Driving gear 1 rotates about axis B of link 2 which turns about fixed axis A. Gear 1 meshes with gear 3 which rotates about axis D of link 2 and is rigidly attached to template d and to blank b. When gear 1 rotates, template d slides along fixed pin a and the cutting tool (milling cutter), mounted at point C, machines blank b whose finished shape is determined by the shape of template d.
$3339$GrC,FD$