Methods, systems, and apparatus, including medium-encoded computer program products, for computer aided design and manufacture of physical structures using subtractive manufacturing systems and techniques include, in one aspect, a method including: obtaining a toolpath specification for a three dimensional model of geometry of a part to be machined from a workpiece; calculating predicted cutting forces to be applied to the workpiece when machining the part; generating a set of holding tabs for the part based on the predicted cutting forces, wherein each of the holding tabs bridge from the part to the workpiece so as to keep the part fixed, in situ, within the workpiece during the machining, and at least one position of a holding tab from the set is determined in accordance with the predicted cutting forces; and providing the set of holding tabs for use by a computer-controlled manufacturing system.