Mechanism for drawing circle 7

Blue, yellow and green bars create a pantograph. Length of blue bars: a b. Here a is larger than b. Length of green bar: b Length of yellow bar: a Length of pink crank: e Length of brown crank: f Pink and brown eccentrics, orange conrod create a parallelogram mechanism to ensure that pink and brown cranks are parallel and rotate in the same direction and with the same speed. A gear drive of transmission ratio of 1 can be used instead. The red pen traces a circle. Its radius R = e (e f).(b/a) Its cen