What is CNC machining?

CNC machining is the most widely used subtractive manufacturing technology. In CNC, material is removed from a solid block using a variety of cutting tools to produce a part based on a CAD model. Both metals and plastics can be machined with CNC.

CNC produces parts with tight tolerances and excellent material properties. CNC is suitable for both one-off jobs and low-to-medium volume production (up to 1000 parts), due to its high repeatability. When compared to 3D printing though, CNC has more design restriction, due to the subtractive nature of the technology.

In this article, the differences between the two main CNC machine setups (milling and turning) are first explained, and then, the characteristics of the process are discussed. After reading this article, you will have an overview of the basic principles of the technology and how these relate to its key benefits and limitations.


How does CNC machining work?

There are two main types of CNC machining systems: milling and turning. Each is better suited for manufacturing different geometries, due to its unique characteristics.

Let’s break down how parts are manufactured using these two machine setups…

How does CNC milling work?

Schematic of a typical CNC milling machine

CNC milling is the most popular CNC machine architecture. In fact, the term CNC milling is often synonymous with the term CNC machining.

In CNC milling, the part is mounted onto the bed and material is removed using rotational cutting tools. Here is an overview of the basic CNC milling process:

  1. First, the CAD model is converted into a series of commands that can be interpreted by the CNC machine (G-code). This is usually done on the machine by its operator, using the provided technical drawings.
  2. A block of material (called the blank or the workpiece) is then cut to size and it is placed on the built platform, using either a vice or by directly mounting it onto the bed. Precise positioning and alignment is key for manufacturing accurate parts and special metrology tools (touch probes) are often used for this purpose.
  3. Next, material is removed from the block using specialized cutting tools that rotate at very high speeds (thousands of RPM). Several passes are often required to create the designed part. First, an approximate geometry is given to the block, by removing material quickly at a lower accuracy. Then one or more finishing passes are used to produce the final part.
  4. If the model has features that cannot be reached by the cutting tool in a single setup (for example, if it has a slot on it back side), then the part needs to be flipped and the above steps are repeated.