There are instances where we are asked to CNC machine parts, product or a prototype that are either hard to machine, too complicated geometrically, will not yield great accuracy or simply cannot be machined. What do we do? For these scenarios, 3D printing the parts can be a great solution. So why not substitute CNC machining with 3D printing every single time? Well, there are advantages and disadvantages, pros and cons for each machine and process. So, which one is best for our needs? In which circumstances do we prefer one over the other? And is there another solution that might combine these two together to create a combo part?
The basic difference between the two processes is that with CNC machining we are reducing material as we start with a foam block for example, carving it away; while with 3D printing we are layering on and adding material until we receive the final product, therefore called additive manufacturing.
The 3D printer uses the same materials that make up the part it is creating, for instance ABS PLA and nylon, but it cannot switch between materials, whereas in CNC machining we can use several types of materials, often adding additional materials at the end. However, machining can be messy – Sometimes we need to use a dust collector while operating a CNC router machine to catch all that excess made in the drilling, carving and milling process, while there is less waste material produces in printing and the whole process is less noisy.
CNC machining can be more precise providing more accuracy because the machines have a higher tolerance for heat. It can also result in a much smoother polished surface finish given the materials to be machined. 3D printers can actually distort a part, bend and warp if using too much heat on the layered material, so if exceptional smoothness is required the 3D printing will fall short.
3D printing is generally an easier more convenient process and not as labor-intense as CNC machining, since with machining we need to program, write a G-Code, set up different tools and speed, decide on cutting path and clean up after. However, the part size plays a role, as larger parts take longer to be printed adding layer by layer. Overall, 3D printing can assist in some cases of prototyping of high geometric complexity where the router tool cannot reach inside the shape.
3D printers can only use the area of the printer bed itself to fabricate the parts. Therefore, if large scale parts are needed they might not be able to fit in there. It is also not recommended for mass production as the materials are much more expensive and take much more time to fabricate. Therefore, 3D printing is more appropriate and more cost-effective for low volume production.
CNC machining can rarely run unattended and requires skilled operator, while with 3D printing we can easily run the process unsupervised and it requires minimal training for its operator. However, CNC machining is an older practice (started in the 40's) and currently still has a stronger position in the manufacturing industry. 3D printing is relatively new and still evolving to be more useful and adaptable and still cannot be a full replacement for machining.
In summary, the most appropriate technique to use will be determined by the material, geometric complexity, manufacturing volume and our budget. As a general guidance, we would switch to 3D printing mostly if fast turn-around in critical, if the part is too complex to be machined, for prototyping small volumes and if we need to use certain materials that cannot be easily machined.
Having named most of the pros and cons for each technique, apparently there is a good solution that actually combines the two together to create one part. We often machine parts of the desired product using a CNC router, while fabricating other small yet more complex parts in the 3D printer, we then glue all parts together to make one unit. Another option is then to coat all glued combined parts with hard coat such as Polyurea, Styrospray or epoxy, then smooth and paint them. That way we both save time using the CNC machining process as well as being able to manufacture more complex parts combining the best of the two worlds together to create a hybrid.