One of my pet peeves as a former physics major. 

CLAMPS EXERT FORCE.   PRESSURE = FORCE / SURFACE AREA

http://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure?utm_source=wdm-newsletter&utm_medium=email&utm_campaign=wood_weeklyupdate_062217&did=155633-20170622

 

If I put 1 pound a square foot block on your stomach, that's 1 PSI.

If I put 1 pound on a pin and put that on your stomach, you're gonna be punctured.

 

If I put a veneer panel that's 20" x 20" (400 sq. in.) and get somewhere near a vacuum,  say 14 psi, that's 14*400 = 5600 pounds of force.  (Atmospheric pressure at sea level ~ 14.7 PSI)

 

I can change any of the numbers in the article by simply changing the size of the clamp pad, Then of course, if I have a 2" x 3/4" pad on an edge glue up, and it's spread over 12" of edge, that reduces the PSI.

 

To add to that, I went to a "Glue U." put on by Titebond a few years ago.   They determined, through actual testing, that was almost impossible to get too much PSI with shop clamps.   In their tests with red oak, the joint was actually a bit stronger with very high pressure.

 

"In God we trust, all others bring data." - W. Edwards Deming

 

 

excellent...

thanks...

I suppose if you squeeze the boards together tight enough, you'll create a vacuum between them, and air pressure alone will hold them together. No glue needed.

I remember reading somewhere that, when clamping, more pressure is better. Guess that's right, then.

And I remember reading that the joint can be starved of glue with too much pressure. Never happened to me though and, I squeeze 'em tight.

I thought the starved joint had been laid to rest as a urban legend. I am in the camp of good fit and just enough pressure to get a small glue squeeze out.

12 minutes ago, Gerald said:

I thought the starved joint had been laid to rest as a urban legend. I am in the camp of good fit and just enough pressure to get a small glue squeeze out.

I like Geralds method. Most of my clamping involves cauls, so I guess there is a formula for those too, and there should be a formula for spacing the clamps too. and there should be a formula for spreading glue on one side or both sides,

Herb

7 minutes ago, Dadio said:

I like Geralds method. Most of my clamping involves cauls, so I guess there is a formula for those too, and there should be a formula for spacing the clamps too. and there should be a formula for spreading glue on one side or both sides,

Herb

They say the "pressure" spreads out at 45 degrees.   A minute's thought will tell you that it can't be the same for a very stiff beam as for a limp one.  

 

Yet another won't die myth.

As much pressure as I can muster, specially with chair making. One of my chair making mentors did a small study on his own for clamping pressure, you cannot torque it too much, ever, the tighter the better, the glue is forced into the fibers and the fibers are forced upon one another. I'm a pretty big guy when it comes to strength, when I am building a sculpted rocker, I turn the clamp handles till I can't turn anymore, typically the clamps are bending by that time.

1 hour ago, John Morris said:

, typically the clamps are bending by that time.

Careful, you will be cited for clamp abuse..........Just joshing.

 

I have had some small irregular pieces that there was no way I could clamp them, so I just held them in place for a minute , and then set them down to dry ,and they set up fine. One I even had to remove because I twisted it while holding it and a day later, it broke out wood around the joint, , after that instead of knocking it off I sawed it off to keep from damaging the piece it was glued to.

Ever tried to get a knob of glue off your workbench after it sets? It hasn't had any clamping and it is tough to remove.

Just saying,

Herb

 

 

 

 

 

Edited June 23, 20178 yr by Dadio

31 minutes ago, Dadio said:

Careful, you will be cited for clamp abuse..........Just joshing.

 

I have had some small irregular pieces that there was no way I could clamp them, so I just held them in place for a minute , and then set them down to dry ,and they set up fine. One I even had to remove because I twisted it while holding it and a day later, it broke out wood around the joint, , after that instead of knocking it off I sawed it off to keep from damaging the piece it was glued to.

Ever tried to get a knob of glue off your workbench after it sets? It hasn't had any clamping and it is tough to remove.

Just saying,

Herb

 

 

 

 

 

Most definitely Herb, I agree.

