Jeff Miller's oval jig features a wild mechanism that produces miraculous results.
Synopsis: One of the jigs Jeff Miller relies on when using his disc sander (“The Versatile Disc Sander,” FWW #296) is this one for sanding ovals. It’s made up of a frame that slides along a center bar and rotates around an off-center disc. It’s a wild mechanism that produces miraculous results.
I first made a jig to create ovals in conjunction with my David Pye–inspired Fluting Engine. Pye wrote of using one, explaining that it was based on “an ancient turner’s chuck.” He then refers the reader to Charles Holtzapffel’s Turning and Mechanical Manipulation (1850). I followed him there.How it works. The workpiece is attached to a frame that that slides along a center bar and rotates around an off-center disc. The amount of offset from the center pin determines the proportions of the oval.
My oval sanding jig operates much the same way as the circle jig in my article, and uses the same baseplate. The heart of the jig is a frame that the workpiece gets attached to. The frame slides along a beveled center bar and sits over a circular disc. As you turn the workpiece, the frame slides on the bar and circles the disc. The bar rotates around a centerpoint—the bolt that penetrates the whole jig. If the bolt is centered in the disc, the resulting workpiece will be round. But if you set the bolt so it is off-center in the disc, then the frame moves elliptically as it slides from side to side and travels around the disc, creating an oval workpiece. How far off center you set the central pivot determines the shape of the oval. It’s a pretty wild mechanism, but the results are miraculous. I have made a few of these jigs in various sizes. Feel free to experiment.Oval-sanding jig. Adjustable jig allows you to sand ovals of just about any shape and size. Determining the oval shape. The farther the pin is offset from the center of the disc, the longer the oval.
Building the jigThe frame slides along a central bar. The beveled sides of the bar are captured between angled slots in the frame sides.
You can cut the disc out on the lathe, or with a fly cutter on the drill press. The fly cutter is more dangerous, so if you do it that way, clamp the blank to the drillpress table, and be sure to keep your hands clear of the cutter.
The frame that rides around the disc consists of two long sides, each with an angled slot cut into the inside edge. The beveled bar rides in those slots. Two shorter crossbars are screwed to the long sides to tie the frame together. The crossbars should be spaced so that the disc just fits between them.Crossbars tie the frame together. Pinch the center bar between the frame sides and screw a crossbar to one end (left). Use the disc to position the second crossbar (right). The frame should be snug to the disc but still able to rotate without too much effort. Assemble the parts. Screw the adjustable base to the disc, then position the frame on the disc. Insert the bolt with its bearing sleeve through all the parts. It threads into a T-nut in the guide bar. The position of the adjustable base in relation to the fixed base will determine the shape of the oval. Add a mounting plate. Screw or bolt on an oval mounting plate to the top of the frame. The shape of the oval will change based on the position of the center pin, so you’ll need to make a fresh plate if you reset the offset distance. Hold a pencil over the blank while turning the jig to determine the oval shape.
To make these components work together, you’ll add a few more parts. First, the adjustable base—a rectangle of plywood that you screw to the disc. Once the disc is attached, extend the center hole in the disc through the plywood. Then rout a slot through the base and the disc running parallel to the edge of the plywood. It should start at the center hole and end within 1/4 in. of the edge of the disc.
Then there is the fixed base, a plywood rectangle with a hole for the bolt that holds the whole mechanism together. The bolt hole gets a shallow shoulder to accept the end of the bearing sleeve.
To change the shape of the oval you want to make, you’ll slide the adjustable base and the disc one direction or the other, then screw them to the fixed base to lock in the setting. The bolt also penetrates the bar beneath the baseplate; I recess a T-nut into the bar from below to receive the end of the bolt. Use some Loctite on the bolt to keep it from loosening as the jig is rotated.Mount the workpiece. After cutting the workpiece to a rough oval, use double-sided tape to secure it to the mounting plate. Measure from each side of the plate to ensure that the workpiece is centered on the jig. Get ready to sand. Add a clamp or a stop to the guide bar to limit the size of the finished oval. Use light pressure as you slowly feed the workpiece into the sander on the side of the disc rotating downward.
To make it easier to determine the shape of the oval that will result from a given setting of the jig, I bolt a mounting plate to the top of the frame. If you rotate the jig while holding a pencil over an oversize plate, you get a rough drawing of the oval. The plate can be sawn out and then sanded to get the exact shape. Use the plate to lay out and trim your workpiece, which can then be attached to the plate with either screws or double-sided tape.
As always with a disc sander, use a light touch.
From Fine woodworking #296
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