Since the ring's dimensions are 12" O.D. (outer diameter) and 10" I.D. (inner diameter), the outer measurement will be 6" (half of the O.D.) and the inner measurement will be 5" (half of the I.D.). In addition, I use carpenter-style markings by putting an "X" on the "scrap" side of the cut to reduce cutting errors.
When making rings with materials less than 11/42" thick, the pilot hole must be drilled through the scrap material, so the pivot point of the Perfect Circle will seat properly.
Since the sample ring is 31/44" MDF, three passes need to be made to cut all the way through. I recommend making 11/44" deep passes per revolution in MDF and 11/48" deep (or shallower) passes with acrylic and ABS. Adjust the depth stop so the first cut will be 11/44" deep. When plunging into the MDF, spin the router slightly, to reduce router bit "burn up." Spinning the router with a smooth and consistent speed will result in a cleaner cut. I usually rotate the router clockwise, placing one hand on the Perfect Circle knob and the other on the router handle to maintain control during the cut. As long as the router bit is surrounded by MDF during the entire cut, the direction of travel will not matter.
I use a digital caliper to measure diameters of small parts and adjust the pivot point for the Perfect Circle. When cutting an outer ring, place the caliper between the pivot point and the router bit. Make sure to spin the router bit to where the cutting edge is contacting the caliper. The formula for the measurement you need is: (Outer diameter/2) - 0.063. This compensates for the offset from the center of the pin. For example, if you need a 2" outer ring, (2.000/2) - 0.063 = (1.000) - 0.063 = 0.937".
Once all of the outer diameters are cut, loosen the knob on the Perfect Circle and position the router bit on the "X" side of the inner diameter cut. (Be sure to disconnect power to the router first!) Remember to spin the depth gauge back so the first pass will be 11/44" deep. Make your second and third passes. It's very important to remember where you start your third pass, so you can stop cutting immediately when the two pieces detach.
For the inside cut of a smaller ring, I place the digital caliper on the outside of the pivot point and router bit. Again, make sure the router bit is rotated so that the cutting edge is contacting the caliper. The measurement for the caliper setting of the inner diameter is: (Inner diameter/2) + 0.063. For example, a 1.5" inner diameter cut would be, (1.50/2) + 0.063 = (0.750) + 0.063 = 0.813".
This is an example of how precise you can cut with this technique. I challenge even the best jigsaw masters to try and reproduce these rings. The Perfect Circle router guide, solid carbide router bits, non-slip router pad, double-sided template tape, digital caliper and other great installation tools are available through Mobile Solutions at www.mobilesolutions-usa.com.
Make sure the router is unplugged. Place the pivoting pin of the Perfect Circle into the center hole, and slide the router bit toward the outer mark. (Always cut the outer diameter first.) Drop down the router bit so it's just above the surface without touching. Line up the edge of the router bit on the "X" side of the mark. Plunge the bit further, so the router bit touches the surface, and spin the bit by hand. If the router bit is positioned correctly, the mark the router bit makes should be exactly on the pencil mark. Tighten the knob on the Perfect Circle.