While visiting a friend a few years ago, I spotted a little wooden gift-shop toy with the catchy label. "Bullshit Grinder," on his desk. When you turned a little crank the two perpendicular dovetail slides ran back and forth endlessly. I was intrigued by the way the circular motion of the crank was transformed into the linear motion of the sliding dovetails. On closer inspection, I found the cranking motion wasn't circular at all, it was oval.
I forgot all about the "grinder" until I spotted a glorified version in the American Woodworker Design Shop. Fred Matlack, our resident mad scientist, had applied the principle to a router jig for cutting elliptical table-tops. It worked great, but you couldn't cut ellipses smaller than 24 in. wide.
Fred and I brainstormed awhile, and came up with a new twist that allows you to rout or draw ovals, in a continuous range of sizes and shapes, from Vi in. by 4}/i in. to at least 48 in. by 96 in. Using the jig is simple, as Fig. 3 shows. You can cut out ellipses, rabbet them for glass—even rout elliptical inlay grooves—as easily as routing a circle.
Just one warning, though: While this jig is easy to use, it isn't very easy to visualize how it operates. I recommend building it, then experimenting to see how it works.
Math books give long-winded explanations of the geometry of ellipses. But the easiest way to picture one is to think of it as an oval, like you'd get if you sliced obliquely through a cone. True ellipses are symmetrical about both their major and minor axes (those ccnterlines of their length and width). Their curves are fair, that is, progressing smoothly, without bumps or flat spots. And for any given rectangle, there is only one true ellipse that fits inside.
Most woodworkers know how to draw ellipses using two tacks and some string. But until now, if you wanted to actually cut out an elliptical form, you'd have to draw the ellipse, make a template, trace it out on your work-piece, handsaw the rough shape, and flush trim to your template. With this jig you can draw or rout a geomctrical-
ly correct ellipse directly. All you need to know is its length and width.
The jig consists of a lyase, a platform and a separate fixture to hold the pencil or router over the platform. (See Fig. 1 and 2.) When you rotate the platform, hardwood channels attached to its underside slide back and forth on two dovetail blocks attached to the base, producing the elliptical motion of the platform. By changing the distance between the dovetail blocks, you can change the shape of the ellipse. The size of the ellipse is determined by where you place the router or pencil.
Make the platform first as shown in Fig. 1. I made mine of Vi-in.-thick birch plywood, but an MDF (medium density fiberboard) surface would work even better as a drawing board. Cut your platform to an exact 36-in. by 48-in. rectangle, and carefully lay out perpendicular centcrlincs on one face. You'll center your channels on these lines later. Continue your centcrlincs up the edges and across the top face of the platform.
I made my channels of cherry but any strong hardwood will do. It's important for the channels to be perfectly straight, so I ripped them slightly oversize, then jointed them and ripped them to their finished dimension.
Before routing the dovetail grooves in the channels, I removed as much waste as possible by cutting a %-in.-widc by yifrin.-dccp dado up the center of each channel blank. Then I followed up with a l-in.-dia. dovetail bit in a router table. If you don't have a 1-in. bit, you can make two passes with a }/t-in. bit, referencing off the same edge to keep the width of the groove constant. The rake of the dovetail isn't critical as long as you cut the sliding dovetail blocks to a matching angle.
Next, I cut the points on the channels with my power miter saw and fine-tuned them with a stationarv disc sander. These points should be exactly 90° and centered perfectly on the dovetail grooves so the opposite channels will line up when you assemble them to the underside of the platform.
Finally, attach the channels with by 1 '/«-in. dry wall screws, making certain the dovetail grooves are exactly perpendicular and centered on the platform's centeriines. An extra chunk of dovetail slide material will help you line up the channels, but I find that the old pool-shooter's eyeball method yields the straightest alignment.
Make the base from a rectangular piece of plywood or other 3/4-in.-thick sheet stock. (See Fig. 1.) I used MCP (melamine-coated particleboard) because it's inexpensive and lias a friction-free surface for smooth operation.
The base provides points of attach-
FIG.l: ELLIPSE JIG ASSEMBLY
Draw centeriines on platform.
Cut channel points to 90° angle.
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