Whether referring drilling, milling, or turning, almost any material can be machined into a part. In CNC machining, a piece of material is clamped into the machine, and a numerically controlled tool removes material until the part is completed. Similar to injection molding, material selection is very broad and can be chosen based on the desired application. Tolerances of machining are very high (25µm) meaning highly accurate parts can be machined. However, it can be very difficult (if not impossible) to manufacture undercuts or internal features with machining. As material is selectively subtracted, CNC milling is often used for one-off units and functional prototyping, as well as for engine and machine components.