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Beam Compass

I needed to lay out some large circles for a project recently. And knowing how important accuracy is, I built a beam compass to meet my needs. This compass is simple, easy to use, and changing circle sizes is as easy as simple sliding a dowel.

Beam Compass

The arm is made from a four-foot length of 5/8″-dia. dowel rod. A slot and 1/4″ hole drilled in one end allow it to hold a pencil. The pencil is clamped in place by drilling a second 1/4″ hole perpendicular to the first to fit a bolt and wing nut, as shown in the drawings.

The compass is adjusted by a 4″-long piece of 1/2″-dia. PVC pipe drilled and tapped to accept a 5/16″-dia. nylon bolt. The bolts holds the dowel at the desired radius.

A 3d nail inserted in a pre-drilled hole in the bottom of the PVC acts as a pivot for the compass, as in the photo. (The bottom of the dowel is sanded flat to accommodate the nail head and prevent rotation.)

Now just slide the dowel in place, adjust it to the radius you want, and you’re ready to lay out a circle.

Good Woodworking,

Phil Huber
Online Editor, ShopNotes

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