CNC milling, a type of CNC machining, is commonly used in the manufacturing industry due to its high cutting efficiency and accuracy with multi-point milling cutter tools.
While most people are more familiar with 3D printing for creating precise physical objects, CNC milling has been around much longer and remains widely used today. CNC milling machines were first commercially used in 1952. Since then, numerous advancements have been made, including the development of more complex machines and automation.
In this article, we'll take an in-depth look at CNC milling—how milling machines work, what products they can produce, and whether milling is the right choice for your parts.
What Is CNC Milling?
CNC milling is a manufacturing process where a cutting tool, mounted on a rotating spindle, selectively removes material from a workpiece. This operation is automated by Computer Numerical Control (CNC) systems. Unlike CNC turning—where the workpiece rotates against a stationary cutting tool—in CNC milling, the workpiece is rigidly mounted to a stationary table. Yet, modern milling machines allow this table to move linearly or rotate on different planes, enabling the tool to work at various angles.
Advanced CNC milling machines, like 5-axis or multi-axis CNC mills, offer enhanced capabilities through tool tilting and workpiece rotation, enabling intricate shapes to be created more precisely and efficiently without moving the workpiece to another machine.
How Do CNC Milling Machines Work?
A thorough overview of the milling process will help you get the complete picture of how it works. Generally, the process can be broken down into 3 steps as below.
Step 1: Create a CAD Model
When provided with 2D drawings or initial design concepts, engineers use CAD software such as Autodesk Inventor or SolidWorks to create a detailed 3D model of the part. This CAD model includes all essential details, such as dimensions, critical features, tolerances, construction lines, and thread indications. CAD software allows designers to visualize, analyze, and refine their designs efficiently before the manufacturing process begins.
Step 2: Convert CAD Model to CNC Compatible Format
CNC milling machines cannot directly interpret CAD models, so it is necessary to convert these models into a format compatible with the machines. This conversion is done by CAM software, which translates the CAD model into machine-readable instructions.
By importing the CAD model into a CAM system, a CNC program is generated. This program is often written in G-code, which primarily focuses on tool operation parameters such as spindle speed, movement direction, and cutting depth, and M-code, which deals with various tasks like tool changes, turning the machine on and off, and other auxiliary functions.
Many CAM software packages include a simulation feature that allows designers or engineers to verify their CNC programs and ensure they work as intended. This helps identify potential issues before actual manufacturing begins, ultimately saving both time and resources.
Step 3: Setup the Milling Machine and Perform the Milling Operation
The operator attaches the cutting tools to the spindle and securely positions the workpiece on the worktable. They will then import the CNC program into the milling machine and begin milling. Modern milling machines are highly automated and often equipped with automatic tool changers (ATC), which allow the machine to switch tools during the operation without manual intervention. This minimizes interruptions and reduces downtime.
Depending on the desired outcome, the process may require multiple passes. Initial operations typically use larger cutting tools to remove material quickly and achieve a rough shape. Subsequent finishing cycles employ finer tools and slower speeds to refine the surface, bringing the part to its final dimensions, surface quality, and required tolerances.
Types of CNC Milling Operations
CNC milling is a versatile process that can perform a variety of operations to create different shapes and features in a workpiece. Here are some common types of operations:
Plain Milling:
Plain milling, also known as slab milling, is used to machine flat surfaces parallel to the worktable. The cutting action occurs primarily on the circumference of a cylindrical cutter, which is well-suited for machining wide, flat surfaces.
Face Milling:
Face milling is similar to plain milling but typically uses a cutter with teeth on both the face(the bottom)and the circumference(the sides). This design allows face milling to efficiently remove material and produce large, flat surfaces with excellent finishes. It can be used independently or after plain milling to improve surface finish or achieve specific contours.
Angular Milling:
Angular milling involves the use of milling cutters with specific angles, such as single-angle and double-angle cutters, to machine angled features on a workpiece. Common features produced by angular milling include dovetail grooves, chamfers, and V-shaped slots, which are oriented at angles that are neither perpendicular nor parallel to the primary axes of the workpiece.
