CNC Machining

The ultimate CNC machining guide

By February 23, 2022
CNC turning machine in action

Computer Numerical Control (CNC) machining is a modern, efficient, and automated subtractive manufacturing process that allows manufacturers to make repeatable parts of consistent quality from a wide range of materials, including plastics, metals, and composites. Today, CNC-machined parts can be found in a variety of different industries, from aerospace to automotive to medical.

The beginnings of CNC technologies can be traced back to a few different points in history. The first modern machining tools appeared in 1775 when industrialists developed a boring tool that allowed manufacturers to produce steam engine cylinders with more consistent degrees of accuracy and precision.

The technological forebears of modern computing appeared earlier in the century, however, emerging from the textiles industry. In 1725, Basile Bouchon began using a system of punched holes in paper tape to encode data. Decades later in 1805, Joseph Marie Jacquard refined the process, swapping out the paper tape for sturdy punch cards. When tied in a sequence, the punch cards directed how the fabric was added to the loom, simplifying the process and providing one of the first examples of programming. This machine, the Jacquard loom, is in a way the forebear of our modern computing and automation solutions.

Hanging jacquard loom
Jacquard loom

This guide to all things related to CNC machining will walk through how modern CNC machining processes work, including material options, design considerations, cost drivers, and more. Keep reading or click here to download the PDF.

Types of CNC machining

Today, there are several technical processes included underneath the CNC machining umbrella.

3-axis machining: Milling and turning

Once the operator enters the machining instructions, 3-axis CNC machines will carry out the input operations by using cutting tools to cut along three axes: X (left to right), Y (front to back), and Z (up-and-down). CNC milling and CNC turning are two common examples of 3-axis machining.

3-axis CNC milling involves fixing the block of material — also called the workpiece or “blank” — with a vice or machine bed. This holds the workpiece stationary as cutting tools and rotating drills, attached to a central spindle, make cuts to remove material and shape the final component. The 3-axis CNC milling machines are easy to program and operate and can be used to create most parts with simple geometric designs.

Since the cutting tools and drills are limited to three axes, some design features or areas of the blank may be difficult to complete or reach (e.g., unconventional shapes or deep, narrow cavities). However, multiple setups can help orient the workpiece for the machines to reach these features. While almost every 3-axis machining job has more than one setup per part, too many setups per part can quickly increase production costs. Processing geometrically complex parts can, for instance, require operators to manually reposition workpieces, which can impact processing speed, increase labor-related costs, and create more opportunities for human error.

The CNC turning process operates a little differently. The blank is fixed to a rotating spindle, and a lathe then shapes the piece’s inner and outer perimeters or bores holes around the workpiece’s center axis. The most significant design restriction with CNC lathes is that they always produce rotationally symmetrical parts. The process is ideal, therefore, for the manufacturing of parts such as screws, bowls, or chair legs. For these types of rounded parts, lathes can produce pieces faster and at lower costs than CNC milling machines, especially when producing parts in high volumes.


5-axis or multi-axis machining

Multi-axis machining processes allow the cutting implements greater freedom of movement while the workpiece is milled or turned. 3-axis processes allow either the blank or the cutting tool to rotate, but not both. In contrast, 5-axis processes allow both the workpiece and the tool to rotate and move along the X, Y, and Z axes simultaneously. 5-axis CNC machining saves time and is ideal for creating complex and precise parts like those found in the medical, oil and gas, and aerospace industries.

The multi-axis CNC machining family includes three processes:

Choosing the right type of CNC machining

When selecting a manufacturing process for your part, you’ll need to evaluate if it’s a better fit for 3-axis or 5-axis machining. Generally, parts with simpler designs can be made quickly and affordably using 3-axis machines, while 5-axis machines are better for creating geometrically complex pieces with increased speed and accuracy.

