CNC Machined Parts: Definition, Advantages, and Design Guides

In today’s world, machined parts are everywhere and it’s not hard to see why. These components offer precision and efficiency affordably, and they’re therefore part of most machinery.

But what exactly are machined parts? And what makes them so essential?

In this article, I’ve compiled extensive up-to-date information on everything you need to know about CNC machining and CNC machined parts. I provide my expert opinion as a professional engineer with over 8 years of experience. Gain valuable insights into CNC machined parts materials, tolerances, advantages, design guidelines, and CNC machining services overall. Let’s dive in!

What Are Machined Parts?

Machined parts

Machined parts are components that are crafted using the machining process. But what exactly is machining?

Machining involves the use of subtractive cutting machines such as mills, lathes, and grinders to create a desired part. Although these machines employ different techniques, the goal is ultimately the same – to remove material from a workpiece and create the desired shapes.

Machining can be a manual or digital process. 

  • Manual machining – a skilled machinist will use manually operated cutting machines to shape components. 

  • Digital machining – as the name suggests, this is an automated cutting method and it involves the use of Computer Numerical Control (CNC) machines. In this digital manufacturing process, pre-programmed instructions are fed into the CNC machine and it cuts parts based on these specifications. The result? Unparalleled precision, uniformity, and efficientmachined parts.  

CNC machines have been a revolutionary technology in the manufacturing industry. And with an online CNC machining service, you can easily get CNC parts that suit your unique business needs. That being said, there is still a place for manual machining and other manufacturing processes such as molding, casting, and forging.   

Materials Used for Machined Parts

CNC Machined Part

CNC machining can handle a wide array of materials ranging from metals to plastics and ceramics. However, all machining materials are not created equal. When choosing the material for your CNC machined part, I’d advise you to comprehensively consider factors such as cost, part weight, electrical conductivity, heat resistance, surface finish, and the intended use of the component.   

Also, some CNC machining materials are easier to work with than others. Materials that are especially hard are challenging to cut through and they may damage the cutting tool. The manufacturing process can also create profound vibrations that affect the accuracy and surface finish of machined parts.

Conversely, materials that are overly soft are prone to warping under the cutting force. Of course, this would affect the quality of CNC machined parts.

A custom CNC machining shop can advise you on the best materials for your specific project. In this section, I’ll cover some of the materials that are often used with CNC machines.


Metals are some of the most commonly used materials in CNC machining services. Metal parts have diverse industrial applications thanks to features such as electrical conductivity, high strength, and good mechanical properties.

Different metals have varying standout properties. For instance, stainless steel is popular for its corrosion resistance, versatility, and intense strength. On the other hand, we’ll use Aluminium for its lightweight, thermal conductivity, and corrosion resistance properties.

Some of the most commonly used metals in CNC machining services include aluminum, stainless steel, copper, titanium, magnesium, zinc, and mild steel. Sometimes, we may use metal alloys in the production of machined parts. Bronze is a popular metal alloy favored for its appealing surface finish, malleability, and chemical resistance to rust.

Metal Material Ultimate Strength (PSI) Fatigue Strength (PSI) Hardness (Brinell) Machinability
Stainless Steel 304 84000-170000 30000-63000 170-360 40%
Stainless Steel 304L 78000-170000 25000-62000 160-350 40%
Stainless Steel 430F 79800 30000 170 75%
Stainless Steel 440C 100000-290000 38000-120000 222 40%
Steel 1018 63000-70000 26000-38000 130-140 78%
Steel 1215 78300 N/A 167 136%
Aluminium 2024 45000 20000 120 360%
Aluminium 7075 83000 23000 150 360%
Brass C26000 46000-99000 14000 210 30%
Brass C36000 48000-77000 6000 78 100%
Titanium 6AL-4V 131000 74000 334 22%
Metal Material Hardness (Brinell) Machinability
Stainless Steel 304 170-360 40%
Stainless Steel 304L 160-350 40%
Stainless Steel 430F 170 75%
Stainless Steel 440C 222 40%
Steel 1018 130-140 78%
Steel 1215 167 136%
Aluminium 2024 120 360%
Aluminium 7075 150 360%
Brass C26000 210 30%
Brass C36000 78 100%
Titanium 6AL-4V 334 22%


CNC Machining Plastic

Another material used in CNC machining is plastic. Plastic parts are lightweight, versatile, and corrosion-resistant.

Commonly used plastics used in CNC machining services include Nylon, Acrylic, Polycarbonate, ABS, PVC, Polyethene, and Delrin.

