This instructable will show how to make an accurate and easy to use jig for cutting perfect circles with a router. There are many pre-made circle cutting jigs on the market, but they are fairly expensive, and many of them don't have a very large maximum radius. Several of them also rely on a series of holes to control the size, which can be a problem if you need a very precisely sized circle that happens to be in between the sizes of two of the holes.

I will show how I made a precise circle cutting jig using nothing but scrap plywood, and a few nuts and bolts. This basic plan can be modified to make accurate circles of almost any size.

Disclaimer:
Please practice shop safety, especially when working with power tools. Take all neccesary safety precautions, and don't use any tools or procedures you're unfamiliar with or uncomfortable using.

Step 1: Tools and Materials

Tools-
router
1/4" router bit
3/4" router bit
straight edge
pencil
drill or drill press
1/4" drill bit
chamfer bit
saw (prefferably a tablesaw or bandsaw, but any will work)

Materials-
Scrap of MDF or cabinet grade plywood
1/4"x20 tpi threaded rod 3" long (or a 1/4"x20 tpi bolt with the head cut off)
1/4" x20 tpi T nut (or pre-made t-handle)
1/4"x20 tpi standard nut
3/4" OD washer

I don't have a cost for the materials, because it was made entirely from items I had on hand. I would think that most of not all of these materials could be found in most workshops. The cost to purchase these items would only be a few dollars. It does of course assume that you have a router to use the jig with.

Step 2: Layout

The base material could be made of any flat sheet good material, but preferably MDF or cabinet grade plywood. Cut the sheet goods to 2-3" wider than your router base, and 5-6" longer than the largest radius you plan to cut. I happened to have a scrap of 3/4" cabinet grade ply that was about 8" wide, and 29 3/4" long, which looked like a pretty good size to me. It will allow me cut circles up to about 46" in diameter.

Draw a line down the center of the base lengthwise, and make marks on this line 1/2" from each end.

Remove the baseplate from your router, and position it centered on the line you drew, and 1/2" away from one end of the base. Use a pencil to mark the screw mounting holes, the center hole, and trace around the outside diameter.

Make a mark on the center line one inch away from the outer edge of the baseplate.

My baseplate was round, but many will have a flat edge. if yours has a flat edge, i would orient it towards the center of the base, which will enable you to cut samller circles than if it were orented otherwise..

Step 3: Routing the Slots

Put the baseplate back on the router, and install a 3/4" bit (or whatever size your washer is). A plunge router is preferable for this, but not neccesary. I already had the fixed base router that will be used in this jig out, and didn't feel like taking my plunge router out of my router table. Unplugging the router anytime you install or change bits is a good safety habit to practice.

Set the depth (or plunge depth) to the thickness of your nut, plus the washer, plus another 1/16". For my parts, this ended up being about 3/8".

Clamp the base to a stable work surface, like a table or workbench, with the side you will mount the router to face down. Then clamp a straight edge (scrap of plywood in this case) to the base to act as a fence, or guide. Line up the straight edge with the outside edge where you marked the baseplate, and parallel to the length of the base. This will ensure that the slot will be centered in the base.

Using the straight edge as a guide, route a slot between the mark 1" from the baseplate, and the mark 1" from the other end of the base. If as you look at it, the fence is to the left of the router, start at the end closest to you. If the fence is to the right of the router, start on the end furthest from you. This will pull the router into the fence, which not only makes it easier to make a perfect slot, but is also safer.

With the router unplugged, replace the 3/4" bit with the 1/4" bit, and set the depth to go all the way through the base.

With the fence still in place, rout the smaller slot the in the same way as the first one.

Step 4: Drill the Holes

Drill holes for the mounting screws, making them just a little bigger than the diameter of your screws. I ended up using a 1/4" bit. Flip the base over, and using the holes as a guide, trace the baseplate on this side too. Choose which side you want to moun the router on, and chamfer the mounting holes on the opposite side. You want the chamfers just deep enough that the screws will sit beneath the surace.

Drill or rout out the center hole. It just has to be slightly bigger than the largest router bit you might use. A hole saw is probably best for larger holes, but I used a twist bit, and it caught and made a pretty messy hole. It won't really effect anything other than aesthetics, but next time I'll take the time to get the correct bit out. I did clean up the hole a little with the router later.

Step 5: Making a Mounting Recess

The mounting screws are most likely much too short to reach all the way through the base material. We could buy longer mounting screws, but unless you have a set of really long router bits, the thickness of the base may limit the depth of cuts you can make with the finished jig.

To solve both problems, I reccomend routing a recess in the base for the router to sit down in. The 3/4" ply I used was actually about 23/32" thick, so I made the recess 13/32" deep, which left about 5/16" of material to mount the router to.

After clamping the base to a work surface with the router mounting side up, rout out the baseplate area, using the line you traced as a guide. It's easiest to start at the center and work around the the hole clockwise, sneaking up on the final size with a few light passes.

Now the baseplate can be removed from the router, and the router can be installed in the jig using the baseplate screws. Make sure all the screws sit flush with the bottom suface of the jig so that they won't catch on anything.

Step 6: Making the Post Assembly

While the easiest thing to do is buy a t-handle or star handle for jigs, but I'm too cheap to do that, so I make my own. It takes a piece of scrap wood, a t-nut which costs just a few cents, and takes just a few minutes to make. I make them in bulk to use in all of my jigs, and I won't go into the process here. It's easy enough to simply cut out a shape you like, drill a hole in the center to accept the t-nut, and hammer it in place.

Cut a piece of 1/4" x20 tpi threaded rod to 3" length. I didn't have any threaded rod, so I just cut the head off of a 3" bolt. Screw the nut onto the center of the threaded rod, and then place the washer on. Slide the portion above the washer up throught the slot in the base, and then screw the handle on to the top.

Alternatively, if you have access to a metal lathe as I do, you can cut the threads off one end of the threaded rod, making a smooth post of a specific size. That way you can use a drill bit that matches the specific size, and always be sure to have a nice tight fit. The threaded rod with an appropriate sized drill bit should work well enough for most applications though.

Step 7: Cutting Off the Corners

This step is entirely optional, but I thought it might make it slightly easier to use. The base had quite a bit of uneccesary wood, so I decided to cut it off, and round the corners. I also slightly beveled the edges on the bottom side with some sandpaper so that they wouldn't catch on anything as they slide around the circle.

Step 8: Using It

The first step to using it is setting the post height. Measure the thickness of the material you'll be cutting a circle in. Adjust the nut on the post so that it sticks out slightly less than the material is thick.

Mark the center of the planned circle, and drill a hole that the threaded rod post will fit in snugly. Set the post at the desired distance from the router bit, and tighten the handle on top to lock it in place. If you are cutting all the way through your material, make sure to hang the portion being cut off the table, and keep rotating the material periodically during the cut to keep from cutting the table. You could also put a sacrificial piece of scrap underneath the material instead.

It's generally safer and easier to cut outer edges of circles in a counter-clockwise direction, and inside edges clockwise.

If you don't want a small hole in the cener of whatever you're cutting, you can use a sacrificial piece on top. Drill a hole in a piece of scrap, and use doublesided tape to secure it to the workpiece with the hole at the center of where you want to cut the circle. Then just increase the depth of cut by the thickness of the sacrificial piece, and cut as usual.

Alternatively, if a hole would be acceptable on the underside, but not on top, you can drill the hole only part way through the material, and cut from the bottom.

Have fun, and be safe!