If you’re on a budget or only need to cut a flat surface, 3-axis machines are the way to go. In addition to being more affordable, 3-axis machines are simpler to program, so you won’t have to incur the cost of working with expensive 5-axis programmers and operators. Plus, prep time is shorter with 3-axis machining.

If you need to produce a deeper part or one with complex geometry, you’ll want to use 5-axis machining. Using 5-axis machines lets you machine the workpiece from all sides with no manual rotation required. With 5-axis machining, you’ll have higher yields, greater accuracy, and increased freedom of movement, as well as the ability to manufacture larger parts faster.

Common CNC materials

One of the advantages of CNC machining is that the process is compatible with many different materials, plastics and metals being the two most common material families. Here are some of the plastics and metals you’re likely to encounter.

CNC machine drilling plastic

High-performance polymers

Various colored plastics
Stock manufacturing plastics
PMMA part
CNC machined acrylic (PMMA) part


Steels and stainless steels, aluminums, and soft metals like brass, bronze, titanium, and copper are among the most popular machinable metals. While steels and aluminums are the most commonly machined, each family of metals offers a different set of physical properties and price points.

Steel blanks
Steel blanks for machining

CNC surface finishes and post-processing

After a part or component has been machined, it may need to undergo one or more finishing processes. These processes can be used to remove aesthetic flaws, improve a product’s appearance, provide additional strength and resistance, adjust electrical conductivity, and more. Common finishing processes for machined parts are anodizing, powder coating, and bead blasting. However, it is also common to leave parts as-machined or as-milled when other finishes are not necessary.

Common CNC finishing options

Anodized parts
Bead blasted parts
Black oxide parts
Powder coated parts
As machined parts

Choosing the right materials and finish

Selecting materials and post-processing options for your part relies heavily on how and where your part will be used. Key end-use considerations for selecting your part material and surface finish include:

Designing for CNC machining

Designing a high-quality part that will perform as expected is only half the challenge. You must also be able to machine the part in an efficient and cost-effective way. Design for manufacturability (DFM) is the process framework that helps integrate how a part will be made with the part design. DFM is critical to manufacturing and affects everything from production timelines and costs to operational efficiency and the quality of the part itself. The most common design considerations for CNC machining include:

Deep pockets:

Features like deep or narrow pockets and slots require longer tools to machine properly. However, longer cutting tools are more prone to breaking and are often less precise due to machining vibrations, or chatter. Deep features also typically require several cuts to machine properly, which increases both machining time and production costs — especially since smaller tools will likely be necessary to complete the finishing passes. If slots or deep pockets must be machined, aim to either reduce the depth of the feature as much as possible or increase the area of the pocket’s cross-section. The depth of a pocket or slot should be no more than 3x the diameter of the smallest tool needed.

Illustation of deep pockets

Narrow regions:

Narrow features limit the size of viable cutting tools. This can present a challenge for manufacturers because of how susceptible long, small-diameter tools are to chatter and breakage. Just as with deep features, narrow regions should be no less than 3x the diameter of the smallest cutting tool. Minimizing the depth of these features allows you to use shorter, larger-diameter cutting tools, which helps to reduce machine chatter.

Illustation of narrow regions

Sharp internal corners:

CNC end mills are circular, which makes it difficult to machine sharp internal corners because the bits will leave behind a corner radius. While there are methods of machining sharp internal corners, these processes tend to be costly. In general, sharp internal corners should be avoided for machined parts. Internal corner radii should also be larger than the radius of the intended cutting tool. By designing internal radii to be larger than the cutting tool, the machine can create a smooth transition between walls. In contrast, an internal radius that is the same size as the cutting tool will force the machine to make a sudden change in direction, resulting in a less smooth finish.

Inaccessible features:

Features like counterbores — those that open inside another pocket or pockets with negative drafts — can pose a significant challenge for production teams. These design features take considerably more time to machine because of how difficult it can be to allow the cutting tool access. It’s critical to ensure that cutting tools can freely access all of a part’s features without being blocked by any other feature.

