Cut perfect circles and arcs with this simple setup.
Make perfect bandsawn circles and arcs with this jig, thanks to an adjustable pivot pin that guides the work as you rotate it through the blade. This jig can tackle circles up to 6 ft. in diameter–a nice size for a tabletop. The sled lets you move the workpiece straight into the blade as you begin sawing a circle–a distinct advantage over other jigs that require you to first notch the work at the cutline to provide entry clearance for the blade. As a bonus, the jig accepts a platform for sawing arcs, allowing you to shape all sorts of items, ranging from curved rails and corner panels to custom trim.
Make the jig
Referring to Figure 1, cut the parts to suit your saw table. Then glue and screw the runner in place, locating it so that the left end of the sled sits 2" from the inner edge of the saw table, with the runner’s front end aligned with the edge of the sled.
Install a bandsaw blade dedicated for use with the jig. Now, lay out the pivot point centerline. To determine its offset from the front of the sled, measure from the front of your saw table to the front of the blade, and add 1⁄2". Next, supporting the cantilevered end of the sled on a stand, cut the kerf as shown in the photo (Next image, below). Then clamp the control bar to the table against the sled.
Remove the sled, and mark the pivot point centerline 1⁄2" from the saw kerf, where shown in Figure 1. Attach a straightedge so that, when a router is placed against the straightedge, the center of its collet is offset 1⁄8" to the opposite side of the pivot point centerline.
Outfit your router with a 14° × 1⁄2" dovetail bit set for a 3⁄8"-deep cut. Then, guiding the router against the straightedge, cut a dovetail slot that terminates at your stop line. Shift the straightedge 1⁄4" further from the centerline, and make a second pass to widen the slot to 3⁄4". (See photo after next, below)
Make the 14° dovetail pin block to fit snugly, but move freely, in its slot. Tap the brass pivot pin into its hole, along with a drop of super glue. Thread the setscrew into its hole and make sure the block locks securely when the setscrew is wrenched against the bottom of the slot.
A Blade/Jig Partnership
A 1⁄2" blade nicely handles circles over 5" in diameter. A 3-4 TPI blade with hook-style teeth works well, but carbide teeth cut even smoother. Whatever blade you choose, always pair it with your jig.
Bandsaw Circle-Cutting Jig
The runner-guided sled includes a dovetail slot that houses an adjustable pivot pin block on which the workpiece rotates. As the sled is pushed, it’s held down by a control bar that also stops its forward travel at the appropriate point. All components except the runner and pin block are made from 3⁄4" hardwood plywood.
Cutting circles
To cut a circle, first size the workpiece panel to about 1" wider and longer than the desired circle diameter. Then drill a 1⁄8 × 3⁄8"-deep centered hole in the underside. For large work, you’ll first need to nip off the corners of the panel in order for them to clear the hold-downs. To do this, set the pin block for a radius 1" larger than the final radius. Then rotate the workpiece into the blade. Alternatively, use a jigsaw.
Reset the pin block to the desired radius, remount the work, and then begin the cut as shown in the photo at right.
When the sled stops against the end of the control bar, begin steadily rotating the work into the blade, as shown in the main opener photo above. Slow down when the blade starts to skim the edge at the end of the cut. Then let the blade stop before lifting your perfect circle off the jig.
Cutting arcs
Cutting a curved rail
For an example of cutting arcs, let’s look at making a curved rail like the one shown in the drawing at far right.
First, make a platform to an appropriate size, and drill a pivot hole in the corner. Mark the panel with any reference lines necessary to orient your rail blank symmetrically. This produces an attractive appearance while minimizing weak short grain.
Prepare the blank by mitering its ends to finished length. Then secure it to the platform with double-faced tape applied to both the rail area and the inner waste section. Set the pivot pin location for the desired radius, position the sled against the control bar in its final stopped position, and slip the platform onto the pin. Turn on the saw, and swing the assembly steadily through the blade, as shown in the photo at upper left.
Readjust the pin block to make the inner cut on the rail, and then repeat the sawing procedure, cutting through both the rail and the platform this time. (See right-hand photo on the opener image above.) Be ready to catch the freed parts as they come off the blade.
Arc-Cutting Platforms
Mount a pivoting carrier platform on the sled to make curved rails, shelves, trim, and other arced cuts. The 3⁄4"-thick plywood platform can be reusable or sacrificial, depending on the job. Size it to suit the dimensions of your workpiece.
