CNC vs 3D Printing: Subtractive vs Additive Manufacturing
3D printing has rapidly hit mainstream usage: from industry to hobbyists at home. Additive manufacturing startups receive billions of dollars a year in investment, and some of the biggest technology firms in the world have turned their eye to the technology.
You could be forgiven for thinking that 3D printing’s ascendancy spells the end for more traditional manufacturing methods. However, that’s not the case, and there will always be an irreplaceable place in the industry for methods like CNC.
But which is better: CNC or 3D printing? There’s no right answer, but here we break down both 3D printing and compare them over each main area of use.
Part 1: CNC vs 3D Printing: What’s the Difference?
In the short-run production of 3D products and rapid prototyping, there are two competing technologies: CNC and 3D printing. Both can produce an identical part, but they both do so differently.
Computer Numerical Control (CNC) is the more traditional manufacturing method. It uses computer-controlled tools and commands to produce 3D objects.
Using a 3D design, CNC starts with a solid block of material called a blank, and uses a variety of tools to remove material to produce the final shape. This can be through milling, cutting, lathing, or using water or laser cutters.
CNC tools can operate through multiple axes, and can produce a range of attractive finishes with many different materials. It is also a multi-scale technology, capable of effectively manufacturing single or small-line products as well as producing identical, large-scale parts for the market.
3D printing also begins with a 3D CAD design, but produces the part in a very different way. Slicer software deconstructs the design into hundreds of layers. Material is then deposited onto the printer bed one layer at a time, gradually building the design.
There are a range of different 3D printing technologies, each compatible with different materials. Fused Deposition Modeling (FDM), also known as Fused Filament Fabrication (FFF), is the most popular method, where a plastic filament is heated until it melts, and then deposited to form the final design.
FDM is most commonly used with polymers like PLA and ABS, the same materials used in resin-based technologies such as Stereolithography (SLA) where light beams cause monomers to join together to form structures, and Selective Laser Sintering (SLS), where high-powered lasers fuse plastic together.
Similar to SLS is Direct Metal Laser Sintering (DMLS), which does the same, but for metal.
These differences in production methods show why alternative names such as Additive Manufacturing and Subtractive Manufacturing are often used.
CNC is often referred to as Subtractive Manufacturing due to the fact that it removes material from a larger block. Various tools remove excess material from the larger mass through cutting, milling, lathing, and other methods. The overall quantity is gradually reduced until the final 3D part is left. This subtraction of material gives CNC this name.
Conversely, 3D printing does the exact opposite. The process begins with no material present, but layers are gradually added to form the final structure. The deposition of material, adding to the starting quantity, is where this term comes from.
CNC vs 3D Printing: The Industry War
As 3D printing’s popularity increases, it is increasingly being incorporated into many high-performance industries. It is rare to find an industry that uses one or the other nowadays, rather a combination of the two. As many in the industry say, it is ‘and’ rather than ‘or’.
Both are primarily prototyping technologies. Which is better is dependent on the individual project. Certainly, for complex parts, 3D printing’s additive process means it is capable of producing more complex parts with more intricate geometry, and can even add color. The reduced waste is also a big factor when manufacturing using expensive materials like metal.
However, CNC methods are far better for functional prototypes. Parts for heavy industries like construction, for example, that must be rigorously tested before use are better made using CNC. CNC is also capable of working with much harder materials than 3D printing, producing a better finish in the process. However, it is far more expensive to manufacture this way.
While it is also difficult to say which is better suited to use in a particular industry, both CNC and 3D printing have their own set of benefits and drawbacks that individual companies take into consideration. They often use each technology for different things, making it difficult to make a like-for-like comparison.
However, here are a few industries that use both methods, and why they use them.
Aerospace is perhaps the biggest industry to embrace 3D printing to date. There is a near endless list of considerations to take into account in the production of aircraft, and this is why CNC and 3D printing are both used extensively.
Among the chief concerns for designers is reducing weight. This exponentially reduces costs through lower fuel consumption. A famous study by United Airlines found that simply using lighter paper in their in-flight magazine saved 643,000 liters of fuel in a year across their fleet, equivalent to $290,000. They also found that the reduced weight of ending duty free sales on board saved a further $2.3 million annually.
This is why the incorporation of 3D printing has been pursued. Low-volume, highly complex parts like brackets for sensors and landing gear can be made much lighter through 3D printing, which has a huge impact on the cost of each flight. And 3D printing has allowed plastics such as PEEK to find a useful place in aircraft manufacturing too.
We also have a ranking of the best PEEK and Ultem 3D printers
On the other hand, CNC methods are preferred for more high-volume parts like anti-lock brakes, door hatches and airflow valves, which can be produced quickly and in higher quantities.
Again, much like in aerospace, the distinction in the military applications of CNC and 3D printing is that CNC is preferable for mass-produced parts, whereas 3D printing is used for short-line produced parts.
Aside from producing plane parts, 3D printing is being adopted by the military to produce drone components, belt buckles, ammunition boxes, and there are several exciting projects that have recently become reality.
In July 2020, the United States Navy launched Optionally Manned Technology Demonstrator (OMTD), the first submarine to have an entirely 3D printed hull. It was not only cheaper and stronger than comparable vessels due to the reduced number of panels, but reduced production time from 5 months to under 6 weeks.
