Did he put a couple of push-pins or nails into the wood, then use a length string and a pencil to make an ellipse, then shorten the string length to make a smaller ellipse, then rout at the drawn marks to cut the frame?
Obviously I didn't watch the video...
That was how my precal teacher did it in high school, and I'm sure lots of people are going to come tromping in here with "everyone knows!" So I'll just leave this here:
Always a pleasure to watch the man work.
He didn't. Probably intentionally since your method would result in a frame that was thicker at some points than at others. The method he does use to draw the second ellipse is entirely non-scientific, but it does result in a (close approximation of a) consistent width.
It looks like he spray paints something on the back right around 6:06.
Obviously Mark Frauenfelder skipped some high school analytic geometry classes. Two ways to make an ellipse. One is the string and 2 nails method as illustrated here. The other is the two perpendicular tracks with sliders joined by a rod of fixed length (known as the distance c and representing the distance from the center to each focus.) Semi major axis squared plus semi minor axis squared is equal to c squared (a^2 +b^2 =c^2).
Nice job on the glass cut! That's quite a bit harder than he makes it look.
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