What is CNC machining?

CNC machining is one of the most widely used subtractive manufacturing technologies, known for its precision, reliability, and versatility in producing custom metal and plastic components. Guided by digital CAD models, CNC machines use cutting tools to remove material from solid blocks with exceptional accuracy. 

This process supports tight tolerances and excellent material performance, making it ideal for prototyping and low-to-medium volume production runs. While CNC machining offers high repeatability, it does have certain design limitations compared to additive manufacturing due to its subtractive nature. 

In this guide, we’ll explore how CNC machining works, its core advantages and challenges, and the differences between its two main methods—milling and turning. 

How does CNC machining work? Let's talk milling and turning

CNC machining works by using computer-guided tools to remove material from a solid block, creating parts with precise dimensions. The process primarily involves two techniques: milling and turning. Each method is tailored to specific part geometries and production goals. 

Let’s break down how parts are manufactured using these two distinct machine setups. 

How does CNC milling work?

CNC milling is the most widely used type of CNC machining and is often what people refer to when they mention CNC. Milling machines use high-speed rotating cutting tools to shape a stationary workpiece mounted on a machine bed. 

Most machines operate along three linear axes (X, Y, and Z), while advanced 5-axis CNC mills offer additional rotational movement, allowing to produce complex geometries and reducing the need for multiple setups. 

Here’s how a CAD model becomes a finished part through CNC milling: 

1. The digital CAD model is converted into G-code, the machine-readable language that guides tool paths. 

2. A precisely sized material block (called a blank or workpiece) is securely mounted to the bed using a vise or clamps. 

3. Proper alignment is critical—tools like touch probes ensure accurate positioning before machining begins. 

4. High-speed cutting tools remove material in multiple stages: a rough pass for speed, followed by finishing passes for precision. 

5. If the part has features that can’t be reached from one side, the workpiece may need to be repositioned or flipped for additional operations. 

After machining, milled parts require deburring. Deburring is the manual process of removing small defects from a finished part. These defects, generally found on sharp edges, come from material deformation during machining. For example, when a drill exits the far side of a through hole, it will leave blemishes that need removal. This is a standard part of the process when you order CNC milled parts from Protolabs Network. 

We also inspect the part’s critical dimensions to ensure the correct tolerances are achieved. After this, the part is ready for use or for post-processing.  

There is a lot to explore when it comes to coatings and finishes for CNC-machined parts (milled and turned alike), so here are some helpful resources:  

• Part marking for CNC machining  

• Types of surface finishes for CNC machining 

• Heat treatment for CNC machined parts 

How does CNC turning work?

CNC turning

is a subtractive manufacturing process that uses a rotating workpiece and stationary cutting tools to shape parts. This makesit ideal for components with rotational symmetry. The material is mounted on a spinning chuck, while precise tools trim it down to the desired geometry. 

CNC turning (or CNC lathing) is commonly used to produce cylindrical parts quickly and cost-effectively. Advanced multi-axis turning centers can also handle complex geometries by integrating milling capabilities—enabling production of parts like camshafts or impellers in fewer steps. Here's how CNC turning works: 

1. The CAD model is translated into G-code, which instructs the machine. 

2. A cylindrical blank is loaded and spun at high speeds. 

3. Stationary cutting tools shape the part's exterior and can drill along the center axis. 

4. For more complex shapes, the part may be flipped or repositioned. 

5. Once machining is complete, the part is ready for inspection or post-processing. 

A quick guide to CNC machining parameters

CNC machining parameters are typically defined during G-code generation and directly impact part accuracy, quality, and build capacity. Two of the most important considerations are machine build size and dimensional tolerance. 

CNC machines offer large build volumes—significantly larger than most 3D printers. For example, CNC mills can handle parts up to 2,000 x 800 x 100 mm (78" x 32" x 40"), while CNC turning can turn parts up to Ø 500 mm (20"). 