Form Milling:
Form milling is a process that uses a specially shaped cutter to machine unique profiles on a workpiece, such as contours, curves, or grooves, in a single pass. It's typically used for creating complex shapes like gear teeth or intricate mold surfaces.
Side Milling:
This refers to the process of milling a flat or angled surface on the side of the part. It typically involves cutting along the side of the workpiece to create vertical, angled, or contoured surfaces.
Groove or Slot Milling:
Slotting involves cut narrow channels or grooves into a workpiece. This operation typically involves using an end mill or slot cutter to create slots of various shapes and sizes, such as straight, rectangular, or V-shaped slots. Slot milling is widely used for applications like keyways, gear teeth, and other features requiring precise and straight grooves, and can also include specialized forms like T-slot milling for securing fixtures or bolts.
Straddle Milling：
Straddle milling is an operation where two parallel surfaces are machined simultaneously using two side milling cutters mounted on the same arbor, spaced apart by collars. It is commonly used to machine opposite sides of a workpiece to ensure they are parallel, and is often applied to create square or hexagonal surfaces.
Gang Milling:
Gang milling is a milling operation where multiple cutters are mounted on the same spindle to machine multiple surfaces or features simultaneously in a single pass. This method significantly reduces machining time and can handle complex part geometries and multiple surfaces.
What Products Can Be Made by CNC Milling?
CNC milling is an automated process that, once set up, can run continuously for days, producing parts consistently and efficiently. With the use of digital templates and autonomous machining, human error is minimized, resulting in exceptionally high accuracy . Modern milling machines have become increasingly advanced, capable of creating square shapes, bevels, angles, slots ,complex curves, and even round shapes.
Additionally, CNC milling can work with a wide range of materials, including metals, plastics, elastomers, ceramics, and composites. This versatility makes CNC milling suitable for fabricating nearly any part. Below are some typical examples of applications:
Aerospace
CNC milling is applicable in making many aeronautic components adopting materials such as Titanium and Aluminum. These materials are lightweight and durable. Milling process can meet the strict requirements for accuracy and precision. The engine components, landing gear components, and fuel panels are produced by this technique.
Automotive
The automotive industry uses milling machining due to the requirement for efficiency without losing accuracy. Therefore, it is suitable for products made using this process including control panels, axles, car molds.
Electronics
The high versatility and preciseness of the CNC milling machine make it ideal for the electronics industry. CNC milling machine not only shapes the conductors and metals required for transmission, but also the polymer boards that house these conductors.
Medical
CNC milling allows manufacturing parts from many grades of titanium and stainless steel, which are often used in making medical devices, such as scalpels and implants. Medical parts such as prostheses require precise and unique design. Therefore, CNC milling is the better method for such parts.
Prototyping and mold making
Milling is well-suited for prototyping and mold making due to its high precision, flexibility, and efficiency. CNC milling machines can handle a variety of complex shapes and materials, providing accurate machining results. This allows designers to quickly transform ideas into physical prototypes or molds, facilitating further evaluation and optimization, thus speeding up the product development process and reducing manufacturing costs.
While CNC milling machines are highly versatile, they do have limitations. Perhaps the biggest drawback is cost. Even the most basic CNC machines suitable for mass production come with a high price tag, and costs only increase as machines become more advanced, both in terms of purchase and maintenance.
Another limitation is related to part size and geometry. The size of the part is constrained by the dimensions of the CNC machine, and the geometry is restricted by the physical capabilities of the cutting tool. For example, features like undercuts or recessed areas beneath a part's surface require special tools or expensive multi-axis machines to achieve.
CNC milling also isn't entirely free from human error. Operating these high-tech machines requires skill, and setting up a milling machine for a specific job can be time-consuming and complex, demanding experienced operators.
Is CNC Milling the Right Choice for Your Parts?
Now that we have a general understanding of CNC milling, the question remains: Is CNC milling the right choice for your project? Let's dive into the key aspects to help you decide:
Part Geometry
CNC milling excels at producing complex, non-symmetrical shapes. If your parts have intricate geometries, sharp angles, or multi-surface features, CNC milling is likely a good fit. However, for round or symmetrical parts, CNC turning might be more efficient.