I guess I should have also prefaced my over torquing of clamps with, it's necessary, for the type of joints found on my rockers and any sculpted rocker for that matter, the joint has to be tight, very very tight. I clamp the way I do more for assurance that the glue line does not look like a glue line, but a beautiful joint of wood to wood, an invisible line. These joints in the chair making business is what we call the "Money Joints". If you blow this joint, the whole chair seat is wrecked, there is no turning back from it, so over torquing is an insurance. My Cherry Rocker below, showing the joint, it has to be perfect.

I'll even take channel locks and put it up on the handle of my bar clamp and torque it even more. Before anyone gets on me about my channel locks method, it's a common practice in this type of chair building, so back off!

 

 

52 minutes ago, Dadio said:

and it is tough to remove

 

Just warm it up...

heat is wood glue's worst enemy...

10 minutes ago, Stick486 said:

heat is wood glue's worst enemy..

Contact cement too! I used to lay laminates quite frequently when I was a maintenance carpenter for Sea World of San Diego, we were always repairing or replacing laminate counter tops. A good heat gun or strong hair dryer and a putty knife, laminate comes up pretty easy.

I know we are talking about wood glue here, but it just reminded me is all.

1 hour ago, John Morris said:

Contact cement too! I used to lay laminates quite frequently when I was a maintenance carpenter for Sea World of San Diego, we were always repairing or replacing laminate counter tops. A good heat gun or strong hair dryer and a putty knife, laminate comes up pretty easy.

I know we are talking about wood glue here, but it just reminded me is all.

 

copy that...

Apple cider vinegar works to soften TBII in a joint. But, DO NOT use any thing metal to pry it apart. Instant ebonizing!

Never did worry too much about clamping pressure, always tightened clamps as much as I possibly could. It's always seemed to work, even when i couldn't get them as tight as I wanted.

Quote

 

A square foot is 12x12=144 square inches, so the pressure from 1# is 1/144 PSI.  But the vacuum math is correct, and shows how quickly a "PSI" can build up.  (Think about a basement wall with water build up behind it:  3' of water column doesn't sound like much until you convert it to 1.3 psi and multiply by lots of square inches.)

I read somewhere that wood glue works by raising the grains at the joint boundary.  The grains intermix and then are frozen in place when the glue dries.  Pressure that causes squeeze out simply pushes the grains into each other and overcomes the very slight non-flat surface matches (microscopic level).  Once you get the grains mixed, more pressure won't improve the bond.  Squeeze out will pretty much continue until the surfaces completely touch, unless you have wide areas that trap glue, which is why lamination uses a different glue, different mechanism.

I also recall a test where joints were found stronger if the glue is "worked" slightly before matching surfaces.  I think what's happening is that brushing glue insures it penetrates the grain, raises the grain, and then squeezing out excess doesn't hurt because the grain already is "up for the job".

The 45 angle propagation of pressure is a good model.  In soil, it's called "angle of repose", so as you dig near a foundation, you want to avoid penetrating the 45 angle from the top of soil / wall intersection.  Most materials will experience the same distribution of pressure.  Extremely rigid materials get a wider "splay", but I'd say wood at 45 is a good approximation, useful for determining caul locations.

Note that when joints are tested, they almost all break just behind the glue/joint surface, confirming that the joint itself is stronger than the wood. 

30 minutes ago, PeteM said:

A square foot is 12x12=144 square inches, so the pressure from 1# is 1/144 PSI. 

 

Oops, you're right.

 

A friend of mine from Oklahoma was telling me about a guy who build a barn and glued together the roof trusses because "glue is stronger than nails."    Worked great until one summer day when the heat softened all the glue and everything fell apart.

There is always the rub joint.  It works too.

 

 

11 hours ago, John Morris said:

Contact cement too! I used to lay laminates quite frequently when I was a maintenance carpenter for Sea World of San Diego, we were always repairing or replacing laminate counter tops. A good heat gun or strong hair dryer and a putty knife, laminate comes up pretty easy.

I know we are talking about wood glue here, but it just reminded me is all.

Left my laminate covered router table in the sun .....once. Will not do that again.

 

Heating a glue joint requires good aim (at the correct spot) so other joints not weakened.