CNC Machining Complex Parts

CNC Machining Complex Parts

Across the manufacturing industry, innovations in technology for CNC milling, turning, and wire EDM have allowed for highly complex geometries to be machined with ease. In this article, we discuss the types of complex parts that may be fabricated and manufactured with CNC — and how Fictiv may be the right company to manufacture your custom CNC parts, on demand.

Like all modern practices, design for manufacturing requires thorough knowledge of the CNC machining equipment’s capabilities. Advanced manufacturing techniques like milling and turning make machining complex parts possible. But best practices must always be considered to keep the project cost-effective. We’ll take you through some CNC machining methods to explain these practices and demonstrate how these advanced machining techniques have become an art within the manufacturing world. 

Why should you use CNC machining for creating complex parts?

To understand “why,” we need to first understand the capabilities of modern-day manufacturing technologies. A majority of CNC machines have been around for decades and their improved efficiency and operation have advanced the capability of CNC machining complex parts. 

In this article, we’ll explain the fundamentals of various CNC machines, including their operation, tolerancing, surface finish capabilities, and best practices for designing. In particular, we’ll cover CNC milling, CNC turning, and electrical discharge machining (EDM). 

What factors should you consider during complex CNC machining?

  1. Machine Cutting Axes

Your machine’s capability dictates the level of CNC cutting complexity. Modern CNC technology allows for multi-varying axis rotation and translation. This permits the machining of highly complex features. For example, a 5-axis machine can cut angles that a 3-axis machine cannot accommodate. Part manufacturing typically requires multiple set-ups to machine all features on a part. But with higher axis capability, we can reduce the number of set-ups and increase the potential to manufacture highly complex CNC parts. And different variations of CNC milling make it possible to maintain design consistency.

  1. Cutting Tools and Part Features

How the part has been designed will define the level of complexity necessary to fabricate the part. Yes, multi-axes machines permit machining complex parts, but we must practice cost-effective manufacturing too. This begins with a preference for the use of standard-sized cutting tools whenever possible. 

Carbide and ceramic coated cutters are the two best options for CNC machining complex parts because of their ability to cut hardened materials and to work for high-speed machining applications. However, these types of cutters come at a premium cost. 

So, product development teams must determine if high complexity features are truly necessary to justify the additional manufacturing requirements and costs.

  1. Tolerancing and Surface Finishing

Tight tolerances and surface finishes are critical attributes of custom-machined parts to consider. Part complexity is not only defined by the geometry and dimensions but also the level of precision and accuracy with which a part can be machined. 

  1. Work Holding

The complexity of a CNC machined part is often dictated by the quality and capability of the fixtures and jigs. The fundamentals of stack-up analysis can further explain how the quality of a fixture controls machining capability. Still, the design engineer must understand how the part will be held within the machine, especially for highly complex parts. Successful workholding is achieved in CNC machining when all fixtures and machining tooling are rigidly held to the CNC machine. The rigidity of a CNC machining setup is essential in ensuring the precision and accuracy of the finished product.

When it comes to CNC turning, CNC milling, and EDM, which are exceptional processes due to technological advancements, we are often asked, “What type of finishes can CNC machines produce on complex parts?” We will cover these topics next.

CNC Milling

CNC milling is the process of using computerized controls and rotating cutting tools to remove material from a workpiece. CNC milling technology ranges from 3-axis machining to 12-axis cutting abilities. 

It’s rare for a CNC milling machine to be able to fully mill a part using a single set-up — difficult angles and profiles make it nearly impossible on a 3 or 4-axis mill. As the complexity increases for a custom part, so does the cost to manufacture. If machining on a 3-axis mill, four or more set-ups may be required to finish a complex part. This can drive up costs rapidly as the design engineer develops the part. But advancements in CNC milling have created a solution — milling machines with a high number of axes capabilities reduce the number of required set-ups for the most complex parts.

Design for manufacturability

Even though CNC milling complex parts is easier than ever, best machining practices should always be considered when designing. A 12-axis CNC mill makes the most complex machined parts seem easy to manufacture, but there are always methods to mitigate manufacturing costs. 

Maximizing the size of internal features of pockets and profiles is critical to designing cost-effective parts. Large internal features permit the usage of a larger-sized cutter. This is beneficial because more material can be removed along the tool cut path, reducing machining time and the possibility of tool breakage. Large internal profiles and features also support larger clearance regions for tools to maneuver through. If a tool must detour around features with small clearances, the machining time will slowly increase. Even increases of just tenths of a second can add up to increased costs. 

Another important practice to understand is the usage of internal corner radii. For example, using a .250″ end mill to machine an internal pocket will generate a .125″ inside corner radii, which should be used in all possible areas where a corner radius is cut. This approach eliminates tooling changes within the machine. 

On the other hand, if we need to change between a .250″ end mill and a .1875″ end mill to accommodate two different corners, the machine will need to stop milling to change tools — driving up processing time and part costs in the process. 

The final aspect of DFM we will touch upon for CNC milling complex parts is the depth of pockets. There is a simple formula to use:

Internal radii =  

Depth of feature



Depth of pocket = 0.500″

Internal ra
dii =  



  = 0.125″


The end mill diameter required is 0.250″

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This formula states that the minimum internal corner radii of a feature can only be ¼ of the feature’s depth. This formula should be referenced because it will ensure there is enough tool clearance within an internal feature, but also allow the usage of standard sized cutters. Standard sized cutters like end mills have standard lengths based on their diameter. If we exceed that depth, we may not physically be able to hit the required feature depth withoug breaking the tool.

Tolerance and surface finish capability

Another valuable capability of CNC milling is tight tolerance machining. For example, a typical tolerance that a CNC machine can hold is +/- 0.004″, along with a 125 micro-inch finish. This is a tolerance and surface finish that can meet the requirements for many applications and keep assembly stack-ups relatively tight. Click here to see a listing of Fictiv’s standard tolerances.

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CNC Turning

CNC turning involves rotating a workpiece at high speeds while advancing a cutting tool along the workpiece surface to remove material. Turning is a conventional method of producing CNC machined parts that has been utilized since 1940 when the first numerical turning machine was invented. Another name for this machine is a CNC lathe. Within a CNC lathe, cutting tools are mounted on a numerically controlled turret, which allows for complex operations. For example, a hexagonal turret allows 6 different cutting tools to be held in one machine set-up — which enables the maching to make 6 different types of cuts in one operation.

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Live tooling v. non-live tooling

The advancement of live tooling has made it more cost-effective to machine complex parts on a static CNC lathe. Live tooling allows a lathe controlled by CNC, along with the turret of differing spindle and sub-spindle configurations, to perform an array of operations while the workpiece is fixed in orientation to the main spindle. 

The operation of a CNC lathe without live tooling limits the types of cuts and tooling that can be used. During the standard turning operation, the tool is in a static turret. The only movement possible is a linear motion along the z-axis of the workpiece. This limits what cuts can be made for one machine set-up. Also, with only z-axis translation, more complex machined features must be performed in a secondary operation on a CNC mill. 

Live tooling permits all standard turning cuts to be made but allows milling features as well. Plus, they can accommodate angular configurations to support both axial and radial cuts. The development of live tooling lathes has allowed significant reductions in lead time and machining cost. The ability to machine features in the radial and axial direction has generously expanded the window of custom machined parts. This allows the designing and prototyping of highly complex CNC machined parts with a much smaller budget.

Tolerance and surface finish capability

A key attribute for the machining of complex CNC machined parts is meeting surface finish requirements without post-processing. CNC lathes have made significant developments in this area, as well as machined tolerancing. Today’s CNC lathes can produce machined finishes up to 32 micro-inches. This is an excellent finish, which would otherwise have to be obtained through post-CNC-machining surface grinding and polishing. The standard finished tolerance on a lathe is +/- 0.005″,  and with the proper tooling and right machine, even higher precision can be achieved.

Designing to finish with a lathe and avoid grinding

There is no question that CNC grinding is a fascinating machining process, which can finish custom CNC parts with high levels of precision, finish, and accuracy. But this process is not always cost-effective for complex CNC parts. With the advancements in CNC turning, engineers can now design parts to have finishes and tolerances equal to CNC grinding without any post-processing. 

So, when designing parts for manufacturing, begin by examining the fit and function of the system. Understanding how the parts will come together in the parent assembly makes it easier to target specific tolerances and finishes. Proper application of CNC lathe tolerances will eliminate the need for grinding. Not only does CNC grinding add cost, but also often creates a bottleneck in many facilities because of the long set-up and processing time, quality inspection requirements, high fallout, and rework requirements. The goal for designs should be a surface finish and tolerance that requires the least amount of hands-on work to perform the part’s fit and function.


Electrical Discharge Machining is a non-traditional method that’s unique because it doesn’t use sharp tooling for material removal like CNC mills and lathes. Instead, EDM removes material using a flow of electricity and thermal energy, which is why it’s referred to as an electro-mechanical process. 

Conventional EDM uses a tool (c
athode) that emits an electrical current. The cathode runs along a workpiece (that serves as an anode) immersed in a dielectric fluid. This closed electrical circuit generates high temperatures and sparks which vaporize the material. For this cutting process to be effective, the workpiece material must be electrically conductive. Fictiv offers various non-ferrous metals, normalized and/or hardened that are great options for cutting complex part profiles.

An advantage of EDM machining is its ability to easily cut hardened metals. EDM machining is better suited than conventional CNC machining processes because tooling costs are significantly reduced — traditional tooling wears quickly when cutting hard materials. Additionally, CNC EDM permits machining of advanced CNC parts like gears, splines, internal gears, keyways, and other difficult-to-cut internal profiles. Many of these features would commonly be manufactured with a CNC mill or broaching machine. EDM allows for this processing with high efficiency and minimal set-ups. 

One enormous advantage of EDM machining is its ability to machine multiple parts at one time. So, not only can you cut one rack gear profile, multiple racks can be stacked linearly and cut at the same time. This capability reduces lead times, part cost, and machine cycle processing time.

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Tolerance and surface finish capability

EDM offers high-quality cuts with tolerances tight as +/- .0001″, which permits the design and manufacture of complex custom CNC parts. Wire EDM can cut materials as thin as .0004″ and thicknesses up to 16″. EDM also offers surface finishes as great as 32 micro-inches, a surface finish close to CNC grinding without the post-processing. To accomplish this level of finish, multiple tool passes are necessary, but it’s still a fractional cost compared to conventional machine grinding.  

Sourcing Simplified

This article has only scratched the surface in the realm of CNC machining, but we hope that it has served as a helpful source of information to better understand the different methods for cutting highly complex parts.If you’re tasked with sourcing and supplying CNC machined parts, Fictiv is your operating system for custom manufacturing that makes it faster, easier, and more efficient. In other words, Fictiv lets engineers, like you, engineer. Fictiv offers a range of CNC machining capabilities that leverage many types of CNC machines to produce both simple and incredibly complex geometries — create a free account and upload your designs today!


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