CNC machining is also known for high precision. Tolerances can reach as tight as ±0.025 mm (0.001"), which is less than half the width of a human hair. If no tolerance is specified in your technical drawing, manufacturers typically default to ISO 2768 standards—usually around ±0.125 mm (0.005"). 

What are the most common cutting tools for CNC machining?

To produce a wide variety of part geometries, CNC machines rely on an extensive selection of cutting tools, each tailored to specific machining operations. In CNC milling, common tools include: 

• Flat head, bull head, and ball head tool are among the most common tools used in CNC milling. These tools are designed to machine features like grooves, cavities, and vertical walls. While flat and bull nose tools are ideal for sharp or slightly rounded edges, ball nose tools are essential in 5-axis CNC machining, especially when working with freeform or contoured surfaces. 

• Drill bits are the go-to tools for quickly and efficiently creating holes. For non-standard hole diameters, a plunging flat end mill following a helical toolpath can be used instead. 

• Slot cutters have a unique geometry where the cutting edge is wider than the tool shaft. This allows them to remove material from the sides of vertical walls—ideal for creating T-slots and undercuts that standard tools cannot reach. 

• Threading taps are used to cut internal threads in pre-drilled holes. Creating accurate threads requires tight synchronization between the spindle's rotation and the tool's feed rate. Despite advances in automation, many workshops still rely on manual tapping for high-precision threading. 

• Face milling cutters are used to machine wide, flat surfaces. These tools have larger diameters than standard end mills, allowing them to remove material more quickly with fewer passes—significantly reducing cycle times. Face milling is often used early in the machining process to square up the block or prepare a smooth surface for further work. 

CNC turning tools serve functions like face cutting, threading, and grooving. These tools are optimized for rotational symmetry and often support high-speed material removal. 

CNC machining parts with geometric complexity: what are the design restrictions?

While CNC machining provides a high level of design freedom and precision, it also comes with inherent limitations—especially when compared to additive manufacturing like 3D printing. The more complex your design, the more expensive and time-consuming it becomes to machine, largely due to additional setup steps, tool changes, and part reorientations. 

Tool geometry constraints 

Most CNC cutting tools are cylindrical with a limited cutting depth. As a result, sharp internal corners are difficult to machine—you’ll often need to design with internal radii that match the tool’s diameter. This geometric limitation can impact feature resolution and part detail. 

Machine accessibility 

Access is another major constraint. In 3-axis CNC machining, tools can only approach the part vertically, which limits the number of accessible surfaces. This restricts the complexity of parts unless they are repositioned manually during the process. In contrast, 5-axis CNC machines offer much greater flexibility, as they allow simultaneous movement along multiple axes—giving the tool better access to difficult geometries. 

Thin walls and fine features 

Parts with thin walls or delicate features are particularly prone to vibration and breakage during machining. To ensure structural integrity and surface quality, we recommend maintaining a minimum wall thickness of 0.8 mm for metals and 1.5 mm for plastics. Designing beyond these minimums improves both manufacturability and durability. 

Understanding these key limitations is essential for designing CNC-friendly parts that meet tight tolerances without excessive cost or rework. For more tips on optimizing your design for CNC manufacturing, check out this article. 

What are the characteristics of CNC machining?

One of the greatest strengths of CNC machining is its compatibility with an exceptionally wide range of materials, including both metals and plastics. Whether you're producing prototypes or end-use parts, CNC machines can work with almost any engineering material, from aluminum and stainless steel to engineering plastics like POM, nylon, and ABS. 

CNC machining predominantly involves metals for both prototyping and larger production runs. Metals like aluminum, brass, and titanium are commonly used due to their high strength, durability, and excellent machinability. 

While plastics can also be CNC machined, they are generally more difficult to work with because of their lower stiffness and melting temperatures. That said, one common and valuable use case is machining functional plastic prototypes. These prototypes allow you to test design performance before moving on to injection molding for full-scale production—helping you avoid costly design changes later in the process. 

