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Panel Clamps

4K views 56 replies 14 participants last post by  BuckeyeDennis 
#1 · (Edited by Moderator)
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.
 
#2 ·
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.
 
#4 · (Edited by Moderator)
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
 
#6 ·
.... 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?
 
#10 · (Edited by Moderator)
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.
 
#11 ·
.... 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.
 
#12 · (Edited by Moderator)
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.

- Robert
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.
 
#13 ·
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.

- Robert
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.
 
#14 ·
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.
 
#16 ·
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.
 
#17 ·
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.
 
#18 · (Edited by Moderator)
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
 
#19 ·
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.
 
#20 ·
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.
 
#22 ·
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.
 
#23 · (Edited by Moderator)
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.

- Robert

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.

- OSU55
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…....................
 
#25 · (Edited by Moderator)
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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