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 center Or is on the line connecting rotary centers of pink and brown cranks OpOb.
Or is on the right of Ob,
ObOr = OpOb.(b/a)