[SML] question for router wizards

[Dale Farmer]

On 12/17/2016 4:40 PM, Bill Conner via Stagecraft wrote:
> 4 different ellipses - the axis changes every 90 degrees -  so the
> ellipsograph seems tricky for a more or less two time use.  It is like
> guide collars - only probably 2 1/2 to 3 inches apart.  I'll have the
> "corners" of the mdf I plan to use the template, grid and jig and belt
> sander, and thought I could make bases out of that.  Rough cu,. attach
> to router, plunge, and then use router with bit locked down as the
> pivot to turn base against on a band saw or another router.  Ought to
> be pretty round and centered.
>
If I'm recalling my woodworking class from 40 or so years ago, you can 
easily draw an ellipse on the wood by putting a nail at each focus, then 
using a loop of string to constrain your pencil, just draw the thing.  I 
don't have a formula to calculate the length of the string, as the class 
was back in 7th grade woodshop, and we were making plaques for something.
   Could scale it up to a loop of rope to constrain the router somehow? 
Or just cut it out with a jigsaw then sand it down to the desired 
dimensions?

--Dale