Creating circle-shaped parts, whether for speaker rings, mounting plates or protective grilles, is a common task in an installer/fabricator's day. During the course of a project, I find myself needing a variety of custom-sized rings and holes. Many installers just draw a circle, grab their trusty jigsaw and cut away. This technique can adequately do the job in some instances, however hand-cut circles and rings are never 100 percent consistent or accurate. A better way to cut holes and make custom rings is with the use of a router and a circle-cutting guide. For those of you who are hesitant to put down your jigsaw, I have some tips and techniques to help you make the process of cutting perfect circles, quick, safe and simple.
One of the most frequently used tools in my shop is the hand-held plunge router, which is permanently attached to a circle-cutting guide. My personal preference (along with many other top fabricators) is the Porter Cable 890 series plunge router with the Perfect Circle router guide. Perfect Circle lets me cut holes as small as 1" in diameter on up to 30" in diameter with infinite adjustability in between. This is because the pivot point has a great range of movement and is able to slide under the base of the router, unlike other designs. (It also doubles as a great offset router base to maintain balance for hand-held routing.) For those who are a little rusty with geometry, a diameter is the measurement of width across the center of a circle.
Whenever a router bit is changed or installed into the router, I reset the "zero point" of the router's vertical movement by plunging the bit down until it touches the surface of a fresh piece of MDF (with the router turned off). This is an important step in setting the cutting depth.
Making a ring or hole that perfectly fits a speaker/subwoofer is only possible if you have a precise measurement of the mounting hole diameter. Accurately measuring the diameter of a speaker or woofer is not possible with just a single ruler or tape measurer. While some owner's manuals have accurate dimensions, many don't. This is why I've made it a habit to measure each woofer before making a cut. A simple way to do this is by using two framing squares and adjusting them so that the bottoms are flush while contacting the sides of the woofer and measuring the distance in between.
Accurately setting the plunge depth of the router is very important so that the router bit cuts just through the MDF/acrylic/ABS but not into the material beneath it. This is done by placing a small scrap piece of the same material you'll be cutting, between the lowest depth gauge setting and the adjustable depth gauge bar (while the router bit is "zeroed"), then tightening the depth gauge bar's set screw. Since the sample ring will be made using 31/44" MDF, I placed a scrap piece of 31/44" MDF between the lowest point of the rotating stop and the bar. (The scrap piece should still be able to slide between the stop and bar.)
Even if your choice in routers differs, I highly recommend a plunge-base router that has a multiple-stop depth gauge. A "plunge router" has a spring-loaded mechanism that allows the height of the router bit to quickly be vertical without having to "spin" the router motor. It also helps make cleaner cuts in the material.