This versatile and easy-to-build jig will have you cutting circles of virtually any size in no time. It’s a second-generation improvement to the original circle-cutting jig shown below. Rather than employing a pivot point, this clever design features a revolving, clear plate to guide the router.
by John English
Many years ago, we featured my “first generation” circle-cutting jig. It was a popular project among readers, with just one glitch: it couldn’t cut a circle with a diameter smaller than the radius of the router’s subbase. This problem certainly isn’t unique; most circle cutting jigs have one thing in common: they cut a radius equal to the distance between the router bit and the pivot. The size of the router base limits how short that distance can be. Being the industrious sort, I went back to the drawing board and came up with the ingenious second generation jig you see here. It doesn’t require a pivot point, which opens up the range of circle-cutting capabilities tremendously.
Creating the Plexiglas™ Disk
The basis of my jig is a large disk that revolves inside a frame. Mounting the router to this disk effectively eliminates the pivot point. To use the jig, the frame is centered on the workpiece and clamped in place. You can change the diameter of the circle simply by changing the position of the router on the disk. Clear Plexiglas™ works best for making the disk: seeing through it is essential for positioning the router bit correctly on the workpiece.
Use a regular circle-cutting jig to cut the disk (piece 1) to the dimensions given in the Material List on page 88 (see Figure 1, above). After routing the disk, gently break the top and bottom edges of the disk with sandpaper. Cut the top and bottom of the jig (pieces 2 and 3) to size from a sheet of sturdy MDF (medium-density fiberboard).
This article is courtesy of “Fox Chapel Publishing’s Best of Woodworker’s Journal: Jigs & Fixtures for the Table Saw & Router.”
Center the disk on the jig top and mark around its edge with a pencil (see Figure 2). Then use your regular circle-cutting jig to plow the rabbet in the middle of the top that supports the disk (see Figure 3). Use the same jig to create the large hole in the center of the top. Look to the Jig Top Elevation below for key measurements.
Figure 2: Center the Plexiglas disk on the sheet of MDF that will become the top of the jig, and scribe around the edge with a pencil.
Finish Building the Base
The safest way to secure work in this jig is to attach it to a sacrificial piece of 1/4" scrap plywood, then clamp the plywood in place. That way, if you cut through the workpiece, you won’t cut into the jig, too.
With that in mind, you’ll need a full inch of clearance between the jig top and bottom (1/4" for the plywood and 3/4" for a standard workpiece). Do it by installing two spacers (pieces 4) on the jig bottom. Secure these with glue and countersunk screws. A pair of cleats (pieces 5) are then glued and screwed in place to add a bit of strength.
Building the Adjustable Top
The odds that the Plexiglas™ disk will fit perfectly in its rabbet (without any play) are very slim. Fortunately, there’s a simple remedy: you can make the top adjustable by cutting a kerf in one of its edges, then adding a built-in clamping device. It makes the jig adjustable so the disk revolves freely, but without slop.
Before attaching the torque adjustment plates to the jig, you need to plow some grooves in the disk (see Disk Groove Elevation). Using the jig as a clamp, set the disk in its rabbet and secure it with a bar clamp. Doing this now makes sense because the torque mechanism would be in the way if it were installed first. Locate the grooves and mill them with a straight bit, guiding your plunge router with a clamped-on straightedge (as shown in Figure 4).
This article is courtesy of “Fox Chapel Publishing’s Best of Woodworker’s Journal: Jigs & Fixtures for the Table Saw & Router.”
Figure 3: Use your regular circle-cutting jig to reveal the recessed rabbet in the new jig, and to remove the waste from its center.
Figure 4: With a kerf cut through one edge of the jig top, clamp the disk in place and use a straightedge to mill the various grooves.
Figure 5: Pre-drill pilot holes for the screws in the continuous hinge that keeps the top and bottom of the jig properly aligned.
Begin building the torque adjustment mechanism by cutting the two plates (pieces 6) to size. Plow a dado in one face of one plate, then temporarily clamp them together and drill a hole through both. Counterbore the hole in the second plate and band-saw both plates to their final shape. Then sand the sawn edges smooth. Attach the plates to the jig top with glue and screws (pieces 7), driven up through the bottom of the jig top into piloted, countersunk holes. After the glue has fully cured, insert the T-bolt and knob (pieces 8 and 9).
