For the top of this art Nouveau style coffee table top which is 60” wide by 80” long, needing to cut a perfect ellipse onto 1” thick 17 ply apple plywood. To make this cutting jig I used 3/4” and 1” thick apple plywood which is very expensive and very durable and hardly ever warps.
A. Starting with the dovetail slotted base that screws to the bottom side of the top when cutting. The base is 1” thick 22” square, then route in two 1” wide 5/8” deep dovetail slots centered to each side.
B. Then using the bandsaw I cut off the corners of the base jig, I do this for making smaller elliptical circles. Next sand the edges straight and ease the sharp edges.
C. Next cut out the dovetail runners by cutting a long piece of hardwood at 1” wide x 5/8” thick and 24” long.
You will only need the runners to be 4” long, it’s best to have a longer piece for running through the router table. Set the router fence to where 14 degree does not remove any material at the 1” width bottom.
D. Using the drill press I dill in center holes for the insert nut, then using a insert nut driver to screw into the hardwood.
E. For the router base top cutting jig, use a 3/4” thick x 5 3/4” wide x 48” long of 13 ply apple plywood. Place the Porter Cable router base on top and trace out the radius at one end, and a 2 1/2” diameter at the smaller end. Then draw connecting lines. I use the bandsaw to cut out the jig and sand the edges flat and ease the edges.
F. Next cut out two pieces of 1” thick plywood to match the thickness of the base, and screw to the bottom face of the top routing jig. For the outer one match up the radius to the base this will help you see exactly where you are going when routing.
G. For the elliptical circle at 60” x 80” divided by 2 will give you your placement for drilling the holes into the base. 30” for your minor axis and 40” for your major axis, this will give you a 10” pitch which will just fit the base jig by 1” if you take the 22” base and divided it by 2 will give you a 11” pitch clearance.
H. The last step before routing out the top is to drill a small 1/2” diameter hole into the center to the jig. This will give you a place to see your center mark on the top. Then just square it up to make sure the cut will work then screw it down to the bottom face of the top.
I. When routing the top turn on the router in the base and set your cutting depth for about 1/4” making around 4 to 5 passes.
With this Elliptical circle cutting jig the maximum size would be approximately 88” 7’-4” at the longest point.
-- Dennis Zongker