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.
Additive Manufacturing (AM) or 3D Printing processes build parts by adding material one layer at a time. AM processes require no special tooling or fixtures, so initial setup costs are kept to a minimum.
Schematic of the 3D printing process
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 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 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).
Recommended process reference table
|No. of parts||1's||10’s||100’s||1000’s|
|Plastic||3D Printing||3D Printing(consider: CNC)||CNC(consider: Injection Molding)||Injection Molding|
|Metal||3D Printing & CNC*||CNC(consider: 3D Printing)||CNC(consider: Investment Casting)||Investment or Die Casting|
*: highly dependent on part geometry