Advantages of prototype CNC machining
There are many reasons why companies use CNC machining to produce prototypes, including speed of production, part quality, material selection and accuracy with the final part.
From file to prototype
As CNC machining is a digital process of creating parts from computer files, engineers know that the machined prototype will closely match the digital 3D design and that the same digital design can later be used to create a final part with the same dimensions. The degree of repeatability is particularly high.
In addition, the use of digital 3D design allows for quick and precise changes. If a machined prototype exhibits physical defects due to poor design, the engineer can return to the CAD software to make the appropriate changes to the next prototype. Different versions can be compared at the same time, and it is even possible to use simulation software to pre-test how the part will behave in the real world.
Quality and consistency
Computers are not perfect, but a computer-controlled machine often works exactly as expected, unless it breaks down. While many prototyping processes rely on human skills, CNC machines follow their instructions to within a fraction of a millimetre.
Importantly, they can also do it over and over again. While a company may only make one prototype, if required, the CNC machining machine can run the same job a second time with minimal deviation from the first job. This is very useful for developing new iterations of a prototype and for transferring to production using the same machine. (Manual processes are important, but it is easier to ensure consistency with automated machines.)
A wide choice of materials
If the prototype has no mechanical properties required, it may be suitable for 3D printing, although it is not known how strong the parts produced will be, but the cost is very low and it can be done in a very short time. However, for most 3D printing processes the material options are narrower than for CNC machining.
CNC machining offers not only a wide range of compatible materials, but also some extremely robust and durable materials, including various metals. It is also possible to 3D print using metals, but not using low-cost FDM printers for 3D printing.
Disadvantages of prototype CNC machining
Despite its advantages, CNC machining has limitations as a prototyping method, which may lead companies to favour alternative methods.
More expensive than 3D printing
An obvious disadvantage of CNC machining prototypes is the cost of the process. Machining centres are large mechanical machines that require more power and greater manual supervision than 3D printers. Machinable metals are more expensive compared to common printing materials such as PLA.
This is one of the main reasons why engineers choose alternative prototyping processes, even if they intend to use the machining for the final part. Development can be resource intensive, which is understandable if companies need to cut costs per item at the (early) prototyping stage.
Some geometric constraints
4- and 5-axis machining centres offer a large degree of geometrical flexibility, but even these machines have their limitations. For fine structures with complex internal geometries, additive manufacturing processes may be more appropriate as they are not limited by the angle of the cutting tool.
But remember that 3D printed prototypes can be misleading: it is not possible to produce a digital 3D design perfectly using a 3D printer using the chosen production equipment, be it a machining centre, injection moulding equipment or other equipment. The geometric flexibility of a prototype is only useful if the final part can replicate the flexibility.
As CNC machining is a material reduction process, it requires more material than actually goes into the part. Some material is cut off and ends up as metal or plastic scraps, which must then be discarded. This is in contrast to the additive prototyping process, which does not produce scrap unless the print fails and must be repeated.
Using machining as prototype manufacturing process will lead to higher material cost due to the increase of material use and waste. However, some can usually be recycled, so the impact of the process on the environment is not necessarily very serious. (selling recyclable waste also helps to recover some material costs.)
The above analyses the advantages and disadvantages of CNC machining prototyping from different perspectives. The selection of different machining processes for different application scenarios can help to speed up product development and reduce costs and risks.