Everything You Need to Know About CNC Milling

CNC (computer numerical control) milling is a metal-cutting method that uses automated machine control to produce very precisely shaped parts at a high rate of production. The CNC milling process begins with the creation of a CAD (computer-aided design) model. This model is converted into instructions that the CNC machine can follow via CAM (computer-aided machining) software. The CNC machine then executes the CNC program to complete the part. However, the CNC milling process is much more complex than this short description implies,  from the technicalities associated with CAD and CAM software to the tools, cutters, and equipment used for CNC operation, to a machine’s cutting parameters. 

CNC milling is used in several industries, including: automotive, aerospace, agriculture, construction, medical, and dentistry. This piece will discuss the CNC milling process, including its history, how it works, and the steps, methods, and equipment used.

What is CNC Milling?

CNC milling is the typical machining process known as milling performed under the control of a computer. Milling is the cutting of a material such as wood, metal, or plastic using a rotating tool. The controls in CNC milling dictate everything from the sequence of tools used and the toolpath of each cutter, to the spindle RPM and feed rate. There are several different types of CNC milling machines that allow varying levels of complexity in the cutting process. The simplest machines have three axes of motion. More complex milling machines can have five or more axes of motion control. These machines are used for making more complex parts. For more information, see our guide on the Types of Machining Processes.

Figure 1 shows a CNC milling machine:

A wide range of tools is available to support variations in the milling process, from standard end mills and drill bits to special face milling and profile milling tools. With so many different types of CNC mills and cutters, almost any shape or design can be produced on a CNC milling machine.

Who Invented CNC Milling?

The first commercially available CNC milling machine was developed in 1952 by Richard Kegg, in collaboration with J. F. Reintjes and his team at MIT (Massachusetts Institute of Technology). Since then, numerous advancements in CNC milling have been developed including producing more complex CNC milling machines and automation.

How Does CNC Milling Work?

CNC milling works by using a set of rotating cutting tools to create parts from blocks of material in one cycle. Material is removed from a workpiece when the cutting edge of the tool contacts the workpiece as it rotates. During a milling cut, a workpiece is held stationary while the rotating cutter removes material. The exact tool paths, cut depth, XYZ and axis travel, and spindle RPM, are all predetermined by the CNC control program.

What Are the Steps of the CNC Milling Process?

To get from the CAD model to the physical part, several tasks must be completed. The steps of the CNC milling process are described below.

1. Making a CAD Model

The first step of the CNC milling process is to create a 3D CAD model for the part to be fabricated. This CAD model will include information pertaining to the size, geometry, and shape of the part. It’s important for designers to develop CAD models that are capable of actually being cut by their CNC milling machines. Parts should be designed with no (or minimal) undercuts. The size of the undercut that can be machined depends on the capabilities of the specific CNC machine. Undercuts are recessed or sunken features in a part that cannot be cut with standard tools. While undercuts can be cut by CNC mills, they typically require specialized tools, multi-axis milling machines, or both, leading to higher tooling and manufacturing costs.

2. Converting CAD to CNC

Once a suitable 3D CAD model has been developed for a particular part, a CNC program must be created to control its fabrication on the CNC milling machine. Using CAM software, the designer can create program instructions for the CNC machine, to direct the movement of the tools and cutters during manufacturing. These programs are often written in G-code or M-code. The portions of the program written in G-code focus on the operating parameters of the tools, such as spindle speed, the direction of movement, and cut depth. M-code instructions focus on miscellaneous tasks such as tool changes, powering on and off the machine, and other auxiliary functions. CAM software will often include a simulator that allows users to verify whether their CNC programs can successfully fabricate the desired part.

3. Installing CNC Milling Machine

Once the CAD model and the CNC program are ready, the CNC milling machine is prepared to fabricate the designed part. A machine operator will import the CNC program into the milling machine and insert a pre-prepared blank with the proper pre-machining dimensions into the workholding device of the machine. The specific tools, spindles, vices, and fixtures, are also set up in the machine. 

