A Simple Mechanical Calculator for Sine

[Ian Curtis]

I always got a huge kick out of watching this video that explains how the massive mechanical calculators they used to solve firing problems worked. It's 40 minutes long, but I've yet to meet an ME who didn't finish the whole thing once they started.

Some of the mechanisms are so cool, I made a really simple one as a Christmas gift. Essentially you move the pin to the angle you want to find the sine of, and by reading the location of the pin you can find the sine of that angle. By picking a location on the scale and then reading the angle you can find inverse sines too. And if you remember your trig identities (probably should've inscribed these on the face, they are hard to remember!) you can really do a whole lot more. I figured there are enough engineers here someone would be bound to get a kick out of it.

Video, in case my explanation doesn't make sense.

[Al Skierkiewicz]

Very cool!

[Ninja_Bait]

Absolutely brilliant.

[Ether]

Equation of track: radius(θ) = a+1 + sin(θ)

"a" is the distance from the pivot to the "dimple" in the curve.

Attached figure "eqn.png" shows track curve for a=0.2

If you make "a" larger, say a=1, you can get a track with virtually no dimple (it may slide a bit smoother). See figure "no dimple.png"