How Much Pressure Is TOO Much?

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Forum topic by Dallas posted 06-14-2012 05:19 PM 2409 views 0 times favorited 28 replies Add to Favorites Watch
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3599 posts in 3544 days

06-14-2012 05:19 PM

In a recent thread it was and is at the moment being discussed about Harbor Freight F clamps.

Some say that nothing less than a Jorgy or a bessy is acceptable and others say that if the HF clamps are used they will stretch/warp/bend or in some other way ruin the glue up or even worse, break leaving the user up a well known creek without the proverbial paddle.

I use a couple of Jorgy Clamps on my glue ups, but most of my clamps are of HF origin. I know that I’m not the most accomplished wood worker, but I’ve never found much difference in the F clamps I use. Am I using too little pressure? Too much pressure? The right pressure?

How much pressure does it actually take to join two pieces of wood together?

How do we know when we’ve applied too much?

Too Little?

Is a Jorgensen or Bessy clamp at 50 PSI any better than an HF or, (God Forbid), a Cheaper alternative store any different in pressure?

I read good, nay, even Great reviews of Shop Fox, Grizzly, etc, but then I also read terrible reviews of the aforementioned mfg’s.

Come on fellow woodsmiths, other than anecdotal information, is there a target pressure we should go for when joining wood with glue?

-- Improvise.... Adapt...... Overcome!

28 replies so far

View DS's profile


3727 posts in 3477 days

#1 posted 06-14-2012 05:38 PM

With HF clamps, if the clamp breaks it is ALMOST enough pressure. ;-D

No hard and fast rule exists. Most will judge the pressure based on experience. You usually want to see some squeeze out of glue from the joints. This lets you know the glue is flowing and will make a good bond.
If you are getting dry joints, you either; didn’t apply enough glue, or, you’ve applied too much pressure and squeezed out whatever glue you did apply.

If you’re having to apply a lot of pressure to close a joint, you might want to rethink the joint and cut it correctly next time.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View tsangell's profile


216 posts in 3750 days

#2 posted 06-14-2012 05:38 PM

The quality of the tool is no substitute for the quality of the operator. If you know what you’re doing, you have control of the process, and the tool does what it is supposed to do, then you’ll successfully complete the task.

Some tools are better made than others, but we can put too much thought into these things…

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465 posts in 3321 days

#3 posted 06-14-2012 05:58 PM

I have a couple harbor f clamps and for what it is worth and it is not much they work fine for me. They probably have a larger area where if you don’t know what your doing it can go wrong vs a more expensive one.

-- so a bear and a rabbit are sitting on a log

View dbhost's profile


5777 posts in 4288 days

#4 posted 06-14-2012 06:12 PM

I have both Jorgensons, and HF clamps. Both will let me dent hardwoods during glue ups if I try my gorilla impression with them… I am still stunned at the guys that claim the HF clamps won’t hold enough clamping force. I think they are trying to accomplish a wood to wood bond by driving the fibers together with clamping force…

I may be wrong, but I honestly think if you are applying enough pressure with the clamp to cause the clamp to flex, you are doing something very wrong…

I think on the question of how much it too much or not enough, I was taught by the late Dee Smith (my Jr. High wood shop teacher), that you wanted just enough pressure, applied evenly, to get a slight bit of squeeze out. If you are denting the wood, chances are excellent you have forced all the glue out of the joint. He demonstrated that gluing together a couple of scraps of 2×4, one set hamfisted, the other with just enough pressure to barely squeeze out. This was back in the day I think the only wood glue I had heard of was Elmers Wood Glue… The following day’s class he showed how slight a pressure with a screwdriver popped apart the hamfisted joint, but he couldn’t get the light squeeze out joint apart with a chisel and mallet… That lesson has stuck with me all these (mind your own business on the # of) years…

I am sure that there is an engineer on this board somewhere that can chime in with #s, and let me stand aside while that happens… Engineers and their numbers bore me to tears. Nothing personal, I just work with them and see them always take the long way around a simple problem…

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View ClayandNancy's profile


532 posts in 4072 days

#5 posted 06-14-2012 06:26 PM

I have Bessey, Jorgies, and a few HF and my experience is the HF’s are pretty decent. I think any clamp if you crank down on then will distort in some manner. I tighten then till I see an even amount of glue squeeze out. I don’t believe there’s any rule of thumb.

View Chris 's profile


1880 posts in 5048 days

#6 posted 06-14-2012 06:37 PM

The only complaint I have ever had with my Hf bar clamps has been that the face of the clamp pad is not flat / level on all of them. Other than a little machining the have worked wonderfully. Especially when you consider the price.

-- "Everything that is great and inspiring is created by the individual who labors in freedom" -- Albert Einstein

View Bearpie's profile


2601 posts in 4075 days

#7 posted 06-14-2012 06:48 PM

dbhost is right, you only need to apply just enough pressure to get a tight joint with a little squeeze out of glue to make a strong joint. I really don’t know why many woodworkers think you have to tighten the crap out of the clamps to the point of bending it! A snug squeeze is all you need!

