How Much Curve to Put on Curved Cauls

Q: Your article "Surviving Glue-Ups" explained how curved cauls can be used to apply pressure where clamps can’t reach. But you didn’t say much about how to make the cauls. Is there a rule of thumb for the amount of curvature per foot of caul? Does the curve have to be symmetrical? What woods are suitable? -- F.H. Lott , Minneapolis , Minnesota

A: You don’t have to be very scientific to make useful curved cauls. Just be sure they’re long enough for most of your glue-ups.

I use ordinary fir construction lumber for my curved cauls. It is inexpensive and less likely to mar hardwood workpieces. I have some cauls that are 16 in. long and some that are 24 in. long, which takes care of most of the cabinet work I do. I’ve found that tapering the cauls by 1/8 in. to 1/4 in. at their ends gives plenty of curvature, but don’t be afraid to experiment.

Frankly, I spend more time thinking about the national debt than I do about making the curves symmetrical. To make a caul, I cut tapers into the fir starting from the center. Then I blend in the high point with a handplane.

Try the cauls when you dry-fit a piece. If they seem to have too much curve to them, plane the middle of the caul to flatten it a bit more. If they don’t press down enough, increase the taper.

		Making curved cauls Use 2x4 fir construction lumber about 24 in. long, depending on the size of the case. Rip the stock approximately in half, then taper each end from the center about 1/8 in. to 1/4 in. A handplane softens the midpoint to equalize the clamping strength and to knock off a corner that could mar the workpiece.

		Curved cauls clamp across wide cases. Getting a clamp to actively apply pressure in the center of a case can be impossible. With a curved caul, pressure can be applied from the ends to the center.