Unlike 3D printing, CNC-machined parts retain the fully isotropic mechanical properties of the original bulk material. This makes CNC machining an ideal choice for parts that require structural integrity and tight tolerances. 

How much do CNC machining materials cost?

With a wide range of materials available for CNC machining, costs can vary significantly depending on the material you choose. Each material not only has a different price per blank but also affects the total cost of production due to its unique physical properties and machinability. 

For metal parts, Aluminum 6061 is one of the most cost-effective options. This is because machinability plays a major role in determining final part pricing. 

Take stainless steel, for example. While it offers excellent strength and corrosion resistance, it's significantly harder than aluminum. This increased hardness makes it more difficult and time-consuming for a machine, which drives up the cost due to longer machining times and tool wear. 

Below, you’ll find a comprehensive overview of the most used CNC machining materials on the Protolabs Network platform, along with key characteristics like strength, machinability, and ideal applications. This information can help you make informed decisions when selecting the right material for your part—balancing performance and price. 

How can I learn more about materials for CNC machining?

Protolabs Network offers a wide variety of materials for CNC machining, each with its unique properties and advantages. Whether you're looking to learn more about specific materials or comparing material sets, there’s a lot to explore. 

For an in-depth understanding of our materials, we invite you to check out our specialized videos available on our YouTube channel. These videos provide detailed insights into the characteristics, applications, and benefits of each material, helping you make informed decisions for your CNC machining projects. 

Post-processing and surface finishes for CNC machining

When CNC-machined parts come directly off the machine, they often have visible tool marks, which may not meet the desired aesthetic or performance standards for certain applications. To improve the appearance, durability, and functionality of your parts, various post-processing methods can be applied. 

Some of the most popular finishing techniques include anodizing, bead blasting, and powder coating. These methods not only enhance the surface finish but also improve key properties such as wear resistance, corrosion resistance, and chemical resistance—critical for parts exposed to harsh environments. 

While this guide offers a general overview, we recommend exploring our detailed explainer on the most common post-processing techniques and surface finishes used in CNC machining. This will give you a deeper understanding of how to achieve the perfect finish for your custom parts. 

What are the advantages and disadvantages of CNC machining?

While CNC machining is a highly versatile and ideal manufacturing process for many applications—from prototyping to medium-scale production of end-use parts—it does come with its set of challenges. In this section, we will explore both the advantages and limitations of this subtractive manufacturing process. 

Advantages of CNC Machining 

One of the primary benefits of CNC machining is its exceptional accuracy and repeatability. Whether using CNC milling or CNC turning, this process is capable of producing parts with tight tolerances, making it ideal for high-end industries such as aerospace, aviation, and automotive. Parts produced through CNC machining exhibit fully-isotropic physical properties, ensuring strength, durability, and precision, which are critical for demanding engineering applications. 

In terms of cost-effectiveness, CNC machining is often the most efficient method for producing low-to-medium volumes of metal parts. It is particularly well-suited for producing single prototypes or parts in quantities of up to 1,000 units. 

Disadvantages of CNC Machining 

However, despite its numerous advantages, CNC machining is not without its limitations. The subtractive nature of the process means that more complex geometries can become costly or even impossible to manufacture. Additionally, the setup costs for CNC machining are typically higher than those of 3D printing. For engineers looking to produce low-cost prototypes, especially in plastic, 3D printing may be a more cost-effective and quicker option due to its lower startup costs. 

Lead times for CNC machining also tend to be longer compared to 3D printing. On average, the lead time for CNC machining is 10 days, whereas 3D printing can often deliver in as little as 2-5 days. Moreover, CNC machines are less widely available than 3D printers, and they require specialized knowledge to operate effectively, which can add another layer of complexity. 

What are Protolabs Network's rules of thumb for CNC machining?

Let's break down the key parameters to consider for CNC machining both metal and plastic custom parts.