Mechanical location of the Fermat Toricelli Point

Given a triangle, the Fermat Toricelli point is the intersection of lines joining the triangle's vertices with equilateral triangles drawn on the opposite sides.
If all angles of the triangle are less than 120 degrees, this mechanical device can find the point.
Why?

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With thanks to Mark Levi for "The Mathematical Mechanic" and Max Levi for recommending it to me.

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