Most carpenters know how to draw an ellipse using string, a pencil, and a couple of nails. But once it’s drawn, cutting and sanding a perfectly smooth elliptical curve takes some time.

Stew Junge of Landmark Finish in Andover, Mass., had a lot of elliptical head casings to make, so he built this router jig. It’s a temporary setup: strips of wood nailed to the floor in a “T” shape, spaced so that blocks of wood can slide between them. The router trammel is attached with a single screw to each block so that the screws can pivot and the blocks can slide in the channels created by the wood strips. The shape of the ellipse is determined by the distance between the edge of the router bit and each of the pivot points.

Using a spiral cutter, Junge cuts the ellipse in multiple passes, lowering the bit gradually into the stock — in this case a sheet of MDF. A block of wood screwed to the end of the trammel supports the router above the work. To change the ellipse’s size and shape, he simply moves the pivot points.

This article first appeared in JLC.