Router Trammel

Hylton's design calls for an 8-inch arm, which is the length I started with. But I eventually decided to cut mine down to 6 inches so that there would be no radius between 1.5 and 22 inches that I could not cut. With a 1/4" bit, a longer arm will have a gap in the range of possible radii between the pin-out and pin-in arm positions.

Use a metal rule to measure six inches from one end of the stock 80/20 rail. Mark the location.

Use a hacksaw with a fine-tooth blade to cut the rail to length.

My cheap plastic miter box was a convenient clamping means, but not dependable for keeping the cut square and true. Fortunately, a perfectly square cut is not necessary for this design. If I were to do it again, I'd just freehand this cut.

The 80/20 channel has a center line extruded into its inside surface.

Position a metal rule as shown and use a hobby knife or other sharp steel tool to nick the centerline one-half inch in from the factory-cut end of the arm.

Drill a 1/8-inch diameter hole through the rail at the nick. Use a sharp twist drill, a drop of cutting fluid, and a slower drill speed.

The router should be turned off and unplugged until the last step of this project.

Chuck the countersink into the router collet.

Position the router in the corner of the 1/4" stock, with the edges of the stock just tangent to the router base on both sides of one corner, as shown.

Trace around the base of the router with a sharp pencil.

To accurately mark the bit location, manually depress the router's height adjustment (or plunge adjustment, if it has one) pushing the tip of the countersink into the plywood. Be sure not to move the base as you do so.

Use a framing square, as shown, to carry the center point to the edges of the stock. The distance to the corner should be the same on each side. In my case, this distance was 3 inches.

Use the square, again, as shown, to carry the diameter of the router base to the edges of the stock. The distance to each corner should be exactly twice that of the smaller square; in my case, 6 inches.

Using a steel rule, measure 6.5 inches from the router's center point, along one edge of the stock, and make a witness mark.

Use the framing square to extend a rectangle that includes the baseplate diameter and the witness mark.

Measure and mark the center of the distant edge, then connect it to the center of the near edge with a line.

To locate the bolt hole, measure 0.75" in from the distant edge, along the base centerline, and make a witness mark first with a pencil, then with a center punch.

Lay the cut 80/20 rail on the stock, open side up, as shown, aligning the rail's centerline with the base centerline at each end.

Trace the long edges of the rail onto the base with a pencil.

Measure out 1 inch from each side of the channel, along the distant edge of the base, connect those points with tangent lines to the baseplate perimeter. I forgot this step until later in the project, but it makes more sense to do it now.

Use a 24-tpi blade in a saber saw to cut out the base along the outermost rectangle. Cut to preserve the outline.

Trace two copies of the cut-out rectangle onto the plywood sheet, and cut them out as well, again cutting to preserve the outlines.

If I were to do it again, I would lay out the rectangles with their short edges along the factory-straight edge of the stock, to save time in truing them up later.