Dividing a Circle Into Thirds, Without a Protractor

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One of the things I'm doing in preparation for the JOTA event on the USS IOWA, is (finally!) building up my new "more portable" antenna mount for my satellite antenna.

I started with a 5' surveyor's tripod I got really cheep on eBay. I have numerous "spare" Yaesu Azimuth/Elevation rotators and control boxes, so I figured I could mount one of my spares to the tripod with an adapter plate.

I bought some good 6061 1/4" thick aluminum sheet, and proceeded to cut out a 6"x6" square. I marked it, and using a set of trammels, laid out a circle the same diameter as the bolt circle for the Azimuth motor. Then I drilled and countersunk some holes to bolt it to the motor with flat-head machine screws.

Now the task was to mount the plate to the tripod, and there was the snag I hit.

The top of the tripod is triangular, and where the legs mount to it there's a lack of space on the underside of the plate to use four mounting screws. So, I decided I'd use three screws instead, and they'd have plenty of room between where the legs meet the top of the tripod.

The problem was, how do I lay out 3 equally spaced holes on the correct 3" diameter circle I just scribed using my trammels?

I remember from way back in high-school geometry (or was it trig?) that dividing a circle into thirds was a very complex, tricky task, and you were better off to just get a protractor, and lay out your marks every 120*.

A quick Google search found this very ingenious method of equally dividing a circle into three sections, WITHOUT the use of a protractor.
I was so amazed that I thought I'd share it with my friends here, in case any of you are metal cutters like I am, or perhaps woodworkers.

Enjoy the little video. I found it quite amazing!



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