# Panel Clamps



## Cincinnati2929 (Aug 26, 2018)

Are panel clamps a better way to go than a parallel jaw clamp for solid wood top glue up ?

I'm referring to those clamps that put pressure on edges as well as top and bottom that keep boards from buckling while edge gluing to create a panel. I prefer solid wood over plywood for countertops, tabletops, workbench tops, etc I was considering a set of panel clamps. But since they essentially a single purpose specialty clamp, perhaps it would be a better investment for my small hobby shop to put that money into more k body parallel jaw clamps that could be used for other clamping needs as well. Unless there is a benefit to the panel clamps other than keeping the boards aligned in the same plane (I could use splines or floating tenons to accomplish that), I'm inclined to go with parallel jaw clamps.


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## RichT (Oct 14, 2016)

If you're talking about the Woodpecker clamps they are advertising, they are grossly over-priced, and that style of clamp isn't very effective at flattening the boards. You're better off spending your money on more clamps and making your own cauls.

The advantage of real cauls is that they have a slight convex face which ensures even pressure across the panel. They're easy to make. I cut a piece of 1/4" MDF to the right curvature and use it with a flush trim bit to shape the cauls. Be sure to put some vinyl or packing tape on the face of the caul so the glue doesn't stick.


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## wichman3 (Sep 12, 2016)

On the other hand are the one's sold by Rockler and Home Depot. Much less expensive and you make the cauls for them out of 2×2 material, to any length (within reason) you need. And you can cut the cauls with a convex shape to flatten the boards.


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## Delete (May 19, 2017)

Panel clamps can be very inexpensive to make, but as Rich states they don't really apply equal pressure across the full face of the panel, they can actually lift up at the center of the panel as pressure is applied. Cauls get around this. For information on both and an idea I haven't tried out yet that combines both to make a panel clamp that costs next to nothing here's a link.

https://hobbyworkshopprojects.blogspot.com/2019/07/panel-clamps-revisited.html


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## GR8HUNTER (Jun 13, 2016)

in the cabinet shop our cauls were rounded with formica on end they worked very nice :<))))))


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## Cincinnati2929 (Aug 26, 2018)

> .... I cut a piece of 1/4" MDF to the right curvature and use it with a flush trim bit to shape the cauls. Be sure to put some vinyl or packing tape on the face of the caul so the glue doesn t stick.
> 
> - Rich


What is the right curvature? Is there a table somewhere that shows a particular wood and the amount of curvature as a function of caul length? Or perhaps instead of curvature, it would show the difference from a straight line tangent to the caul center to the end points at each foot from the center point?


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## RichT (Oct 14, 2016)

For a 24" long caul, if you laid a straight edge along it, there should be around 1/16 to 1/8 inch gap at each end. If you want to get technical, the radius of curvature for 1/8" at each end would be 576 inches.


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## AlaskaGuy (Jan 29, 2012)

> in the cabinet shop our cauls were rounded with formica on end they worked very nice :<))))))
> 
> - GR8HUNTER


What does rounded mean? Does that mean curved? Formica on the left end, right end or both.


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## OSU55 (Dec 14, 2012)

How to make cauls


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## rwe2156 (May 7, 2014)

Well, I never use cauls. My feeling is if you start right and the wood is ready, you don't need them. If you need to use cauls to push boards flat, the panel is in trouble from the start.

In a few days after the cauls have "done their job", the panel is going to achieve homeostasis (relax and move to its least stressed shape and position). Having acclimated wood with equal moisture balance are the factors.

IMO if the edges and clamping are done correctly, the panel will be flat when you clamp it - no need for cauls.

I do the "face in/face out" approach on the jointer that way any error off 90° is cancelled out by complimentary angles.


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## Cincinnati2929 (Aug 26, 2018)

> .... If you need to use cauls to push boards flat, the panel is in trouble from the start.
> 
> IMO if the edges and clamping are done correctly, the panel will be flat when you clamp it - no need for cauls.
> 
> - Robert


Robert

I agree with the importance of having straight, parallel edges that are perpendicular to the face. I was not using cauls to "spring" boards into position, just as I wouldn't use a gang of clamps to accomplish that. I am interested in using curved cauls to distribute clamping force, to minimize the number of clamps needed for a panel glue-up, plus to get clamping force where it's difficult if not impossible to use clamps.


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## OSU55 (Dec 14, 2012)

> Well, I never use cauls. My feeling is if you start right and the wood is ready, you don t need them. If you need to use cauls to push boards flat, the panel is in trouble from the start.
> 
> In a few days after the cauls have "done their job", the panel is going to achieve homeostasis (relax and move to its least stressed shape and position). Having acclimated wood with equal moisture balance are the factors.
> 
> ...


The purpose of the cauls is to keep flat properly prepped boards aligned as they are pushed together by clamps, not force warped boards into position. If you dont need them great. I find edge alignment significantly improved in the glued panel needing less wood removal for a flat panel.


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## RichT (Oct 14, 2016)

> Well, I never use cauls. My feeling is if you start right and the wood is ready, you don t need them. If you need to use cauls to push boards flat, the panel is in trouble from the start.
> 
> In a few days after the cauls have "done their job", the panel is going to achieve homeostasis (relax and move to its least stressed shape and position). Having acclimated wood with equal moisture balance are the factors.
> 
> ...


Like OSU55 said, they keep the boards aligned during glue up. Even straight boards with perfect edges can slip during glue up. Even a 1/32" is a pain to deal with.

I use cauls for every glue up. Long ones for panels and little ones for things like door frames to ensure a perfectly flat joint.


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## WoodenDreams (Aug 23, 2018)

I don't use panel clamps but I do use cauls if feel they're need on a glue-up. most of the time I don't need the cauls. Panel clamps add extra time to the glue-ups, if your time sensitive they're a waste off time. If you need to, glue up your panels in sections, then glue-up the sections together to finish out your width of the project.


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## farmfromkansas (Jun 16, 2019)

I have just been using 2×4's that I run through the jointer, and then use some clear tape to seal the clamping surface. The panels are better than just using clamps and gluing up a bunch of pieces. Less offset on boards, and flatter panels. The radius seems like a cool idea.


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## Lazyman (Aug 8, 2014)

In the manual for the circa 1950 Delta jointer I used to have, it had instructions for using the jointer for tapering a leg for example. I use the same technique to make cauls. You basically just run though the jointer making very light cuts and lift when you hit the center. The next pass lift an inch or so sooner and repeat until you have the taper you need. Then flip it around and do the same thing with the same number of passes from the other end. I usually start with 2×2, or with a 2×4 ripped in half. I don't do this often but it only takes a couple of minutes per caul when I need them.


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## Cincinnati2929 (Aug 26, 2018)

It seems to me if you use a straight tapered caul, essentially a "V", you have pressure at the center, then pressure at the end where the clamp is positioned; but in between, you are virtually putting a reverse bend in the wood, as you try to straighten out the "V".

This is The reason I plan to create a caul with a curved clamping edge to create clamping pressure across the entire surface. With this geometry, a curved caul only requires 2 clamps - one at each end. As those clamps are tightened, the caul becomes an infinite number of "springs" as you pull the curved caul surface flat across its length. This clamping action will squeeze the glue starting at the caul center, and moving outward as you clamp the caul ends toward the panel.

I have read about tapered cauls from many sources, but I may be missing something as I can't see that they would accomplish this even pressure profile.


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## Lazyman (Aug 8, 2014)

Cinci, you may be confusing a caul for applying veneer for example with one for holding boards flat for a panel glue up where the glue is between the boards so there is no glue to squeeze out from the action of the caul. You don't necessarily need an even pressure profile as long as they make contact all the way across and hold all the boards flat against whatever is on the bottom, in my case, my assembly table top. My table top may not be able to handle the type of clamping pressure you are describing so I really don't want huge PSI, just enough to hold everything flat. As you clamp down the tapered caul it flattens out enough so that it holds everything down. It has worked for me anyway. Frankly, a straight board may work well enough as long as it is thick enough that it can resist the little bit of bowing of the panel that might occur as you apply pressure from the side but a very slight taper adds just a little bit of extra pressure and allows you to use thinner stock for the caul


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## Cincinnati2929 (Aug 26, 2018)

I'm wanting a caul to clamp the boards together - ie to clamp the boards perpendicular to the strips. The clamping action of the cauls will cause the glue to evenly and slightly squeeze out of the joint. To register the top surface and keep the boards from shifting, I'll use splines or floating tenons. With square and perpendicular joints I hope to not need cauls to keep the panel from bowing, but if so, I will follow the counsel on using thick straight boards as top cauls. I hope this clarifies what I'm attempting to accomplish.


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## Lazyman (Aug 8, 2014)

I'm not following your description. "Perpendicular to the strips" would be boards or cauls across the top to hold it flat, which is the primary reason to use a panel clamp as you mention in the OP. Dowels, biscuits and tenons will keep the edges aligned but it can still bow a little if you aren't careful.