-- Erwin, Jacksonville, FL

View lumberjoe's profile


2902 posts in 3305 days

#8 posted 06-14-2012 06:54 PM

I posted this another thread (and probably the thread that spawned this one). In college, our strength in materials professor was an avid woodworker. In demonstrating mechanical bonds, we used wood, glue, and pressure as our example. The class was polled first and the unanimous answer was “there isn’t a limit on pressure, hammer the clamps home!” We should have known by the Dr.’s smug grin that we were in for a surprise.

First we tested the clamps. 36” jorgensen bar clamps. We found that with minimal deflection we could apply between 300 and 500 PSI of pressure. This also depended on the hand strength of the operator.

We then tested on some oak and titebond II. We did 3 glue ups, light, moderate, and “drive it home” clamping pressure. They were glued, clamped for the recommended time on the glue bottle, unclamped, and left to cure for 3 days
We then put the pieces on a 20 ton press with an anvil right at the glue joint

First test Light pressure (`100PSI) – Instant fail. The glue joint came apart easily
Second test – moderate pressure (190-200 PSI) – unbreakable. The joint held but the wood splintered off around it
Third test – Max pressure (`400PSI) The joint failed. Not as cleanly as the 100PSI, but it did separate

There are a lot of other forces at work when you clamp wood together. Pressure makes heat, glue escapes, glue sets up before it can be pressed into the wood fibers, etc. This is what happened under “too much” pressure. We had a glue-to-glue joint. The glue did not penetrate deeply enough into the wood grain before setting up.

As with all experiments, this doesn’t exactly simulate real world conditions, however it was enough to convince me to go easy on the clamps. More clamps with less pressure is better than a few clamps hammered down.


View Fuzzy's profile


299 posts in 5045 days

#9 posted 06-14-2012 07:06 PM

I’ll go against the grain on this one … most of the glue manufacturers today say that the IDEAL glue joint is one molecule thick … that is based partly on the premise that one should never glue glue to glue … stated differently, most modern glues are NOT gap filling in nature. With that thought in mind, I clamp everything as tight as I can by hand, using Bessey’s & Jorgensens. I doubt the average shop has clamping pressure available that could even possibly approach what would be necessary to starve a joint. Once the surfaces are coated, it is nearly impossible to force out all of the glue. Now, I don’t use pressure to offset poor joinery … but,I do apply much more pressure than most folks consider adequate, and have had no bad experiences thus far. The added advantage is that I never seem to get glue “creep” as some describe it.

Any & all comments/criticisms welcome for discussion … ... ...

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

View lumberjoe's profile


2902 posts in 3305 days

#10 posted 06-14-2012 07:12 PM

I don’t think there is a right answer to this question, it’s whatever worked best for you. As I stated, the “over pressured” joint did fail, but who puts their work in 20 ton arbor presses? Interestingly, where the joints held together on the “over clamped” example where the furthest points between clamps; implying ideal pressure was made there.

My only take away was I do not need superman clamps to get a strong joint. Your average quality bar clamp is more than adequate. As long as you can put 200PSI to the joint, you are going to be good.


View MrRon's profile


6006 posts in 4300 days

#11 posted 06-14-2012 07:20 PM

If the surface of the wood joints are smooth and flat, a thin application of glue applied to both surfaces, clamping pressure is correct when an even amount of glue oozes out. Too much ooze; too much pressure. When the surfaces are not in complete contact with each other, clamping pressure is used to get the ooze and that’s when the clamps perform badly. I have HF F clamps and no problem. The bars bend, but they still work ok.

View lumberjoe's profile


2902 posts in 3305 days

#12 posted 06-14-2012 07:34 PM

I always make sure glue is spread thin and evenly on the entire surface of both pieces I am mating. The glue doesn’t really travel and spread itself out when you clamp it. Try it. Run a bead down one side, don’t spread or even it. Clamp the pieces, then pull them apart. You will see a nice, single line of glue. That’s your joint – not the entire surface area as it should be.


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Mainiac Matt

9861 posts in 3385 days

#13 posted 06-14-2012 08:43 PM

In college, our strength in materials professor

definately my all time favorite class in school… grinning ear to ear as we shot samples accross the lab floor with the Charpy V-notch toughness tester.

Oh to break things in the cause of knowledge :^)

-- Matt -- I yam what I yam and that's all what I yam

View lumberjoe's profile


2902 posts in 3305 days

#14 posted 06-14-2012 08:46 PM

Jonathan, what kind of glue is that? Titebond has never done that for me. I “waste” a lot of time spreading glue evenly.


View Mainiac Matt 's profile

Mainiac Matt

9861 posts in 3385 days

#15 posted 06-14-2012 08:47 PM

For 20 some years… Norm A. has poked a couple brads into his pieces and then slapped on two pipe clamps and seems to always warn against over tightening…. and he got pretty good results.

Keep in mind though, that clamps are kind of all purpose tools and though glueing up wood may be their major purpose in the wood shop, on the construction site, or laying a floor, you may need a fair amount of force to pull the bow out of a board.

-- Matt -- I yam what I yam and that's all what I yam

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