Those of you who have made large table tops and similar projects requiring circles or arches know the value of an adjustable circle-cutting and routing jig. Although it is possible to cut a large circle using a bandsaw, it is much easier doing this task with a jig-mounted power tool. This article will show how you can make yourself a very precise, adjustable circle-cutting jig that will accept jig saws and routers alike. It will also cover using this jig to cut out circles where you don’t want to drill an unsightly pivot hole in the top of the piece you’re cutting.

Making the Tool Saddle

Start by first determining the size of the tool saddle (A) you will need to properly mount your jig saw or rout­er. The leading edge of the jigsaw blade lines up with the centre line of the tool saddle and the edge of its base is perpen­dicular to that line. This location allows the blade to track most efficiently.

Making a mounting template that is custom-made for your jigsaw is as easy as placing your jigsaw onto a stiff piece of paper and pushing the blade through until the base plate bottoms out. Trace the out­line of the base plate onto the template, ensuring you indicate clearly the location of the blade. Transfer the location of any mounting holes you’ll be using to securely mount the tool, then go ahead and re­move the jigsaw from the template.

You can also do the same thing with your router. However, this time make sure to align the centre of the bit with the centre line before tracing the outline of the base, the location of the bit and the mounting holes. It’s a good idea to use different colours for these two separate tracings, to account for the unique size base plate mounting holes of each tool.

Transfer your mounting template information to the tool saddle ply­wood using carbon paper, then using a certain amount of artistic license, go ahead and draw in the full outline of the tool saddle. Err on the side of hav­ing the tool saddle at least ½” larger all around than required to securely mount the tool, as you may eventual­ly end up replacing your original tools with larger ones.

Proceed to cut out the tool saddle, then drill out the mounting holes, being careful to use the correct size diame­ter bits. In all likelihood, you’ll have to buy longer mounting bolts in order to get completely through the ½” thickness of the tool saddle and firmly engage the threads in the base plate. These hard­ware items are available through most specialty hardware or fastener suppliers.

Two 5/16″ holes must be made in the tool saddle, however, countersink them with a Forstner bit before drilling these holes. These holes are for the two bolts (E) used to join the tool saddle to the bridge (B).

The Bridge and Arm Parts

The two remaining plywood compo­nents are relatively easy to make. The bridge has a 7″ x 5/16″ wide slot ma­chined down the one end that clamps to the tool saddle. Used in concert with the bolt holes, this 7″ slot allows up to 1 ½” of travel for fine-tuning the radius of the circle or arc, which is usually much more than necessary. Drawing a line right down the middle of the arm (C) will provide you a point of refer­ence for drilling pivot pin holes.

The arm is by far the simplest ply-wood component to produce. Aside from cutting it out and slightly chamfer­ing its edges, it is simply fastened to the bridge using glue and screws (F) Now that you’ve complete the two main components, all that is needed to assemble your jig is to insert the two bolts (E) into the tool saddle with their heads set flush into the Forstner drill holes.

Align the bolts with the slot in the bridge (B) and screw on the cam bolt clamp. Depending on the length of the bolts, you may need to cut them shorter so they don’t interfere with action of the cam clamp arm.

Test Driving Your New Jig

Let’s start off by cutting a simple circle. After mount­ing your jigsaw so that everything lines up properly, set the cam clamps so there is approximately ¾” of space between the ends of the arm and that of the tool saddle. Select a test piece of ½” or ¾” plywood, then carefully measure along the arm and mark the location of where the pivot pin should to be located. Careful is a relative term, for as mentioned above, you’ll have quite a bit of travel to play around with. Err on making the circle somewhat larger than required, that way, you can be sure that you don’t cut the radius smaller than the plans call for.

Many use a 10-penny, 1″ common nail for a pivot pin, however you could use an awl just as easily. Rather than measuring carefully for the proper size drill bit to make the pivot pin hole in the arm, simply chuck the nail into your drill and use the nail to drill the pilot hole. Remove the nail, then push the pivot pin through the hole until it bottoms out enough so that you can clearly see it. Position the pin over the center of where you want the radius to be and tap it in ap­proximately ¼” with a hammer before proceeding to cut out the circle. If your test piece is wider than the given radius, reposition the pivot pin so the blade has a starting point that just barely scrapes one edge of your test piece of plywood. Now, go ahead and cut out the circle. Be careful that you don’t cut into your workbench top, as few things in wood­working make you feel as silly as that.

Cutting or Routing Circles Without Making a Pivot Pin Hole

Some projects require you to rout from above the top, which usually means you can’t nail your pivot pin into the top surface of the tabletop or whatever piece you’re machin­ing. In such cases, you’ll need an auxiliary pivot pin plate (G). This plate is held firmly in place on top of the piece by two-sided tape, and then a pair of support blocks (H) are screwed to the underside of the tool saddle to bring every­thing to an even keel. Be careful not to exceed the maximum depth of cut for your blade or bit, and if this is the case, make the auxiliary pivot pin plate and support blocks of ¼” material.

As you can see, this jig provides you with many options to cut or rout circles of in­finite sizes. Cutting circles in your shop doesn’t need to be a source of aggravation. Using this jig will make it easy as π.