Clamping Pressure

[Cody Colston]

There was a thread a while back discussing how much clamping pressure was required on glue-ups. I believe the "just finger tight" crowd was the majority.

The Dec issue of FWW has an article that might change that method. It seems that most woodworkers use too little clamp pressure. The author puts some science into the equasion, recommending various clamping pressures for various wood species.

The most interesting thing to me was the illustration showing how the pressure radiates at a 45 degree angle from the clamp, thus dictating how many clamps are necessary on a given glue-up. I've always just kinda guessed at that...now I know.

[Gary Keedwell]

Ya, according to them, most of us are underclamping, and our fear of starving the joint is unfounded.
Gary

[Lance Norris]

And I guess all the things Ive made in the last thirty years are going to explode apart soon because I didnt use tons of clamp pressure. I hope Im not standing next to one when it happens;).

[Rick Thom]

Most experienced wwkrs I know consider the FWW recommendations quite frankly out to lunch.

[Mike Cutler]

Cody
Did they break the adhesives dowm by type? Or was "yellow glue" the adhesive used for the article
Ya' know. It seems like one of the wood magazines was the catalyst for the last discussion on wood clamp pressure.
How many times can the magazine folks rewrite/reformat the same material? They have to know that they're making folks second guess themselves.

I'm going to keep using epoxies. I know the answer to that one.

Thanks for the headsup Cody. I'll try to find the mag' this weekend and check out the article.

[Rick Gifford]

So I wonder if they are going to create a tension device for our clamps that tells us when they are "just right"?

I don't know if my clamping is too strong or too soft, all I know is the stuff stay together when done. I just guestimate as I go. :rolleyes:

[Gary Miller]

26 clamps on a 3' board!!! Come on.

[frank shic]

gary, don't you see the potential for bargaining with SWMBO? but honey, FWW says that i need at LEAST 26 clamps to make sure that this table top doesn't explode in the future, you wouldn't want that to happen now, would you? ;)

[Cody Colston]

Cody
Did they break the adhesives dowm by type? Or was "yellow glue" the adhesive used for the article

Mike,

I believe all the clamping pressures recommended assumed PVA glue.

[Todd Jensen]

In laminated handrails, I've always used enough clamps to avoid gaps and enough pressure to eliminate/minimize glue lines, which can be SUPER tight. I get a ton of squeeze out but have never had a problem with de-lamination. With curved treads and cove molding blanks, I often get away with a lot less clamps or clamping pressure than I would think necessary when compared to the rails. I think ultimately this may be a 'situation dictates' thing, but if you use plenty of glue to begin with and 'size' your endgrain surfaces, I think it'd be really tough to over-clamp a joint. If your making 'nickel holders', i.e. crushing the wood with the clamps, you might want to ease off.

[Tom Henderson2]

Most experienced wwkrs I know consider the FWW recommendations quite frankly out to lunch.

In fairness to FWW, I believe they were testing per the glue manufacturers specs and recommendations.

Don't shoot the messanger....

[Eric Wong]

I think the take-away message for me was "don't worry about it". I've always been worried about putting too much pressure on and starving the joint. After reading the article, I said to myself, "self, just clamp 'em down hard. No need to worry about how much to tighten any more."

My wife worries when I start talking to myself like that.

[Todd Jensen]

Eric, I told myself, I like your style.:D

[Gary Keedwell]

In the February 2008 FWW #196 on page 10 there is a question from a Alan Schaffter ( Is that our Alan from the Creek?) about clamping pressure. Plus there is a big window block on the same page called Clamping Pressure: Reality Check. I highly recommend you check it out. I got my issue in the snail mail yesterday;)
Gary

[Lee Schierer]

I think the take-away message for me was "don't worry about it". I've always been worried about putting too much pressure on and starving the joint. After reading the article, I said to myself, "self, just clamp 'em down hard. No need to worry about how much to tighten any more."

My wife worries when I start talking to myself like that.

