For example, in the picture below, I'm using some special
cauls to make sure the sides of this cabinet are straight.
I put glue in the groove that the back fits into and I
wanted to make sure the sides were not bulging outward.
A more common use of cauls is during a panel glue up - the
cauls are used to keep the boards aligned during glue up.
In this glue up, six cauls are used to keep the boards of
the panel aligned. The cauls also keep
the panel flat during glue up and minimize the amount of
work (and lost wood) to get it flat later.
I'll demonstrate the use of cauls in a panel glue up later,
but now, let's talk about how to make the cauls.
The first issue is the material the cauls are made from. You
can use almost any kind of wood, but many people use a good
grade of construction lumber. Here, I'm using eight foot 2
by 4's. The reason for selecting 2 by 4's is cost.
The ones I show here were less than $3 each. When you go to
select your 2 by 4's for cauls, don't buy the cheap 2 by
4's. Good ones are not expensive, you don't need a lot of
them, and you'll have them for many years.
Go to the stack
marked "premium" (or some name like that) 2 by 4's. Those
will probably be dry (most cheap 2 by 4's are "green" and
still have a lot of water in them). If you select those,
you'll have to let them dry out before using them - which
could take a year and they might warp in the process.
In selecting your 2 by 4's, try to find those with the
minimum of knots, or at least only small knots. Also look
for straight grain. Examine the end to make sure this 2 by 4
didn't come from the center of the tree (you'll see the
grain in a circle around the center pith of the tree).
Premium 2 by 4's sometimes have the end grain sealed with
As Martha Stewart would say, "That's a good thing." Make
sure there's no splits in the end of the boards.