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“Cutting Circles” by: Chris Yato Overview Creating circle shaped parts, whether it’s 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. Although this adequately does the job in some instances, hand-cut circles and rings are never 100% consistent or accurate. A much 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 out there that have been hesitant to put down the jigsaw, I would like to share some tips and techniques that will 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 out there) is the Porter Cable 890 series plunge router with the Perfect Circle router guide. The Perfect Circle allows me to cut holes as small as 1” in diameter up to 30” with infinite adjustability in between 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 great balance for hand-held routing.) For those that are a little rusty with geometry, the diameter is the measurement of width across the center of a circle.
Even if your choice in routers differs, I would 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 be quickly adjusted vertically without having to “spin” the router motor. It also helps make cleaner cuts in the material.
A ¼” “downcut” spiral bit (pictured on the right) is recommended for MDF because it helps hold down the part and creates smoother cuts. A ¼” upcut spiral bit is necessary for acrylic and ABS so that the shavings are ejected upward and do not melt back together.
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 on a fresh piece of MDF (with the router turned off). This is an important step in setting the cutting depth.
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 will 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 our sample ring will be made using ¾” MDF, I placed a scrap piece of ¾” 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.)
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 ¾” MDF with non-slip mat glued onto both sides with contact cement. The non-slip mat keeps the pad from sliding on the benchtop and the material from spinning when being routed. This cutting pad also keeps the benchtop 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 benchtops.
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 measure. While some owner’s manuals have accurate dimensions, many do not, so 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.
For our sample ring, we will 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 1/8” hole in the center (9/64” for acrylic/ABS) before marking the cut lines.
Next, using an accurate ruler, we will mark the cutting points on the MDF. I usually shove a pencil into the center hole so that the edge of the hole is clearly visible and line up the 1/16” marks at the edges of the hole.
Since the dimensions of our ring 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. This is a useful way to reduce cutting errors.
Before this step, make sure that 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 is just above the surface without touching. Line up the edge of router bit on the “X” side of the mark. Plunge the bit further, so that the router bit touches the surface, and spin the bit by hand. If the router bit is in the correct position, the mark that the router bit makes should be exactly on the pencil mark. Tighten the knob on the Perfect Circle.
For those that don’t have a lot of experience at 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 is the correct size. Make any adjustments if necessary.
Since our sample ring is ¾” MDF, we will be making three passes to cut all the way through. I recommend making ¼” deep passes per revolution in MDF and 1/8” deep (or shallower) passes with acrylic and ABS. Adjust the depth stop so that the first cut will be ¼” 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 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.
After the first pass is made, raise the router bit and spin the Perfect Circle counter clockwise. This will help keep the electrical cord from getting too twisted up. Adjust the depth stop to cut another ¼” (1/2” total) and make another spin. Making all passes in the same direction will help make a cleaner cut.
For the final pass, adjust the depth stop so it can cut the last ¼”. Make sure that 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 an outer pieces will be separate. Stop rotating immediately after the two pieces become detached.
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 are making multiple rings with the same outer diameter, make all of the outer cuts at the same time without adjusting the Perfect Circle.
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 that the first pass will be ¼” deep. Make your second and third passes. It will be very important to remember where you start your third pass, so that you can stop cutting immediately when the two pieces become detached.
At this point, you should have your perfect ring. You can use a piece of 80 grit sandpaper to clean up the edges if necessary. Although it may seem like a lot of steps to make a simple ring, this technique becomes quite simple with practice and the accurate and repeatable results are awesome.
When making smaller diameter rings like these for projects like tweeter and instrument gauge mounts, the non-slip mat isn’t able to 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.
When making rings with materials less than ½” thick, the pilot hole must be drilled through the scrap material, so that the pivot point of the Perfect Circle will seat properly.
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 inch outer ring, (2.000/2) – 0.063 = (1.000) – 0.063 = 0.937 inches.
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 that 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 inches.
This is an example of just how precisely you can cut with this technique. I would challenge the even 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.