I asked in the Tool Talk forum and at an Audio DIY forum about the best way to route nice circles for building speakers.
Got some great replies and saw some honest ingenuity and home made jigs to accomplish this task.

A few guys mentioned this Jasper Circle Guide and I purchased it at Parts Express. Set it up in no time and was cutting perfect circles up to 18" with no fuss and no fancy adjustments between size switches.



Place your material on a backer, drill a 1/8" hole in the center through and into the backer, select the circle size on the index in 1/16" increments and insert the pin in the guide hole coresponding to the circle size you want to cut. Place router and guide on material and the pin through the 1/8" hole and into the backer.
Router requires a 1/4" UP-CUT spiral bit, (Other straight bits will probably work as well).

I cut circles for 4" and 14 1/16" in less then 2 minutes in MDF and was very happy the dust stayed down and under the plate for easy clean up.

This is a nice tool and makes my work alot simpler then how I was doing it before.

Gary