CNC Milling Explained: Process, Types And Benefits

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CNC Milling Explained: Process, Types And Benefits

If you are just getting started with computer numerical control (CNC) milling or any other form of CNC machining, it might first appear confusing and challenging. The use of computer technology in the manufacturing industry has grown tremendously over the decades. Advanced technologies are controlling how machinery operates, adding many benefits to the manufacturing processes.

The best manufacturers use CNC machines or services to produce intricate parts with repeatable results. CNC milling is one of the most prominent processes in these businesses. Others include CNC drilling, CNC turning, and CNC routing. In this comprehensive guide, we delve into the world of CNC milling. We hope that the milling machine insights offered here will help you undertake your next CNC milling project satisfactorily.

What is CNC Milling?

CNC milling is the removal of material from a work piece using a combination of special cutting tools and computer programs. A milling machine is used in this process to achieve the desired shape and size of the part with great accuracy.

As one of the modern advanced manufacturing processes, CNC milling has revolutionized how parts are made. It is a fast and precise subtractive manufacturing process that requires very minimal operator input.

With this advancement comes increased versatility. CNC machining can involve a wide range of materials including composites, plastic, and different types of metals. Varieties of milling tools are also endless. The machine and machine tool combination produces different features on the workpiece – slots, chamfers, grooves, sockets, and so forth.

To achieve face milling and other results accurately and fast, the CNC system features different components. The machine has a work table, spindle, column, saddle, machine tool, knee, and other parts.

Let’s go into the details of how CNC milling works, as we also discuss more about these parts.

How do CNC milling machines work?

We will cover how the CNC milling machines work by dividing the process into four steps:

Step 1: Computer-Aided Design (CAD) Software Design Generation

Considering that the CNC milling process is computer-controlled, the process begins with the design in a Computer-Aided Design (CAD) software. Machinists have a variety of software at their disposal to undertake this step. Popular ones are Creo, SolidWorks, Catia, and Autodesk Inventor.

Additionally, there are different ways of generating the CAD model design. The machinist could create the design from zero. He or she does all the work of building the shape, which involves a lot of data inputs.

A second option on how to design CNC milling parts is where reverse engineering is applied. The engineer begins with an object and works backward to acquire the CAD model details. There are all manner of ways to get to the CAD model, but we can cover them in a different coverage. For now, let’s look at the second step of CNC machine operation.

Step 2: CAM (Computer-Aided Manufacturing) software for Design Translation

With the CAD design ready, the next step is to change it to machine language or instructions. The CAD design instructions need to be changed to a language that the machine can understand – CNC machine program. The typical CNC machine uses a CNC program or language called G-code. The G-code helps the machinist aspects of the machining process such as spindle speed and feed rate.

Some CAD software options are integrated with CAM software while others come separately. Either option is applicable in this case. Having specified the parameters for the CAM – the tool, machine type, cutting speed, and feed rate – the stage is now set for G-code generation to create a CNC program. The CNC program is editable so one can get whatever output they want from the machining program.

There are special considerations when creating the CNC program. For instance, the CNC program should allow for slow cutting speeds around the corners. This would work perfectly with the corner rounding cutters.

Transferring the G-code or CNC program to the CNC milling machine is the final step of translating the CAD design to CAM parameters.

Step 3: CNC Milling Machine Setup

The machine may be categorized as automatic, but it still requires the input of the operator. The level of this intervention will depend on the advancement of the machine. Newer models don’t require much human role. Some can change their milling tools automatically depending on the demands of the milling process.

The basic steps of setting up the CNC mill are as follows:

  1. Start with the pre-start activities – checking the coolant level, machine service details, cleanliness of the working area, the compressor for air supply if necessary, machine tool

  2. Start the machine/power it from the main breaker. Switch the machine on from the button that is typically close to the control panel.

  3. Load the CNC milling machine cutting tools as indicated in the CNC milling program

  4. Set the tool length offset (TLO)

  5. Find the XY datum for the work piece

  6. Find the offset Z for the fixture

The CNC milling equipment is now all set for the loading of the CNC milling program.

Step 4: The CNC Milling Process Begins

This marks the last step in how CNC mills work. We now have a stationary workpiece as the cutting tool does its job on the flat surface. It cuts the preset shape into the work piece in what is called slab milling.