Illustration of Inaccessible features

Outside fillets:

Outside fillets, or rounded corners along the top edge of pockets, bosses, slots, and other features, require custom cutting tools supported by a precise machining setup — a pairing that can quickly become expensive. Beveling or chamfering the outside edges can help avoid these costs.

Illustration of fillets

Wall thickness:

Thin walls can create issues for metal and plastic parts alike and should be avoided wherever possible. Thin metal walls are more prone to chatter, for instance, which negatively impacts the accuracy of the part and its surface finish. Plastic parts with thin walls are also more likely to warp or soften. The minimum wall thickness for metal parts should be 0.03” (0.762mm) and 0.06” (1.524mm) for plastic parts.

Illustration of wall thickness

Flat-bottomed holes:

Holes with flat bottoms are not only difficult to machine, but they also tend to create difficulties for any subsequent operations. In general, product teams should avoid blind holes with flat bottoms in favor of standard twist drills, which create holes with cone-shaped bottoms.

Illustration flat holes

Threaded parts:

When machining internal or external threads, there are a few different design considerations.

For internal threads:

For external threads:

Illustration of threads

Drill entrances and exits:

If the surface of a workpiece is not perpendicular to the axis of a drill, the drill tip will wander when it comes into contact with the material. To minimize uneven exit burrs and streamline the burr-removal process, ensure that hole features have start and end faces that are perpendicular to the drill axis.

Illustration of drill entrance

A checklist for reducing costs

1. Keep parts simple
Parts with complex designs or manufacturing processes can have a number of downstream effects. Complexity can increase machining time and cost, introduce opportunities for error, and make it difficult for end-users to understand the exact use of components or parts. Separating which characteristics and design features are vital to the viability of the part from those that are desirable yet non-critical helps remove unnecessary complexity from a part’s design and machining operations.

Metal parts

2. Design with tool geometry and setup orientation in mind

While most CNC machining jobs require more than one setup orientation, you’ll want to keep the number of orientations per part to a minimum. For example, if you need to flip a workpiece three times for the cutting tools to be able to access all of the features, this is going to increase costs because each setup requires re-mounting the part, zero-ing the machine to establish the correct axes, and running a new G-code program. 5-axis machines are going to be more capable in this aspect as they can reach more features without requiring multiple setups, though this is dependent on the geometry of a particular part.

Minimizing the number of cutting tools required is another way to reduce costs. As much as possible, try to keep radii consistent so that fewer tools are needed to machine internal corners. Likewise, avoid very small details where possible. Smaller cutting tools aren’t able to cut as deep into the workpiece and are also more prone to breaking. Typically, the smallest internal features that can be machined are 0.0394” (1mm), though holes can go as small as 0.0197” (0.5mm).

Cutting tools

3. Balance your tolerances

Choose your critical dimensions thoughtfully. Not every dimension requires inspection or is critical to the viability of the part, so focus your attention on those that are most essential. Determining how precise specific features need to be will allow you to adjust tolerances accordingly, reduce costs, and streamline the manufacturing process. Make sure that tolerances are still within machineable limits, adhere to given standards, and account for tolerance stacking.

Avoid over-dimensioning your part, as well. Unclear part drawings defeat the purpose of geometric dimensioning and tolerancing, which is to clarify and streamline communication. At Fast Radius, we recommend that you align drawing datums with the CNC coordinate system, as using consistent datum reference frames between machining and measurement systems is more likely to ensure parts are accurately located.

If possible, ensure all datums — points, surfaces, or axes used as references for measurement — reference the same setup. If a datum was machined in a previous setup, it becomes more difficult to hold tight tolerances after a setup change. Finally, be conscious about your datums. Choosing reference datums that are easy to measure from will ease manufacturing.