4. Executing the Program

With the CAD model developed, the CNC program created, and the CNC milling machine prepared, the machining program can be executed. When a CNC program is running, human intervention is seldom needed. The CNC milling machine will follow the program line by line,  completing all of the specified machining operations on the workpiece. Once the full program has been executed, the part can move on to any subsequent planned manufacturing steps.

What Are the Various CNC Milling Methods?

There are several different forms of milling that can be used for the fabrication of parts. Plain milling, or the act of cutting flat surfaces parallel to the rotating axis of the cutting tool, was the first type of milling developed. The introduction of CNC machining has led to several other types of milling, including:

1. Face Milling: Cutting material to create a surface perpendicular to the rotating axis of the cutter.
2. Angular Milling: Removing material from a flat surface of the workpiece at an angle.
3. Form Milling: Cutting material to make irregular surfaces, like curves.
4. Gang Milling: Removing material with two or more cutters to increase the production rate.

What is the Equipment Used in the CNC Milling Process?

Several pieces of equipment must be used in tandem to have a successful CNC milling operation. The equipment that is commonly needed for CNC milling is listed below:

1. Worktable: The worktable is used to hold the workpiece in place during machining.
2. Saddle: The saddle is located under the work table. It helps provide additional support and guides the movement of the worktable parallel to the rotating axis of the tool. 
3. Knee: The knee is located under the saddle and helps provide support to both the saddle and worktable. Its position can be adjusted vertically to accommodate different part thicknesses.
4. Spindle: The spindle is used to hold the cutting tool and directs the translational and rotational movement of the tool.
5. Arbor: The arbor is a shaft that is assembled through the spindle. Tools are fixed to the arbor.
6. Ram: The ram is an optional piece of equipment and is usually used in vertical milling or angular milling machines. It is used to help support the spindle.
7. Machine Tools: Cutting tools such as end mills and other tools are needed for CNC milling.
8. Interface: The interface is the point at which the operator can communicate with the computer controlling the CNC machine. This will usually consist of a keyboard and display screen, at a minimum.

What Are the Applications of CNC Milling?

CNC milling is used in several different applications across various industries for the fabrication of parts, including: the automotive, aerospace, agriculture, construction, electronics, and consumer products industries. For example, in the aerospace industry, CNC milling is used to fabricate aircraft engine components, fuel tank panels, and landing gear components. In the medical industry, CNC milling is often used to produce medical devices such as scalpels and implants.  

CNC milling can be used to create parts from a wide array of materials, including metals, plastics and elastomers, ceramics, and composites. Because of this, CNC milling is a highly versatile manufacturing process that can be used to fabricate seemingly any part — with limitations.

What Are the Benefits of CNC Milling?

CNC milling has several benefits. For one, CNC machining offers an unprecedented degree of precision for machined parts. This allows tight-tolerance parts to be made both easily and efficiently. Because CNC operations are computer-controlled, little to no human intervention is required. This leads to not just unmatched high production rates, but also the consistent quality and reduced labor costs.  

What Are the Limitations of CNC Milling?

Perhaps the biggest limitation of CNC milling is its cost. Even the most basic CNC machines capable of mass production are not cheap, and the machines only get more expensive as they become more advanced. Another limitation of CNC milling is part size and geometry. Part size is limited by the size of the CNC machine. Geometry is limited by the ability of the tool to physically make the desired cut. For example, undercuts, or features that are recessed beneath the outer surface of a part, cannot be cut except with special tools or expensive multi-axis machines. CNC milling is not immune from human error, either. Operators can execute incorrect programs or set up tools incorrectly which leads to bad parts and scrap.

How Much Does CNC Milling Cost?

CNC milling cost varies depending on the size of the machine. Small horizontal or vertical milling machines can cost as little as $10,000. As machines become more complex, such as with a 5-axis machining center, costs can go as high as $300,000. 

What Are the Available CNC Milling Services?

Xometry offers custom CNC milling for a wide range of materials and industries. With 3-axis, 4-axis, and 5-axis milling machines, Xometry can complete rapid prototyping, tooling, and end-use production for your product. 

Summary

This article presented CNC milling, explained what it is, and discussed how it works and the various methods. To learn more about CNC milling, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including CNC machining and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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