Most woodworking power tools struggle if the workpiece you want requires something other than a straight edge. When I need a circle, I turn to my hand-held router and a good trammel jig. I’ve made my share of one-off jigs only to throw them away between uses.
RULING CIRCLES.
For my last project, I invested some time (not that much, frankly) and materials to make a circle-cutting jig that works better. The main components are shown in the drawing at left. It’s designed around an aluminum rule.
A sliding block holds a pivot pin for the jig. The block grips the ruler with a pair of knobs, machine screws, washers, and an acrylic panel. This panel has a hairline scribed in it. In this way, you can set the radius of the circle accurately using the markings on the ruler.
The router mounts to an auxiliary baseplate. I prefer a plunge router for this jig. It gives you better control for starting and ending a cut. Another block attached to the baseplate anchors the ruler.
SETTING UP.
In order for the jig to be accurate, you’ll need to calibrate it. Start by attaching your router and installing a bit. (I used a 1⁄2" bit.) Measure from the inside edge of the bit to the block on the baseplate. Cut off the ruler at this dimension. Just make sure to use this same router bit each time.
ROUTING CIRCLES
You’ll master the jig in short order. The steps are outlined in the photos below. You need to drill a pivot hole on the workpiece. I do this on the bottom side. When making rings, a fixed pivot block is required.
Place the workpiece on a sacrificial surface. This can be MDF, plywood, or a sheet of foam insulation like I’m using here. Set the jig to match the radius of the circle you wish to create. Place the pin into the center hole on the workpiece. Power up the router and make the first pass about 1⁄4" deep.
You can lower the bit and continue making more passes. However this creates a blizzard of dust and chips. Instead, I grab a jig saw to follow the path in order to remove most of the waste. Then clean up the edge with a final routed pass.
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Jig Saw. The path left by the first router pass creates a lane to guide a jig saw to cut away most of the waste. The closer you can stay to inside (for circles) means there’s less material for the router to trim away in the next step.
Cleanup Pass. The final step is to make one more pass with the router and trammel jig. Set the depth stop on the router for a pass that trims the full thickness of the workpiece. Move the router counterclockwise to avoid backrouting. You’ll be routing against the grain in two of the quadrants, so a slow and steady feed rate is necessary to minimize tearout in these locations.
