Grey gear of Z1 teeth is stationary. The blue frame is pivoted to the grey gear.
Pink gear of Z2 teeth has key sliding joint with the green shaft to which is fixed green gear of Z3 teeth.
Yellow vertical shaft to which is fixed yellow gear of Z4 teeth has yellow tracer.
Tooth numbers are chosen so that 1 rev. of the blue frame corresponds 2 rev. of the yellow shaft. (here: Z1 = Z2 = 20; Z3 = 32; Z4 = 16)
When the blue frame turns, the point of the yellow tracer will trace an ellipse (in green).
Semi minor axis = A - R
Semi major axis = A + R
A: center distance of the grey and yellow gears (adjusted by moving brown slider).
R: distance from the point of the yellow tracer to yellow shaft axis (adjustable).