Clamping pressure question

oh…...nm, i just read what i typed and answered my own question

I’ve always been curious about those Titebond numbers. Do you really think you need 7000 lbs to glue two 3” x 12” boards together? Really??

They also say “enough to bring joints tightly together”. 10 psi will bring joints tightly together. I use Titebond Cold Press veneer glue in my vacuum bag. The absolute maximum you can ever get in a vacuum is 14.7psi. I think 10-12psi is far more common when vacuum veneering. The Cold Press Glue recommends 100-250psi, but I’ve used several gallons at 10 psi with no problems.

I guess what I’m saying is you don’t need 225 psi to clamp boards with good fitting joints.

-- Gerry, http://home.comcast.net/~cncwoodworker/CNC_Woodworker.html

If you needed that kind of pressure everyone’s joinery except large factories would fail.

-- W James Brokenbourgh Custom furniture maker http://artisticwoodstudio.com/

I’m also curious as to how you plan on making clamps that can apply 60 tons of pressure. :-) Or do you only need 30 tons now??

-- Gerry, http://home.comcast.net/~cncwoodworker/CNC_Woodworker.html

I’m from the school that says “tight enough for a little squeeze out is tight enough”. Too tight and you’ll squeeze too much glue out of the joint.

-- Charlie M. "Woodworking - patience = firewood"

It all depends on the specific gravity of the wood and grain being clamped. In hardwoods, flatsawn boards require twice as much force as quarter sawn boards. And yes Ger21 it can take that much pressure. This is to get an ideal glue joint. You can join two boards together with less pressure, but the joint won’t be as strong and will have the possibility of failing over time.

60grit. 540 sq.in. is a huge glue joint. If you are laminating mutiple boards together, you only have to calculate one glue joint. This is because the pressure (if clamped right) is spread equally throughout the lamination. Oh, it’s also impossible to squeeze out too much glue from a joint with hand clamps. try it.

-- Childress Woodworks

There’s WAY too much ‘Rocket science’ flying around here! LOL!

Ive glued up, literally thousands of softwood and hardwood ‘plates’ to be used for stair treads, and trained many people to do so. One of the most common questions, is ‘how much clamp pressure’. Basically, I tell them to apply just enough pressure to squeeze the glue out of the joint to the point that…no more glue will squeeze out. Tighten till you THINK it’s tight enough…wait 5 or ten seconds and give the clamp another small turn. Did more glue squeeze out? If it did, you didn’t have enough pressure. It doesn’t take long to get the hang/feel of it.
I also agree with childress…with today’s modern glues, it is IMPOSSIBLE to squeeze too much glue from a joint. But with that said, if you’ve done the millwork properly to begin with(one of the most critical aspects of glue up)...TOO MUCH pressure should never become an issue, because it doesn’t take A LOT of pressure to close up a properly milled joint.

-- Come to the dark side....we have cookies...

LOL, you are right…There is a lot of rocket science going on, but this is going to be a big investment for me, so therefore, in order to make clamps that, where the clamping faces do not bow is important. I can only figure this out by knowing the forces acting in the system

-To childress

there will be multiple faces, each face having 540 sq in

Thanks for all the input guys. Will report back later today.

You know, I think they mean 175-250 psi applied with the clamps. It has nothing to do with the size of the project. Kinda like all those clamp tests where they crank down on a meter to see how much psi they can get from each clamp. That’s my thought anyway.

-- Karen - a little bit of stupid goes a long way

After reading a couple of comments from guys with a lot more experience than me who disagree with my statement that too much clamp pressure can starve the joint of glue, I was prompted to do a little internet research.

Tony and Childress, I defer to your real-world experience. But if the idea of glue starvation from too much clamping pressure is an old wives tale, it is certainly one that is repeated often. I found articles in Fine Woodworking, American Woodworker, and numerous other sources that warned against squeezing too much glue out of a joint.

-- Charlie M. "Woodworking - patience = firewood"

Well then, I guess there is no one right answer here. Tony is right when saying “if you’ve done the mill work properly to begin with”. This is the most important aspect of a glue up. If you have to apply more pressure to get the joint together, then something is wrong.

Charlie, I have also read an articlein FWW that stated flatsawn maple requires 1200psi to glue and that is the least amount of pressure needed. It continued on to say that starvation can only occur with hydraulic clamps…never with hand clamps. So, it seems to depend on the author and thier experience.

Doing lamination’s on a regular basis for the cutting boards I sell, I have tested this theory. Sometimes, when gluing a large maple block, I don’t have enough room for all the clamps needed to achieve the optimum pressure. But I have not had any joint failures. I guess time will tell on those. Also, with smaller boards, I have gone to or above the optimum clamping pressure, and guess what. No starved joints which lead to failure. I have come to the conclusion that with todays glues and proper milling (jointing or planing the material the same day as glue up) you can follow Tony’s technique.

Not trying to confuse any one with “rocket science”. Just think that it’s important to covey what I’ve learned from a technical standpoint. One of my downfalls

-- Childress Woodworks

So say i have a clamp with a surface area of 100 sq inches and apply 225 psi on a glue face of 25 sq inches, then the actual pressure on the glue face is 900 psi. That just seems way too high. That means, that if your clamps surface is a consistent 100 sq inches and applies a pressure at 225 psi then the smaller the glue face surface area, the more pressure that is going to be on that glue face area. Meaning pressure will increase on the glue face as surface area on the glue face decreases. It just seems like it has to be based on the surface area of your glue face. Kaybee, i’m glad you brought that up though. I didnt even think about that. I’m going to keep thinking about it because i’m not convinced i’m right

nice responses guys. They are greatly appreciated!

I go back to what Tony said about too much rocket science.

In researching the question of glue starvation, I read quite a few articles on clamping pressure. None of them got into psi to any extent. The one thing I read over and over is that there should be some squeeze-out, and it should be even. If you are getting beads in one area, but not in another, you either didn’t apply the glue evenly, or you need another clamp where there is no squeeze-out. (That’s how I judge if my clamps are tight enough, and I’ve never had a glue joint fail.)

I think we can all agree that you shouldn’t need a degree in physics and a torque wrench to make a successful flue-up. :-)

-- Charlie M. "Woodworking - patience = firewood"

you are right :(, ....my engineering intuition is getting the better of me. I should just over engineer the clamp so it can provide godly amounts of force, but only apply as much as needed using the technique you mentioned. I needed to know so that i could design the clamps so that the clamp faces, which stretch the entire length of the glue face will apply an even distributed force. If they cannot resist bending then my clamping force will be uneven. And i have to design the clamps in a certain budget range so i can afford to buy the wood ;)

Here’s my simple logic on it .. right or wrong.

It is pretty much accepted that you cannot glue glue to glue. If you have a piece of furniture with a failed joint, it is accepted wisdom that the old glue must be removed for this reason.

Taken a step further, the ideal glue joint is one molecule thick .. that is, the bonding molecule is in contact with both surfaces. Once glue is applied sufficiently to wet both surfaces, I don’t believe it’s even possible for the types of clamps most of us own to “starve” the joint .. it would take an enormous amount of pressure to squeeze that last molecule out of place. I clamp until I can’t clamp anymore, UNLESS it’s a smallish surface, and I fear that more pressure might disfigure the stock.

-- - dabbling in sarcasm is foolish … if you’re not proficient at it, you end up looking stupid … ... ...