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 and data specifying a starting element for each of multiple different subsets of elements; processing starting elements having milling depths associated with layers below a top most layer, the processing including, for a current starting element for a current layer, identifying other starting elements that have milling depths associated with a layer above the current layer and are closer to the current starting element than an amount at least equal to a radius of a smallest available milling tool, calculating a maximum angular difference, and moving the milling depth for the element subset of the current starting element to a layer above the current layer, responsive to the maximum angular difference being greater than a threshold, to remove a non-manufacturable corner.