Yet, CNC is still a popular manufacturing method for the military, primarily in the production of firearms. An invaluable tool for soldiers, and one that requires enormous trust, mass producing weapons components and bullets to the highest possible tier of accuracy is paramount.
Guns and ammunition need to be highly mass produced, and well enough that they can be reliably used in a war zone. CNC is preferable as it has a much higher production speed and capacity than 3D printing.
Read more in our feature story on 3D printed guns
There are two key elements to the production of robotics: the skeleton, and the electronics. 3D printing is now almost ubiquitously used for the former, and CNC entirely for the latter.
The metal framework and plastic or metal exterior used in some of the most advanced robotic projects currently in operation are almost entirely made using 3D printing.
For more information, check out our feature on 3D Printed Robotics
3D printing allows for developers to produce parts with a minute degree of accuracy which improves the overall efficiency of the machine and gets the best results.
On the other hand, the electronics powering the devices are produced through CNC machining, as 3D printing is incapable of manufacturing individual circuit components, let alone entire circuits. The wires and motors are also machined traditionally for the same reason. However, new 3D printing technologies are emerging for producing 3D printed circuit boards.
CNC vs 3D Printing: Which is better?
Equipment and Material Cost
This is the hardest single area to assess because of the variety in machines, brands and quality that you are going to find. Generally speaking, 3D printers are on average cheaper than equivalent CNC machines, which is remarkable considering the relative infancy of the 3D printing industry.
However, the materials are much cheaper in their raw form than those engineered for use in 3D printing. Many 3D printing filaments, such as ABS and PP, suspend the material in resin which inflates the price. Depending on the polymer, a filament could cost 10 times more than the raw material per kilogram.
The same is also true for metals, especially metal wires designed for use in newer metal 3D printing techniques like Directed Energy Deposition (DED), and the metal powders used in DMLS. Adapting a material for use in 3D printing is far more expensive than the raw material used for CNC manufacturing.
Assuming that you have the machine already, there are a few factors that determine the production cost of CNC versus 3D printing. Energy, labor, material cost and material waste all affect this. Luckily, 3D printing giant Stratasys have conducted a useful study to compare.
Stratasys manufactured two pairs of items: a pocket tray, and an industrial robotic adaptor. They manufactured identical parts, one using 3D printing, the other using CNC.
They found that while the material cost was higher for 3D printing, the labor costs were so low that the finished product was 60% cheaper for the tray, and 30% cheaper for the adaptor.
The flip side of the study is that the production time for CNC was vastly shorter than with 3D printing. The tray had a total production time of 2.3 hours with 3D printing, but only 1.3 hours with CNC.
The robotic adaptor was a similar story, with 3D printing producing the part in 3.8 hours, with CNC only taking 2.5 hours. CNC evidently has the edge when it comes to speed, yet 3D printing triumphs in cost.
However, it is widely acknowledged that this is for simpler, less complex parts. For parts with more intricate and complicated designs, 3D printing makes up that lost ground.
CNC milling has an edge over 3D printing when it comes to production capacity, both in terms of scale and part size.
CNC is the method of choice for manufacturers who need a steady and reliable production flow on a larger scale. 3D printing is not capable of operating at those levels under the current limitations.
In addition, 3D printers have much smaller build volumes than CNC machines, which can be designed to produce much larger parts.
Ease of Use
3D printing is recognized as the easier of the two technologies to use, especially for the less knowledgeable. Both rely on 3D modeling software to produce product designs, but 3D printers manufacture the part automatically, whereas CNC machines require attention and experience to operate effectively.
While 3D printing certainly comes with its own unique set of pitfalls that have to be overcome, getting a high-quality product with an ideal finish is comparatively easier with additive manufacturing.
3D printing is by far the superior technology when it comes to part complexity. Additive manufacturing is capable of making structures with high accuracy, and is far superior when it comes to the manufacture of parts with complex geometry.
By comparison CNC is less adept at this. While it is capable of making incredible accurate and can repeatedly produce complex parts, it does not have the same skills as 3D printing at producing parts with complex geometry.
CNC has the edge when it comes to diversity of materials. 3D printing is currently able to print directly only metal and plastics, whereas CNC is workable with metal, plastic, acrylic, machining wax, modeling foam and even wood.
It should also be noted, however, that while 3D printing has a smaller range of compatible materials, it is far more efficient at using them.
Versatility is measurable mainly by how quickly the technology can be adjusted from one design to another. And in that sense, 3D printing is better than CNC.
When changing designs with CNC machining, you have to get through an arduous retooling process and adjust your equipment based on the next project. There is no need for this with 3D printing. The printer is capable of printing any new design instantly.
Conclusion: CNC vs 3D Printing: Which is Better?
Overall, it is difficult to say whether CNC or 3D printing is better as they both excel at different things. Broadly, CNC is quicker but more expensive, 3D printing is slower but cheaper.
When choosing which design to use for your own process, it is vitally important to assess what kind of production you are looking to establish. If you are looking to print dozens or hundreds of identical parts a day, then CNC is by far the clear choice. If instead you have a more limited run planned, or your part is more complex in its design, then 3D printing is for you.
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