CNC machining has been the major method for producing high-quality precision metal parts in industries for a long time and will still continue to be. With the developments in metal 3D printing, it is a common question where metal 3D printing will stand out against the existing CNC machining. The answer to that is not straightforward as both of the methods are not competing on the same grounds. In this article let’s understand in more detail about metal 3D printing versus CNC machining. And though there are many metal 3d printing technologies under development such as Metal Fused Filament Fabrication (MFFF), we refer to Direct Metal Laser Sintering (DMLS) or Selective Laser Melting (SLM) in mind. Firstly we look at the process itself and then move on to comparing both techniques from other factors.
Process
Fundamentally it is already known that CNC machining is a subtractive manufacturing process while metal 3d printing is an additive manufacturing process. Subtractive manufacturing refers to all forms of machining that involve removing the unwanted portion of the material from the stock with cutting tools till desired shape and finish are obtained. Additive manufacturing refers to adding and fusing thin layers of material one on top of another till the desired shape is obtained.
Production Volume
If the parts need to be produced in large numbers, CNC machines are capable of handling a large number of parts due to the automation available whereas metal 3d printing is not suitable for large-scale production due to the additive nature of the technology.
Size
Part size is another factor. Metal 3D printers have a typical build volume of 250 x 250 x 250 mm but there are CNC machines that can machine as large as a marine engine crankshaft, of course, such large CNC machines are built for such purposes alone.
Speed
CNC machines can achieve faster material removal rates to achieve the part geometry while metal 3d printing is a layered manufacturing process hence build time is always more and hence the process is slow. But it all depends on the part geometry. If there are a number of holes and slots that have to be machined from a plain blank it may turn out that 3D printing could be a faster approach than CNC machining. The final verdict cannot be made purely by looking at the time per single part but the overall batch size.
Costs
For CNC machining the same part if quantity increases, cost per part reduces as once the initial machining setup is done it need not be changed. But in 3D printing, the machine setup has to be performed for each small batch hence cost per part does not reduce with the number of parts.
However from another perspective, if the part geometry is complex, CNC machining costs can increase with quantity if the number of operations and tools required are more. But in 3D printing, there is no such need for changing any tools as printers can produce any geometry hence cost per part remains the same.
Tolerances and Surface Finish
CNC machines can achieve precisely tight tolerances of +/- 0.05 mm but metal 3D printers can only achieve a tolerance of +/- 0.1 mm. CNC machining can produce smooth surface finishes (standard Ra 3.2 micrometers) requiring no further finishing operations for most applications but metal 3D printed parts in DMLS/SLM have rough finish similar to that of sand-cast parts (Ra 16 to 50 micrometers) and might require further finishing operations.
Design Complexity
Although there are 5-axis CNC machines to produce complex 3D contours, there is still a limitation on machining internal cavities, deep grooved pockets, and holes as thin cutting tools cannot reach greater depths and even if they do, they are prone to vibrations and tool breakage. Hence such complex geometries with all internal holes and pockets have to be 3D printed. This is the advantage of 3D printing and parts that are topology optimized and designs obtained by generative design methods can be easily printed in metal.
Materials
CNC machines can handle almost all forms of metals, materials, and alloys as long as a harder tool is available for machining them. Soft materials like aluminum are machined by high-speed steel and harder materials like steel and titanium are machined by cemented carbide, tungsten carbide, cubic born nitride, and polycrystalline diamond.
When it comes to 3D printing, there is no need for selecting the cutting tool material. However, the material limitation comes with the ability of the particles to get welded together under the heat of a laser. Hence only metals like aluminum alloys, stainless steel, titanium, cobalt chrome, and nickel alloys are 3d printable.
Conclusion
The suitability of CNC machining or metal 3D printing depends on the application at hand. So far, metal 3D printed parts are used in experimental aerospace parts, experimental automotive parts, medical implants, and custom parts that have complex geometries (having internal cavities) while CNC machining remains a method for mass-producing standard machine parts and fixtures that are more or less shaped in a rectangular or cylindrical shape.
| CNC Machining | Metal 3D Printing | |
| Cutting Tools Required | Yes | No |
| Raw Stock | Solid blocks or bars | Metal powders |
| Surface Finish | Ra 3.2 micrometers | Ra 10 to 50 micrometers |
| Geometry | Limited to 5-axis machining | No limitation |
| Materials | Many | Limited to those developed and specified by printer manufacturers like Aluminum alloys, Stainless steel, Titanium, etc |