Thanks to their versatility, plastic components are used in consumer electronics, medical equipment, and automotive industries. Plastic parts are also used to make electrical insulation components.

Besides metal and plastic, CNC machining centers also use ceramics and composite materials to manufacture parts. Composite materials consist of two or more constituent materials with varying properties. They have a matrix material such as fiberglass and epoxy resin.

Machined Part Tolerances

CNC Precision Machining

Machined part tolerances refer to the acceptable deviation from the specified dimensions. Some of the factors affecting CNC machine parts tolerances are:

  • Machining method

  • Material used

  • The complexity of the design

  • Cost implications

CNC machining is known for its remarkable capability to produce parts to tight tolerances. This comes in handy, especially with crucial dimensions such as diameter, lengths, and hole sizes. Abiding by the specified machining tolerances is critical to the functionality of parts. Tighter tolerances call for accurate CNC machining procedures which can increase the cost of production. 

Conversely, looser tolerances allow for a more laid-back machining approach but they can compromise component performance. These more relaxed specifications are common with prototypes and non-mechanical components. 

Machined Part Surface Finishes

CNC turning parts

After the CNC machining process, we may treat machine part surfaces to enhance their appearance and texture. The surface finish plays a functional and cosmetic role. 

As-machined Surfaces

As the name suggests, as-machined surfaces are not treated after the CNC machining process. Instead, they are left as is and they maintain their natural surface finish. As-machined surfaces are commonly adopted for internal parts since they only contribute to functionality and not aesthetics. 

Bead-blasted Finishes

Bead blasting involves the use of abrasive media to create a matte surface finish on machined parts. The resulting surface finish will vary depending on the level of roughness. 

The bead-blasting process is not compatible with delicate machined parts. This is because it can alter their geometry by removing material.  


After CNC machining service, we may perform anodization on the resulting component. Anodization is commonly used in the treatment process of aluminum machined parts. This procedure creates a strong corrosion-resistant finish. The coating also provides wear resistance for the component.

Powder Coating

This surface treatment method entails coating the surface of machined parts with powdered paint and baking it in an oven. The resulting wear-resistant and corrosion-resistant layer holds up better than a paint coating. As a bonus, powder coatings come in a variety of beautiful colors and, therefore, allow for beautiful custom machined parts.

The Advantages Of Machined Parts

CNC Machined Part

CNC machined parts offer a variety of benefits compared to parts that have been manufactured using other production processes. In this section, I’ll cover some of these advantages. 

#1 No MOQ

One of the most notable perks of CNC machined parts is that there is no MOQ or minimum order quantity. This means that we can efficiently create a small batch or even one-off custom parts at relatively low costs. 

This is not always possible with other manufacturing methods. With molded parts, for instance, there is a need to fabricate metal tooling first. This is an expensive process and, therefore, manufacturing a few custom parts would not be cost-effective. 

Given that machined parts do not have a MOQ, the CNC machining process is suited to businesses with varying quantity demands, customized designs, and prototyping. This brings me to the second point. 

#2 Good prototypes

A standout feature of CNC machining is its precision and speed. We can create high quality prototypes fast and with a high level of accuracy. Such prototypes are similar to the final product and this comes in handy during assessment and design verification. 

Rapid prototyping also makes it possible to create multiple variations of a part for the testing and validation process. We would then manufacture CNC machined parts based on the best-performing prototype. 

This is not always possible with other manufacturing methods. With molded parts, for instance, there is a need to fabricate metal tooling first. This is an expensive process and, therefore, manufacturing a few custom parts would not be cost-effective. 

Given that machined parts do not have a MOQ, the CNC machining process is suited to businesses with varying quantity demands, customized designs, and prototyping. This brings me to the second point. 

#3 Design freedom

CNC machining offers unmatched design freedom. We can manufacture machined parts that feature intricate details and elaborate geometries with a high degree of accuracy.

This makes it possible to actualize imaginative concepts that would be otherwise tricky or unachievable. The flexibility associated with CNC machining services facilitates the creation of custom machined parts.

#4 Quality

CNC machining allows us to create top-tier machined parts. This production process caters to tight tolerances, excellent surface finishes, precision, and the highest quality standards. Therefore, you can expect components that completely satisfy design specifications.

This aspect makes CNC machining the go-to production process in industries such as aerospace, medical, and military where high quality parts are vital. 

#5 Lead times

CNC machining boasts a very brief lead time. The design and prototyping process can be completed relatively fast, especially since there is no lengthy tooling process like with other production methods. 