If you are talking about boards along the edge (parallel to the strips) they are usually only needed if you do not have enough clamps, especially if the strips along each side of the panel are too narrow to get even distribution of clamping pressure. Think of the clamping force from each clamp acting in a wedge from the point where the clamp hits the edge. If you draw lines at a 45 degree angle from those clamping points, theoretically, you should have enough clamps so that the lines from each clamp overlap by the inside edge of the first board. If the edge board is too narrow, you can either add more clamps or add an edge strip that is wide enough so that the lines intersect. This is all theoretical of course, you may be able to get by without doing that but if you have 6" gaps between the wedges, you may want more clamps.

Here is a good diagram that shows what I am talking about.


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## Cincinnati2929 (Aug 26, 2018)

I see my last post was incomplete and confusing. The cauls will run parallel to the laminations, sandwiching them together, creating a clamping force perpendicular to the strips. And my intent was to use cauls so I could do the 60" top glue-up with 4 clamps. Instead of 12. Thanks for the diagram.


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## Lazyman (Aug 8, 2014)

I have never seen anyone use a bowed caul for the side clamps. That makes me wonder if there is a reason for that. Maybe someone will chime in who has tried it. I would definitely do a dry run before adding glue into the mix to make sure that it doesn't cause problems.


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## rwe2156 (May 7, 2014)

> Well, I never use cauls. My feeling is if you start right and the wood is ready, you don t need them. If you need to use cauls to push boards flat, the panel is in trouble from the start.
> 
> In a few days after the cauls have "done their job", the panel is going to achieve homeostasis (relax and move to its least stressed shape and position). Having acclimated wood with equal moisture balance are the factors.
> 
> ...


Not to be argumentive, but you're making my point!! I would suggest properly prepped, flat boards would be the least likely candidate for cauls ;-)

Please understand guys, if you're taming boards with cauls in order to get them flush, then when you take the clamps off, you're more like to get an unflat panel, as the boards will be fighting each other & want to spring back to their destressed state.

My system is very simple: Start with properly prepared boards. Gradually increase clamp pressure and use a judicious whack with a rubber mallet to fine tune any slight discrepancies.

That said, in the real world I'm not always using perfect lumber. When I need an alignment aid, I use biscuits or Dominoes.

Just sayin', use cauls if you want but this works great for me…....................


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## waho6o9 (May 6, 2011)

http://www.bowclamp.com










Good stuff.


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## Lazyman (Aug 8, 2014)

The price of those bow clamps is why I make my own. $50 for one 4' bow clamp so $100 for a 4' panel glue up (clamping from the edges). You can get almost 9 48" harbor freight aluminum bar clamps for $100 if you use a 20% off coupon for each of them. If the strips near the edge are 6" wide, you would need at least 8 clamps for a 4' long panel based upon the link I shared above.

At the risk of taking this to a tangent…I've been thinking about the physics of the bow clamp so for any engineer types out there here is are some questions: 
Wood magazine says that a pipe clamp can apply up to 1000 lbs of pressure. I doubt that much is needed to bring a 4' bow clamp flush but let's assume it does. First, is the total pressure 2000 lbs because there are 2 clamps? Assuming a 1" thick panel glue up 4' long, my main question is this: do you compute the PSI by simply dividing the clamping pressure by 48"? That would be only about 42 PSI assuming 2000 lbs of clamping pressure. If it only takes 500 lbs total to pull if flush, there is not going to be much pressure if it is spread out over the entire length. I have to wonder how uniform it actually is. Can someone fill me in on the physics. I have to wonder if this is really a good way to clamp a panel?


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## waho6o9 (May 6, 2011)

I prefer to support fellow woodworkers.

Cheers


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## Cincinnati2929 (Aug 26, 2018)

> The price of those bow clamps is why I make my own. $50 for one 4 bow clamp so $100 for a 4 panel glue up (clamping from the edges). You can get almost 9 48" harbor freight aluminum bar clamps for $100 if you use a 20% off coupon for each of them. If the strips near the edge are 6" wide, you would need at least 8 clamps for a 4 long panel based upon the link I shared above.
> 
> At the risk of taking this to a tangent…I ve been thinking about the physics of the bow clamp so for any engineer types out there here is are some questions:
> Wood magazine says that a pipe clamp can apply up to 1000 lbs of pressure. I doubt that much is needed to bring a 4 bow clamp flush but let s assume it does. First, is the total pressure 2000 lbs because there are 2 clamps? Assuming a 1" thick panel glue up 4 long, my main question is this: do you compute the PSI by simply dividing the clamping pressure by 48"? That would be only about 42 PSI assuming 2000 lbs of clamping pressure. If it only takes 500 lbs total to pull if flush, there is not going to be much pressure if it is spread out over the entire length. I have to wonder how uniform it actually is. Can someone fill me in on the physics. I have to wonder if this is really a good way to clamp a panel?
> - Lazyman


It will help to think of the units. Force is in pounds, and pressure is pounds per square inch. Clamps exert force. In the case of your example 1000 pounds of force. So two clamps apply 2000 pounds. If you could get it evenly distributed across the glue surface - like with a caul, the pressure would be 2000 pounds divided by the area of one glue joint. If the boards were 1" thick and 40" long, the area would be 40 square inches, so the pressure would be 2000 pounds per 40 square inches or 50 psi across each glue joint.

I spoke with the bowclamp rep and he said they measured about 400-450 pounds of force at the center of the 4ft cauls in a glue up. At this force we should get even squeeze-out of glue and therefore a fairly even pressure distribution across the panel. There's no need to keep cranking down clamps once you get the same amount of squeeze-out at the ends as you're getting in the middle.


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## RichT (Oct 14, 2016)

> So two clamps apply 2000 pounds.
> 
> - Cincinnati2929


No, they are applying 1000 lbs at each end of the board, so the two clamps are applying 1000 lbs. It's not additive. We studied these things in static class in engineering school. You could put ten clamps down the board at 1000 lbs each, and the total force would still be 1000 lbs.


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## Cincinnati2929 (Aug 26, 2018)

> So two clamps apply 2000 pounds.
> 
> - Cincinnati2929
> 
> ...


You're claiming 1000 + 1000 = 1000 and 1000×10 = 1000?
Must be some new math that I've missed.

What if I put 3 clamps 1000 , 1500, and 500? How would you total that?


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## RichT (Oct 14, 2016)

> Must be some new math that I've missed.
> 
> What if I put 3 clamps 1000 , 1500, and 500? How would you total that?
> 
> - Cincinnati2929


It's not math, it's physics. Mechanical statics to be exact. Like I said, it's not additive.


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## Cincinnati2929 (Aug 26, 2018)

> Must be some new math that I've missed.
> 
> What if I put 3 clamps 1000 , 1500, and 500? How would you total that?
> 
> ...


I'm asking for you to explain the mechanical statics to me. If 3 clamps at 1000 lbs each Exert 1000 total pounds, what do 3 clamps at 500, 1000, and 1500 pounds exert totally?


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## RichT (Oct 14, 2016)

> I'm asking for you to explain the mechanical statics to me. If 3 clamps at 1000 lbs each Exert 1000 total pounds, what do 3 clamps at 500, 1000, and 1500 pounds exert totally?
> 
> - Cincinnati2929


What part of "it's not additive" don't you understand? There is no total force. Those three clamps are exerting different forces at different locations. If you had something like weights applying the force, and you stacked them, then you could add their weights and get a total force.

Here's a fun one. If you have two tractors chained together back-to-back and they start pulling in opposite directions with 1000 lbs of force each, how many pounds of tension is in the chain?


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## Cincinnati2929 (Aug 26, 2018)

Rich, I never imagined this thread Would diverge to this discussion.

Someone asked about the difference between clamping force In Pounds and pressure in psi. I do understand your position that the force of multiple clamps cannot be added to arrive at a total force, But I am asking you to explain the mechanical statics principle to which you were referring. It's been a long time since I took physics. From the perspective of the wood, there are three forces acting. It seems you are claiming if those forces are generated by applying weights, you can use the add the forces to get the resulting pressure across the joint. But if those same forces are generated by turning a screw, then you cannot add the forces to get a resulting pressure.

But in the clamping example, the pressure at the glue joint is the total force applied to the joint divided by the area of the joint. If you can't total the forces from each clamp, then by default you're also implying the pressure will not increase even if you add 10 clamps at 1000 pounds each because as you claim the forces do not increase. If you are correct in what you're claiming, I'm not seeing it.

The tractor pull is an excellent analogy. The tractors are like the opposing heads on a clamp. So both heads exert the same force in opposite directions on a board, and you would not add those forces. In your tractor pull example, if you used 3 chains instead of 1, (making a few assumptions) the total force from the tractors would be divided across the 3 chains so there would be 333-1/3 pounds of tension in each chain. This is the reverse of the clamping scenario. Put the tractors in reverse an put a solid link in between, and you have the clamping example. In this case, the links would be in 333-1/3 pounds of compression. It's additive. The tractors are still exerting 1000 pounds of force on the link(s) between them. And that total force is divided across the linkage.

Also, if you put 2 more tractors pulling at 1000 pounds each at the ends of that same chain, The forces are additive and the chain tension increases from 1000 pounds of tension to 2000 pounds of tension.

Are we getting closer to being in agreement?