Production Volume
CNC milling is ideal for small to medium production runs. If your project involves custom parts, prototypes, or limited quantities, CNC milling is often considered an economical and efficient option. However, for larger production volumes, other methods like injection molding or stamping may be more cost-effective.
Material Selection
CNC milling is versatile and works with a wide range of materials, including metals, plastics, ceramics, and composites. If your project requires precise cutting across different materials, CNC milling can be a reliable solution.
Budget Considerations
While CNC milling offers high accuracy and flexibility, it can be more expensive, particularly for simple designs or large production volumes. It's important to weigh the cost-benefit ratio based on the complexity of your project and the required quantity.
Tolerance Requirements
One of the key strengths of CNC milling is its ability to achieve high precision and tight tolerances, even down to ±0.01 mm (±0.0004 inches). However, it's important to consider whether your project actually requires this level of precision, as higher tolerances demand more engineering resources and come with increased costs.
When evaluating if CNC milling is the right choice for your parts, think about the specific application. For instance, if your parts are standalone and do not need to integrate with other components, they may not require extremely tight tolerances. In such cases, choosing standardized tolerances with a 3-axis milling machine can be a more cost-effective solution.On the other hand, if your project involves industries like aerospace, medical, or automotive—where precision is paramount—even the smallest deviations can lead to functional failures. In these cases, using multi-axis milling machines may be more appropriate.
If you're uncertain about the tolerance requirements for your parts, collaborating with an experienced CNC machining partner can help you make the right decision. By discussing your project objectives and specifications in detail, you can determine the ideal tolerances without overspending on unnecessary precision.
Chiggo is a trusted bespoke manufacturing partner for custom CNC milling and machining. With 3-axis, 4-axis, and 5-axis milling machines, we provide rapid prototyping, tooling, and end-use production at competitive machining costs. Request a CNC milling quote today, and our team will respond within 12 hours.
Types of CNC Milling Machines
There are several CNC milling machines to choose from, each suited for different tasks. Based on design and functionality, there are main two types:
Vertical Milling Machine
In a vertical milling machine, the spindle, which holds the cutting tool, is oriented vertically. This configuration provides better visibility during machining, making it ideal for working on smaller parts or tasks that require detailed precision. Vertical milling machines are the more common type of CNC mill and are frequently used for slot cutting, drilling, and contouring.
They have a smaller footprint and lower costs, both in terms of initial purchase and ongoing maintenance. However, vertical mills may struggle with very large or heavy parts, and while they offer high precision, they can be slower than horizontal mills when it comes to large-scale production.
Horizontal Milling Machine
Horizontal milling machines can perform many of the same operations as vertical milling machines, but with a different configuration. The spindle is positioned horizontally, and these machines are better suited for processing heavier workpieces. The stability of the horizontal spindle allows for heavy-duty cutting, enabling the rapid removal of large amounts of material. This makes horizontal milling machines ideal for high-volume production of relatively simple parts. Additionally, their superior chip removal capabilities help achieve better surface finishes and extend tool life. However, horizontal milling machines are more expensive and can be challenging to install in smaller machine shops.
CNC mills can also be categorized by the number of translational and rotational axes they can move along—often referred to as degrees of freedom. The common types of CNC milling machines include:
3-Axis Milling Machines
3-axis milling machines allow the cutting tool to move along the X, Y, and Z axes. This type of machining is the most common due to its lower start-up costs and ease of use. It can produce most standard shapes and is simpler to program and operate. The relative simplicity of motion makes it suitable for many projects that do not require complex geometries but benefit from high part throughput and process efficiency.
4-Axis Milling Machines
The 4-axis milling machine owns all the capabilities of a 3-axis milling machine, with additional one axis known as A-axis. This enables the rotation of the workpiece for cutting around the A-axis, which proves particularly useful when parts need to be cut around a cylinder or the side of a piece.
