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How much crown should be on a clamping caul? What species and over what distance?

I have made and used these things over the years but I did it by the guess-by-gosh method and I have no idea if they were advantageously effective. The stuff that I glued is still sticking together but I guess I'd like to know that the cauls that I use are optimum.

Thanks very much!

Tim
 

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To my understanding, the only reason to have any crown at all in cauls is to make sure there is no slack in the middle when you clamp the ends. So the amount of crown needed is really very slight.

And, again to my understanding, the only purpose for cauls is to keep the parts of a glue-up aligned. So when you say that you're not sure if your cauls were effective, I'd say that should be obvious. If things were aligned when you took the clamps off, your cauls did exactly what they were supposed to do.

When I use cauls, it is generally on small panels. I don't even bother with a crown… I just make sure they are straight by laying them on the saw table. If I was gluing up something large, like a dining table, I'd want to have a little crown to be safe.
 

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Another name for creowned cauls is 'bow clamp'. The premise is that they allow for equal clamping pressure along the entire length of a glue line. Can't comment one way or the other as I have never used them.

I have a set of Douglas Fir cauls (4 of them 2" x 2" and about 24" long each) that are jointed on one side. I wrap them in plastic and put one of each side of each end of a panel. They keep the boards in the glue-up aligned while the parallel clamps draw the panel together.
 

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Discussion Starter · #4 ·
Thanks for the feedback guys.

I'm a born worrywart. I'm mainly thinking about these things in terms of even distribution of clamp pressure when glueing panels.

As CharlieM indicated I've got to think of cauls more often in terms of flat alignment. I get to relying on biscuits and the planer and I think that in spite of their popularity biscuits aren't really that effective in terms of real close tolerances. I'm going to joint up a few sticks and have them ready.

Tim
 

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Biscuits can help with alignment … they don't do much for strength of the glue-up.

Charles Neil uses bow-clamps on some projects, and there is a thread on bow-clamps (or bowclamps) over on Sawmill Creek, and there is a commercial product (http://www.bowclamp.com/) available. There are a couple of videos about bowclamps on YouTube … kind of interesting.

Good Luck!
 

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Hi TLE,

I have used cauls on some glue ups and was also concerned with the pressure distibution. What I did on my cauls is to joint one edge and make a mark in ~ 1/3 of the way in from each end. I then took my hand plane and took off 3 or 4 passes from the 1/3 mark to the end on each side. This gave me a slight crown and puts pressure on the glue up when you bring the clamps home. You can experiment with how much crown you like but try not to overthink it.

TheDane's suggestion about wrapping them in plastic is good too. I used clear packing tape on the side of my caul that touches the glue up. I didn't do this at first and what a mess. Once I started using the tape things come out much less messy.

Hope this helps.
 

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Discussion Starter · #7 ·
Thanks WWilson for some guidelines on the crown and thanks to TheDane for the plastic wrap idea. I always overlook things like this until panic is setting in, the glue is setting up and I'm all thumbs and can't find what I'm looking for. It's really nice to have things ready and convenient, isn't it.

To TheDane - are you looking forward to some snow tonight? I'm down in Madison - I think you'll get more than me. After 31 years on the old snowblower I bit the bullet and bought a new beast (the old one still ran). I figured I was old enough that I deserved one that did more of the work than me. Then I figured that since I spent all that money it it actually wouldn't snow and the neighbors would be greatful. That didn't work.

Do good, everybody and stay warm!

Tim
 

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TLE-Yup … they are saying 1 to 3 for our area. I have a Craftsman 26" snowblower … worst POS I have ever owned. I keep threatening to dump it and buy an Ariens or Brute, but just never seem to get it done. We are still covered in frozen slush from the Christmas Eve storm.
 

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OK. Even though this thread is a one year old I will update it with my info on how I make my bow cauls.

I use 3 sizes 60cm (30") - 90cm (36") and 120cm (47")

The lumber I use is kiln dried Swedish Pine with straight grain 2×2s or 2×4s
I make sure that at least the opposite edge to the crowned is made flat on your jointer.

The crowned at the ends varies according to the length of the cauls.
For the 60cm I give a 5mm crowned on both ends.
For the 90cm I give a 7mm crowned on both ends.
For the 120cm I give a 9mm crowned on both ends.

First I will get a 6mm (1/4") MDF and cut it in size… 60×5cm (30×2") or 90×5cm (36×2") or 120×5cm (47×2") and by using a bending stick or something else I will draw my curve according to which template I am doing, 5mm crowned for the 60cm, 7mm for the 90cm and 9mm for the 120cm.
Then I will cut close to the line on my band saw and sand it up to the line creating a smooth even curve.

Use this template with carpenters tape and stick it o your lumber and using a flash trim bit on your router create your smooth even curve. Then on my router table I groove (dado) in the center of straight opposite site of the crowned cauls wide enough to fit my F shape clamps. You can also do this groove with a stack dado blade on your table saw.

And there you have it… Bow cauls for any application

Oh yea. Drill a hole at one end and tie a loop so you can hang them from the wall and use some clear tape on the crowned to prevent glue up.

Enjoy
 

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A1Jim
"The ones I have are 30 " long and Mine have about a 3/16" bow in the middle."

This is on my TODO list also. I make that out to a 100' radius, about what I was figuring to use . . my thought was to make ONE 8' template out of mdf or
and route an edge using 100' of piano wire or perhaps mount the router to an 8' long strip of plywood and use 92 feet of wire., seems like that should keep everything constant. just need to find space to do it without someone running into the wire!

once I get a template can use a router with bushing or template bit to do any length that I need
 

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I've seen them commercially available made from ash - really expensive. I made some from scots pine and tested them by seeing if glue applied by roller squeezed out evenly in a trial glue up. After a few attempts, 3/8 was the crown that worked best. Used a drawing program to create the arc, printed it off in sections and stuck it down on mdf to make a master template for use on the router table. It's worth marking a C where the clamps go and covering in tape on glue side.
The squeezout test was not very scientific, but they work really well. If it gives you an idea of the force they can apply along their length, you wouldn't be able to bring together the unclamped ends by hand, not unless you were Hulk Hogan.
 

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well I worked in a custom cabinet shop we just used 2×2 oak as far as the center we just hammered in some shims the joint closed up very nice …they were used on bottom only as we were able to clamp the top
 
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