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## Lazyman (Aug 8, 2014)

Rich, your comment about 10 clamps versus a bow clamp is sort of why I asked the first part of my question. If I compare the bow clamp to a seesaw balanced on a fulcrum with 1000 pound weights on each end, the total force applied to the ground under the fulcrum would be 2000 pounds, correct? However, it also occurred to me that with a clamp applying the force at each end, only when the second clamp is there is there any force on the first one and vise versa so it didn't seem logical that it would be additive and made me wonder what the total force that is transferred through the bow clamp to the glue joint.

So the real question is: if the bow clamp is applying 1000 lbs of pressure and assuming that it evenly distributes the force across the edge of the 1" thick panel as they claim, is the PSI derived by simply dividing the 1000 by the area of the edge? If that is true, I just have to wonder if that is enough clamping pressure for the average panel glue up, especially since it is unlikely that you would actually apply 1000 lbs to the bow clamp. It certainly doesn't seem like it would be effective at closing any gaps from imperfections in the joints or bowed boards. (Admit it, we've all done that).

Comparing that to using 10 clamps of 1000 lbs each (not that 1000 lbs per clamp would ever be needed) is the PSI greater on the surface of the glue joint? Assuming that the pressure emanates down as depicted in the article I linked to above, it would seem like the PSI on the glue joint under each clamp would be the 1000 lbs. divided by the area of the projected 45 degree cone? With a 6" outside strip, that would be 1000/12 or 83 PSI.

Sorry to hijack your thread Cinci. The discussion got me wondering just how effective a bowed caul is at applying pressure along a long panel glue up and got me curious about the physics of it.


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## Cincinnati2929 (Aug 26, 2018)

Nathan, no worries. I was hoping some others would chime in. The rep at Bowclamp said the 4ft caul straightens out at about 450 pounds. I will contact him for clarity, but I assumed he meant it applied 450 pounds of force at the center, using 2 clamps. I know the Bowclamp was designed for use with the economy f style Jorgensen clamp. So there is no need for say a Bessey K body Revo capable of exerting 1700 lbs.


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## Delete (May 19, 2017)

Yes Cinci, he means with a clamp at each end, you will develop 450 pounds of pressure at the center before you start to actually start to apply clamp pressure on the ends when the cauls straighten out. This is why they are not normally used for clue up clamping but rather used on the face of the glue up to keep it flat. Many panels will arc as you apply clamp press. even if they are well jointed, this problem is obviously greater for thinner panels, less of a problem for a thick panel like a bench top. The cauls apply the most pressure at the center where the most deflection will happen as you increase clamp pressure. Great for keeping the panel flat.

This is not so great if you are using the cauls to apply glue up pressure, you will probably squeeze out most of the glue at the center of the panel before you even start to apply clamp press to the ends.

But hey guys don't stop now this was very interesting.


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## Lazyman (Aug 8, 2014)

> Nathan, no worries. I was hoping some others would chime in. The rep at Bowclamp said *the 4ft caul straightens out at about 450 pounds*.
> 
> - Cincinnati2929


That is sort of what worries me about using them for panel glue up. It would seem that once the clamp is straight, addition pressure would not be distributed along the entire length but would be concentrated at the ends where the clamps are so really, 450 is the maximum. If my assumption is correct and you simply divide 450 by the 48" surface area, that would only be 9.375 PSI for a 1" thick board. Maybe that is okay but that just sounds really low to me.


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## RichT (Oct 14, 2016)

This has gotten complicated 

Yes, in the tractor example, the tension is 1000 lbs. The way to picture it is if the chain were connected to a wall, the tension would be the same, which is 1000 lbs.

It's important to realize pressure is the important value. Cincinnati2929 figured that one out right away. Getting back to the clamps, multiple clamps don't increase the pressure, they spread it. Does that help make more sense?


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## Cincinnati2929 (Aug 26, 2018)

> It would seem that once the clamp is straight, addition pressure would not be distributed along the entire length but would be concentrated at the ends where the clamps are so really, 450 is the maximum. If my assumption is correct and you simply divide 450 by the 48" surface area, that would only be 9.375 PSI for a 1" thick board. Maybe that is okay but that just sounds really low to me.
> 
> - Lazyman


You at correct. And it does seem low to me too. But once the caul is straightened, it has applied pressure across the entire 4 ft of length causing the glue to squeeze from the center outward. Properly prepared joints are the key - meaning the glue face is flat, perpendicular to the faces, and all glue faces are parallel, apparently a small amount of pressure is all that should be required. The visual clue is even glue squeeze-out. My understanding is to tighten the two end clamps until the squeeze-out at the ends is the same as the squeeze out in the center of the cauls.

I don't think these are intended to be used to force boards together. I don't plan on building in that kind of internal stress in my laminated panels.