I agree. I've known for a long time that lots or pressure was needed and that getting too much was hard to do without crushing the wood. I've also known that the only way to starve a glue joint was by applying too little glue to start with. My HS shop teacher from years ago taught me how to glue up boards and how to place clamps. Clamps every 3-4" on opposite sides of the boards. Basically you have to leave enough room between clamps for the handles to swing. And there is no such thing as too many clamps.

Now about talking to yourself, sometimes that is the best way to have an intelligent conversation....:D

[Gary Keedwell]

In fairness to FWW, I believe they were testing per the glue manufacturers specs and recommendations.

Don't shoot the messanger....
In the new issue, the Editor says in retrospective that their recommendation was more geared for optimum rather then minimum pressure. They are saying that the higher the pressure the thinner the glue line: and the thinner the glue line the stronger the joint.
They also admitted that they should have done a better job of explaining.
Gary

[Dan Barr]

LOL :D

IMO: When you can get plenty of pressure, do it. just put whatever pressure you can on the pieces you are working with. if you hear something cracking, maybe that's too much. :D lol Once the clamp starts getting really hard to tighten any further, thats about where i stop.

(Obviously there are exceptions to the rule of thumb of max possible pressure. miters for example can only be subjected to so much pressure anyway due to the fact that there arent a lot of systems to clamp them that allow extreme pressure. Also, the end-grain to end-grain glue up will not necessarily benefit form extreme pressure either. small crushable pieces and delicate work are also not to be maxed out either.)

I know i can get my bessey pipe clamps extremely tight. i could probably crush pretty much anything except maybe lignum vitae. I always use clamp pads that are softer than the wood im clamping. that way i can use as much pressure as i want without damaging the pieces.

Just my .02

dan

[Gary Keedwell]

Another thing is that their sticking to their guns when they say that the Quick- Grip clamps exert more pressure then the parallel-jaw clamps. 470# VS 370#.
Gary

[Cliff Rohrabacher]

I've always squeezed the livin' begeebus outta my work.

Then one day I got some of that runny west epoxy and learned that over-clamping starvation is a reality.

Then I got some Resourcinol and learned that over-clamping starvation may be an impossibility.

Go figure.
The lesson: One must use one's head.

[Brett Baldwin]

My biggest revelation from that article is that putting glue on both sides of the joint is about allowing the wood to soak some up, not about getting enough glue in the joint. It made a lot of sense when they mentioned that the compression of the wood may exclude some glue from moving from one side to the other.

[Thom Sturgill]

If your making 'nickel holders', i.e. crushing the wood with the clamps, you might want to ease off.

Or just use a caul to distribute the pressure, like the article suggests.

[glenn bradley]

Tight enough to hold the parts in place while the glue sets. Too tight and you'll get starved joints. There is no residual strength left behind following clamp removal. Crushing the material is not required.

[Gary Keedwell]

Tight enough to hold the parts in place while the glue sets. Too tight and you'll get starved joints. There is no residual strength left behind following clamp removal. Crushing the material is not required.
They say if you put on enough glue to start with...it is virtually impossible to starve the joint. As a matter of fact they say the tighter you get ..the thinner the glue line..the stronger the joint.
Gary

[Bill Huber]

I will add my 2 cents on this because I have been doing a lot of clamping in the last few months with these $@# cutting boards.

When I glue up the boards I clamp them as tight as I can turn the handle on my Cabinet Maters. I do this on both the first and the second glue up.
I do have a lot of squeeze out from the joints that I have to scrap off.

After I have the glue ups done and have set for a day I often have to cut some off to get to the size I want. These may be from almost nothing to 1 inch strip.
Just for the heck of it I always try and break the strips in half just to see how good the glue is, to date I have never broke a glue joint in half, I always break the wood first.

I use 3 Cabinet Masters on a board that is 12 inches wide, so that is a clamp about every 3 inches and again I turn the handles as tight as I can.

[Gary Keedwell]

I will add my 2 cents on this because I have been doing a lot of clamping in the last few months with these $@# cutting boards.

