CNC-milling is a production process in which a computer controlled robotic arm mills out e.g. a negative shape digital 3D-model in physical material by stepwise drilling off substance from a material block (typically polysterene, steel or wood) using a cutting device. Concrete is then cast into the block,

using the negative shape as a mould. For larger shapes, several blocks are assembled to form a combined mould.
The image above shows traces from the cutting process of the milling robot. At high step-over values, the cutting process accentuates the shape by stepping over at certain

intervals, This production method allows for a high degree of geometrical complexity to the design, making the realization of topological optimized shapes possible.