Abstract In this work, we develop an approach for the level set based topology optimization of millable 3D structures. We focus on the 3-axis machining with realistic formed milling tools. The basis of the method lies in the identification of surface areas that cannot be reached by a given milling tool during optimization. For this purpose, we present an interpolation method that identifies these areas by an interpolation of the level set function along the outer contours of realistic milling tools, considering available machining directions. To minimize inaccessible surfaces, we define a potential field whose values decrease linearly into the outer normal direction of the structure. The inaccessible boundaries are pushed outward by minimizing their respective potential and therefore become accessible. Manufacturability is integrated into the optimization problem as an explicit constraint.