When I glue up the boards I clamp them as tight as I can turn the handle on my Cabinet Maters. I do this on both the first and the second glue up.
I do have a lot of squeeze out from the joints that I have to scrap off.

After I have the glue ups done and have set for a day I often have to cut some off to get to the size I want. These may be from almost nothing to 1 inch strip.
Just for the heck of it I always try and break the strips in half just to see how good the glue is, to date I have never broke a glue joint in half, I always break the wood first.

I use 3 Cabinet Masters on a board that is 12 inches wide, so that is a clamp about every 3 inches and again I turn the handles as tight as I can.
Well Bill, it sounds like that is the way to go. Apply enough glue and tighten the heck out of those clamps:eek::)
Gary

[Chris Padilla]

I dunno, Folks. I'm pretty manly and strong. I curl 8/4 ebony, bench press a house, and open beer bottles with my eye sockets so I'm libel to turn two 5" wide boards into 1 5" wide board with the pressure I can apply to my special order Bessey "Strong-Man" K²-body clamps!

;)

One thing with all this pressure to really pressureize your clamps: don't crank so hard to buckle two edge to edge boards. There is a time and place to crank 'em hard...and times not to.

[Gary Keedwell]

I dunno, Folks. I'm pretty manly and strong. I curl 8/4 ebony, bench press a house, and open beer bottles with my eye sockets so I'm libel to turn two 5" wide boards into 1 5" wide board with the pressure I can apply to my special order Bessey "Strong-Man" K²-body clamps!

;)

One thing with all this pressure to really pressureize your clamps: don't crank so hard to buckle two edge to edge boards. There is a time and place to crank 'em hard...and times not to.
:DGee, I hope that isn't sarcasm. You could hurt a fellow's feelings ya know;) Actually, FWW concedes that most cases don't require optimum strength and they admit that it wasn't clearly explained in their original test results.
Gary

[Bill Huber]

I dunno, Folks. I'm pretty manly and strong. I curl 8/4 ebony, bench press a house, and open beer bottles with my eye sockets so I'm libel to turn two 5" wide boards into 1 5" wide board with the pressure I can apply to my special order Bessey "Strong-Man" K²-body clamps!

;)

One thing with all this pressure to really pressureize your clamps: don't crank so hard to buckle two edge to edge boards. There is a time and place to crank 'em hard...and times not to.

Now I don't want to do any toppers here but I don't have any car jacks or lug wrenches in my cars. I just use one hand to lift the car and the the other one you take off the lug nuts.:D

I agree with the edge clamping... You clamp to much then and they will buckle right up.

[Jeff Norri]

I think the take-away message for me was "don't worry about it". I've always been worried about putting too much pressure on and starving the joint. After reading the article, I said to myself, "self, just clamp 'em down hard. No need to worry about how much to tighten any more."

My wife worries when I start talking to myself like that.

I am in the same boat, I always thought that I was clamping too hard. According to that article I wasn't clamping hard enough. Now I figure I can do no wrong, as according the the science I can't clamp too hard and accoridng to real life testing I am not clamping too soft.

[Gary Keedwell]

I am in the same boat, I always thought that I was clamping too hard. According to that article I wasn't clamping hard enough. Now I figure I can do no wrong, as according the the science I can't clamp too hard and accoridng to real life testing I am not clamping too soft.
I know what you mean....I can walk away from a glue up without any doubts....not that I had any:eek::D

Gary

[Chris Barnett]

Would like to read the specifics on pressures radiating at 45 degrees. This just seems to go against logic. If the item being glued is a perfectly inelastic body, the pressure would be directly in line with the clamp. If elastic, seems the force would spread out over 180 degrees but not equally. But again, who cares, it has nothing to do with clamping pressure, or does it? Maybe someone will post the details; I don't take mags and don't intend to start, so I might just remain uninformed :).

[Eric Wong]

Would like to read the specifics on pressures radiating at 45 degrees. This just seems to go against logic. If the item being glued is a perfectly inelastic body, the pressure would be directly in line with the clamp. If elastic, seems the force would spread out over 180 degrees but not equally. But again, who cares, it has nothing to do with clamping pressure, or does it? Maybe someone will post the details; I don't take mags and don't intend to start, so I might just remain uninformed :).


