CNC milling, or computer numerical control milling, is a machining process which employs computerized controls and rotating multi-point cutting tools to progressively remove material from the workpiece and produce a custom-designed part or product. This process is suitable for machining a wide range of materials, such as metal, plastic, glass, and wood, and producing a variety of custom-designed parts and products.
Several capabilities are offered under the umbrella of precision CNC machining services, including mechanical, chemical, electrical, and thermal processes. CNC milling is a mechanical machining process along with drilling, turning, and a variety of other machining processes, meaning that material is removed from the workpiece via mechanical means, such as the actions of the milling machine’s cutting tools.   
This article focuses on the CNC milling process, outlining the basics of the process, and the components and tooling of the CNC milling machine. Additionally, this article explores the various milling operations and provides alternatives to the CNC milling process with production fixture.
Milling Definition
What is milling? It's is a type of machining that uses cutters to shape a workpiece, often on a moveable tabletop, although some milling machines also feature movable cutters. Milling started out as a manual task performed by humans, but most milling these days is done by a CNC mill, which utilizes a computer to oversee the milling process. CNC milling offers higher precision, accuracy, and production rates, but there are still some situations when manual milling comes in useful. Manual milling, which requires a lot of technical skill and experience, offers shorter turnaround times. It also has the added benefit that manual mills are cheaper and the user doesn’t need to worry about programming the machine.
Overview of CNC Milling Process
Like most conventional mechanical CNC machining processes, the CNC milling process utilizes computerized controls to operate and manipulate machine tools which cut and shape stock material. In addition, the process follows the same basic production stages which all CNC machining processes do, including:
Designing a CAD model
Converting the CAD model into a CNC program
Setting up the CNC milling machine
Executing the milling operation
The CNC milling process begins with the creation of a 2D or 3D CAD part design. Then the completed design is exported to a CNC-compatible file format and converted by CAM software into a CNC machine program which dictates the actions of the machine and the movements of the tooling across the workpiece. Before the operator runs the CNC program, they prepare the CNC milling machine by affixing the workpiece to the machine’s work surface (i.e., worktable) or workholding device (e.g., vise), and attaching the milling tools to the machine spindle. The CNC milling process employs horizontal or vertical CNC-enabled milling machines—depending on the specifications and requirements of the milling application—and rotating multi-point (i.e., multi-toothed) cutting tools, such as mills and drills. When the machine is ready, the operator launches the program via the machine interface prompting the machine to execute the milling operation.
Once the CNC milling process is initiated, the machine begins rotating the cutting tool at speeds reaching up to thousands of RPM. Depending on the type of milling machine employed and the requirements of the milling application, as the tool cuts into the workpiece, the machine will perform one of the following actions to produce the necessary cuts on the workpiece:
Slowly feed the workpiece into the stationary, rotating tool
Move the tool across the stationary workpiece
Move both the tool and workpiece in relation to each other
As opposed to manual milling processes, in CNC milling part, typically the machine feeds moveable workpieces with the rotation of the cutting tool rather than against it. Milling operations which abide by this convention are known as climb milling processes, while contrary operations are known as conventional milling processes.
Generally, milling is best suited as a secondary or finishing process for an already machined workpiece, providing definition to or producing the part’s features, such as holes, slots, and threads. However, the process is also used to shape a stock piece of material from start to finish. In both cases, the milling process gradually removes material to form the desired shape and form of the part. First, the tool cuts small pieces—i.e., chips—off the workpiece to form the approximate shape and form. Then, the workpiece undergoes the milling process at much higher accuracy and with greater precision to finish the part with its exact features and specifications. Typically, a completed part requires several machining passes to achieve the desired precision and tolerances. For more geometrically complex parts, multiple machine setups may be required to complete the fabrication process.
Once the milling operation is completed, and the part is produced to the custom-designed specifications, the milled part passes to the finishing and post-processing stages of production.
