How much pressure do you need on a glue joint?  Apparently, not as much as you might think.  Titebond recommends 150~250 pounds per square inch (psi) [range depends on soft/medium/hard wood].   I dug into some university reports, and wood glue (PVA, on maple, “tangential” grain match [flatsawn] ) maxes its strength at about 2400 psi clamp pressure, with strength maxing at 38 MPa (ignore what the letters mean); Titebond’s 250 psi clamping gives 27 MPa; and zero clamp pressure (just rubbing the blocks together) gives 26 MPa:  that is, ZERO pressure gives 96% of the Titebond standard and 68% of max.  IOW, you don’t get much added for all that clamping to 250 psi or higher.    With pine (PVA, flatsawn), max strength is at 250 psi, giving 9 MPa; Titebond’s 150 psi gives about 8 MPa; “zero” clamping is 7 MPa (87% of T’b).  The conclusion I draw is that clamping until you see a bit of squeeze out is enough.    Another oft-cited caution:  too much pressure will rob the joint of glue, reducing strength.  The report did not show that.  The peak glue strength in the testing was achieved just before the clamp pressure got so high that the wood deformed, and deformation (not glue loss) characterized maximum strength.   [1 MPa = 145 psi]

{ https://wfs.swst.org/index.php/wfs/article/view/1395 }

I continue to tighten the clamps as much as I possibly can, despite the various recommendations (more, or sometimes less). I can't say that I've never clamped my joint, but I'm also certainly not going to try. Lab tests sometimes do not duplicate real world circumstances. That said, I do agree that clamping until you squeeze is plenty....it's just not what i do.

I usually use a rub joint and leave it at that.  Most of my stuff is small and doesn't require much strength.  I have experimented and found a rub joint to have plenty of strength.  It is more about the mating surface prep than the glue in my opinion,

I prefer to see some squeeze out.  But not buckets of it ya know?  Don't really want to spend 30 days sanding and such to get rid of the glue line.   

 

 

Small parts I usually just rub together, pull apart, put back together and hold for a few seconds before leaving the glue to dry.  Panel glue ups I use clamps mostly to make sure any desire of the strips to bend is dissuaded.  Big thick glue ups to make a lathe blank aways get more glue than necessary and squeezed until there is a consistent line of squeeze out all along the edges.  Don't like it when lathe turnings fly apart due to weak glue joints. 

4D 

  Must be too early in the morning, that went over my head with that plane at 10,000 ft. 

 

I clamp until the joint is tight, then wipe the squeeze out away with damp paper towels.

Must be doing it good enough because if I find that it's not quite right after it sets, it's a fight to get it back apart without destroying one part or another.

I have always wondered how good small glue ups using blue painters tape were.  Thanks for the info

Kinda makes me wonder if you just have a close fitted joint and hold the glued joint a few seconds we could get rid of our clamps.

On 8/8/2023 at 12:15 PM, Gerald said:

Kinda makes me wonder if you just have a close fitted joint and hold the glued joint a few seconds we could get rid of our clamps.

A few minutes with small pieces works.