What Is CNC Milling? A Basic Guide
- Written by: Gavin Leo
- Updated By: CoCo
- Published:
- Updated
CNC milling has gained significant dominance in different industries due to its versatility, consistency, and high precision. I will help explain these attributes in detail through a description of how it operates and the different CNC milling machines that exist.
What Is CNC Milling
CNC milling is a subtractive manufacturing process, utilizing computer-guided multi-edge cutting tools attached to a high-speed rotating spindle to accurately shape workpieces.
Initiated from digital CAD designs translated into machine-readable G-code instructions, the system executes automated precision machining operations to convert stock materials into completed components.
This process is mainly used for the rapid prototyping of plastic and metal parts as well as the efficient and large-scale manufacturing of components.
CNC milling is a part of the broader CNC machining services, which also include other aspects such as CNC drilling, CNC turning, and CNC routing.
How Does CNC Milling Work?
Four stages of CNC milling:
Design (CAD): CNC milling starts with the development of a 3D computer-aided design (CAD) file that contains information on how the final output part should look.
Programming (CAM): The design is then transformed into a machine-readable format using a computer-aided manufacturing software (CAM software). The machine program is typically in G-code format, which helps outline the movements that the machine should make during its operation, taking into account high accuracy and precision.
Setup: The next process entails machine setup, where the operator places the material on a worktable and secures it in place.
Machining and Finishning: The CNC machine will then execute the G-code instructions, ensuring the cylindrical cutting tool (milling cutter) removes a desired product based on the initial CAD design. Depending on the milling machine used, we can achieve different angles and axes for the final product. Lastly, the finished product will undergo deburring and polishing to ensure a smooth surface finish.
What Is a CNC Milling Machine? Key Components, Types
A CNC milling machine is a computer-controlled milling machine used during the CNC milling process. The milling machine consists of various parts and components. It also comes in different types and variations. This section will describe the key components and types as follows:
Components of a Milling Machine
Though the components of a milling machine vary based on machine type, the key ones include the following:
Column: The primary support structure of the machine, which helps to reinforce the other components.
Knee: This is an adjustable part fixed to the column. Its role is to support the saddle and the worktable. It also facilitates movement across the Z-axis.
Saddle: Situated over the knee and below the worktable, the saddle helps move the workpiece horizontally. It can also move parallel to the spindle axis.
Worktable: This area holds the workpiece during the milling process. It’s adjustable based on the machine type, hence it can be used both vertically and horizontally.
Spindle: This refers to the rotating part that holds the machine tool or the arbour, normally driven by an electric motor.
Arbour: This is a shaft along which the cutting tool moves in horizontal milling machines.
Ram: This component connects the spindle and supports vertical movement within the Z-axis. It’s mostly common in vertical milling machines.
Machine tool: This is the essential component performing milling operations. It’s kept in place by the spindle and ensures the efficient removal of material from the workpiece. Machine tools exist in different forms and types.
Types of CNC Milling Machines
We can classify CNC milling machines into different types based on their size, build, or cutting orientation. However, the most common classification is that which entails the number of axes. Based on that, we can have 3-axis, 4-axis, and 5-axis milling machines. Below is a description for each of them:
3-Axis Milling Machines
For 3-axis milling machines, the cutting tool moves along the X, Y, and Z linear axes. Specifically, “X” represents a side-to-side movement, “Y” represents a front-to-back movement, and “Z” represents an up-and-down movement.
3-axis milling machines are mostly used for simple parts that don’t need intricate details. They are used to create basic contours, slots, or holes.
4-Axis Milling Machines
In addition to the X, Y, and Z linear axes, the 4-axis milling machine has an extra rotational axis, known as the A-axis. The extra axis helps rotate the workpiece while cutting around a cylinder or along part sides, for example, holes or grooves.
The advantage of a 4-axis CNC milling machine is that it lets you work on multiple sides without the urgency to reconfigure any part, thus boosting efficiency.