The CNC machining tool rotates at the set speed and removes matter from the work piece. This subtractive manufacturing process continues until the work piece acquires the shape and size envisaged in the CAD model.

As intimated earlier, the CNC program and the components used in the milling process should adhere to best practices and engineering principles. For instance, the use of finer toothed cutter is necessary for the lighter cuts.

Types of CNC Milling Machines

The market has many varieties of CNC milling machines. There are also different ways of classifying these machines. It is worth noting that the basic working principle of the milling machine doesn’t change a lot. Only a few modifications give rise to the different CNC mills.

In this article, we look at the types of CNC milling machines as classified based on the type of axis. We will typically be referring to the three main axes – the X-axis, Y-axis, and Z-axis. These axes refer to the side-to-side, forward-to-back, and up-and-down movements respectively.

3-axis Milling Machines

This is the simplest form of the CNC multi-axis milling machine. With the work piece in position, the spindle of this machine can move in the X, Y, and Z linear directions during the CNC milling process.

The 3-axis milling machines are mostly applicable for machining 2D geometry. They could machine all directions of a work piece, but this is expensive and time-consuming. One has to set up the fixture for each side.

It makes more economic sense to restrict the 3-axis CNC equipment to simpler tasks. Examples of such tasks for CNC machines are drilling, cutting edges, threading, and creating milled profiles.

4-axis Milling Machines

To add to the X-axis, Y-axis, and Z-axis, the 4-axis CNC machine has the A-axis. The effect of the A-axis is the incorporation of rotation around the X-axis. With this additional movement, the milling process can occur on four sides of the work piece without the need for multiple machine setups.

By removing the need to change the fixture, the 4-axis CNC machine is often more economical than the 3-axis CNC milling machine.

5-axis Milling Machines

Adding an extra two rotation axes results in the 5-axis CNC milling machine. That means the milling machine now has the A axis, B axis, and C axis. Notably, only two of these rotational axes are functional, based on the type of 5-axis milling machine. This advanced CNC milling machine is popular in precision engineering for the manufacture of aircraft parts and such intricate parts.

From the simpler 3-axis milling machine to the more advanced 5-axis milling machine, each class of milling machine offers a set of pros and cons. Understanding the capacity of each class is essential in identifying the most suitable one for your project requirements. For even more details about the CNC milling machine, let’s now look at the components.

CNC Milling Machine Components

CNC Milling Machine Components


This is among the most fundamental machine parts, whether you are looking at vertical milling or horizontal machines. Its role in the setup is to facilitate the rotation of the milling tool through the tool holder.

In both horizontal milling and vertical milling operations, the spindle can rotate at varying speeds as set by the operator and depending on the material and type of machining processes. It can also tilt and swivel for those complex machining results. The common varieties of spindles are self-contained, cartridge, and fabricated.

Of note is that the orientation of the spindle in a horizontal milling machine is horizontal. Vertical milling is different from horizontal milling machines in that the machine spindle is oriented vertically.


This is the vertical component of the CNC milling machine that supports the spindle and other parts. Carrying the Z-axis setup, this part is fixed to the bed of the vertical milling machines. For the horizontal milling machines, the column lies in the horizontal direction and attaches to a sizeable frame.

Tool Holder

During the milling process, the cutting tool holder keeps the tool firmly in place. The cutting tools of the vertical and horizontal milling machines are attached tightly to the tool holder to prevent vibration during the machining process.

It is important to have the highest stability for the precision of the work. This component is the interface between the spindle and the cutter.

Machine Bed

A CNC milling machine comprises a bed, which is a sizeable cast iron support for other components. Given that it supports so many parts, the machine bed is typically built for strength and damping capabilities.

It is designed to prevent any form of deflection from the milling forces in horizontal and vertical milling machines.

Work Table

The work table of the CNC milling machine is the platform on which the cutting operation takes place. You will find in this component the mechanisms (T-slots) for holding the work piece.

It is typically rectangular and of cast iron. The CNC machine controller (to be covered in a short while) bears the code that prompts the motor to drive the work table. The work table can move longitudinally and horizontally.

Drive Components

Milling components include drives, which provide controlled movement of various CNC machine elements. The main drive components are ball lead screws and motors.

The drive motors receive signals from the control unit. Consequently, the motors activate the ball lead-screws leading to the rotation of the spindle or movement of the CNC machine table.

Coolant Tank

The CNC machine comes with a coolant system to facilitate the cooling of the work piece and machine tool, as well as the removal of debris from the area. Part of this system is the coolant tank, which stores the coolant for continual supply during the milling process.


The CNC milling machine also contains the controller, which is central to all the operations. When it receives the CNC code as input by the user, the controller interprets it. Therefore, it is behind the automatic movements of various parts of the CNC milling machine.

This computing unit performs the following functions:

  • Explains the G-code instructions

  • Generates the right signals

  • Controls the motion of axes

  • Detects errors in the system, for instance, extreme speeds

Machine Interface

The interface of the CNC machine enables the operator to connect with the machine. This interface is either a computer screen or a control panel. There are also instances where it combines both the computer screen and control panel. With the interface, the operator can feed CNC machining commands, follow the process, and manipulate the machining parameters as required.

Types of CNC Milling Operations

The CNC milling machine is capable of slotting, threading, chamfering, and many other operations. The combination of these operations enables the machine to generate a wide range of complex designs. There could be other milling processes, but here are the most popular milling operations of the CNC machine:

Face Milling

In this milling operation, the surface of the work piece sits at a right angle to the axis of the milling cutter. The cutting tool has teeth on the end and the sides. You want to perform face milling following the surface milling operation (we will highlight it here).

Face milling generates accurate contours and a smooth surface finish. Notably, the side teeth do a bulk of the cutting. The teeth at the end of the miller tool are dedicated to the surface finish.

Plain Milling

Plain Milling Cutter

The other name for plain milling is surface milling. The work piece surface and the rotational axis of the cutting tool are parallel to each other. In other words, the mill is horizontal. There are all manner of milling cutters for plain milling whose application depends on the type of results the operator wants.

For instance, a narrow cutter is good for the slow removal of work piece material. Of course, you can play with several other parameters for varied results – feed rate, cutting speed, teeth coarseness, etc.

Angular Milling

Angular Milling

If you are interested in grooves or chamfers on the work piece, this is the CNC machining operation to use. Angular milling is achievable in different ways on the CNC milling machine. For the typical 3-axis milling machine, it is more practical to use different cutters to achieve the profiles or shapes.

Say, you want to create angled grooves on the work piece. Using dovetail cutters can give excellent results. If you want to create the reverse of that (chamfers), use a conical cutting head on the milling machine. In the case of angular milling, the rotational axis of the cutter can be parallel or perpendicular to the work piece surface.

Form Milling

Form Milling

Form milling is a CNC milling process that results in complex contours on the work piece surface. As you can already guess, the kind of tools used here is more complex. Examples of these tools are concave and convex cutters. What’s most intriguing about form milling is that the complex contours are produced in a single cut.

A most important requirement on the part of the machinist is to ensure that the tool parameters for the milling machines are on point. Form milling can also entail gang milling (covered below) and other types of operations whose goal is to create complex contours on the surface.

Profile milling

A multi-step CNC machining process, profile milling is renowned for semi-finishing or finishing concave and convex surfaces. It usually starts with the roughing operation to remove irregular surfaces before going to semi-finishing or finishing.

Profile milling removes the biggest proportion of the material using roughing. To do the finishing and semi-finishing process, the CNC milling machine uses other special tools such as ball-nose end mills. Doing this CNC milling work is worthwhile, with the multi-axis machining operations hastening the work and offering a high-quality finish.

Gang Milling

If you have wondered if it’s possible to machine multiple surfaces simultaneously, yes, it is. This type of milling does exactly that. A single arbor carries multiple cutters of varying diameters. You want to consider this machining process for milling a large number of work pieces.

There is a basic principle for setting up the gang milling cutters. The cutter with the fewest number of teeth determines the feed rate. The largest diameter cutter determines the peripheral speed for calculating the spindle revolutions per minute (RPM). This process is different from straddle milling in that the number of milling cutters in the latter is typically limited to two.

Side Milling

A side milling cutter is the result of adding teeth on two or one side of the plain cutter. If the teeth are present on only one side, it is referred to as a half-side milling cutter. So, this milling operation entails milling on both the face and side.

It means that side milling can also be used in the formation of grooves, particularly the deep narrow ones. The side mill cutters could also be interlocked to machine wider slots.

Groove or Slot Milling

Talking of groove or slot milling, this is a CNC milling process that results in grooves or slots in the work piece. This milling operation is popular in the generation of pockets, keyways, and so on.

It is distinguishable from other operations, say end milling, by the fact that its grooves or slots are deeper. The surface finish also tends to be cleaner and smoother.

Hole Drilling

Finally but not least, the CNC milling machine can perform the hole-drilling operation. There is no problem locating the holes accurately because the feed screw sequence can help with that. The machine spindle can secure the twist drills via the drill chucks. Angle plates or clamps can hold the work piece for the hole-drilling operation.

Benefits of CNC Milling

The manufacturing industry is drawing a lot from the benefits of computer numerical control milling. Most CNC milling machines present these benefits:

Manufacturing Complex Parts

Compared to traditional methods, CNC milling is capable of manufacturing highly complex machine parts. Modern CNC machines are designed to operate in many axes. The machine tools and the work piece can interact from varied angles. The details of the contours that the CNC machining process produces are not achievable on traditional machines.

When the CNC milling process is merged with CAD and other design software, the capability to produce complex machine parts is even greater. Transferring digital designs to tangible items is seamless. The accuracy of the parts is not an issue because the CNC setup allows for prototyping and simulation.

Fast and Efficient CNC Milling Process

Production processes using CNC milling are fast and efficient, making manufacturing highly advanced. Consider the following features and how they impact on the milling process and CNC milling cost:

  • Simultaneous multi-axis operation

  • Fast changeover of milling machine tools

  • Fewer steps for complex designs

  • Manual operation is minimized

  • Continuous milling operation

Cost-Effective Milling Process

Efficiency alone is not enough for a manufacturing process to have a competitive edge. Cost-effectiveness is another element that a business can benefit from. With most CNC milling operations, this benefit is easily achieved.

One area where this benefit emerges is in material wastage. Conventional milling processes are infamous for the high wastage of materials. Due to the manual nature of the process, miscalculations are rife.

However, the CNC milling process reduces wastage through more precise cuts. Instances of human error are also minimal because most of the milling processes are automated. There are also fewer variations in the output. With consistent results, wastage is increasingly less. Other areas of cost-effective effect are reduced labor costs and enhanced energy efficiency in CNC machining.  

Material Versatility

Before the arrival of the CNC milling methods, machine tools were unique depending on the material. CNC milling machines are different because they work with a wide range of materials. This includes metals such as tungsten, silver, gold, stainless steel, and copper.

Under the composites category, CNC milling can handle Hybrid Composites, Graphite Composites, Carbon Fiber Reinforced Plastics (CFRP), and Glass-Reinforced Plastic (GRP) among others. The milling capability extends even to plastics and polymers – nylon, ABS, Polypropylene, and so on.

Teflon (has excellent chemical and temperature resistance) and other polymers may also suit the CNC milling process.

Accuracy and Precision of CNC Machine

Accuracy and precision go hand-in-hand with CNC machining processes. This technology produces consistent and accurate results to the minutest detail. Human error is not part of the CNC milling process. The final product often meets the design expectations because issues like fatigue or lack of concentration are eliminated.

With an error rate not exceeding 1%, the modern milling technology is miles better than the traditional alternative. The average tolerance is also much better and the design flexibility unrivalled. Putting all this together, the user gets reproducible and cost-effective CNC machining results that promote customer satisfaction.


As we have highlighted here, CNC milling is a popular automated machining process. With the replacement of manual processing with automation, the CNC miller machine reduces errors, enhances production, and minimizes waste, among other benefits.

Sometimes, the types of CNC milling machines out there are overwhelming. Users have challenges distinguishing the best type for their machining processes. Economic priorities can also mean that you are not ready to invest in an advanced milling machine immediately.

In case you are considering subcontracting your CNC milling services, we have your back at Aria Manufacturing Ltd. We have modern multi-axis CNC milling machines, the best machine tools/multi-point cutting tools, and expert machinists to complete all custom manufacturing services with finesse.

Call us for more information on CNC machining today!


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.