Once the prototype is approved, we move on to the production of parts. Again, the manufacturing process is fast and efficient and this means a shorter lead time. Businesses that opt for CNC machined parts tend to see shorter time-to-market which is a competitive edge.  

#6 Alterations

Sometimes, it becomes necessary to adjust part design. Since CNC machined parts are manufactured using digital CAD files, it’s relatively easy to make these changes. All we have to do is update the technical drawings and then produce a new part based on the modified specifications.  

Effortless alterations effectively lower the CNC machining service cost and duration. These easy adjustments also give CNC machining an edge over other production processes. If enhancements are required with molding, for instance, it could mean extensive retooling. Of course, this has significant cost and time implications. 

#7 Strength

With the manufacture of parts, performance and durability are always a concern. Machined parts tend to be stronger and more resilient compared to parts produced using other processes. This is because CNC machining does not cause internal stresses typical of other production methods. 

Also, machined parts are derived from solid material blocks. This results in parts that are structurally sound and more durable.  

The Design Guide of Machined Parts

CNC Machining Design Guide


Undercuts are features that can’t be created using standard cutting tools. The part is typically obstructing some of the surfaces making them inaccessible from the top.

There are two types of undercuts: T-slots and dovetails. To machine such cuts, we’ll need to employ a specialized cutting tool. The width of an undercut will range between 3 and 40mm. The undercut depth should be no more than twice its width. 

Also, you need to consider clearance for the cutting tool if the design has undercuts on internal walls. The general principle is to ensure that the space between the machined wall and the internal wall is at minimum four times the undercut depth. 

I’d recommend sticking to standard sizes (i.e. whole millimeters) for the undercut dimensions. This is because cutting tools come in standard sizes. 

If the design features a non-standard undercut, the manufacturer would need to first create a custom cutting tool. Of course, this adds to the lead time and production cost. If speed and cost are a concern for your business, I’d recommend avoiding such undercuts. 

Wall thickness

Injection molded parts are likely to warp if walls are overly thick. The opposite is true for CNC machined parts. Very little thickness will cause a decrease in the stiffness of the part and more vibrations during CNC machining processes. Overall, this tends to decrease the attainable precision. 

For metal machined parts, the recommended wall thickness is 0.8mm. Walls of plastic machined parts should be at least 1.5mm. If you opt for a lower wall thickness, the plastic part tends to deform (because of residual stress) and soften due to higher temperatures.

If possible, opt for a larger minimum wall thickness for your design. However, if this is not feasible, you can settle for a different manufacturing method such as injection molding.  


Protruding features are hard to execute through the machining process. Just like with thin walls, these features are prone to damage due to vibrations during machining. Additionally, the vibrations may decrease the achievable accuracy. 

A good rule of thumb is to ensure that the height of protruding sections does not exceed four times its width.

Cavities, holes, and threads

CNC Milling

The holes and cavities on a part will vary depending on your tools. The recommended cavity depth is 4 times its width. More shallow cavities result in heightened vibrations. You’re also likely to end up with rounded rather than potentially sharp edges for deeper cavities. 

CNC technology can cut threads down to M6. When designing thread features, remember that the thread length should be at least 1.5 times its diameter. The recommended thread length is however 3 times its diameter. Any length longer than this value is virtually pointless. This is because the first few teeth take on most of the load. 

Holes are usually created using drill bits or end mill tools. Since drill bits come in standardized sizes, I’d recommend incorporating hole diameters that match standard drill bit sizes. Also, remember that the maximum recommended depth of a hole is 4 its diameter. 


The size of parts that can be machined is defined by the machine work envelope, which varies depending on the type of material and CNC machine deployed. 

The size of milled parts is typically limited to a maximum of 400 x 250 x150 mm. The maximum dimensions for turned parts are ⌀ 500 mm x 1000 mm.

It is possible to manufacture larger parts if you’re using a bigger machine. You can seek the advice of your manufacturing partners before proceeding with the custom CNC machining process. 


CNC machining has been a game-changer in the industrial production of parts. It comes with a host of benefits ranging from no MOQ and rapid prototyping to a shorter lead time and improved quality control.

Online CNC machining services have made it even more convenient to obtain quality CNC parts.

Aria is your one-stop shop for custom CNC machining services, sheet metal fabrication, 3D printing, and much more. Contact us today!


Q: How much does CNC machining cost?

A: The cost of CNC machining will vary depending on the complexity of the design and the number of required parts. Get an instant quote for online CNC machining services by simply submitting a CAD model.

Q: How does CNC machining work?

A: CNC machining is a subtractive process. It utilizes computerized controls and cutting tools to remove materials from a workpiece and form the desired custom part.


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.