Two thumbtacks and a closed string. Calculate the foci's points
from the major and minor axis. Simple algebra.
Once you know - then any size can be done.
x^2/a^2 + y^2/b^2 = 1
where b^2 = a^2 - c^2.
where a is 1/2 long diameter and b = 1/2 short diameter
and c is 1/2 the length between pins.
I'd get a piece of MDF to use for a template. After
deciphering Morris' instructions on how to draw an oval, cut
it out close with your jigsaw. Smooth it up with a drum sander
in your drill press. Cut the real material with the jigsaw as
close as you can to the correct shape. Using double-stick
tape, adhere the template to the real material. Then use a
flush trim router bit to copy the shape of your template. If
you can get this far the rabbet is the easy part.
In my opinion if you are going to do it freehand, then skip the
template step and just free hand the blank. My whole reason for a
template is to use a trammel and\or other precise methods to form the
shape and it is easier and cleaner to hog it out from MDF. Then you
can free hand the blank down to very close to the actual finish shape
and use the template to guide a trim pass.
If I recall many years ago either ShopNotes or Woodsmith magazine had
plans for a router jig to make elliptical frames. I believe it used
1/4" threaded rod to allow adjustment to various sizes. The router
mounted directly to the jig.
I tried a Google search and came up blank. Anyone have their index(s)
(I'm pretty sure it was ShopNotes).
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