Circle Sanding Jig

Smooth disk edges with a spindle sander

Myriad methods abound for cutting circles. (See page 44 for a router trammel or the last issue for a bandsaw jig.) But whichever approach you take, it’s likely to require some clean-up sanding afterward. As with cutting, there’s more than one way to sand your circles—typically involving a jig attached to a disk or belt sander. But this jig is different; it attaches to an oscillating spindle sander and allows for incrementally advancing the work into the spindle without introducing divots.

In operation, your workpiece pivots on a pin at the inner end of a sliding bar while you rotate the disk against the sanding drum. At the outer end of the bar, a knurled knob allows fine adjustment for incremental sanding, having locked in the coarse-adjustment slide underneath the jig. My jig is sized to handle disks from 2" to 22" in diameter. If you need to sand larger-diameter work, simply alter the dimensions provided to lengthen the pivot bar, the deck, and the slide. Each additional inch increases the diameter capacity by 2".

3 steps to smooth circles

Install your sander’s largest drum. Clamp the jig to the sander, with the half-circle cutout concentric to the drum. For small disks, use the pivot hole near the end of the bar.

Load the work and set the coarse adjustment as shown. Pull the work away from the drum, and turn on the sander. Hold the work, and reengage the drum while rotating the work counterclockwise. Retract it when you’ve knocked down the high spots and are barely contacting the drum. Set the fine adjustments as shown and repeat the process. Small increments here will help you to creep up on your exact diameter.

1 Mounting the work

Lower the bar. It’s easier to install the pivot pin and bar onto your disk than the other way around. Place the pin in a shallow hole in your disk, and drop the assembly into the slot, with the piece kissing the drum. Make sure the sander is unplugged.

2 Coarse adjustment

Set it and forget it. Pull the coarse adjustment slide out until the stop reaches the bolt, and tighten the slide’s thumbscrews, which will remain locked during the sanding session. A few revolutions of the disk against the drum here will do the bulk of the sanding.

3 Fine adjustment

Sneak up on it. Loosen the knurled fine adjustment knob, turn the bolt counterclockwise a small amount, and retighten the nut. This allows the piece to incrementally advance into the sanding drum. Make a few more revolutions and repeat the process until the entire edge is smooth.

A plethora of parts add up to precision

This jig is designed to fit the WoodRiver spindle sander (Triton, Rockwell, and Shopfox make similar models.) but it can be adapted to fit other spindle sanders.

To make the jig, you’ll need 3/4" and 1/2" plywood, some 1/4" hardboard, and hardwood stock. The hardware includes two toggle clamps, a Kreg “Jig and Fixture Bar,” a steel pivot pin, set screws, and fasteners. There are a lot of parts, but each one is a simple shape that easily installs to the underside of the jig.

Buyer’s Guide

See page 70 for tools and supplies used in this article.

Order of Work

  • Cut parts to size
  • Shape deck
  • Rout slots in slide
  • Assemble jig
  • Make tailpiece

Prepare all the pieces

Cut all the parts except the leg to the sizes listed in the drawing. For safe handling, cut small parts from the ends of longer pieces.

Bandsaw the semi-circular drum opening, and then rout the pivot bar slot on the deck to intersect the circle’s center. Rout the slide’s slots, and install the threaded inserts in the deck as shown. Next, make the aluminum angle stop, and install it at the location shown in the drawing.

Router setup. After drilling four 5⁄16" starter holes for the slide slots, chuck a 5⁄16"-diameter straight bit in your table router, and raise it to just over 1⁄4". With the router off, lower a starter hole onto the bit, and lock the fence in place against the slide’s long edge. Then, with the starter hole at the slide’s leading end placed over the bit, clamp a stop block to the fence against the trailing end of the slide as shown. The opposite stop block is set up in reverse fashion. Use a featherboard to hold the workpiece tight to the fence.

Rout the slots. Use a push block to hold the piece steady, and turn on the router. To make your first pass, push the slide right to left and against the fence until you hit the stop. Turn off the router, lift and rotate the piece to make the first pass for the second slot. Finally, flip the piece over and repeat the process.

Insert the threads. Position the holes for the inserts using the slide’s routed slots and where shown in the drawing, then drill 3⁄8" countersunk holes through the deck bottom. These inserts hold the thumbscrews that tighten the slide for the coarse adjustment.

Bring it all together

Glue the rails to the rail tops. While the glue sets, attach the locator dowels, clamp pads, and toggle clamps to the deck. Test the fit of the jig against the sander, as shown. Measure the distance from the deck bottom to the workbench to determine the length of the leg.

Now, square the slide to the deck bottom, and tighten the thumbscrews. Shim the slide’s edges, and shim and install the rail assemblies as shown.

Fasten the leg support to the leg using dowels, and trim the leg to the appropriate length. Attach the leg assembly to the deck with a butt hinge, and install the leg catch and catch pad where shown.

Drill two holes in the bar for the pin centered at 1/2" and 6-1/4" from the inside end, and cross-drill and tap for #8-32 setscrews to lock it in place. Install the insert into the tailpiece and attach the tailpiece to the bar.

Lastly, make a support spacer and attach it to the slide to prevent the bar from tipping out of its slot when sanding large disks. 

Check for fit. Position the deck on the sander table with the locator dowels against the table edge, and adjust the toggle clamps for tightness. The jig should snugly grip the sander with no slop.

Shims for the slide. Place a narrow strip of paper on each edge of the slide, and a wide piece under each rail assembly. Install the rail assemblies tight against the slide, and remove the thin strips from between the rails and slide. The shims under the rails stay put.

Catch the leg. Use a round-head screw, a lock washer, and a flat washer to attach the leg catch. The catch swivels over the leg support to secure the leg when the jig is in use.

Attach the tail. Drill and countersink the back of the pivot bar, and screw it to the tailpiece.

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