is definitely dependent on the preparation--a pair of boards that aren't well fitting may be brought together by force and the glue will hold them together when dry and the clamps removed but that residual strain will still be there and be a source of movement and/or even joint failure w/ time.

So, the answer is that yes, you should try to make joints fit and yes, best results are still obtained when clamping pressures are fairly high.

But, you generally don't need to bend a stout clamp, either... :)

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[-MIKE-]

I've seen and read these reports, but what we really have to ask ourselves is, "is it really applicable to the project?" For your typical woodworker's panel project, a table or a cabinet door, how strong does the joint really need to be? We already know that most panels will split in the middle of a board instead of the glue joint. We already know that most edged glue joints aren't being used for any kind of real load bearing.... so again, why bother with the ridiculously high clamping pressures?

It's akin to the arguments we get into about accuracy. Do we need accuracy in woodworking that gets down to the width of an atom? No, we need accuracy somewhere around the amount the measured distance will increase or decrease to to expansion and contraction of the wood. We're not building o-rings for the space shuttle. :-)

[dpb]

...

Well, if I didn't put enough caveats in for you the last sentence above should have done.

But, my take is if you're bothering to do a glueup for anything that's not just throw-away craftwork, why not make it be "the best it can be" or why bother? I'm convinced from the difference between my early and later work's longevity (and I'm enough of an ol' phar... by now that early stuff is past 40 which is starting to get enough to see what did/didn't work) that glue joints last longer w/ higher pressures and better fitting than without as well.

When I first began, all I had was some pretty light Big Lots-type bar clamps and while the old Craftsman jointer was set up well, some of those panels have developed problems w/ age whereas later don't show signs as of yet...

So, my general advice to OP is my stand on the subject...I'll agree that the extremely high pressures studied under some of the conditions by US FPL for commercial manufacturers are beyond the need of individuals, the general trends shown there are worthy of noting.

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[-MIKE-]

The subject comes up again and again and for a beginner reading this thread, it's worth presenting both sides, again.

Because you reach a point (I would argue, quite soon) of diminishing returns. Using your argument, we should all be using a micrometer for all our measurements. We should be be sanding down to 1500grit on everything. Why not? It's smoother.

I'm not saying glue joints don't last longer with higher pressures. I'm saying, you reach a point where it doesn't matter at a much lower pressure than you'd think.

Maybe it's not the gluing pressure you used. Maybe it's all the other factors that come into being when you're less experienced.

... and only noting. :-)

[tiredofspam]

First of all, your boards should not be straight to join them. They should bow in a little toward the center. So that the ends touch first. The reason is that during seasonal changes shrinkage happens at the ends first, and if perfectly jointed the boards will split at the ends.

After jointing on the joiner, I hand joint the bow in..

You always need to apply proper pressure to glue up boards. Not light pressure. Heavy pressure. I use light pressure to align... Then heavy to clamp.

[-MIKE-]

Acceptance or practice of that theory is hardly ubiquitous in the woodworking community.

[tiredofspam]

I would guess that depends on where you live.

If you live in Nevada not an issue (arid). If you live in the North East... ISSUE.

The humidity here in the summer and the cold dry air in the winter makes it a bigger deal.

If you live somewhere that its always humid no big deal either.

[CW]

Acceptance or practice of that theory is hardly ubiquitous in the woodworking community. ======================================================T Quite true. I have seen some people do it that way but the vast majority get them strait as possible before glue up. My sister has a table that my father built when I was 1 year old in Germany. We had it with us in Ethiopia, Turkey and Japan. It has also been from coast to coast in the U.S. It now resides in Texas. Beside the dings it has picked up over the years, it is in perfect shape. Nothing loose and no splits or cracks. I have a coffee table and end table that he built in Japan in the mid 70s. They went from there (humid) to eastern Washington state (very dry) to western Washington (humid). They too are perfectly sound. My father believed in strait and tight fitting joints. So do I.

[CW]

I would guess that depends on where you live.

If you live in Nevada not an issue (arid). If you live in the North East... ISSUE.

The humidity here in the summer and the cold dry air in the winter makes it a bigger deal.

If you live somewhere that its always humid no big deal either. =========================================================================== See my post on the subject.

[tiredofspam]

That's what's beautiful about this hobby and country. You can believe or do what you want.

To each his own.. I'll still prepare mine as I do. It works for me, and has never split. I have refinished things that have split (none that I made)

A student's desk. A cutting board. (really gets wet then dry quickly) A table.

I chose to believe that my repairs by cutting down the split, then jointing and springing helped. They have not failed since.

Sorry if I misspelled or are unclear, I have >

[-MIKE-]

That's the the issue I'm talking about. No one argues RH changes cause wood to expand and contract. What is not universally excepted in woodworking is the practice of joining bowed instead of straight boards to keep them from cracking at the ends.

[-MIKE-]

I would credit the glue and not the cut.

[Larry W]

Couldn't have said that better myself! :)

(Though I usually do aim for a very slightly sprung joint)

[Larry W]

Besides, a panel that opens up in the middle but the with the ends staying intact is not unheard of (should that be unseen of?)