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## Cincinnati2929 (Aug 26, 2018)

> This has gotten complicated
> 
> ... It's important to realize pressure is the important value….Getting back to the clamps, multiple clamps don t increase the pressure, they spread it.
> 
> - Rich


If that were true, the maximum amount of pressure you could get on a glue joint would be what you could obtain with one clamp. According to that premise, each successive clamp would not increase the pressure. So what would be the reason to use multiple clamps?

Pressure is force per unit area. If you "spread out the pressure",and by that you mean it stays the same over a larger area, you'd have to add force to keep the pressure constant. You do this by adding more clamps. If you don't believe the additional clamps add to the total force, then if you could spread the force of that first clamp over a larger area, it would result in a lower pressure across the glue joint,

Clamps exert force. This is the critical value. Pressure changes according to the area over which that force is applied. With the same force, Clamp a 1" thick board and the pressure is 2x what it would be for a 2" board. Of the same length.


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## Delete (May 19, 2017)

I would be careful using cauls for glue up press. You might get away with it for longer panels and cauls. but for shorter panels I think you will end up with weak center glue ups on your panels. It is deceptive to average out the pressure applied. When the center of the panel is at 450 lbs. press. and the cauls have straightened, you will still be close to 0 press at the ends. Lets say you apply 50 lbs. at the ends, you will be at 500 lbs across the center, you won't have any glue left and end up with a weak center on the panel. Everything I have read and many personal glue-ups, has driven the point home to me that you don't want to apply excessive press., and pressure should be relatively even. Using cauls can violate both those rules.


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## RichT (Oct 14, 2016)

> If that were true, the maximum amount of pressure you could get on a glue joint would be what you could obtain with one clamp. According to that premise, each successive clamp would not increase the pressure. So what would be the reason to use multiple clamps?
> 
> Pressure is force per unit area. If you "spread out the pressure",and by that you mean it stays the same over a larger area, you'd have to add force to keep the pressure constant. You do this by adding more clamps. If you don't believe the additional clamps add to the total force, then if you could spread the force of that first clamp over a larger area, it would result in a lower pressure across the glue joint,
> 
> - Cincinnati2929


Just when I thought you were getting it….

One clamp would apply X lbs of force, creating Y PSI based on surface area. That pressure would be confined to the zone around the clamp, decreasing with distance. If you had some magical non-flex caul to put along the edge, then, assuming the clamp was centered along its length, it would be equal down its length.

That's not going to happen though. So, we have to consider the pressure curve surrounding the center of the head of the clamp. What about one clamp in the middle? Say, it's 1000 lbs. If you put clamps on either side applying 1000 lbs of force, all you've done is spread out the pressure. You've got three times 1000 lbs of force, applied over three times the area, so it all evens out.

To go a little further, understand that clamping is a static situation. It doesn't matter if the board is clamped in traditional clamps, or pressed against a wall. Same thing.


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## Cincinnati2929 (Aug 26, 2018)

> ....
> 
> .... You ve got three times 1000 lbs of force, applied over three times the area, so it all evens out.
> 
> ...


Yes!!! Now we're in agreement. Increasing the clamps from one to three gives you three times the force over three times the area. The forces are additive and that's why we need multiple clamps because each one is effective for a "zone" extending 45 degrees from the center of force At the clamp head.


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## RichT (Oct 14, 2016)

> Yes!!! Now we're in agreement. Increasing the clamps from one to three gives you three times the force over three times the area. The forces are additive …
> 
> - Cincinnati2929


No, we're not in agreement.


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## Cincinnati2929 (Aug 26, 2018)

This is confusing. I quoted you. And I'm in agreement with your words. If I knew how to attach a diagram to this, It would confirm what I think were both saying.

If the clamps are spaced "correctly"creating slightly overlapped "clamping zones", I agree that you have three times the force over three times the area. According to your words, Both the force and the area increase. Apparently you seem to object to my describing the forces as being additive. Maybe you object to the area being additive as well. If the clamps are spaced correctly, the pressure across the glue joint stays relatively even.

I'm not seeing how we could say the same thing yet have a misunderstanding on the increase in force but I'll have to leave it there.

I'm not sure if we have a body left on this thread. But let's go back to our concern as to whether 10 psi is enough at the glue joint. Because I will be using these cauls to laminate 3-1/2" strips of 8/4 maple into a workbench top. 10 psi seems enough on a flat surface. But increasing the area 3.5x drops that pressure to maybe 2.5 psi. Enough for some glue squeeze out, but enough for integrity in the finished joint? I'm not sure of that? I've never seen any data on pressures required at the glue joint. A lot of this is theoretical since without some pressure and force transducers, we can't be sure how much force were applying. But I was using the average number I got from Bowclamp.

