# Jimmy DiResta makes an oval mirror frame

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 didnâ€™t watch the videoâ€¦

That was how my precal teacher did it in high school, and Iâ€™m sure lots of people are going to come tromping in here with â€śeveryone knows!â€ť So Iâ€™ll just leave this here:

Always a pleasure to watch the man work.

ETA:

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He didnâ€™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! Thatâ€™s quite a bit harder than he makes it look.

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