To make a new wheel for my 2 x 72 belt grinder project I figured I would put together a circle jig that is used on the disk sander to make circles that are perfectly round.
The circle jig is simple and I made mine from scraps of plywood:

There is no right or wrong size, you can make it as big or as small as you need. I made mine to shape small to mid-sized circles, up to about 20″ diameter.

The base is 3/8″ plwood and the top is made up of three parts, with the two on the sides fixed. These are glued and clamped to the base:

The strip in the middle is movable and I have that in place as a spacer until the glue dries.

The strip was cut about 3″ longer than the base and I drilled a 5/8″ hole for a piece of shaft to fit into:

For each new circle size with a different centre hole, new holes can be drilled into the strip.

A block of plywood is clamped to the strip and this acts as a stop:

Something more ellaborate can be made, with a knob and slot, but this works fine.

With the jig clamped to the table on the disk sander, the disk blank is mounted and sanded to size:

In no time at all I have a stack of perfectly sized MDF disks to make a new drive wheel. Not only are they perfectly round, they are concentric with the 5/8″ hole in the centre.

I made a video showing the jig in action to make the drive wheel: