What Is CNC Machining?

Subtractive manufacturing

Where 3D printing builds a part up layer by layer and injection molding forms it in a mold, CNC machining starts with a solid block and cuts the part out of it, removing everything that isn't the part. Because there's no tooling to make first, you can get accurate parts fast and in real production materials, which is why CNC is a mainstay for functional prototypes and low-to-medium production.

How it works

The path from design to part has three stages:

1. CAD: you model the part in 3D.
2. CAM to G-code: software converts the model into G-code, the instructions that specify tool movements, speeds, and toolpaths, along with M-codes that control functions like coolant and tool changes.
3. Machining: the machine follows the code, moving the cutting tool and the workpiece across its axes to slice away material until the part is done.

Early machinists wrote G-code by hand; today CAM software generates it from the CAD model automatically. (The name and history of "numerical control" are covered in what does CNC stand for.)

Mills, lathes, and axes

Two machine types cover most work:

• Mill: the cutting tool spins in a spindle while the workpiece moves beneath it. Mills suit boxy, prismatic, and complex shapes, and are the most common CNC machine.
• Lathe: the workpiece spins while a stationary tool cuts into it. Lathes suit round, symmetric parts, and the operation is called turning.

Machines are described by their axes. A basic mill moves in three (X, Y, Z); a five-axis machine adds two rotational axes, letting the tool reach a part from almost any angle to cut geometry a three-axis machine can't.

Materials and tolerances

CNC works in a broad range of real materials: aluminum (light and easy to machine), steel and stainless, titanium, and engineering plastics. It holds tighter tolerances than most other processes, which makes it the right pick for structural parts, threads, sealing faces, and precise fits. That accuracy, in the actual production material, is a big part of why machined parts can be trusted for functional testing.

The one geometry rule to remember

Because the cutting tool is round, every inside corner a mill cuts ends up with a radius; you can't machine a perfectly sharp internal corner with a round tool. Designing with that in mind (adding radii, avoiding deep narrow pockets) keeps parts machinable and cheaper, since simpler geometry means less machine time.

When CNC is the right process

CNC has no tooling cost but a per-part price that stays roughly flat with volume, so it shines from one to a few hundred parts, especially when you need real metals or tight tolerances. For very small quantities, 3D printing can be cheaper; above several hundred to a thousand simple plastic parts, a mold's low per-part cost wins. CNC is also the classic choice for a bridge run, a first batch in the real material before you commit to tooling. See how products are made for how CNC fits among the processes, and how to manufacture a product for the full path.