from this doc , 18th Century mirror making, short strokes with a small amount of overhang to parabolize the mirror. Simulation: full size tool 460, stroke 60, x offset 140 but from stellafane, the parabolizing stroke is "w center" stroke which is a combination of these 2 strokes( chordal short stroke+ CoC 3/4D stroke) s 60, y 110 s345, y 0: from bbastro , if sphere is a flat surface: