For the sample ring, make the outer diameter 12" and the inner diameter 10". When cutting/finding material for the ring, the dimensions should be at least 1" larger than the outer diameter, 13" x 13" in this case, so that the router bit can cut smoothly. Start by marking the center of the MDF by scribing lines across opposite corners with a ruler. Drill a 11/48" hole in the center (91/464" for acrylic/ABS) before marking the cut lines.
One of the most important aspects of cutting circles and rings is to keep the parts from moving and sliding. In the past, I would use clamps, double-sided tape or brad nails to attach the material to a scrap piece of MDF. While effective, these methods were time consuming when cutting many rings and wasted MDF. To keep my material from sliding around, I now make a simple circle cutting pad, which is just a 24" x 24" piece of 31/44" MDF with non-slip mat glued onto both sides with contact cement. The non-slip mat keeps the pad from sliding on the bench top and the material from spinning when being routed. This cutting pad also keeps the bench top from being damaged in the event of "depth stop error." I've found that this size and thickness of MDF is necessary in order to keep the pad from sliding on smooth bench tops.
A 11/44" "down-cut" spiral bit (pictured on the right) is recommended for MDF because it helps hold down the part and creates smoother cuts. A 11/44" "up-cut" spiral bit is necessary for acrylic and ABS so that shavings eject upward and don't melt back together.
Next, using an accurate ruler, mark the cutting points on the MDF. I usually shove a pencil into the center hole so the edge of the hole is clearly visible and line up the 11/416" marks at the edges of the hole.
For those who don't have a lot of experience cutting circles, I recommend taking a test spin. I also strongly recommend the use of eye and ear protection when using any power tool. With the router bit barely touching the surface, start the router and spin the Perfect Circle one revolution. This should make a very shallow groove in the surface. Remove the Perfect Circle, and measure the diameter (across the center hole) to check to see if it's the correct size. Make any adjustments if necessary.
For the final pass, adjust the depth stop so it can cut the last 11/44". Make sure the router is plunged all the way down against the depth stop and that firm pressure is being applied downward during the last pass. Remember where the final plunge took place, so that you can anticipate when the inner and outer pieces will separate. Stop rotating immediately after the two pieces detach.
When making smaller diameter rings like these for projects like tweeter and instrument gauge mounts, the non-slip mat doesn't provide sufficient grip for safe and accurate cutting.
For rings under 4" in diameter, the material should be attached to a piece of scrap MDF using double-sided template tape before being placed on the non-slip router pad. The tape must be placed in a way that all pieces (inner disc, ring and outer scrap) will remain adhered to the scrap material after being cut. Make sure that both the material and scrap MDF are clean and free of dust and use adequate pressure when attaching the two.
If the router bit height was properly "zeroed" and the appropriate piece of scrap was used to set the depth stop, the disc should easily separate and the non-slip pad should only be slightly etched by the router bit. If you're making multiple rings with the same outer diameter, make all of the outer cuts at the same time without adjusting the Perfect Circle.