What is CNC? What types of machines are there? Which are their key benefits & limitations?
What is CNC machining? What are the different types of CNC machines? How do they work?
In this section, we answer all these questions and we compare CNC machining to other manufacturing technologies to help you find the best solution for your application.
CNC (Computer Numerical Control) machining is a subtractive manufacturing technology: parts are created by removing material from a solid block (called the blank or the workpiece) using a variety of cutting tools.
This is a fundamentally different way of manufacturing compared to additive (3D printing) or formative (Injection Molding) technologies. The material removal mechanisms have significant implications on the benefits, limitations and design restrictions of CNC. More on this below.
CNC machining is a digital manufacturing technology: it produces high-accuracy parts with excellent physical properties directly from a CAD file. Due to the high level of automation, CNC is price-competitive for both one-off custom parts and medium-volume productions.
Almost every material can be CNC machined. The most common examples include metals (aluminum and steel alloys, brass etc) and plastics (ABS, Delrin, Nylon etc). Foam, composites and wood can also be machined.
The basic CNC process can be broken down into 3 steps. The engineer first designs the CAD model of the part. The machinist then turns the CAD file into a CNC program (G-code) and sets up the machine. Finally, the CNC system executes all machining operations with little supervision, removing material and creating the part.
In this guide, we will focus on CNC machines that remove material using cutting tools. These are the most common and have the widest range of applications. Other CNC machines include laser cutters, plasma cutters and EDM machines.
CNC milling and CNC turning machines are examples of 3-axis CNC systems. These “basic” machines allow the movement of the cutting tool in three linear axes relative to the workpiece (left-right, back-forth and up-down).
3-axis CNC milling machines are very common, as they can be used to produce most common geometries. They are relatively easy to program and operate, so start-up machining costs are relatively low.
Tool access can be a design restriction in CNC milling. As there are only three axes to work with, certain areas might be impossible to reach. This is not a big issue if the workpiece needs to be rotated just once, but if multiple rotations are needed the labor and machining costs increase fast.
CNC lathes are extensively used, because they can produce parts at a much higher rate and at a lower cost per unit than CNC mills. This is especially relevant for larger volumes.
The main design restriction of CNC lathes is that they can only produce parts with a cylindrical profile (think screws or washers). To overcome this limitation, features of the part are often CNC milled in a separate machining step. Alternatively, 5-axis mill-turning CNC centers can be used to produce the same geometry in one step.
Multi-axis CNC machining centers come in three variations: 5-axis indexed CNC milling, continuous 5-axis CNC milling and mill-turning centers with live tooling.
These systems are essentially milling machines or lathes enhanced with additional degrees of freedom. For example, 5-axis CNC milling centers allow the rotation of the machine bed or the toolhead (or both) in addition to the three linear axes of movement.
The advanced capabilities of these machines come at an increased cost. They require both specialized machinery and also operators with expert knowledge. For highly complex or topology optimized metal parts, 3D printing is usually a more suitable option though.
Indexed 5-axis CNC milling systems are also known as 3+2 CNC milling machines, since they are using the two additional degrees of freedom only between machining operations to rotate the workpiece.
The key benefit of these systems is that they eliminate the need of manually repositioning the workpiece. This way parts with more complex geometries can be manufactured faster and at higher accuracy than in a 3-axis CNC mill. They lack though the true freeform capabilities of continuous 5-axis CNC machines.
Continuous 5-axis CNC milling systems have a similar machine architecture to indexed 5-axis CNC milling machines. They allow, however, for the movement of all five axes at the same time during all machining operations.
This way, it is possible to produce parts with complex, ‘organic’ geometries that cannot be manufactured at the achieved level of accuracy with any other technology. These advanced capabilities come of course at a high cost, as both expensive machinery and highly-trained machinists are needed.
Mill-turning CNC centers are essentially CNC lathe machines equipped with CNC milling tools. A variation of the mill-turning centers are swiss-style lathes, which have typically higher precession.
Mill-turning systems take advantage of both the high productivity of CNC turning and the geometric flexibility of CNC milling. They are ideal for manufacturing parts with 'loose' rotational symmetry (think camshafts and centrifugal impellers) at a much lower cost than other 5-axis CNC machining systems.
Use the table below for a rough estimate of the cost per hour of the different CNC machines. The cost is presented relative to that of a 3-axis CNC milling machine, which is typically $75 per hour.
|CNC machine type||Machining cost|
|CNC milling (3-axis)||$75 ( Baseline for comparison )|
|CNC turning (lathe)||$65 ( - 15% )|
|Indexed 5-axis CNC milling||$120 ( + 60% )|
|Continuous 5-axis CNC milling||$150 ( + 100% )|
|Mill-turning CNC centers||$95 ( + 25% )|
Here's a list of the key strengths and limitations of CNC machining. Use them to help you decide whether it is the right technology for your application.
One of the greatest things about CNC machining is the wide range of applications it has found over the years.
Here, we collected some recent examples to illustrate how professionals have exploited the benefits of CNC machining to get the best results in different industrial situations. Use them as inspiration for your projects.
CNC machining is one of the very few manufacturing processes that is suitable for creating parts for space applications. Not only because of CNC parts have excellent accuracy and material properties, but also due to the wide range of surface treatments that can be applied to the parts after machining.
For example, KEPLER used CNC machining and space grade materials to go from a sketch on a napkin to a satellite in space in 12 months.
Aerospace was one of the first industries to use CNC machining. This is due to its ability to manufacture lightweight parts with excellent physical properties and very tight tolerances. CNC machining is used both for aircraft parts and also during the development stages.
For example, Tomas Sinnige is a PhD researcher at the Delft University of Technology. With his team of researchers, they used CNC machining to manufacture scaled-down versions of their prototype engine, aiming to increase the efficiency of modern propeller engines.
CNC machining has applications in the automotive industry when manufacturing of high-performance custom parts is required.
For example, the Dutch company PAL-V, designs Personal Air and Land Vehicles. These are essentially the world's first flying cars. During the development stages, they chose CNC machining to prototype and manufacture key components.
The ability of manufacture quickly custom metal parts with great dimensional accuracy, makes CNC machining an attractive option for producing functional prototypes. This is essential during later stages of design and development.
The design team of DAQRI, for instance, used CNC machining to prototype their professional Augmented Reality (AR) hardware. They selected this process, as it was the most cost-competitive solution that was capable of producing custom metal parts with the required level of detail and at the small-scale needed for their designs.
CNC machining has many applications in the electrical and electronic manufacturing industry: from the prototyping of PCBs to the manufacturing of enclosures.
TPAC, for example, used CNC machining to manufacture an enclosure for their high-power electronic sensing systems. Heat dissipation and electrical insulation were the main design requirements in this case. So, CNC machined anodized aluminum was ideal for their one-off custom enclosure.
A very common industrial application of CNC machining is the fabrication of tooling for other processes. For example, the molds in Injection Molding are commonly CNC machined from aluminum or tool steel.
Precious Plastic, for instance, developed a system for the developing world that turns waste plastic into iPhone cases! For this purpose, they used a low-cost manual injection molder and custom CNC machined molds.
High-performance sports & motorsports manufacturers always try to increase the performance of their products by reducing their weight.
CAKE is a Swedish company that designed and developed the first off-road electric motorbike. Since it is the first of its kind, every single component of the motorbike was custom-made with CNC to achieve the intended level of quality and durability.
Both CNC machining and 3D printing are exceptional tools in the arsenal of an engineer. Their unique benefits make each more suitable for different situations though.
When choosing between CNC machining and 3D printing, there are a few simple guidelines that you can apply to the decision making process.
As a general rule of thumb parts with relatively simple geometries, that can be manufactured with limited effort through a subtractive process, should generally be CNC machined, especially when producing metal parts.
Choosing 3D printing over CNC machining makes sense when you need:
CNC offers greater dimensional accuracy and produces parts with better mechanical properties than 3D printing, but this usually comes at a higher cost for low volumes and with more design restrictions.
If high volumes are needed (1,000’s or more), neither CNC machining nor 3D printing are likely to be suitable options. In these cases, forming technologies, such as investment casting or injection molding, are more economically viable due to the mechanisms of economies of scale.
For quick reference, use the table below. In this simplification, it is assumed that all technologies are able to produce the geometry of the part in question. When this is not the case, 3D printing is generally the preferred method of manufacturing.
|No. of Parts||Plastic||Metal|
|1-10||3D printing||CNC machining (consider 3D printing)|
|10-100||3D printing and CNC machining||CNC machining|
|100-1000||CNC machining (consider Injection molding)||CNC machining (consider Investment casting)|
|1000+||Injection molding||Investment or Die casting|