So for decades of hobby woodworking I’ve gone along my merry way gluing up panels from time to time with little thought to calculated glue pressure. Based on just watching how others seem to do it. See photo below of one of my recent glue ups. I’d put a parallel clamp every 6″ or so with decent squeeze out and call it good. Then I started reading articles on calculating glue pressure needs and amount of psi various clamps can produce. Now I feel like an idiot. Take 3 board 3/4″ thick cherry panel glue up, 40″ long in photo below. That’s two glue lines 40″ x 3/4″ x 2 = 60 sq. in. of glue area. Titebond apparently recommends 175 psi to 250 psi for its glues. Ok, 60 sq. in. x say 210 psi = 12,600 total psi needed. From another FW article a parallel clamp can produce average of 370 psi. So how many parallel jaw clamps do I need 12,600 psi / 370 psi = 34 clamps. No this can’t be right! Is it? If it is what good are parallel jaw clamps? Pipe clamps produce average 1,050 psi so with those you need 12 clamps. Unfortunately, it looks like my happy go lucky “perfect panel glue ups” have been so wrong. Have I got this right?
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Given that you have been at this for decades as you say... I think a better question to ask yourself is have you had any glue ups fail? If yes, then reevaluate... if no... move on :)
Agree with Eideann, don't over think the glue up. Glue, clamp, go.
Glue up fails... I had one definite failure years ago immediately after releasing the clamps. Joint just cracked open on one end. I think there were other factors involved there too. I probably removed clamps to soon or maybe didn't joint the boards well also. I've had some panel edge joints where 6 months or years later have slightly realigned along the glue joint and you can feel one board being slightly higher or lower than the other by running your fingernail along the wood surface. I don't know. I think I'll switch to pipe clamps from now on and call it good.
I also have doubts about the math involved, Bessey claims 1,700 lbs of pressure for their current K-body clamps I don't know how that gets reduced to 370 psi and I doubt very much that a 3/4" pipe clamp is going to apply more.
Now if you really want to crush your wood I have some solid steel bar clamps that will get the job done.
Your calculation of pressure is wrong, for a same panel, regardeless of the number of individual boards, the area to be considered is one glue joint ie 40 X 3/4.
You're right. Thanks for pointing that out.
You are wayyyyyyyy overthinking it. There is absolutely no need for math in gluing up panels. None.
Parallel clamps for panels are great. When I bought my first set, my pipe clamps just started gathering dust.
Dry fit your panels. If they look off when dry fit, do the edge/edges over. When the fit, spread glue evenly on both edges and clamp it up. You'll never have a problem. And no math.
Thanks all. My confidence in parallel clamps is sufficiently restored. Here's the FW article that got me started on this, if interested..
https://www.finewoodworking.com/2007/11/01/get-serious-about-clamping
Your units are also wrong but to get to the point, we have 30 square inches that need to be seing 210 psi (this varies according to the wood type) . We then need to apply evenly 6300 pounds of force. This 40 inches panel would normally require 5 pipe clamps in my shop, 3 on one side and two on the other, each developing 1050 pounds this gives me 5000 pounds of force, close enough to the 6300 required. Someone in designing those pipe clamps had their maths right.
Agree with the overthinking thing. A clamp of reasonable power every 6-8 inches is enough. If the joint needs more than that to come together then the joint needs improvement.
I should like to see evidence that a particular pressure is required. As far as I can find on Google there is no scientific data to support any particular glue pressure.
Here's Wood Mag.com Q&A where I picked up Titebond manufacturer, Franklin International's recommended clamp pressure.
https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure
Jim, this article is wrong again, it mixes pressure and force ! Wrong units again. And misunderstanding of basic physics.
You're right again :). I checked it out. Pressure = Force/Area. Thanks for pointing that out. Boy, did I incorrectly use PSI for everything in the original post.
I shamelessly admit taking physics in college a little over 50 years ago, not that it did me any good. I don't remember a thing at this point. We used slide rules back then. Calculators were not commonly available for a few more years. Horrible back of the neck pain using those for exams, I do vividly remember that :).
Jim, I took physics 50 years ago with a slide rule, and then astrophysics sophomore year. I saw my first calculator then -- the original HP. Woodworking is like physics -- theorems that demand falsification.
Your photo of the panel glueup just came in... I am stealing those little end blocks that straddle the glueline. Thanks!
Happy to share :). I think I picked up the design on one of Norm Abram's New Yankee workshop shows.
Never in my life have I sat down with a calculator to figure out how many clamps and where to put them....
Honestly, if you get a touch of squeeze out and your joint closes nicely, that's when to stop cranking on the handle. The more accurate I am with my work before assembling the joint, the less pressure I need and the better the joint will be.
I think you have enough experience to know. I mean, it sounds like you've had good success. If it works, don't fix it.
If you have ever done a sprung-joint panel, you will realize that one clamp can be plenty for a panel up to 24 inches or so. Put the two pieces edge up. Use your hand plane to make a short pass in the middle of the joint, a longer pass after that, and then one long pass for the whole joint. Then all you need is one clamp at the middle of the panel to apply all the pressure that is needed.
A rare find about glueing solid wood panels , I happen to have two clamps from these rotary clamp carriers that I use as a base to lay panel glue up and can they develop force. Back in the days we used to run two 40 stations semi automated carriers.
https://www.jamesltaylor.com/wp-content/uploads/2018/08/Book_on_Gluing.pdf
https://www.jamesltaylor.com/product_category/edge-gluing-laminating/
Those clamp frames sound ideal. You have all the clamps in place and ready to go. Thanks for posting "Book of Gluing" link. Some good info in there.
I took your advise on glue pressure and now my violin is in splinters! What do I do?
Watch Bob Van Dykes video on the spring joint. One clamp for a panel glue-up that looks to be maybe 3 ft long. The joint is stronger than the wood.
https://www.finewoodworking.com/2020/02/07/video-creating-a-spring-joint-with-a-handplane
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