Clamping Pressure

[Chris Barnett]

Would like to read the specifics on pressures radiating at 45 degrees. This just seems to go against logic. If the item being glued is a perfectly inelastic body, the pressure would be directly in line with the clamp. If elastic, seems the force would spread out over 180 degrees but not equally. But again, who cares, it has nothing to do with clamping pressure, or does it? Maybe someone will post the details; I don't take mags and don't intend to start, so I might just remain uninformed .

[Eric Wong]

Would like to read the specifics on pressures radiating at 45 degrees. This just seems to go against logic. If the item being glued is a perfectly inelastic body, the pressure would be directly in line with the clamp. If elastic, seems the force would spread out over 180 degrees but not equally. But again, who cares, it has nothing to do with clamping pressure, or does it? Maybe someone will post the details; I don't take mags and don't intend to start, so I might just remain uninformed .

Actually, the pressure radiates at a factor of 5.68 degrees per point of flexibility quotient of the material, which is then divided by the sum of the tangential silliness, then take the integral of the "I've got too much time to think about this" coefficient. I've done the calculation with 17 different wood species. It always equals exactly 45 degrees.

[Steve Milito]

One key statement is "The force applied by each clamp varies greatly on the strength of the operator". To paraphrase a west coast govenor, the manly man exerted 60% more force than average, and the girly girl 40% less than average.
Another issue is that the clamps with spindles were harder to tighten. My guess is that if you modified the handle on the parallel clamp so it was T shaped, like the I-beam clamp, it will perform much better. The cranks on three of the clamps provide a mechanical advantage, and squeezing a pistal grip is easier than the squeeze and turn movement needed for the P clamps.

[Dennis Peacock]

And I guess all the things Ive made in the last thirty years are going to explode apart soon because I didnt use tons of clamp pressure. I hope Im not standing next to one when it happens.

I hear ya Lance.!!!!
Same goes for all the stuff I've made.

[Dan Barr]

but,

i have heard a wood-joint failure that actually startled me. it had a nice loud crack to it.

beware...

lol

dan

[Rick Christopherson]

And I guess all the things Ive made in the last thirty years are going to explode apart soon because I didnt use tons of clamp pressure. I hope Im not standing next to one when it happens.

It isn't necessarily about joint longevity. Yes, I know that Lance's is the typical response from the "barely tight" viewpoint, but there is more to this than just making joints not fail.

Dig back through forum postings and other discussions about the multitude of woodworkers asking how they can hide their visible glue lines. Heck, this is such a predominate problem that glue manufacturers have even developed colorized glues just so the joints are less visible. They don't do this because they know glue lines are inevitable, they do it because they know people will buy it.

The shame of it is, is that many beginning woodworkers have assumed that visible glue lines were inevitable because Norm Abram (and others) have told them that they should only apply a small amount of clamping pressure to their joints, lest they starve the joint. This is the fallacy that needs to be dispelled. You can't starve a PVA glue joint using standard clamps.

That's the message that needs to be sent, but unfortunately, people focus on the numbers and become defensive about it. You don't need to crush the wood to achieve a proper joint, but if you daintily tighten your clamps for fear of starving the joint, then don't complain about having a visible glue line.

Tantamount to a Jeff Foxworthy punchline, if you have ever been compelled to buy a colorized glue, then you might be under-clamping your joints. Personally speaking, a visible glue joint is not tolerable in my shop, and none has ever left my shop. If there is even a hint of a glue line, the joint is cut and re-glued. When I glue two boards together, the only evidence of a joint is a change in grain direction.

I have seen "gloat" postings on this forum from well known and respected woodworkers where the glue lines were visible from the low-resolution photographs, yet these same people are the ones claiming to have no problems with their clamping (not specifically referring to anyone in particular). It isn't the experienced woodworkers that need to pay heed to clamping pressure, it is those woodworkers who have jointing problems, whether they see it themselves or not.

[jim chambers]

In my experience, glue starved joints have only occurred when the joint was diddled with AFTER it had initially been clamped - such as to re-adjust the face alignment of edge glued boards. Or to re-align mating profile details of miter joints by sliding one piece up or down. And the joint needn't have been clamped to full pressure before the diddling occurred for this to happen either - especially if the temperature is warm.

If a joint needs such action, then new glue needs to be applied before re-clamping.

[Gary Keedwell]

In the new issue, the Editor says in retrospective that their recommendation was more geared for optimum rather then minimum pressure. They are saying that the higher the pressure the thinner the glue line: and the thinner the glue line the stronger the joint.
They also admitted that they should have done a better job of explaining.
Gary

Again, this needs to be repeated from FWW: The higher the pressure--the thinner the glue line. The thinner the glue line--the stronger the joint.
Gary