How often do you need to cut a perfect circle? Often or only rarely? When you need to cut one, nothing beats a good circle cutting guide for the router.
Over the years I’ve made a few different versions of this jig. Some very rudimentary and others more elaborate. This one takes lessons learned from all of the previous efforts and applies that to a design that is simple yet very functional and convenient to use.

I started with the requirement that I don’t want to have to screw the jig to the base of the router or use some other means to clamp it on. I want to quickly attach and just as quickly detach it when I’m done.
My router is equiped with baseplate for a guide bushing set. This is great for quickly putting in a guide bushing for template work and I highly recommend finding a good set to have on hand. The type I have lets me leave the baseplate for the template set on the router at all time, and I thought a good way to mount the router on the circle jig was to use the correct bushing, and it would fit in a hole in the jig. Quick, easy and accurate.

For most circle cutting, the bit used would be 1/2″ diameter or less, so I chose a guide bushing that has an outside diameter of 5/8″. I drilled a 5/8″ hole in the plywood of the jig for it to fit in:

The main part of the jig is made from 1/4″ plywood. After drilling the hole, I cut a 3/8″ slot down the middle and trimmed the base to size:

The router mounted. It’s just sitting there, the guide bushing holds it in place and it’s just a matter of lifting it up to remove it. No screws, no clamps, no hassle.

The next step is to add the guide rails for the radius adjustment. These are 1/2″ thick solid hardwood with a rabbet on the inside edge. As shown, there are spacers temporarily installed in each end in the rabbet to ensure it’s lined up properly:

The pivot pin is the shank of a 1/8″ drill bit, glued in a piece of hardwood that was cut 3/8″ wide and 1/4″ thick. This is the first part of the slider that fits in the 3/8″ slot that was cut in the plywood base.

A piece of 1/4″ plywood is then glued to it. This is cut to size and rides in the space created by the rabbets in the guide rails:

Another piece of wood is added to the slider to make it an exact fit for the recess in the guide rails:

A hole is drilled for a 1/4″ bolt to go through and the hole is countersunk, to make the head of the bolt flush with the bottom.

This is the bottom of the jig, showing the guide pin and the head of the clamping bolt:

The bolt is glued in (the nut holds it until the glue sets) and a strip of plywood is used to close the end of the track. The end is just screwed on, in case the slider needs to be removed.

A knob and fender washers are used to clamp the slider in place. With the knob loosened, the slider is free to move back and forth, to adjust the cut radius. The maximum size circle that this can cut is about 36″ diameter. The minimum is about 5″:

The jig finished

Here’s a video of the jig in action. It works great. Infinite variability within its range, accurate, convenient to use, easy to build and easy to store: