What is CNC Milling: A Comprehensive Overview of the Milling Process

cnc milling definition

CNC milling is an essential process when it comes to the production of complex components. As one of the capabilities under the broad umbrella of precision CNC machining, this process helps create precise parts for many industries. With its wide range of applications, this technology continues to grow and evolve rapidly. 

So, we will take a detailed look at the milling process, examining the various operations involved and their applications. This will help you understand the various facets of the process and how to get the best from it. Without further ado, let’s get into it. 

Definition: What is CNC Milling?

CNC milling is a computer-controlled process that involves removing parts of a workpiece with a cutting tool. Its basic setup includes a machine table and cutting tool(s) attached to the spindle. The workpiece is placed on the machine’s work table while the cutting tool(s) rotate to shape the workpiece.

The rotation of the cutting tool will depend on the type of CNC milling machine and its level of sophistication. Most CNC milled parts can reach high tolerances between +/-0.001 inches to +/-0.005 inches. Some machines can even achieve tolerances up to +/-0.0005 inches. 

As such, this process is highly versatile and can work with an extensive range of materials, including metals, plastics, wood, and glass. It can also help create a wide variety of custom products to meet unique requirements. 

Analyzing the Milling Process: How Does CNC Milling Work?

It is vital to give a complete overview of the milling process. This will help you get the complete picture of how it works. Generally, the CNC milling operation occurs in three phases:

1. Designing the CAD Files 

The first phase involves creating a virtual representation of your intended product using CAD software. You can use different CAD-CAM programs to develop the needed G-code for machining. It is essential to translate the CAD design into a CNC programming language because that is what the machine will understand. 

You can check and amend the code to meet the machine’s capabilities. The software also enables manufacturing engineers to simulate the entire milling process. As a result, they can check for design errors and avoid creating complex models that are too difficult to machine. 

The operator also requires a technical engineering drawing that contains information like: 

  • Dimensions and critical features of a part
  • Tolerances
  • Construction lines
  • Indication of threads
  • Finishing preferences

2. Setting Up the Milling Machine

Although the CNC milling machines automatically cut the workpieces, some aspects of the process require a machine operator. For instance, an operator has to fix the workpiece to the worktable and attack the cutting tools for the spindle. 

Modern milling machines have live tooling capabilities. Therefore, they have the ability to change tools during the milling process. This reduces the number of stops that will occur during the operation. However, there’s still a need for initial setup and program checks before starting the process. 

milling process

3. Producing the Components

As mentioned earlier, CNC milling uses a rotating tool(s) that cut off chips when it contacts the workpiece. Continuous rotation of the tool gives the desired shape. The process may comprise different operations depending on the type of raw material and desired shape of the final product. 

The first few operations use larger tools to carve out materials and get an approximate shape. Then, a tool change will help create more accurate machined parts. The precision CNC milling is achieved in the last stages, involving unmatched levels of surface roughness and engineering tolerances. 

Various CNC Milling Operations

A CNC milling center can produce several complex features with high accuracy. Such features include threads, chamfers, slots, etc. However, creating these features requires various milling operations such as: 

plain milling

Plain Milling

It is also known as surface milling, this operation uses a horizontal mill. Thus, the cutting tool has a rotational axis parallel to the work table. Plain milling uses different cutters of different shapes and orientations based on the desired outcome. A combination of a wide cutter (with fast feed rate) and finer cutting teeth (with faster cutting speed) is a great choice for cost-effectiveness. It also ensures better surface finishes and high-quality end products.

face milling

Face Milling

This type of milling uses cutting tools with teeth on the sides and the end. The side teeth do the major cutting work, with other teeth on the tip helping with the surface finish. Face milling usually comes after surface milling because it can create intricate contours and leave a high-quality finish. 

angular milling

Angular Milling

This is one of the best milling operations to create grooves, chamfers, slots, and other intricate features. The traditional 3-axis mills use different cutters — dovetail cutters for angled grooves or conical cutting heads to produce chamfers. The cutter’s axis is often perpendicular or in line with the cutting surface.

form milling 

Form Milling

This milling operation uses special tooling for creating more complex contours. Concave and convex cutters are the most common ones here. They help create surface contours, round edges, and round recesses in just one cut.

Other Milling Operations

Asides from the aforementioned milling operations, CNC milling machines can accomplish other types of specialized milling operations like:

Profile milling: Here, the tool creates a cutting path along an angled or vertical surface on the stock bar. The process uses a profile milling tool that may be parallel or perpendicular to the surface of the workpiece. 

Gang milling: This is a subcategory of form milling that uses several cutters to create patterns simultaneously. The cutters typically have varying shapes, sizes, or widths. They may perform similar or different operations to create more complex features or intricate parts within shorter periods.

Straddle milling: This milling operation allows a machine tool to work on two or more parallel workpieces with a single cut. Two cutters are arranged on either side of the workpiece to mill both sides on the same machine arbor.

Pros and Cons of CNC Milling in Part Manufacturing

The CNC milling process is essential for various industrial processes because of its advantages. Nevertheless, there are disadvantages to note. Here are some of the benefits and downsides of this manufacturing technique. 

custom milled part

Pros of CNC Milling

Accuracy and Precision

The nature of CNC milling operations leaves minimal room for errors, thereby ensuring higher accuracy and precision. This technique can produce parts that meet exact technical specifications and tolerance requirements.

Most CNC milled parts can reach high tolerances between +/- 0.001 inches to +/- 0.005 inches, while some can be up to +/-0.0005 inches. The automation of the process also limits human errors. Even if you require surface treatment, micro-milling allows the removal of some thickness to prevent affecting the part’s dimension.

Efficiency and Repeatability

CNC milling centers offer high production output because of the automation involved. Advanced milling machines can produce parts faster with more consistent quality and finish. 

Wide Material Compatibility

This process is suitable for an extensive array of materials, including metals, plastics, glass, and wood. This further increases its versatility and makes it perfect for various applications.

Less Labor Intensive

The automation and high output of CNC milling machines significantly reduce the labor used for the process. These machines also allow live tooling, ensuring tool changes without stopping the operation. Very few operators would be required for the process.


CNC milling machines efficiently produce complex products faster and more accurately. This reduces the time spent in the production stage. Moreover, the lesser labor requirements further reduce the costs of the process.

milling operation

Cons of CNC Milling

High Machine Cost

The initial investment cost for CNC milling machines is higher than traditional manual equipment. This may drive up overall manufacturing costs.

Requires Intensive Training of Operators

Not just anyone can operate CNC milling machines. From designers to programmers and machine operators, there’s a need for adequate training to get the best from the process.

High Level of Maintenance

Milling machines must be constantly maintained to ensure they function properly and increase tool life. This increases the downtime of using the milling machines.

Common Materials for CNC Milling Operations

CNC milling works on a wide variety of materials. Your choice of material will depend on the requirements and specifications of your applications. The most common materials include:

Metal Materials for Milling

  • Aluminum alloys
  • Stainless steel
  • Mild steel
  • Tool steel
  • Brass
  • Copper alloys
  • Titanium alloys

Plastics Materials for Milling

  • POM
  • ABS
  • Nylon
  • Polycarbonate
  • PEEK
  • PTFE (Teflon)
  • Polyethylene
  • PMMA (Acrylic)
  • HDPE
  • PVC
  • Polypropylene

Four Classifications of Milling Machines

Multiple milling machines are available in today’s market, with various ways to classify them. While the basics remain the same, a few modifications make each mill unique.

The categories of milling machines available are: 

1. Bed-type Milling Machines

Their design includes a standard long machine bed that allows the attachment of multiple workpieces. This decreases idle times and increases efficiency. The worktable attaches to the machine bed and can move in two directions. The orientation of these mills allows the spindle head to move axially, determining the depth of the cut.

There are vertical, horizontal, and universal bed-type milling machines. The most common is the CNC universal milling machine, providing more flexibility. Using a two-machine stand for these mills will help increase productivity. It allows the operator to mount one large part or several parts onto the worktable for simultaneous processing.

2. Knee-type Milling Machines

These machines help produce small to medium-sized components. However, they provide less stability than bed-type machines. Traditional knee-type mills are great options for creating one-off components for preparatory tasks, maintenance works, etc. 

Manufacturers commonly use them to prepare workpieces for further refining on CNC stations. The cutting head moves in only one direction, limiting the possibility of accidents. However, knee-type mills require manual changing of tools after each operation. This makes the process relatively slow. 

ram-type milling machines

3. Ram-type Milling Machines

These mills have their cutting heads mounted on a ram sliding back and forth. As a result, the tool can easily move in two axes – X and Y. There are both vertical and horizontal options available, and they often allow swiveling of the cutting head. 

4. Planer-type Milling Machines

These milling machines are pretty similar to bed-type mills. They use larger worktables, and their spindles can move in three directions. However, the planer-type mills can accommodate more tools at the same time. They can take up to four tools at once. This increases their flexibility and efficiency while reducing the need to change tools during the milling process. 

Different Types of CNC Milling Machines

There are three major types of CNC milling machines used by manufacturing companies. They are: 

Vertical Milling Machines

These tools have vertical axes, making them better suited for end milling. The machines have their cutting heads at the end of the ram while their spindles are attached to the end. Modern vertical milling machines can rotate parts for better access and faster operations. They also enable the automation of movements for more precision and repeatability.

Vertical milling machines are considerably cheaper than their horizontal counterparts. They are also smaller, requiring lesser space in CNC machine shops. 

horizontal milling machines and vertical milling machines

Horizontal Milling Machines

In contrast to vertical mills, these machines have axes lying horizontally. The cutting tools attached to the arbor, and the tools can be easily replaced by removing the spacers and arbor brackets. Horizontal milling machines can have multiple spindles with many tools for faster processing. In addition, the table and tools can move in different directions. 

These types of CNC milling machines can produce more complex parts with fewer operations. They are much faster, and they deliver better surface finishing. 

Multi-axis Milling Machines

These machines can move along different axes and mill at different angles. Multi-axis milling machines can be:

3-axis Mills

3-axis vertical mills allow the table to move in the X and Y directions. With these machines, the workpieces can be positioned relative to the cutting tool. The operator can then add the Z-axis by lowering the cutting tool. CNC tools ensure simultaneous movements of the three axes, offering the needed flexibility for your needs.

4-axis Mills

These mills have all three axes, as mentioned above. But one other axis comes in here — the A-axis. The addition of another axis allows the table to rotate around the X-axis. This provides room for face milling without repositioning the workpiece. 

5-axis Mills

Everything true for 4-axis mills applies here, but with an addition of the B-axis. This addition helps the table rotate around the Y-axis. 5-axis CNC milling machines are more expensive than the above-discussed options, but they allow the production of complex parts in a single operation. They do not require extra setups and are compatible with live tooling. 

6-axis Mills

6-axis milling machines are not yet standard due to their high cost. They have stationary worktables with complete freedom of movement for the cutting head. That is, the head can move in and around the X-, Y-, and Z-axis. 

Components of A CNC Milling Machine

Here, we will analyze the components that makeup CNC milling machines. Although new mills are a little complex, we will stick to the traditional milling centers to give a proper overview. The essential components of milling machines are:

components of cnc milling machine

1. Frame

This main structure supports the machine and gives it rigidity and stability. It has a base and detachable column(s). The machine headstock is an essential part of the frame for mounting the main spindle, preventing vibrations or chattering during machine operations. This prevents machining errors and prolongs the lifespan of the cutting tool. 

2. CNC Control Panel

You can refer to this as the “nervous system” of milling centers. It consists of the various electronics needed to control the different cutting actions using programming functions. The main components of the control panel are the CNC monitor and programming buttons. Most CNC mills shave easily accessible control panels for easier machining. 

3. Axes

CNC milling machines have X, Y, and Z axes. Depending on configurations, they also come with additional rotational axes (A, B, and C). They are often programmed with the G-code in the CNC controller. 

4. Column

This machine component offers structure and support to other components of the mill. It comes with an affixed base and may include internal components that aid the CNC milling operation. Such internal features include coolant and oil reservoirs.

5. Worktable

This component is the solid base that allows the fastening of the workpiece. In other words, it holds the stock bar, vice, or fixture in place. Depending on the milling machine used, the component can be adjusted vertically, horizontally, or in both directions. Most worktables use T-slots to ensure easy clamping of the workpiece.

6. Spindle

The spindle comprises a tapered section for positioning tool holders. It also has a rotating assembly and a shaft for attaching the cutting tool. Depending on the type of milling machine, the spindle may be horizontally or vertically positioned. 

7. Arbor

This is the shaft component put in the spindles of horizontal milling machines. They allow the mounting of multiple machine tools. Arbors are available in different lengths and diameters based on the specifications required.

8. Ram

You would typically find this machine component in vertical milling machines. It is usually attached to the column supporting the spindle. Operators can adjust the ram to take different positions during the milling process.

9. Machine Tool

This is the component that carries out the material removal. There are several tools available for milling depending on milling specifications. They vary based on arrangement, number, teeth spacing, diameter, length, and geometry.

How Much Does a Milling Machine Cost?

The cost of a milling machine is one of the factors affecting the CNC machining cost. However, this cost varies depending on the sophistication of the machine. For example, entry-level or toolroom 3-axis vertical milling machines cost between $50K to $80K. In contrast, entry-level 5-axis milling machines cost between $100K to $400K.

Likewise, production-level 3-axis vertical mills cost between $150K to $300K, while the production-level 5-axis mill is over $500K. These are rough valuations that will depend on factors like:

  • Weight and size of the Machine (measured in the X/Y/Z travel)
  • Machine configuration (3, 4, 5, or 6-axis)
  • Machine speed
  • Power
  • Other options available, e.g., coolant pressure, chip conveyor, automatic tool changer, robotic part loader, etc.

Which Industries Use CNC Milling for Their Applications?

Several industries enjoy the advantages and multiple applications of CNC milling. Here are some of its industrial applications: 

Aerospace Industry

The milling process is also suitable for making aerospace parts with materials like aluminum and titanium. These materials are durable and lightweight, and they ensure accurate machining of desired components. 

precision milling part for automotive industry

Automotive Industry

Products made for the automotive industry must be efficient and precise. Thus, CNC milling is suitable for producing interior panels, drive axles, cylinder heads, gearboxes, exhaust parts, suspension components, etc. 

Medical Industry 

Medical and surgical tools often have unique designs that require optimal precision. This makes CNC milling one of the best methods for creating such components. This way, the designs are retained with increased efficiency and productivity. 

Consumer Products

CNC milling operation also helps produce general-purpose components. These items include nuts and bolts, gears and shafts, flanges, etc. The process is suitable for both short- and large-scale production. 


CNC milling is suitable for several high-quality applications, regardless of the complexity. This cost-effective manufacturing process also saves plenty of time, ensuring operational efficiency and faster product to market.

We have outlined the CNC milling basics, the various operations involved, applications, and the factors affecting its cost. Contact WayKen today if you’re unsure whether this process is the optimal solution for your application. We are always ready to assist. 


What is the difference between CNC milling and turning?

Their difference lies in the mode of operation. CNC milling involves holding the workpiece in place as the cutting tool rotates to shape the part. On the other hand, The definition of CNC turning uses a stationary cutting tool and a rotating workpiece. 

What products can you make with CNC milling machines?

Milling machines can create any product so long it can be designed. You can get regular and custom parts, regardless of the complexity of their geometries. 

What is the cost of CNC milling?

Several factors affect CNC milling cost, including raw material, part design, type of milling machine, production volume, and post-processing. Each element plays a unique role in determining the cost of your CNC milling project. 

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