5-Axis Milling Machines
The 5-axis milling machine combines the X, Y, and Z linear axes, a rotational axis, plus an additional tilt axis. This complex 5-axis setup makes producing advanced geometries like gas equipment parts, aerospace components, and medical devices easy.
The main advantage of 5-axis milling machines is that they support continuous and indexed machining of intricate geometries, minimizing the need for multiple setups, thus enhancing accuracy.
What Are the Differences Between 3-axis, 4-axis, and 5-axis Milling
The key difference between 3-axis, 4-axis, and 5-axis milling is the number of directions moved by the cutting tool and the workpiece relative to each other to help develop complex part geometries. A 3-axis machine moves on the X, Y, and Z planes; the 4-axis adds an extra rotational axis to the three, while the 5-axis machine adds two rotational axes to help develop more intricate and high-precision components.
CNC Milling Techniques/Operations (Table)
The most common CNC milling operations include:
Plain milling.
Angular milling.
Face milling.
Form milling.
I will discuss each of them as follows.
1. Plain Milling
This CNC milling technique removes material from the surface of a workpiece parallel to the cutter axis, with the aim of producing broad but flat surfaces with uniform thickness. The common tool used for this process is a cylindrical milling cutter. The operation is commonly used to develop square or rectangular surfaces on large metal plates.
2. Angular Milling
This form of CNC milling entails cutting surfaces at an angle (other than 90 degrees) to the cutting axis. They aim to create angled features like grooves, chamfers, or dovetails. The tools commonly used in this process are the angular milling cutter or the single-angle cutter.
One application area for this milling technique is machining components that need angled edges or precision joints.
3. Face Milling
One of the most popular CNC milling techniques, face milling entails cutting a flat surface perpendicular to the milling cutter’s axis. The process is mainly used to create flat and smooth surfaces, but it can also be used to finish the face of a part.
The common tool used for this process is a face mill cutter containing multiple cutting edges. Face milling is commonly applied in metal/plastic surface finishing before secondary machining occurs.
4. Form Milling
This milling operation is mainly used for machining contours, curved surfaces, or irregular shapes. It entails the use of cutters with specially designed profiles that help in the creation of complex 3D profiles. Common application areas of this CNC milling process include the design of gears, turbine blades, and decorative material.
Table: CNC Milling Techniques Comparison
| Milling Technique | Main Purpose | Tool Used | Common Applications |
| Plain milling | Flat surfaces parallel to the cutter axis | Cylindrical cutter | Steel plates |
| Angular Milling | Angled cuts | Angle cutter | V-grooves, chamfersr |
| Face Milling | Flat surface finishing | Face mill | Surface finishing |
| Form Milling | Complex shapes | Form cutter | Gears, molds |
Advantages and Disadvantagesof CNC Milling
Computer numerical control milling has become a widely used machining process in modern manufacturing. It comes with the following benefits and limitations:
Advantages of CNC Milling
Efficient and automated operation since they require minimal operator involvement.
High precision and accuracy due to the use of computer-guided commands.
Can produce complex geometry, such as intricate shapes, angles, or 3D contours.
Can work on a variety of materials, including metals, plastics, and wood.
Supports consistency and repeatability, allowing for the reproduction of identical parts.
Disadvantages of CNC Milling
Requires a high initial investment.
Comes with high maintenance costs.
Has high energy consumption.
Requires skilled programming and setup.
It may lead to material waste since it’s a subtractive process.
How Much Does CNC Milling Cost?
CNC milling costs vary depending on machine type and axis count, machine time, material choice, part complexity, and production volume. For example, running a simple 3-axis milling machine is less expensive than a high-end 5-axis one.
On the other hand, easy-to-machine materials attract lower costs than harder materials. Also, it may cost you more to just manufacture a single item than to batch-produce it. Therefore, there’s no one-size-fits-all cost for performing a CNC milling task.
Click here for an elaborate CNC milling quote from our dedicated team at Aria Manufacturing. You can also learn more about our CNC machining services here.