People bend wood with 12-14 psi, so perhaps 2 psi is adequate. That translates to 288 pounds per sq ft and that seems more than enough. Makes me wonder if many of us are using too much clamping force in glue ups.


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## therealSteveN (Oct 29, 2016)

LMAO

Well I am certainly glad this is all cleared up.

It's clear as mud to me what the answer is. Me, I'm going to go with sufficient force to get a decent squeeze out, at which time I quit tightening, and nope I have never put a scale on them to find out how many pounds of pressure that is. I think the stated amount of pressure with many clamp types is ludicrous. It's so far above what is actually needed to do the job.

Someone referred to it earlier, but here is from the Titebond website.

Dale Zimmerman of Franklin International, maker of Titebond woodworking glues, recommends 100 to 150 pounds per square inch (psi) for clamping softwoods and 175-250 psi for hardwoods.

Back to the initial question I've found that the ShopSmith style wooden clamps work very well on glue ups. If you are ready to do the glue up.

I've always felt most people when they show them aligning boards to glue up, do everything in a fast and frenzied fashion, where if they just slowed down a little bit, they could help themselves immensely. Sure glue starts to tack, but you still have a minute or 3. Use that time, and "clamping problems" go away.

I think much of that frenzy comes as a result of no pre-planning. Clamps aren't open and ready. Boards to be glued aren't on a lift allowing you to easily clamp them, nothing underneath for glue mess to land on. The biggest is the actual application of the glue, why in God's name it takes some people all that time to put the glue on?


> ?


Just a collective of a bunch of little ohhhh crap's that add up, next thing you see is frenzy. If you don't have that frenzy, you lose the misalignment problems, or at least you can. Once the boards are clamped, I have not seen any of them "slipping" all of that occurs in doing the glue up. It need not be that way.

I'd invite anyone who doesn't know what a good glue up looks like, that they watch an MTM cutting board glue up. He is the King of cool glue up's. No cauls, at most he is smart, and will use a scraper to allow clamping pressure to go evenly along the sides being clamped. His alignment is what you should strive for.


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## BuckeyeDennis (Mar 24, 2019)

> This is confusing. I quoted you. And I'm in agreement with your words. If I knew how to attach a diagram to this, It would confirm what I think were both saying.
> 
> If the clamps are spaced "correctly"creating slightly overlapped "clamping zones", I agree that you have three times the force over three times the area. According to your words, Both the force and the area increase. Apparently you seem to object to my describing the forces as being additive. Maybe you object to the area being additive as well. If the clamps are spaced correctly, the pressure across the glue joint stays relatively even.
> 
> ...


I've been reading this thread with interest, and drawing free-body diagrams in my head. Per a classic free-body analysis, you are correct, and the clamp forces are additive. But after Rich's last post, I think I see where the confusion and disagreement lies.

If you had an infinitely-rigid caul, you could put one clamp in the center and obtain even pressure along the full length of the panel. But the panel is actually compliant, and the caul is not perfectly rigid either. Thus the material deforms, and the pressure is localized in the vicinity of the clamp head. In Rich's model, ALL of the pressure in a given section comes from it's local clamp. Thus adding another clamp some distance away does not increase the pressure at that point.

The BowClamp is presumably engineered to provide an equal clamping pressure along its length. But it would be able to do so at only one specific clamping force, with equally-tightened clamps at each end. And yes, in this scenario the "resultant" (total) clamping force would be the sum of the two individual clamp forces, and the pressure could be accurately calculated by dividing the resultant force by the cross-sectional area of the panel edge.

But per Rich's point, absent the pressure-equalizing capability of the BowClamp, it's nonsense to model the panel as a single rigid body. Because it is compliant, multiple clamps are necessary in order to get a reasonably uniform pressure along the entire length of the panel. To predict the pressure at a given point with reasonable accuracy, you'd have to model the panel as multiple rigid bodies interconnected by springs. Which is exactly what a finite-element analysis software package would do.


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## Cincinnati2929 (Aug 26, 2018)

Thanks for getting in on this. I don't have more than an elementary understanding of finite element analysis. So maybe you can shed some additional light. Using a curved caul - like the Bowclamp - as you tighten the ends, straightening the curve, aren't you essentially adding multiple clamping forces along the face of the caul?

Part of my problem I want to solve, which I don't remember if I actually stated somewhere before our diversion, is I am looking for a clamping solution to avoid purchasing a dozen Bessey K body clamps to accomplish this glue-up. I was hypothesizing that 2 clamps capable of exerting a total 3400 pounds ought to be more than enough if I could distribute that force over the 5 ft top. Now that I've come full circle, I think that hypothesis is correct. So the challenge becomes, can I practically spread that force over the glue joint?