Actually, the pressure radiates at a factor of 5.68 degrees per point of flexibility quotient of the material, which is then divided by the sum of the tangential silliness, then take the integral of the "I've got too much time to think about this" coefficient. I've done the calculation with 17 different wood species. It always equals exactly 45 degrees. :)

[Steve Milito]

One key statement is "The force applied by each clamp varies greatly on the strength of the operator". To paraphrase a west coast govenor, the manly man exerted 60% more force than average, and the girly girl 40% less than average.
Another issue is that the clamps with spindles were harder to tighten. My guess is that if you modified the handle on the parallel clamp so it was T shaped, like the I-beam clamp, it will perform much better. The cranks on three of the clamps provide a mechanical advantage, and squeezing a pistal grip is easier than the squeeze and turn movement needed for the P clamps.

[Dennis Peacock]

And I guess all the things Ive made in the last thirty years are going to explode apart soon because I didnt use tons of clamp pressure. I hope Im not standing next to one when it happens;).

I hear ya Lance.!!!! ;)
Same goes for all the stuff I've made. :D

[Dan Barr]

but,

i have heard a wood-joint failure that actually startled me. it had a nice loud crack to it.

beware...

lol :D

dan

[Rick Christopherson]

And I guess all the things Ive made in the last thirty years are going to explode apart soon because I didnt use tons of clamp pressure. I hope Im not standing next to one when it happens;).It isn't necessarily about joint longevity. Yes, I know that Lance's is the typical response from the "barely tight" viewpoint, but there is more to this than just making joints not fail.

Dig back through forum postings and other discussions about the multitude of woodworkers asking how they can hide their visible glue lines. Heck, this is such a predominate problem that glue manufacturers have even developed colorized glues just so the joints are less visible. They don't do this because they know glue lines are inevitable, they do it because they know people will buy it.

The shame of it is, is that many beginning woodworkers have assumed that visible glue lines were inevitable because Norm Abram (and others) have told them that they should only apply a small amount of clamping pressure to their joints, lest they starve the joint. This is the fallacy that needs to be dispelled. You can't starve a PVA glue joint using standard clamps.

That's the message that needs to be sent, but unfortunately, people focus on the numbers and become defensive about it. You don't need to crush the wood to achieve a proper joint, but if you daintily tighten your clamps for fear of starving the joint, then don't complain about having a visible glue line.

Tantamount to a Jeff Foxworthy punchline, if you have ever been compelled to buy a colorized glue, then you might be under-clamping your joints. Personally speaking, a visible glue joint is not tolerable in my shop, and none has ever left my shop. If there is even a hint of a glue line, the joint is cut and re-glued. When I glue two boards together, the only evidence of a joint is a change in grain direction.

I have seen "gloat" postings on this forum from well known and respected woodworkers where the glue lines were visible from the low-resolution photographs, yet these same people are the ones claiming to have no problems with their clamping (not specifically referring to anyone in particular). It isn't the experienced woodworkers that need to pay heed to clamping pressure, it is those woodworkers who have jointing problems, whether they see it themselves or not.

[jim chambers]

In my experience, glue starved joints have only occurred when the joint was diddled with AFTER it had initially been clamped - such as to re-adjust the face alignment of edge glued boards. Or to re-align mating profile details of miter joints by sliding one piece up or down. And the joint needn't have been clamped to full pressure before the diddling occurred for this to happen either - especially if the temperature is warm.

If a joint needs such action, then new glue needs to be applied before re-clamping.

[Gary Keedwell]

In the new issue, the Editor says in retrospective that their recommendation was more geared for optimum rather then minimum pressure. They are saying that the higher the pressure the thinner the glue line: and the thinner the glue line the stronger the joint.
They also admitted that they should have done a better job of explaining.
Gary
Again, this needs to be repeated from FWW: The higher the pressure--the thinner the glue line. The thinner the glue line--the stronger the joint.
Gary