One of the most popular low-volume manufacturing processes, computerised numerical control (CNC) machining has been around since the early 1950s. Starting with a large block of raw material, a tool mills along a computerised path to create a part. In a variety of plastic and metal materials, these programmed toolpaths can cut incredibly precise and repeatable geometries.

CNC machining is traditionally used to produce low-volume, end-use parts. However, it has also been adopted as a secondary process for additive manufacturing technology. Companies often 3D print plastic or metal parts and then CNC machine them for the following reasons

• Dimensional accuracy – Repeatable, tight specifications must be met in industries with high functional and tolerance requirements, such as automotive, medical and consumer goods. Most additive manufacturing technologies, such as FDM, can achieve up to ±0.005in, but that’s not enough for some critical part features. That’s a big difference when making an assembly tool, production jig or any durable process tool.
• Speed – The second reason companies combine the two technologies is speed. Including printing time, CAD/CAM setup and machining, the process is still much faster than designing and making injection mould tools. And 3D printing and machining gives engineers more flexibility in the schedule to make design improvements. Simply update CAD/CAM files, print and machine a new part, whereas making changes to an injection mould is almost impossible and expensive, causing major production delays.

The most commonly used machining technique for additive manufactured plastic parts is 3-axis milling, either horizontal or vertical, due to its ability to move in the X, Y and Z planes. For more complex features and geometries, 5-axis milling is recommended. 5-axis milling can rotate and tilt on the A and B axes to reach undercuts and small features.