Wow! I just looked at some of the circle cutting jigs and feel a little intimidated. If you need something adjustable and simple, however, this may interest you. It is just 3/4” veneered plywood from a dismantled entertainment center that probably 30 years young. The plywood is flat and both sides are smooth. The hardest part was deciding what the locking pin and what the pivot pin would be. I decided to make a double locking mechanism and tie the two together with a metal tie bar. I picked the router and traced the base plate onto the plywood strip, also marking the mounting holes. The holes must be counter-bored since the plywood is 1/2” thicker than the router’s base plate if the same mounting screws are to be used.
I clamped a couple of wood guide strips to the plywood and used a 1/4” straight bit to cut a slot through the 3/4” plywood. I widened the guide strips and cut a slot that is only 5/16” deep, just enough to recess the hardware and wide enough to allow the flat washers of the locking and pin assembly to slide easily.
I chose two small plastic knobs that are threaded through for 1/4” bolts. The 1/4” bolts just fit through the holes in a 6” tie bar (standard hardware) so I cut roughly 1-1/2” off each end with a hacksaw and used the middle two holes.
One knob sits atop a hex bolt inserted from beneath the jig with a washer and a “star” or friction washer.
The other knob has a round head stove bolt inserted through the top of the knob and has a washer, star washer, and a jam nut (thinner than standard) beneath the jig. The 2” stove bolt extends about 1/2” beneath the plane of the plywood jig, serving as the pivot pin. The locking assembly is easily moved when both knobs are loosened a turn, but hold very securely to the jig in two places. This locking assembly can be inserted into the plywood base slot with the pin close to the router or with the pin farther from the router to increase the range of circle sizes.
This simple jig allowed me to cut the plywood top and bottom of my dust separator, the “drop slot” in the bottom, and the circular dado for the trash can rim.
The jig is a keeper and worth using again.
-- Like Guy Clark sez - "Sometimes I use my head, Sometimes I get a bigger hammer"