3D Printing vs CNC for Rapid Prototyping

When it comes to rapid prototyping, there’s a straight choice between 3D printing and CNC (Computerised Numerical Control) machining – but which one is right for you?

To make the right choice, you first need to understand the differences between the two technologies and how they compare according to what you’re trying to accomplish:

• 3D printing works by adding layers, or slices, of material to form solid objects, as directed by a CAD (Computer Aided Design) file.
• CNC machining also relies upon CAD for its instructions, but that’s where the similarity ends. CNC manufacturing involves gpainstakingly removing material from an initial ‘blank’ to arrive at the desired outcome – aided by programmed toolpaths that use a range of precision cutting tools, including laser cutters, lathes, and grinders.

Rapid Prototyping

Rapid prototyping demands through the life cycle of product development often include initial mock-ups, sample parts and scale models. These can be rapidly assembled for rigorous testing across a range of metrics, including durability, size, and shape.

Additive technology – 3D printing – is often regarded as the ‘go-to’ technology for rapid prototyping, although this will vary on a case-by-case basis.

When choosing between 3D printing and CNC machining for rapid prototyping there is no one-size-fits-all solution. To make a decision you need to look at the following variables:

Accuracy and Repeatability

Rapid prototyping demands an ‘accurate-as-possible’ representation of what the user is attempting to create, either as a standalone product, or an individual part within a larger innovation.

CNC machining is regarded as being the more accurate of the two technologies, including being able to ‘finish off’ products initially created using 3D printing. But this will depend on the material being used.

Repeatability – the process by which a product can be produced at scale instead of in smaller volumes – usually becomes a consideration once the prototyping stage has passed into full production. The rigid framing typical of CNC machinery ensures fewer vibrations, resulting in more consistent finishes. On the other hand, the continual layering of material in 3D printing can result in occasional inaccuracies.

Speed

Rapid prototyping demands a quick solution to develop a better understanding of component parts within a larger product. It enables developers to arrive quickly at their conclusions, but a perceived lack of achievable detail within a narrow timeframe could be regarded as a downside.

Again, much will depend on individual demands, such as initial intricacy and whether individual or multi-part production is required, as well as the size of piece being produced.

CNC machines are considered to be quicker at cutting or forming shapes than 3D printers, as well as being the better option for larger items.

Range of Materials

This again depends on the demands of the overall project. Thermoplastics and thermosets are frequently used in 3D printing and, while some metals and even wax and ceramics can also be used in additive manufacturing (AM) additive manufacturing (AM), CNC is more versatile. It can handle a wider range of materials, including a far broader array of metals, although some exotic alloys necessitate specialist tooling that might be beyond the ability of conventional CNC operators.

Financial Cost

This again depends on the individual case. The type of material used – and whether prototypes are being created in-house or by a contractor – are major economic factors; the cost of skilled/knowledgeable labour as well as the cost of purchasing and maintaining the necessary machinery must be considered.

Finish, quality and compatibility with other components

Much depends on having the appropriate tooling to achieve the desired finish, and the material being used. The very concept of 3D printing of layering material may require hand-finishing before final refinement. CNC machining uses a minimum of three axes – vertical, horizontal and depth – to achieve the desired shape.

Many projects will demand components that smoothly interface with the wider product in which they will be used, with CNC technology arguably better placed to achieve this.

Waste And The Environment

Manufacturing processes produce physical waste, both during production and in packaging the finished product. Correctly disposing of or recycling waste is important for the ‘green’ credentials of an organisation and can also impact on the overall unit cost.

In theory, 3D printing creates less waste than the accepted CNC practice of cutting away of a blank/block, but the former predominantly uses plastics, while CNC works with a far greater array of materials, many of which are more sustainable. This alone does not automatically equate to environmental soundness, but plastic waste is an environmental issue best dealt with by using (and producing) less of it.

Durability and Tolerance

Much depends on the material being used. However, the basic premise of layering additive material that is fused together (3D printing) against one that subtracts from an original building block (CNC) would suggest the latter has greater strength – but that’s not always true. The cutting away of some materials could fundamentally weaken the structure.

Generally speaking, however, it can be assumed that a tighter tolerance is achieved from working with an individual block of material, rather than one that is created by layering.

Individual Project Considerations

From all this, it can be concluded that there are many factors at play that make it difficult to set 3D printing apart from CNC machining for rapid prototyping. Much depends on the individual project, with the desired material chosen for strength and tolerance determining which technology will be best. Also remember that suitability for use during initial prototyping may not go hand in hand with ultimately producing at scale for the commercial market.

Considerations such as sustainability and environmental impact are increasingly becoming more relevant, with both technologies arguing cases for and against their use within a ‘green’ remit.

The operation of CNC machining can involve the use of potentially costly skilled labour, but that is mitigated by the broader range of materials it can work with.

There will be some occasions where either discipline can be harnessed, but in the majority of scenarios it should be obvious whether 3D printing or CNC machining will be the best choice for the project you have in mind.