CNC designs

4. Tailor inspection levels

Similar to balancing your tolerances, strategically applying the right inspection levels can help reduce costs and optimize the production processes. Typical inspection options may include a visual inspection of the part, checking part dimensions using hand tools, or a first article inspection (FAI) where the first manufactured part is checked to ensure all requirements have been met. More in-depth inspections require more time and labor, which increases manufacturing costs.

Increasing the number of parts that you inspect will also increase the cost of production, so it’s important that you select the right sampling plan to ensure confidence across the entire order. At Fast Radius, our default sampling rate is based on ANSI ASQ Z1.4 Level 2.

Person inspecting part

5. Design to common stock sizes

Designing parts to common stock sizes can help streamline the manufacturing process by minimizing the amount of material that needs to be removed from the workpiece. For example: if it’s acceptable for a part surface to retain its stock finish — that is, if the surface can be rougher than 125 μin Ra — and it does not need to be machined, you have an opportunity to reduce machining time without sacrificing part quality.

This is why knowing the common stock sizes for various materials is important. Metals, for instance, are typically available in a number of stock forms, including sheets, rods, bars, or tubes, which can be purchased according to dimensions like length, width, thickness, and diameter.

Keep in mind that metric sizes are more commonly available in Asia, while imperial sizes are more commonly available in the U.S.

6. Minimize deformation risk

Removing a large amount of material from a workpiece can cause the material to deform. This requires additional processing during machining to prevent deformation, leading to higher costs. Plastics have a higher risk of deformation than metals.

CNC machining plastic

7. Choose your material carefully

The material you pick has a significant impact on the overall cost of your part. For example, metals are generally more expensive than plastics. Avoid over-engineering your parts and consider whether a specific grade or material certificate is required for the part. If a generic material can fulfill the part’s requirements or multiple materials can be accepted, select the less expensive material to keep costs low.

The machinability of the material should also be taken into consideration. Steels, for example, are typically more difficult to machine, which leads to higher costs because steel parts take longer to machine and increase wear-and-tear on cutting tools.

CNC machining metal

Common applications for CNC machining

Many industries rely on CNC machining processes to produce reliable, accurate parts that meet precise specifications and regulatory requirements, including the aerospace, automotive, medical device, electronics, and commercial parts manufacturing sectors.

Close up of CNC machine in action

Aerospace parts are held to high quality, testing, and regulatory standards in order to ensure proper function, fit, and safety. CNC machining is an excellent fit for creating plastic and metal parts with extremely tight tolerances that satisfy the aerospace manufacturer’s need for an incredible degree of precision and a range of highly specialized parts. The same is true for automotive components.

CNC machining can also be used to create a variety of medical devices, from implants to surgical implements to components for medical electronics.

Care and precision are critical when producing these sorts of devices and tools, as they are held to additional safety standards and requirements. Semiconductors and electronics components also have incredibly stringent precision requirements and tolerance standards, given their size and complexity. As a process, CNC machining has few material limitations, allowing it to be used with conductive materials like silicon. Many commercial parts — from aluminum castings and extrusions to steel and plastic parts — can also be machined.

Another common application for CNC machining is tooling, or the process of creating the various components, tools, machinery, and master patterns that will be used in production. Tooling is an integral part of every manufacturing and molding process and encompasses items like molds, jigs, and fixtures.

Starting your CNC machining project with Fast Radius

At Fast Radius, we make it simple and straightforward to get quality parts through our CNC machining service. To get started, visit, upload your part designs, and you’ll receive instant DFM feedback. The site also allows you to manage designs and orders from a single, intuitive interface.

When you choose Fast Radius, you’re choosing to work with a seasoned group of experts. You’ll receive the full support of our team of engineers, customer success managers, account executives, and others across the business. We’ll also leverage options for domestic and international CNC machining to ensure that your project is carried out as efficiently as possible, without sacrificing quality.

Contact us today to experience the Fast Radius difference or click here to download our comprehensive guide as a PDF.

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