CNC vs. 3D Printing: What’s the Best Way to Make Your Part?

3D printing is a rapidly growing technology with many advantages over traditional manufacturing methods. It however has its problems, including that 3D printing cannot be effectively used for mass production, and is limited in speed and accessibility. Another manufacturing technique for low-to-mid production is CNC machining (computer numerical control), a fairly common subtractive technique for part creation. We decided to compare the advantages and disadvantages for each method to establish where these technologies are most appropriate.
Feature Size: Both CNC and 3D printing processes are constrained by tool size; in the case of CNC, the tool diameter dictates the smallest negative feature that can be created. In the case of 3D printing, however, the nozzle diameter dictates the smallest positive feature that can be produced. The nozzle diameter for extrusion-based 3D printers is typically between 0.25mm and 0.8mm, and the minimum feature size for those machines is 4 times that, resulting in a minimum feature size between 1.0mm and 3.2mm.
Surface Finish：With the right tools, CNC machines are capable of producing much smoother surfaces than 3D printers. 3D printers are capable of producing parts for fit and finish, but for parts that require exceptional smoothness for mating with other precision components, CNC machining may be preferable.
Tolerances：Some of the best composite 3D printers can hold dimensional tolerances down to +/- 0.005”, and typically have a compliant surface for press fitting. Tight tolerance features on 3D printed parts can be post-machined. However, depending on the other features on the part, it may be easier to machine the whole thing. Your results may vary based on machine, material, and part geometry.
Loading: Non-structural parts are typically easy targets for conventional 3D printing. Structural parts that need to withstand significant physical loads can be produced with continuous fiber reinforcement, or by CNC machining. While continuous fiber reinforcement can provide significant strength improvement over other 3D printed parts, composite parts are stronger in two axes (X, Y) than in Z, and do not exhibit isotropic properties like metal parts.
Environment
3D printing and CNC machining are both capable of producing parts in metals and polymers, so the choice will revolve around which process is more readily available for forming the material that you need.
Temperature: 3D printing and CNC machining are both capable of producing parts in metals and polymers, so the choice will revolve around which process is more readily available for forming the material that you need. The service temperature of a particular metal typically has some relation to its melting temperature. “Pure metals usually weaken seriously at about half the melting point on the absolute temperature scale (melting point in degrees F plus 459).” Alloys can typically push the useful range higher, in some cases to about 65% of melting temperature.
3D printed composites and polymers have service temperature limits lower than metals. GPT precision materials typically should not be used for extended periods at elevated temperatures above 150 °C.
Moisture: Some polymer filaments absorb moisture and may lose strength through prolonged exposure or submersion. A coating, such as, may be required. Moisture typically does not affect aluminum, but may cause steel to rust.
Chemicals: If your part will be subjected to any chemicals, check the chemical compatibility of your material with the chemical. While many metals are suitable for use with a variety of chemicals, always check for compatibility before introducing a new material or environment. GPT precision nylon-based materials are chemically resistant and are unaffected by most petrochemicals. They are not suitable for use in strong acid or base environments.
Economic Considerations
When do you need it?
Machining in-house: If the part is needed immediately, there are no other holdups to the operation, and you have a machine, operator, materials, and workholding available, you should CNC machine your part. If there are more urgent jobs, or the part can wait, 3D printing can usually get you a part in hand the next day, freeing up operator time for more critical tasks.
CNC machining usually is capable of removing material much faster than 3D printing can deposit it. Size usually does not play into the time constraint as much for machining; the amount of material removal required is much more critical. If the part volume/stock volume ratio is very low (material removal is high), 3D printing may be a good option.
With 3D printing, the part size does affect time; larger parts take longer to print. if a part fits in the palm of your hand, you can usually have it the same day. With smaller parts, it can often be faster to 3D print than to machine. With a CNC machine you have to spend time getting stock, writing G-code, figuring out work holding, setting up tools, and cleaning up after. Some parts can print in the time it takes just to get a CNC machine ready.
Machining out-of-house: The process of outsourcing machined components to CNC machining services typically requires several days at minimum, between part review, creating drawings, sending out for quote, and reviewing with purchasing. This, plus shipping and lead time from the shop, can sometimes result in a very lengthy process. Printing, in this case, may be a suitable way to check fit and finish or have a part ready the next day while a permanent part is being cut.
How many do you need?
When you break down the cost of low-volume machined parts, the bulk of the cost comes from the time required for programming and setup; the actual time for cutting metal is generally fairly short. Scaling up production volume is generally achieved by making larger setups to cut more parts unattended. As part complexity and the number of features increases, programming time and the number of required setups may also increase. However, the cost of additional units drops off fairly quickly. Depending on geometry, CNC per unit cost scales well into orders of hundreds or low thousands of units per month since the programming and setup can be reused.
With 3D printing, the programming (slicing) happens in a matter of minutes, and complexity has little effect on the programming time. While the first unit cost and effort is low, the per unit cost is not affected much by volume. Scaling up production volume is generally achieved by bringing more machines online.
What is your budget for equipment and operators?
CNC machines can run unattended when properly set up, but typically trained full-time operators and programmers are critical to success. Machines are not always easy for owners to service, requiring expensive maintenance plans.
3D printers can easily run fully unattended, operators require minimal training, and programming is made easy by software. Machines are typically easy to service and have far less expensive maintenance costs.
CNC machines and 3D printers come in a range of prices based on features and build quality, but typically 3D printers can be owned for a significantly lower investment than a CNC machine.
The upfront cost and operating cost of additive manufacturing is low, making it ideal for low volume applications such as prototyping and tooling. However, when scaling to higher volumes, subtractive manufacturing and forming are more cost effective.
Conclusion
One size does not fit all when it comes to fabrication technologies. Injection molding and CNC machining have been the most cost effective mass production methods available to industry, while 3D printing adoption has grown significantly within the past decade. Amongst other variables, it’s important to compare size, quantity, functionality and purpose to establish what makes the most sense for you.