Attach the jig top to the bottom with a piano hinge (piece 10), securing it with screws (piece 11) driven into pre-drilled pilot holes, as shown in Figure 5. Align the top and bottom perfectly and clamp them in place as you set the hinge.
At this point, it’s a good idea to sharpen your pencil and draw a series of concentric circles on the base (see Figure 6). Make them 1" apart to help you center stock under the disk. This will minimize waste as you use the jig.
Mount your Router in the Jig
Virtually any plunge router will work with this jig. Mid-sized models are perfect choices. You can use a standard machine as long as the bit height is set by screwing the motor housing into the base. Routers with a motor housing that slides freely up and down in the base until locked in position are not a safe option here.
This article is courtesy of “Fox Chapel Publishing’s Best of Woodworker’s Journal: Jigs & Fixtures for the Table Saw & Router.”
The long edge of the router mount (piece 12) matches the diameter of the disk, while its back edge is shaped to match the router base. On a piece of Plexiglas™, lay out the basic design shown on the Router Mount Elevation, then make any necessary changes accommodate your specific router. Band-saw it to shape, sand the edges and drill the two large holes shown on the drawings: these house the knobs, bolts, washers and lock washers (pieces 13 through 16) that make the mount adjustable and lock it in position.
Figure 6: Draw a series of concentric circles 1" apart on the base, to center the workpiece under the router and minimize waste.
Mark, drill and countersink holes in the mount for the screws that attach your router’s subbase, then remove the subbase and install the router on the new Plexiglas™ mount. Attach the mount to the disk with T-knobs.
The long edge of the router mount (piece 12) matches the diameter of the disk, while its back edge is shaped to match the router base. On a piece of Plexiglas™, lay out the basic design shown on the Router Mount Elevation, then make any necessary changes accommodate your specific router. Band-saw it to shape, sand the edges and drill the two large holes shown on the drawings: these house the knobs, bolts, washers and lock washers (pieces 13 through 16) that make the mount adjustable and lock it in position. Mark, drill and countersink holes in the mount for the screws that attach your router’s subbase, then remove the subbase and install the router on the new Plexiglas™ mount. Attach the mount to the disk with T-knobs.
A free-turning guide knob (piece 17) makes revolving the disk a lot easier. Drill a hole for the knob and install it with a bolt, two washers and a nut (pieces 18 through 20). The knob should spin freely after the nut is tightened.
Taking the Jig for a Spin
Now it’s time to fire up your router and take this jig for a spin—literally! You can use any fluted straight bit for cutting circles, and spiral carbide up-cut bits work particularly well. Cut the circles by making a series of passes, setting the bit depth a little deeper each time. Start each pass with the bit raised out of the cut, then plunge it in and turn the router slowly counterclockwise.
Open up the jig as necessary to clear the chips inside or if they impede the movement of the router. For other important usage tips and a few project ideas, see the sidebar at end of article.
Different routers will vary to some degree. Mount your router to a Plexiglass™ plate. Holes drilled in that plate will match the slots created in the circular disk.
This article is courtesy of “Fox Chapel Publishing’s Best of Woodworker’s Journal: Jigs & Fixtures for the Table Saw & Router.”
Putting Your Jig To Work
You can cut inside or outside circles with this jig, in stock up to 3/4" thick for a through cut or 1" thick for a stopped cut. The outside diameter depends on the size of your router’s base, but most routers will allow a circle up to about 12" in diameter.
The key to safe, accurate results is securing the workpiece properly. Judicious use of double-sided tape and drops of hot-melt adhesive will keep your material stationary throughout the machining process (see photo at right).
John also uses a piece of scrap sheet stock (secured in place with hot melt or tape) under the workpiece. This allows him to cut completely through the stock without accidentally routing into the bottom of the jig. As with many shop jigs, the creativity of the user is the only limit to its usefulness. The jig works well with straight bits to cut circles, but switch to a standard router table to mill decorative edges.
Locate spots of hot-melt glue under each of the parts you are making, so that when a part is cut free it can’t drift into the spinning router bit.
Routing perfect circles allows you to create an unlimited number of projects and decorative motifs.
This article is courtesy of “Fox Chapel Publishing’s Best of Woodworker’s Journal: Jigs & Fixtures for the Table Saw & Router.”