5-Axis Milling Machines
These machines operate along the three linear axes (X, Y, Z) and add two rotational axes (chosen from the A, B, and C axes), which can rotate the machine bed, the tool head, or both. This capability eliminates the need for multiple setups and allows for machining on five faces in a single setup. As a result, 5-axis milling machines can produce highly complex geometries with high precision, making them ideal for aerospace components, titanium parts, medical devices, and gas turbine components.
What Are the Rotational and Linear Axes in CNC Milling?
Linear Axes:
The linear axes in CNC milling machines are responsible for the movement of the cutting tool along straight paths. Imagine you're the operator facing the machine:
The X-axis runs parallel to your body, moving left to right.
The Y-axis is perpendicular to you, moving forward and backward.
The Z-axis, parallel to the spindle, moves up and down.
Rotational Axes:
The A-axis refers to the rotational movement around the X-axis, allowing the cutting tool or workpiece to rotate horizontally.
Rotating around the Y-axis, the B-axis provides vertical rotation, enabling more flexible machining angles.
The C-axis adds the ability to rotate around the Z-axis, giving the tool or workpiece full rotational capability in the plane perpendicular to the Z-axis.
Components of A CNC Milling Machine
Basic Parts of CNC Mill
Regardless of the type of milling machine, the following are the critical components that make up a standard CNC milling machine:
Machine Bed：The machine bed is the base where all other components of the machine are mounted. It’s typically made of heavy steel or cast iron to provide a stable foundation and absorb vibrations.
Worktable：The worktable of a CNC mill sits on the machine bed. It fastens the workpiece in place during machining. Standard CNC mills have a worktable that moves in the X and Y directions, whereas some advanced 5-axis CNC mills can rotate the bed to facilitate more advanced machining operations.
Drive components：Drive components are responsible for powering and controlling the movement of various machine elements. These components include servo motors or stepper motors, ball screws, and linear guides. Servo motors convert electrical energy into mechanical motion through drivers, enabling precise movement of the worktable or spindle along the machine's axes. Ball screws and linear guides ensure that this movement is accurately transmitted to the worktable and cutting tool.
Coolant System：The coolant system is designed to regulate temperature during machining by delivering coolant—typically a liquid such as water-soluble oil or cutting fluid—to the cutting area. This system helps reduce the heat generated by the cutting tool and workpiece, preventing tool wear and thermal deformation of the workpiece. Additionally, the coolant aids in flushing away chips and debris from the cutting zone, improving the overall machining quality and prolonging tool life.
Spindle ：The spindle includes a tapered section where tool holders are positioned. It also contains a rotating assembly and a shaft for attaching the cutting tool. The spindle holds the cutting tool in place and directs its movement during operation.
Tool Holder：The tool holder is clamped to the spindle using hydraulics and is designed to clamp onto the tool.
Interface：The interface of a CNC mill typically consists of a screen and input devices (such as a keyboard or touchscreen) attached to the machine. This interface is used to control the machine directly, and some basic CNC programming can also be performed through it, although more complex programming is generally done offline.
CNC Mill Add-Ons
In addition to the basic components of a CNC mill, advanced CNC milling machines often include a variety of add-on equipment that can significantly enhance efficiency. Some common add-ons include:
Automatic Tool Changers (ATC):Automatic tool changers are devices that automatically switch between different cutting tools during the machining process. Instead of manually stopping the machine to change tools, the ATC selects the appropriate tool from a tool library and installs it in the spindle as needed. This speeds up the machining process, reduces downtime, and allows for more complex operations to be completed efficiently without manual intervention.
Chip Management Systems:Chip management systems, including conveyors and air blowers, are often added to efficiently remove chips and debris from the cutting area. This helps keep the work area clean and allows the CNC machine to run continuously without frequent stops for manual cleaning, resulting in higher machining uptime.
Robotic Arms:Robotic arms mimic the movements and functionality of a human arm and are used to automate various tasks, such as loading raw materials, removing finished parts, repositioning workpieces, and even changing fixtures or tools. By integrating robotic arms into CNC operations, manufacturers can automate repetitive tasks, increase production efficiency, enable lights-out manufacturing, and reduce the need for manual intervention.