Vacuum bags are routinely used to laminate as well as to bend wood. Depending on location and system efficiency, those systems exert maybe 12 psi. That's a far cry from the 150-200psi others have stated. (200 psi is Over 14 tons per sq ft.). That amount seems ludicrous to me. It seems just a small amount of pressure, say 2 psi which would be 288 pound per sq ft, would be adequate to create a strong joint. This would be the scenario if I were to glue two 1ft square boards together by standing a 288 lb man on the stack. I'd think the joint would be very adequate.


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## Delete (May 19, 2017)

14 tons per square foot will crush wood fibers, let alone squeeze every ounce of glue out of your joint. The best book I have read was Wendell Castles book on Wood Lamination . He devotes a chapter to wood lamination. If you don't know who he was I suggest you Google his name to see some of his work.


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## Cincinnati2929 (Aug 26, 2018)

I'm familiar with Mr Castle. Do you recall if he addressed any of the issues of this thread?


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## Delete (May 19, 2017)

Not regarding caul use, I don't recall any examples of caul use. Mr. Castle was a clamp man, I was influenced by his early work right away, which is why I have as many clamps as I do, I have a few plan ideas I would like to try out before I hit the ground. Mr. Castle preached the use of many clamps, not so you could apply mega tons of pressure but so you could even out the pressure across the whole surface rather than having pockets of high pressure and pockets of lower press. which makes sense since many of his creations involved carving deeply into his laminations.


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## Cincinnati2929 (Aug 26, 2018)

His book is available on Amazon for $226 if anybody is interested that much. Me, I'll watch the glue joint squeeze-out and make my best judgement call.


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## BuckeyeDennis (Mar 24, 2019)

There's a fairly simple way to spread clamp force evenly to multiple points. The windshield wiper below illustrates the principle. If you lay your glue-up down on a flat surface, you can replicate the windshield wiper linkage by laying appropriate-length boards on the surface in the same configuration. Some profiling of the boards could get you the pivoting action without building actual pinned joints.










Of course, with two clamps, you'd need two of these contraptions on each side of the panel, and that's a lot of stuff. And with thin panels, they might need some sort of restraint to keep them lying flat. All of which makes just using a lot of clamps look pretty sensible. But it would be a fun experiment.


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## Cincinnati2929 (Aug 26, 2018)

Ingenious example! This is like the bowclamp in reverse. Instead of touching in the center and bowing away at the ends, this touches at the ends and bows away at the center. It also has the advantage of only needing one clamp. Probably need a mechanism to keep it centered on the board thickness.


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## BuckeyeDennis (Mar 24, 2019)

I have my doubts that a multi-level load spreader, as in the windshield wiper above, would be practical. But a single-level spreader might work just fine. Basically just put one spreader board under each clamp, either with the center relieved, or with pads with attached at each end. That would cut the number of clamps required in half, and it shouldn't be all that fussy in use.


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## therealSteveN (Oct 29, 2016)

Growing more confused as I read on. It would seem to me you fellas would find clamping most important on the tops of the boards, and all along I though it was best done from the sides. If you are thinking a caul is needed along the edges of a boards sides to get the job done I can assure you successful glue ups have been done for centuries from the sides, with as little as 2 or 3 clamps, or in your examples 2 or 3 contact points along the entire length of a project.

This clamping over and under, and placing clamps every few inches is a variable much more recently thought up. I think it is just an extension of you can't have too many clamps. Somehow they felt the need to prove that.

This










Doesn't get anything better done than










Proper prep of the wood, proper application of the glue, and proper setting of the clamps.


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## BuckeyeDennis (Mar 24, 2019)

> Growing more confused as I read on. It would seem to me you fellas would find clamping most important on the tops of the boards, and all along I though it was best done from the sides. If you are thinking a caul is needed along the edges of a boards sides to get the job done I can assure you successful glue ups have been done for centuries from the sides, with as little as 2 or 3 clamps, or in your examples 2 or 3 contact points along the entire length of a project.
> 
> This clamping over and under, and placing clamps every few inches is a variable much more recently thought up. I think it is just an extension of you can t have too many clamps. Somehow they felt the need to prove that.
> 
> ...


We have indeed been talking about clamping from the edges. As it happens, your second photo is exactly how I do it, using Shopsmith double-bar clamps-I have three of them.

But in this case, if I understand correctly, Cincinnati2929 is laminating 3-1/2" wide strips of 8/4 maple to make a 6' long workbench top. So more than three edge-clamping points probably makes sense. It's not as extreme as applying edge banding, but the "aspect ratio" (board length/width) is pretty high.


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