When it comes to making prototypes, the two manufacturing technologies most likely to be used are CNC vowin.cn/en/News/news1251.html' target='_blank'>machining and 3D printing. We recently blogged about the differences between CNC machining and 3D printing so in this present article we focus on CNC machining for prototype parts.

What is a prototype?

Everyone has their own understanding of what a prototype is, and this can vary significantly from one person to the next. For a word commonly used by engineers and designers, it is surprisingly ambiguous!

We find customers use ‘prototype’ in a number of different ways. To summarise, we are happy to work with any of these common interpretations and others:

• An early iteration of a design: as part of the product development process, the design will evolve through several iterations. Here, a prototype is any physical embodiment of a design iteration.
• The first physical embodiment of the final design: if the product design has been developed digitally and/or with the aid of physical models, the designer may have a high degree of confidence. A prototype is made to prove the design or identify flaws that must be addressed before committing to series production.
• The first and (likely) only physical embodiment of a part: this definition tends to be applicable in situations such as when building special-purpose machinery, or one-off test rigs or pieces of research equipment.

CNC machining can be ideal for producing any of the above types of part. With no need for specialist tooling and a wide choice of materials and finishes, the versatility of CNC milling and CNC turning comes into its own for prototyping.

How many prototypes?

You might expect a prototype part to be a one-off but that is not always the case, depending on your interpretation of ‘prototype’. Maybe a prototype of a product is required for usability trials by a number of individuals or in a number of geographical markets. If so, multiple prototype products could be required.

As well as CNC machining being versatile for one-offs, it also lends itself very well to the production of small batches. In particular, the technology’s inherent accuracy means part-to-part repeatability is excellent, which can be invaluable when prototyping.

Our two newest CNC machines are five-axis Haas vertical machining centres (VMCs), which are equipped with automatic tool changers, programmable vices and robots. Consequently, they can machine parts without an operative having to reposition the workpiece in the vice, and the robots can load blanks and unload finished parts automatically so the machines can produce batches of parts unattended.

Materials for CNC machined prototypes

A major benefit of CNC machining is the extensive range of materials available. We hold stocks of aluminium alloy 6082, stainless steel 304 and 316, acetal (natural and black) and acrylic. However, we can also procure and machine almost any metal, alloy or engineering plastic, plus composites such as Tufnol. If parts are too large for our machining centres, or if customers need parts machined from exotic alloys requiring specialist tooling, we outsource this work.

This choice of materials means customers can have prototype parts in the material they want. If the prototype is a part that will ultimately be injection moulded, we can machine it from the same type of plastic. However, customers need to be aware that injection moulded parts can have residual stresses that will not be present in parts machined from solid. For more on this, see our blog post CNC machining: does it mimic production parts?

Accuracy of CNC machined prototypes

In general, CNC machining is far more accurate than 3D printing. The exception is PµSL (projection micro stereolithography), which achieves tight tolerances, fine details and exceptionally smooth surfaces, but is only suitable for small parts.

We quote a general tolerance of ±0.1 mm on CNC machined parts, compared with ±0.5 mm for 3D printed parts. However, the inherent accuracy of CNC machining means we normally achieve much tighter tolerances than ±0.1 mm. Customers are welcome to talk to us if they need finer tolerances on critical features, as we can usually meet the requirements if we know about them in advance.

If we only receive a 3D model (step file) to quote from then a general tolerance of ±0.1 mm is considered, if a drawing is then provided, we will revise our quote if finer tolerances are required and if an inspection report is required.

Not only is this accuracy achieved when machining individual parts, but part-to-part variability is minimal for small batches.

Variants and alternatives

When designing a part using 3D CAD software, it is simple to alter the design or create multiple variants of the same part. CNC machining can easily produce those same design variants in metal or plastic.

Visibility for additional insights

A physical prototype always provides more insight than a CAD model. For example, CFD (computational fluid dynamics) analysis can predict fluid flows, but a physical model provides higher quality data (witness how much money Formula 1 teams spend on wind tunnel testing!).

Another way to gain even greater insights into a product’s behaviour is to make all or some of the prototype from clear acrylic, then polish it to a glass-like appearance. This permits the prototype’s behaviour to be viewed by eye or with high-speed video.

If it is impractical to make an entire component or assembly from acrylic, another option is to CNC machine clear windows or portholes for inserting in the prototype so critical areas can be viewed.

Finishing choices for CNC machined prototypes

We offer a range of finishes for CNC machined prototype parts, depending on the material and the part’s intended use and operating environment. Aluminium alloy parts can be bead blasted and we can outsource clear, black or coloured anodising. Stainless steel parts are often bead blasted or hand polished, or we can outsource electropolishing. Mild steel parts are suitable for priming and painting but, upon request, we can use local specialist finishers for electroplating, powder coating and chemical blacking.

CNC machined plastic parts are often left as machined or given a light bead blast. Depending on the material, we can prime and paint parts, or apply a blackout/RFI/EMC coating to internal surfaces. If necessary, specialist finishes such as vacuum metallisation can be outsourced.

Speed

With no need for specialist tooling, CNC machining can be very quick. Our Express CNC Machining service enables parts to be shipped in as little as three working days. We do not compromise on quality to achieve this fast delivery, but part designs need to meet certain criteria.