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Prototype Machining: Pros and Cons of CNC for Prototyping

CNC Machining Guide

What is CNC machining?

Computer Numerical Control (CNC) machining is a manufacturing process in which computer inputs are used to control machining tools such as drills and lathes. It is used across many industries for a variety of prototype and end-use parts.

The process starts with a digital 3D design, created using CAD software, which a computer can translate into a series of instructions to the machine’s cutting tools. These instructions are known as G-code. Once the G-code is sent to the machine, very little manual supervision is needed, since the machine knows when and where to cut and performs the machining autonomously. This results in significant time and cost savings when compared to traditional machining, in which a skilled machinist cuts the workpiece using manually operated cutting tools.

Machining is a subtractive process, which means the equipment removes existing material instead of introducing new material. Unlike additive manufacturing, in which a 3D printer deposits material in layers to form an object, CNC machining involves cutting sections away from a formless block known as the “workpiece.” Excess material is discarded or recycled, eventually leaving behind a completed part. More complex CNC machines, those with a greater number of axes, are capable of cutting the workpiece in more complex ways, producing parts with more intricate geometries.

CNC machining is a widely used manufacturing process thanks to its versatility, accuracy, consistency and wide range of compatible materials: although aluminum alloys are the most common material for machining, numerous other metals and plastics can also be used.

Is CNC machining good for prototyping?

Although many now consider 3D printing to be the dominant form of rapid prototyping, CNC machining is also an important process for creating prototype parts. To understand why, it is beneficial to consider the different forms a prototype can take, and to consider why these prototypes exist.

Prototypes can have many different functions. At their most basic, they act as placeholders or props — loose representations of a final part used to convey visual information about how the final part will look or behave. These looks-like prototypes may be used to guide the R&D process or provide proof of concept. If made to a high aesthetic standard, they may also be used to pitch a new product to potential investors. Such prototypes can be very important, but they do not necessarily need to be made using high-end professional equipment: they may be crafted by hand or printed with a low-end desktop 3D printer.

Some prototypes, however, are used for more than just visual representation. Depending on the stage of product development, companies may need to create engineering prototypes or production prototypes: prototypes that not only look like the final part, but function like it too, acting as a substitute as near as possible to the real thing. While 3D printing may be a great option for looks-like prototyping, CNC machining is often preferable for these functional prototypes that require strength, mechanical stability or other characteristics not afforded by additive processes. After all, not many end-use parts are made with a 3D printer.

CNC machining can be an excellent choice for prototyping, but its suitability depends on the nature of the prototype. For what purpose will the prototype be used? From what material will it be made? And from what material will the final part be made? These and other questions will ultimately guide the user to the most appropriate method of prototyping.

Advantages of prototyping with CNC machining

There are numerous reasons why a company may use CNC machining to produce a prototype, including speed of production, part quality, material options and similarity to the final part.

From file to prototype

One of the biggest advantages of CNC machining is the “CNC” element. Since CNC machining is a digital process that creates a part from a computer file, engineers know that a 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 identical dimensions. The degree of repeatability is particularly high.

Furthermore, using digital 3D designs allows for quick and precise alterations. If a machined prototype exhibits a physical flaw resulting from bad design, the engineer can return to the CAD software to make suitable changes for the next prototype. Different versions can be compared side by side, and simulation software can even be used to preemptively test how a part will perform in the real world.

Quality and consistency

Computers aren’t perfect, but computer-controlled machinery tends to function exactly as it’s supposed to — unless it breaks down. While many prototyping processes rely on human skill (and are therefore susceptible to human error), CNC machines follow their instructions to within a fraction of a millimeter.

Importantly, they can also do it over and over again. Although a business might only be making a single prototype, a CNC machine can, if needed, run the same job a second time with minimal deviation from the first. This is incredibly useful for developing new iterations of a prototype, and for moving to production with the same machinery. (Manual processes are incredibly important, but it’s easier to guarantee consistency with an automated machine.)

