The Almost Cube - A Close Shave and a Flat Top

Caul: [kawl]

noun 

A form or plate for pressing a veneer or veneers being glued to a backing or to each other. Origin- French [cale] shim  

It's finally time to glue that nice, flat walnut board stock into something that resembles a table top and not a potato chip. This should be easy, right? Not so fast - there are a ton of options and techniques to get it done right, unless you like Pringle shaped table tops..

Clamping with cauls after the jump.

The problem with clamps, pipe clamps in particular, is that they aren't perfectly square. If you're like me and are fairly cheap or just starting out, you've only got pipe clamps in your repertoire. A fine furniture maker would turn their nose up at the thought of using pipe clamps but with some help they are a great tool for the home shop.

The secret to pipe clamps is knowing how to defeat their weaknesses. Like this:

Weakness exploited.

Enter Underpull and Overpull the evil sisters that are born when your clamping stock (the wood) is not perfectly in line with the clamp's line of force. Allow me to illustrate this:

You can avoid inviting the sisters over for crumpets by perfectly lining up your work with the line of force. Easy, right? Of course - it's made even easier when the clamp's jaws aren't perfectly parallel to each other, which is the case more often than not.

Are you seeing the Pringle's shape that I mentioned earlier? That's what we want to avoid. Easy enough to do if we use some clamping cauls.

The Caul

A caul is just a "clamp" that restrains any vertical movement in the piece. It should span the entire work piece and is best wrapped with duct or clear packing tape so that any glue squeeze out won't glue the caul to the table top. You can make your own out of a hardwood or, like me, use some random maple scraps and steel bar that you have laying around.

Cauls are the boards and steel bar on top and bottom.

Mine aren't fancy, and are held in place with some Irwin quick clamps and C clamps. Remember that the cauls are just used to keep the boards flat while the pipe clamps do the heavy lifting. They aren't a substitute for biscuits, dowels or other jointery types, just an aid to keep you from having to plane/sand your arms off from a warped top.

Above is a closeup of them in use. Before I forget, note how the walnut edges are "wild" and aren't cut to the final length. That's done once everything is pulled out of the clamps.

Glue Up

Now that you have your cauls handy, it's time for glue up. I like to start by laying the boards out how I want them with the pipe clamps already in place and roughly set underneath them. This eliminates you from running around like crazy trying to set your clamps before the glue dries.

Then I bust out the Titebond II wood glue and a Rockler silicone glue brush. It's a lifesaver and allows you to spread the glue out as seen in the first photo without getting your fingers covered. Apply a light coat to both edges to be joined. You don't want a dry joint.

Line the boards up and slap on the cauls. Snug them up tight enough to keep the boards from lifting when you put the screws to them.

Idioms. So useful for blogs.

Then use the big clamps. I still like to alternate one on top, one on bottom to mitigate over and underpull as much as possible. You don't have to go crazy with the pressure, just snug it up and maybe make another half turn if you can. The glue will squeeze out everywhere. Don't try to clean it just yet - wait for it to harden up a bit and it will be easier to remove.

Let the thing sit for a day and start removing the clamps and cauls. You should be rewarded with a fairly flat top. Here's where I cheated - I still had a tiny bit of twist in it and used a friend's thickness sander to get it perfectly flat. If you don't have one of these - use a belt sander sparingly, a hand plane (#4 or #5) or a block and some sandpaper.

Once the piece has been removed and flattened, you should cross cut the lengths to fit the table. I laid mine on top and marked from the bottom, like so:

Use a circular saw and a guide or a table saw and cross cut sled if you have it. I really recommend sealing the top now that it's to your flatness standard to prevent any kind of moisture ingress or egress and the associated wood movement. You don't want to spend the time shaving the top to get it nice and flat only to have it curl back up because you let it sit in your dank basement for 8 months. Yes, I'm talking to you, slacker.

I'll get to finishing the top next. I used a water "borne" polyurethane for the first time and really liked how it turned out. Start deciding what you want to finish it with now - there's way too many options to choose from. Best of luck making and using your clamping cauls!