CNC machining for prototypes

CNC machine

Although 3D printing may be the fashionable face of prototyping, CNC machining remains the better technology for many components. Prototype Projects explores how...

Whatever you need parts for – perhaps a proof-of-concept prototype, visual model to show backers, usability trials or any other stage in the product development process – CNC machining holds tight tolerances on metals and plastics, as well as being fast and cost-effective. CNC machining is also excellent for low-volume production parts required by machine builders and companies manufacturing small quantities of high-value items.

Here at Prototype Projects, our CNC (computer numerical controlled) machining suite has produced tens of thousands of parts for hundreds of customers, both locally in the Cambridge area and further afield. In most cases we are not told what the parts are, what they will be used for or for which market sector they are needed. But we do know that over the last 40 years, we have supplied parts for use in sectors ranging from medical devices, pharmaceuticals and life sciences, through to industrial automation, transportation, consumer goods, electronics, IT and many other sectors.

Benefits of CNC machining

One of the main benefits of CNC machining is the tight tolerances that can be held. We quote a general machining tolerance of ±0.1mm whereas 3D printing tolerances are usually ±0.5mm. In reality, most CNC machined features have tolerances considerably tighter than ±0.1mm, depending on the material and type of feature. Often the cost for CNC machining is less than for 3D printing and, if functional parts have features in different orientations, CNC machining from solid material avoids the problem that can occur with additive manufacturing, in which the material properties are not isotropic.

Furthermore, CNC machining can be used with most solid materials, whereas the material choice for each 3D printing technology is always limited. Small threaded holes can be produced automatically during the CNC machining operation, rather than being tapped manually after 3D printing. As with 3D printing, parts produced by CNC milling can incorporate text, either raised or recessed.

If the part needs to withstand moisture, temperature extremes, contact with chemicals or heavy loads, then it is most likely that machining from metal will be better than 3D printed. Together, the functional requirements, material and geometry will determine whether a part should be CNC machined or 3D printed but customers are always welcome to discuss it with us. Unlike many prototyping companies, we have both technologies in-house and can provide a truly unbiased opinion on which would be best for any given component.

Machines and materials

If a customer concludes that CNC machining is the right approach, our workshop is equipped with three Hardinge-Bridgeport GX 480 vertical machining centres, one Bridgeport VMC 600 vertical machining centre, three Haas Mini Mills, and a Haas ST 10 turning centre. In addition, we have a semi-CNC Haas TL-1 toolroom lathe and a workshop lathe if CNC is not necessary.

For our three-working-day Express service, the maximum material stock sizes for CNC milling are 250 x 200 x 50mm for aluminium and acetal, 250 x 200 x 30mm for acrylic, and 100 x 100 x 12mm flat bar or 50 x 50 x 25mm square bar for stainless steel. Materials up to 60mm diameter can be CNC turned.

If Express dispatch is not required, larger parts can be machined. The maximum milling machine bed size is 450 x 350mm and we can turn parts up to 100mm in diameter, or larger diameters can be milled.

To minimise turnaround times, we hold extensive stocks of commonly used materials including aluminium 6082, black and natural acetal, clear acrylic, and stainless steel 316 and 304, all in various sizes.

For the Standard or Economy service, other materials are stocked in limited quantities or can be procured as required, including PEEK, PTFE, self-lubricating plastics, brass, phosphor bronze, mild steel and tool steels; machining of exotic alloys is outsourced to one of our trusted specialists. CNC machined parts are left ‘as machined’ but finished by deburring, cleaning and degreasing. If specified, we can paint parts using our in-house facilities, or we use our network of specialists for additional finishing operations such as clear or coloured anodising of aluminium, polishing or vapour blasting of stainless steel, electroplating of mild steel or hardening of tool steels.

Inspection and delivery

We have an inspection room, with a three-axis Etalon Derby CMM (co-ordinate measuring machine) as well as handheld measuring instruments including callipers, micrometers and thread gauges. Customers can request detailed inspection reports or just ask for specific critical dimensions to be checked.

CNC machined parts are shipped in seven working days with our Standard service but we also offer an Economy service with shipment in 12 working days. In 2020, we recruited an additional five CNC machinists and introduced a two-shift work pattern, which enabled us to launch a three-day Express service. Whatever service level customers choose, we adhere strictly to our ISO 9001 quality assurance procedures and the parts are always machined to the highest standards. However, to achieve the rapid turnaround for the Express service we stipulate maximum material stock sizes, depths of cut, all machining operations to be perpendicular to the machine bed, no use of fixtures, stock materials only, some limitations on treaded holes and internal radii, and no additional finishing operations. Key dimensions are checked manually but the short turnaround time of the Express service does not permit parts to be inspected on the CMM.

Order fulfilment

When we receive an enquiry for CNC machining, the CAD file (normally STEP or IGES) is assessed by a skilled and experienced member of our team so we can prepare a quote based on the material, number of set-ups and machining time, while also taking into account any special requirements. If we have questions, we contact the customer by telephone, email or video call as appropriate so we can be certain we understand the requirements in full before providing a fixed-price quotation. When making the initial enquiry, the customer might request our three-day Express service or, if not, we check whether the Express, Standard or Economy service is required.

Once the customer has accepted the quote and placed an order, we schedule the job and start off-line programming using Mastercam software. A full simulation is run on Mastercam to check the tool paths before the CNC program is output to the machine tool.

Recently we dispensed with using printed paper for machine setups; instead, each machine has a touchscreen terminal for accessing digital copies of all relevant documentation. This improves production efficiency and streamlines the entire process from order to delivery.

We do not normally ask for 2D drawings because all the information we need for CNC machining is contained in the STEP or IGES file but we do ask for drawings if, for example, there are critical dimensions with tight tolerances.

When the machining operations have finished, the part is deburred, cleaned and degreased. For Express parts, key dimensions are checked, whereas other parts are taken to our inspection room for further checks as necessary.

Finally, the packing department wraps and boxes the part ready for free local delivery or, if the customer is further away, the order is dispatched using a courier. At this point, we email the customer to say the order is complete and on its way.

Complementary prototyping technologies

As we said at the start, CNC machining is still the optimum production technology for many prototype parts but it is not ideal for everything. In contrast to conventional CNC machine shops, Prototype Projects has most prototyping technologies in-house, so we can meet all your prototyping requirements. If you need 3D printing, we have stereolithography (SLA), Polyjet, Figure 4, selective laser sintering (SLS) and fused deposition modelling (FDM) processes, as well as SLS dyeing and SLS & SLA texturing facilities. We also have in-house laser cutting and vacuum casting, and offer a full range of finishing and assembly processes. A technique that works particularly well is vacuum casting to overmould a CNC machined metal insert. If customers need a prototyping process that we do not have in-house, we entrust it to one of the specialist suppliers we use, most of which we have worked with for ten years or more.

Whatever your need for CNC machined prototype parts, contact us on 01763 249760 (or email to find out how we can meet your requirements quickly and cost-effectively.


Prototype Projects, established in 1980, is a specialist in the manufacture of prototypes. The company is experienced in 3D Printing, CNC Machining, Rapid Prototyping, Model Making and Additive Manufacturing. The company is located in Royston, Hertfordshire.

Prototype Projects Ltd