Range of strong materials

If a prototype does not have a mechanical purpose, it may be suitable for 3D printing, which — although not known for producing high-strength parts — costs very little and can be carried out in a short space of time. For most 3D printing processes, however, the material options are narrower than they are for machining.

CNC machining not only offers a wide range of compatible materials, it also offers some extremely strong and durable ones, including a wide range of metals. It is possible to 3D print with metals as well, but not with a low-cost FDM printer.

Common CNC machining materials include:

Metals

Plastics

Aluminum

ABS

Steel

PC

Stainless Steel

PP

Magnesium

PS

Titanium

POM

Zinc

PMMA

Brass

PAGF30

Bronze

PCGF30

Copper

Teflon

DHPE

HDPE

PPS

PEEK

Similarity to final part

Another major advantage of using CNC machining for prototypes is the ability to create prototypes similar to the final part. Since machining centers are fully capable of producing end-use parts as well as prototypes, it is possible to create prototypes that are close to the end product — a feat that is rarely possible with 3D printing or other methods.

Part of this is down to materials. Many engineering metals are highly machinable, which allows engineers to make prototypes from the same (or similar) materials they will use for the final part. But the quality of the process itself is also a factor: machined parts are strong and do not exhibit weakness along certain axes like printed parts, while the machining process itself can even be used to replicate other processes like sheet metal forming.

Furthermore, creating a prototype that is close in appearance and behavior to the final part makes it easier to move to bridge production, since there are fewer significant changes to make.

Disadvantages of prototyping with CNC machining

Despite its advantages, CNC machining has certain limitations as a prototyping method, which may lead businesses to favor an alternative approach.

More expensive than 3D printing

One of the obvious drawbacks of CNC machining prototypes is the cost of the process. Machining centers are large pieces of machinery that require significant power and greater human supervision than 3D printers. Machinable metals also come at a higher price point than common printing materials like PLA.

This is one of the main reasons why engineers may choose alternative prototyping processes, even if they intend to use machining for their final parts. Development can be a drain on resources, and it is understandable if businesses need to cut every expense during the (early) prototyping stage.

Some geometrical restrictions

4-axis and 5-axis machining centers offer a large degree of geometrical flexibility, but even these machines have their limitations. For elaborate structures with complex internal geometries, additive manufacturing processes may be more suitable, since they are not restricted by the angles of cutting tools.

Bear in mind, however, that 3D printed prototypes can be misleading: a digital 3D design that comes out perfectly from a 3D printer may be impossible to fabricate using the chosen production equipment, whether that is machining centers, injection molding equipment or something else. Geometrical flexibility for prototypes is only helpful if that flexibility can be replicated on the final part.

Waste material

Since CNC machining is a subtractive process, it requires more material than what actually goes into the part. Some of the material is cut away and ends up as metal or plastic chips, which must then be disposed of. This is different to additive prototyping processes, which do not produce waste material unless the print fails and must be repeated.

Using machining as a prototyping processes can incur higher material costs due to increased material usage and wastage. However, chips can often be recycled, so the environmental impact of the process does not have to be severe. (Selling recyclable waste material can also help to recoup some material costs.)

Rapid tooling: Injection molded prototypes via CNC machining

We have seen how CNC machining can be an excellent prototyping process. But machining can also be used indirectly to create injection molded prototypes.

By CNC machining tooling or molds, businesses are afforded a more cost-effective way of creating the apparatus required for injection molding. This CNC machined tooling can be created more quickly than traditional tooling, and is therefore a shortcut to molded prototypes. (For the final molded parts after the prototyping stage, traditional tooling methods may be employed.)

Using the rapid tooling process is much more cost-effective when ordering larger volumes of molded parts, since the machined tooling costs a lot more to fabricate than the molded resin parts themselves. And although large quantities may not be particularly desirable during the prototyping stage, the authentic molded prototypes will be more representative of a molded part than, for example, a 3D printed alternative.

Contact 3ERP to discover whether CNC machining is the best prototyping process for your project.

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