[SML] question for router wizards

[Dougherty, Jim]

The two-nails-and-a-string technique has never worked well for me because
of string stretch.  I¹m with Mike Katz regarding the ellipsograph-style
jig (e-jig?).  I fake it sometimes with a stick and pins (one pin is half
the minor axis from the bit or pencil, the other is half the major axis
from the bit) and run the pins along a right angle such as the corner of a
sheet of MDF. Start with one pin at the corner, stop when the other pin
reaches the corner.  That gets you one quarter of what you need.  Make the
e-jig once, though, and you can make all of your ellipses to whatever size
you need.  All you need to know are the major and minor axes.

One thought, comparing it to the orginal idea of a fixed offset from a
given ellipse, is that only the first one may be a true ellipse with the
others slightly off from the true geometric form.  Based on a quick look
via CAD, that would be the case.

If these are to be out of MDF, I would absolutely lay them out
concentrically via pencil on the sheet, cut them out slighty larger with a
jigsaw then sand to the line or apply the jig and router.  Routing out
that much MDF, full width, is guaranteed to wear your bit due to the
abrasive material.

One last technique - draw your first ellipse (inner or outer).  Set a
compass to the offset you want.  Swing many arcs along the ellipse.
Connect the arcs with a flexible batten placed tangent to the arcs.  This
works well for offsets from irregular curves as well; you know, for that
show where you have to build a boat and need to take the thickness of the
planks into account so you can layout the frames.  I hope I get a show
like that some day, for which I volunteer to ³dispose" of the boat
afterwards...

- Jim Dougherty
ATD, Middlebury College Theatre Dept.