Methods, systems, and apparatus, including medium-encoded computer program products, for computer aided design of physical structures include: obtaining a density-based representation of a modeled object, the density-based representation comprising multiple different subsets of elements, each subset having a starting element at a milling depth, which is one of three or more milling depths for three or more corresponding discrete layers that are each perpendicular to a milling direction for the modeled object; and reassigning milling depths of at least a portion of the multiple different subsets of elements, the reassigning for a current starting element in a current layer for an element subset comprising moving the milling depth for the element subset of the current starting element when an angular difference between other starting elements, which are associated with the current starting element and are located in a different layer, meets or exceeds a threshold value.