The example I've posted previously seems to be taken from page 215 of the book "The Complete Technical Illustrator" by Duff/Maxson and I agree there's some personal choice by the illustrator involved about what lines get thick or thin.

Actually the way I would do it and also the way that seems to be most logical is the attached result by Cinema4Ds Sketch & Toon renderer. This is also similar to mattj example from post 6201.28 but without the "overshoot" which again is a personal style.