Quick and Dirty Router Circle Jig
I needed to cut some fairly accurate circular holes and arcs in some 0.220″ acrylic sheet and MDF. Ordinarily, I’d use my bandsaw circle cutting jig to cut arcs and circles, but in this case, the waste piece was the disc in the centre and the ring left behind was the part I wanted, which I needed to be solid. The bandsaw jig would have required a cut through that ring to remove the disc.
A router circle jig is the perfect tool for this, because it can plunge cut the circle. Commercial router circle jigs are often built as elongated base plates with the router mounted at one end and a series of holes drilled at intervals that accept a pin that goes in to a hole drilled in the work piece to pivot around. I was about half way through making such a base plate when I realized that there was a better way.
This super-simple trammel is a block of maple that mounts to the edge guide rods for the edge guide that was supplied with the router. The empty hole in the centre accepts a pin that fits into the pivot hole. To adjust the radius of the circle this cuts, I loosen the bolts at the router base, insert the pin into the pivot hole, and slide the router along the rods until the bit is lined up with a mark on the work piece for the desired radius.
The maximum radius with the supplied rods is around 18″, but if I were to get some longer rods, this design can handle circles and arcs of any radius up to the practical limits imposed by the rigidity of the rods. I’m guessing that’s somewhere around a 3 or 4 foot radius before the sag in the rods would start to affect the accuracy of the radius – although at that wide an arc, that little bit of inaccuracy might be tolerable.
One interesting point – the rods aren’t mounted in the centre of the block, which means that the block can be flipped over to leave a gap under it if needed. This allows me to affix a block of wood with glue or double sided tape to the piece I’m routing so that the pivot hole doesn’t need to do permanent damage whatever it is I’m cutting.