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The lastest issue of Fine Woodworking has an entire article on this subject with charts of how much pressure to put on different woods even quarter sawn versus flat sawn.

I too have heard that too much pressure can strave a joint, but the article in Fine woodworking contradicts that theory assuming you are using normal woodworking clamps. The claim is that pressure forces glue into the wood's cells.

As for me, it's all too much science. Assuming yellow glue, I've always clamped the joint snug until glue escaped and in 40 years I've never had a well constructed joint fail because of the glue (unless it got wet).

On the other hand, if I'm joining two boards edge to edge to make a panel and haven't done a good job getting both edges straight and true; excess clamping will appear to solve the problem, but the wood will likely crack over time.

[quote="Greenvilleguy"]The lastest issue of Fine Woodworking has an entire article on this subject with charts of how much pressure to put on different woods even quarter sawn versus flat sawn.

I too have heard that too much pressure can strave a joint, but the article in Fine woodworking contradicts that theory assuming you are using normal woodworking clamps. The claim is that pressure forces glue into the wood's cells.

As for me, it's all too much science. Assuming yellow glue, I've always clamped the joint snug until glue escaped and in 40 years I've never had a well constructed joint fail because of the glue (unless it got wet).

On the other hand, if I'm joining two boards edge to edge to make a panel and haven't done a good job getting both edges straight and true]

I can remember reading articles in Fine Woodworking and other magazines warning of glue squeeze out and glue starvation. I'm a firm beleiver in "all things come around". There is really nothing new under the sun and to have a publication and to fill it with articles sooner or later your going to reverse your stance.

It is the same thing in all fields, I remember a few years ago they warned that if you ate an egg you were insuring a short life and early death, now they say if you don't eggs you will die. Salt no salt, fat no fat, the list is endless.

I'm with you Greenville guy, I first make sure I have a good joint, I use plenty of glue and enough pressure to close the joint with an even squeeze out. Since doing this I haven't had a failure.
Ed

The old adage about over-tightening clamps is true, but not for the reasons usually given. You cannot "starve" a joint -- that is, squeeze most of the glue out -- by tightening the clamps. You can, however, tighten the clamps to the point where you collapse the wood fibers and weaken the wood in the joint. This is apparently what happened to Ed's cradle.

The fact is, all woodworking glues with the exception of epoxy need pressure to properly cure. At a microsopic level, when you look at a glue joint there is an area on both sides of the glue line where the glue penetrates the wood, forming both mechanical and chemical bonds with the wood fibers. This is called the interface and it only forms under pressure. If there is not enough pressure, the interface will not form (the glue won't penetrate the wood) and the resulting joint will be weak.

Those lathe turners that claim they can glue up turning stock by rubbing two wooden surfaces together to evenly distribute an alaphatic or polyvinyl adhesive then let "surface tension" hold the parts together while the glue dries are kidding themselves. The resulting joint is in danger of flying apart on the lathe. You need clamping pressure, period. But not so much as the wood begins to collapse.

Ideally, the strongest joints for those of you who work with white and yellow glues are those that cure under pressure and leave glue lines no more than .003" thick -- a little thinner than a dollar bill. I haven't the slightest idea how you would measure this glue line, but that's what the folks at the Franklin Glue Company told me when I wrote my book on "Gluing and Clamping."

With all good wishes,

Glad to see that Nick agrees with my old "wood utilization professor" at Univ. of Montana. What Nick said, concurs with my old notes from 1958. Boy that was a long time ago! I had always thought glue starvation was caused by partially cured adhesive prior to clamping - or - glue that was too dry - not clamping pressure.

Ah Ha! - - It can't end here! Compulsively needing more study of the matter, I went to the "Wood Handbook" from USDA, Madison Wood Products Laboratory. Here's what they say how starvation can occur from clamping.

But first - they report "Starvation" only on more porous woods. This quote is taken out of context - so be careful in interpretation until you read the whole chapter and verse from the "Handbook".

"...Pressure serves several useful purposes: it forces entrapped air from the joint; it brings adhesive into molecular contact with the wood surfaces; it forces adhesive to penetrate into the wood structure for more effective mechanical interlocking; it squeezes the adhesive into a thin continuous film; and it holds the assembly in position while the adhesive cures. But if pressure is too high, the adhesive can over penetrate porous woods and cause starved joints that are inferior in bond strength (Fig. 9–7). The strongest joints result when the consistency of the adhesive permits the use of moderately high pressures that are consistent with the recommended pressures for the density of the wood. Low pressures near 700 kPa (100 lb/in2) are suitable for low density wood because the surfaces easily conform to each other, thus ensuring intimate contact between adhesive and wood. High pressures up to 1,700 kPa (247 lb/in2) are required for the highest density woods that are difficult to compress...."

I think some folks (editors and/or other woodworkers) read this (or similar) and assumed starvation would occur with "over clamping" on all woods. Guessing that's how the misconception got going. It's pretty hard to get over 1,000lbs/in2 unless you are using a 3/4" pipe clamp or a heavy bar clamp.