Easy Arcs

Here's a tip for marking large arcs.

Instead of using a beam compass, or a nail, string, and pencil, use a thin piece of stock or rip a thin strip. I like to keep thin strips ripped from 8-foot stock because I design many projects that call for 2 1/4-inch parts. Once a 1 x 3 (which measures 2 1/2 inches wide) is ripped to 2 1/4 inches I'm left with a 1/8-inch strip. (This will vary depending on the kerf of your table saw blade). Keeping 8-foot lengths of these strips gives me plenty to work with in nearly any size arc I might need. And of course, I cut some of them shorter – 8 feet gets a bit unwieldy.

I mark my end points and the peak of the arc on the workpiece. Then I drive brads near these points (allowing for the strip and pencil thicknesses). I fit the strip against the end points, then "bow" the strip to reach the arc peak. Then I simply use the strip as my guide for marking the arc.

I make sure to apply even pressure on the strip at the arc peak, and I don't put any additional pressure on the strip when I make the mark. Sometimes I’ll add a third brad at the arc peak to hold the strip steady.

If you don’t want the brad holes on your workpiece, place the part on a large piece of scrap plywood and drive the brads into the scrap instead of the workpiece. You’ll need to make several adjustments to get the positioning right. If you’re handy enough, I’ve bet you’ve already figured out that you could make jigs out of scrap material to accomplish the same thing.

It will take some practice, but it creates a quick arc when you've got the hang of it. Then all you have to do is have a steady hand when cutting the arc.