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The
completed jig |
Woodworkers
have used versions of this ellipse-cutting jig for generations. The one I built went through several modifications, with
a few mishaps along the way.
The distance from A to the center of the router bit is equal
to 1/2 of the minor axis of the ellipse. The distance from B
to bit center is equal to 1/2 of the major axis. The distance
between A and B determines the differential, or ratio, between
the axes. The size of this base (14 sq.ft.) limits the adjustment
range from 3-1/2" to 6". The adjustment range is logically
1/2 the finished oval. In plain English, the closer A is to
B, the fatter the ellipse and vice versa. Finally, the distance
the router is from the center of the base determines the overall
size of the ellipse.
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| Dovetail
slots and keys move and control the trammel's two axes
and rotation. |
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Dovetail
slots and keys control the two axes of rotation. |
For
the base, I started by laminating a 15" square of 3/4"
MDF and then trimmed it to 14" square. Using a sharp knife,
I inscribed two perpendicular lines on the back and over the
edges. These lines must be accurate, as they are used for alignment
when employing the jig. I cut two dadoes at right angles and
1/2" deep. This makes it easier when cutting the dovetail
slots, which is done using a 3/4", 1 in 7 slope dovetail
bit set to 9/16" depth using a router table. I overlapped
the cuts, rotating the stock in order to center them. I cut
the square board round and then rounded over the top edge. Using
a table saw, I made two dovetail keys, each 2-1/2" long.
It's necessary to fiddle with this process and make several,
as the tightness of fit determines the overall accuracy. In
the center of each key, drill a pilot hole to take a 10-24 threaded
insert, which is installed from the back of the key to prevent
it from being pulled out during use. |
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