CNC Milling Machine Operations
CNC milling is a machining process suitable for producing high accuracy, high tolerance parts in prototype, one-off, and small to medium production runs. While parts are typically produced with tolerances ranging between +/- 0.001 in. to +/- 0.005 in., some milling machines can achieve tolerances of up to and greater than +/- 0.0005 in. The versatility of the milling process allows it to be used in a wide range of industries and for a variety of part features and designs, including slots, chamfers, threads, and pockets. The most common CNC milling operations include:
Face milling
Plain milling
Angular milling
Form milling
Face Milling
Face milling refers to milling operations in which the cutting tool’s axis of rotation is perpendicular to the surface of the workpiece. The process employs face milling cutters which have teeth both on the periphery and tool face, with the peripheral teeth primarily being used for cutting and the face teeth being used for finishing applications. Generally, face milling is used to create flat surfaces and contours on the finished piece and is capable of producing higher quality finishes than other milling processes. Both vertical and horizontal milling machines support this process.
Types of face milling include end milling and side milling, which use end milling cutters and side milling cutters, respectively.
Plain Milling
Plain milling, also known as surface or slab milling, refers to milling operations in which the cutting tool’s axis of rotation is parallel to the surface of the workpiece. The process employs plain milling cutters which have teeth on the periphery that perform the cutting operation. Depending on the specifications of the milling application, such as the depth of the cut and the size of the workpiece, both narrow and wide cutters are used. Narrow cutters allow for deeper cuts, while wider cutters are used for cutting larger surface areas. If a plain milling application requires the removal of a large amount of material from the workpiece, the operator first employs a coarse-toothed cutter, slow cutting speeds, and fast feed rates to produce the custom-designed part’s approximate geometry. Then, the operator introduces a finer toothed cutter, faster cutting speeds, and slower feed rates to produce the details of the finished part.
Angular Milling
Angular milling, also known as angle milling, refers to milling operations in which the cutting tool’s axis of rotation is at an angle relative to the surface of the workpiece. The process employs single-angle milling cutters—angled based on the particular design being machined—to produce angular features, such as chamfers, serrations, and grooves. One common application of angular milling is the production of dovetails, which employs 45°, 50°, 55°, or 60° dovetail cutters based on the design of the dovetail.
Form Milling
Form milling refers to milling operations involving irregular surfaces, contours, and outlines, such as parts with curved and flat surfaces, or completely curved surfaces. The process employs formed milling cutters or fly cutters specialized for the particular application, such as convex, concave, and corner rounding cutters. Some of the common applications of form milling include producing hemispherical and semi-circular cavities, beads, and contours, as well as intricate designs and complex parts with a single machine setup.
Other Milling Machine Operations
Besides the aforementioned operations, milling machines can be used to accomplish other specialized milling and machining operations. Examples of the other types of milling machine operations available include:
Straddle milling: Straddle milling refers to milling operations in which the machine tool machines two or more parallel workpiece surfaces with a single cut. This process employs two cutters on the same machine arbor, arranged such that the cutters are at either side of the workpiece and can mill both sides at the same time.  
Gang milling: What is gang milling? Gang milling refers to milling operations which employ two or more cutters—typically of varying size, shape, or width—on the same machine arbor. Each cutter can perform the same cutting operation, or a different one, simultaneously, which produces more intricate designs and complex parts in shorter production times.
Profile milling: Profile milling refers to milling operations in which the machine tool creates a cut path along a vertical or angled surface on the workpiece. This process employs profile milling equipment and cutting tools which can be either parallel or perpendicular to the workpiece’s surface.
Gear cutting: Gear cutting is a milling operation which employs involute gear cutters to produce gear teeth. These cutters, a type of formed milling cutters, are available in various shapes and pitch sizes depending on the number of teeth necessary for the particular gear design. A specialized lathe cutter bit can also be employed by this process to produce gear teeth.
Other machining processes: Since milling machines support the use of other machine tools besides milling tools, they can be used for machining processes other than milling, such as drilling, boring, reaming, and tapping.
CNC Milling Equipment and Components
The CNC milling process employs a variety of software applications, machine tools, and milling machinery depending on the milling operation being performed.
CNC Mill Support Software
Like most CNC machining processes, the CNC milling process uses CAD software to produce the initial part design and CAM software to generate the CNC program which provides the machining instructions to produce the part. The CNC program is then loaded to the CNC machine of choice to initiate and execute the milling process.
CNC Milling Machine Components
Despite the wide range of milling machines available, most machines largely share the same basic components. These shared machine parts include the:
Machine interface
Column
Knee
Saddle
Worktable
Spindle
Arbor
Ram
Machine tool
 
There are also other machining parts, such as automation and semiconductor part, medical part, etc.