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The Sawdust Factory Presents:

Kurt's Circle Jig for the Router

There are at least a million circle jigs for routers that woodworkers may buy, or build in the shop. They are all based on two basic designs: the trammel stick, and the two bars-and-adjustable-block thing. While these work okay, they are not excellent. This page is about one that is...

Typical trammel bar type of thing available in woodworker's catalogs..

Sells for $20?!?! Good grief!

The jig I am going to detail was mined from a library book I checked out a long time ago, about jigs for table saws, band saws, etc. (I need to find that book again so I can give it credit). Most of the jigs were commonly known, but this router circle jig really caught my attention. I soon built one, and have since found it be one of the most valued tools in my shop. As a maker of custom large-aperture astronomical telescopes, I can tell you without equivocation that I cut a lot of circles. I have tried every circle jig cutting there is, and this one is WAY ahead of the pack!

It can cut circles or arcs as big as you like, since making extra slides is easy and cheap (though I have yet to outgrow my 12" slide). And on the small end, it goes right down into the range of common drill bits, so you're always covered. And it is wonderfully easy and convenient to set up and adjust. Easy to build, too!

Ta-da! Here it is, ready for action. Mine is set up for a Porter-Cable 2hp Model 7829 plunge router (a fine machine, by the way), although any router may be fitted to this design.

The jig consists of the wood body, a Plexiglas slide, a T-nut, a thumb knob, plus a couple of mounting screws to fit your router's base and a nail to act as a pivot pin. That's it!

The wood body is built in two halves: the top half and the bottom. This allows you to easily fashion the recess which seats the router, and also to cut the channel for the Plexiglas slider piece. Use 3/8" or 1/2" plywood, the dimensions are not critical. 1/4" or 3/8" thick Plexiglas makes a perfect slide.

As mentioned, the big hole in the top fits your router's base. Here you see the bottom hole, which ought to be as big as you can manage without getting extreme about it. The idea is to make bit changes easy, and to see what you're doing when you line up for a cut.

Note the Plexiglas slide: it is a 1/4" thick strip of Plexiglas cut 1 1/2" wide by 12" long, with 1/4" rabbets cut along either side, on the router table, to half its thickness. They keep the slide from falling out of the jig.

I made this jig from 3/8" plywood, by the way. Use 3/8" thick Plexiglas when making the jig from 1/2" ply.

A bottom view: check out the slide from here.

Also note the 1/4-20 T-nut visible in the glare of the slide, near the top of the wood body, centered over the slide channel. The thumb knob goes into it to lock the position if the slide.

Further down we see the four mounting screw holes; two with screws in place, and two without. The machine screws fit the existing threaded holes in the bottom of the router, and the holes in the wood body are counter bored to keep things flush on the sole of the jig. I only use two screws to mount my jig.

Finally, that one last hole down at the bottom: it's for hanging the jig on a peg for storage.

The black plastic is, of course, the bottom of my router. I prefer carbide spiral up-cut bits in 1/4" size for routing holes in plywood or hardwood.

A close-up of the slide. Clearly seen are the rabbets along the edges, how it sits flush with the sole of the wood body, two pivot pin holes, several marks where the thumb screw has been tightened down in the past, and the T-nut (right center) under the slide.

Here's a view of the jig off the router, which clearly shows the recess the machine fits into, and also the thumb knob that cinches the slide in position.

The little bitty pins sticking out the top of the wood body are pieces of nails. I just picked a box of finish nails that looked a good size, and then selected a drill bit that makes snug fitting holes for them. I drill holes in the slide where I think they will work the best (so far I have three holes in the slide; one on either end and one not quite in the middle, all located on the slide's centerline). Use the same drill bit to make center holes in the workpiece for the jig to go 'round on. Stick a nail-pin in, lower the jig in place to engage a hole in the slide, adjust to the diameter you wish to cut, tighten the thumb screw, and let 'er rip!

Make your cuts in 1/8" passes so you don't dog your router or dull your bits prematurely (I take four passes to go through 1/2" baltic birch). Be extra careful to cut on the correct side of your mark, it really sucks to go to all that trouble only to have to start over again.

How to make this jig in 10 Easy Steps:

Reference Drawing

1. Cut two pieces for the wood body. Label one "top" and the other "bottom". The outside shape of the thing isn't important; so make it round if you prefer. But don't cut the final shape yet! Do that after the halves are glued together in Step 9. Working with square pieces in the meantime will make your life ever so much easier.

2. Cut a hole in the top to fit your router's base. I highly recommend using plunge routers for this sort of work.

3. Drill a hole, and, with a Forstner bit, a recess for the T-nut. It will be located on the slide's centerline, flange down. You want the thumbscrew to bear on the slide. Insert T-nut. That's it for the top half.

4. Cut a hole in the bottom half to fit your router base inside diameter. You want this hole to be as big as possible for easy bit access, and so you can see what you're doing when lining up and cutting. If your router has a small opening in the base, try removing it. The jig will take its place.

5. Cut a channel for the slide. Start with a slot one inch wide, then cut 3/8" rabbets along either side facing towards the top half, and half the workpiece thickness deep. The router table will be handy for cutting rabbets.

6. Locate and drill mounting screw holes to match those on your router. Counter bore the holes so the screw heads do not protrude from the sole of the wood body. Don't want to scratch stuff.

7. Cut your slide out of Plexiglas. 1 1/2" wide, then rout the 1/4" rabbets along either side of it. Adjust rabbet cuts to a depth that corresponds with the channel in the wood body, so that the slide comes flush to the body's sole when the thumbscrew mashes it down. This will become clearer when you start fitting the pieces together. If you're the nervous type, just cut the rabbets a bit shallow, then sneak up on the correct depth.

8. Test fit the slide to the channel in the wood body. Clamps the halves together to make sure it doesn't bind, and that the bottom face of the slide rides flush with the sole on the wood body.

9. Happy? Good. Glue the top half to the bottom half.

10. Insert the slide, thread in the thumb knob, mount the jig to the router (or the other way around), and go try it out. This makes the easiest-to-use, most well-behaved circle jig on the planet.

Kurt's other websites:

The Sawdust Factory It's about making big telescopes in a tiny shop.

Adventures in Boat Building Construction and general fun in his-and-hers 13-foot wooden kayaks.

Kurt's model Airplanes The title just about says it all on this one.

Delta Contractor's Saw Modifications Improvements made to my new table saw.

Kurt welcomes email at NGC704@aol.com

This page last updated 1-19-04