CNC machining is a common subtractive manufacturing technology. Unlike 3D printing, the process typically begins with a solid block of material (blank) and removes material to achieve the required final shape, using a variety of sharp rotating tools or cutters.

CNC is one of the most popular methods of manufacturing for both small one-off jobs and medium to high volume production. It offers excellent repeatability, high accuracy and a wide range of materials and surface finishes.

 

Selecting the right technology

When choosing between CNC and Additive Manufacturing (AM), there are a few simple guidelines that can be applied to the decision making process.

As a rule of thumb, all parts that can be manufactured with limited effort through a subtractive process should generally be CNC machined. It usually only makes sense to use 3D printing services in the following cases:

• When traditional methods are not able to produce the part, for example for highly complex, topology-optimized geometries.
• When a fast turn-around time is critical; 3D printed parts can be delivered within 24h.
• When low-cost is essential; for small volumes, 3D printing is generally cheaper than CNC.
• When there is a small number of identical parts required (less than 10).
• When materials are required that cannot be easily machined, like metal superalloys or flexible TPU.
• CNC offers greater dimensional accuracy and produces parts with better mechanical properties in all 3 dimensions, but this usually comes at a greater cost, especially when volumes are small.

If higher part quantities are needed, (hundreds or more), then neither CNC nor AM may be a cost-competitive option. Traditional forming technologies, such as investment casting or injection molding, are generally the most economic option, due to mechanisms of economies of scale (see figure).

 

Model complexity of CNC machining

There are a number of limitations that must be considered when designing parts for CNC machining, including tool access and clearances, hold or mount points, as well as the inability to machine square corners due to tool geometry.

Some geometries are impossible to CNC machine (even with 5-axis CNC systems) as the tool cannot access all the surfaces of a component. Most geometries require the rotation of the part to access the different sides. Repositioning adds to the processing and labor time and custom jigs and fixtures may be required, affecting the final price.

3D printing has very few geometry restrictions compared to CNC. Support structures are required in most technologies, like FDM or SLM/DMLS, and are removed during post processing.

Plastic freeform, organic geometries can be easily manufactured with polymer-based powder bed fusion processes, such as SLS or Multi Jet Fusion (MJF), as they require no support. The ability to produce highly complex geometries is one of the key strengths of 3D printing.

 

Rules of thumb

Selecting the right technology for your application is crucial and can be boiled down to the following rules of thumb:

• CNC machining is best suited for medium to high quantities (less that 250-500 parts) and relatively simple geometries.
• 3D printing is generally best for low quantities (or one-off prototypes) and complex geometries.
• When considering metals, CNC can be price competitive even for low quantities, but geometry limitations still apply.
• When quantities are high (more than 250 – 500 parts) other forming technologies are more suitable.

 

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