CNC machining has been around in some capacity for over half a century, but with material choices becoming ever wider and more specialist, and the technology undergoing continuous development, CNC machined parts have become the go-to solution for manufacturers. According to FNF Research , the global CNC machine market is expected to be valued at $115 billion by 2026, and with the drive for competitive pricing, improved quality and bespoke options that so many manufacturers face, we are all looking for ways to reduce manufacturing costs on both prototypes and large-scale production volumes. We take a look in more detail at how you can reduce your CNC manufacturing costs.
Volume of parts
CNC machining can be used for prototype parts because, unlike manufacturing methods such as injection moulding, there are no tooling costs – and, as highlighted by the British Plastic Federation , mould tools and equipment can take weeks or even months to manufacture and are therefore both costly and slow to produce finished parts. Selecting CNC machining as the solution for mid-volume plastic parts, therefore, is a no-brainer, and for metal parts it can be a great solution for both mid-range and large scale production. However, there is a set-up cost which can make CNC machining prohibitive for a one-off or prototype manufacture, in which case 3D printing may make more sense until the part has been tested and signed off. CNC machining really comes into its own for orders of quantities of hundreds or more – particularly where the order is to repeat, because the set-up is saved as a programme and therefore does not need to be paid for on each occasion. Where CNC machining is the right manufacturing method, there are ways to ensure that your product design allows the part to be produced in as cost effective a way as possible; the tips below will help you to design for manufacturability and keep your costs to a minimum.
Material selection
The choice of material affects CNC machining costs in two different ways. Firstly, there is a cost to the raw material, and the choice of material is dependant largely on your application. The second price impact is via the machinability of the raw material; the easier the material is to machine, the lower the cost – and the harder the material, the greater the wear on consumable items such as tools, and therefore this will push the cost up. Again, materials selection is always a balance between cost versus performance. Be clear about the level of functionality and properties that your material requires and seek advice if you are unsure; for example, whilst aluminium only has around 60% of the electrical conductivity of copper, it is lightweight and cheaper to buy – it may be that it will perform just as well for your application but without the added costs that copper brings with it.
Finishes and treatments
Finishes can include the smoothness of parts as well as any treatments to enhance the material performance. CNC machined parts are capable of creating much smoother surface finishes than 3D printing, however, additional processes are required to achieve this. If the component is purely functional, then it is more cost-effective to accept a part with the machined finish and deburr them in-house if you have the facilities. Plastic CNC machined parts are generally supplied as-machined, but with metal parts deburring is common – and where the product design includes an edge break, this requires an additional tool to machine these corner sections. Product designs for CNC machined parts are often drawn with all corners chamfered, and corner radii in place – understandable in terms of making the part smooth and avoiding sharp edges, but it does add machining time and therefore cost.
When considering treatments such as chem film, anodising and blacking, each of these calls for an additional process; multiple treatments require multiple processes and therefore incur multiple costs. If the treatment is necessary for the application then it is an unavoidable cost, but it is worth considering whether an alternative material might provide a more cost-effective solution when combined with the machining time.
Part geometry
This is probably the biggest influence on CNC machined part cost, because the part geometry dictates both the volume of material and the machining time required. By focusing on design for manufacturability, you will be able to cut costs and ensure that you balance aesthetics and functionality against machining time and costs.
#1 Use rounded internal corners
CNC machining tools naturally leave rounded internal corners as a result of their shape. The narrower the internal radius, the smaller the tool required and the higher the number of passes that will be needed at a slower speed. For the most cost-effective result, ensure your product design has an inside corner radius has a length to diameter radius of 3:1 or less.
#2 Avoid deep internal cavities
Deep pockets within your product design will create a need for an end mill to be used in progressively small increments, or alternative specialist tools. Either of these options add costs as extra time is taken and specialist tools are employed. The recommended ballpark is to design CNC machined parts of a length which is up to 4 X its depth.
#3 Keep walls as thick as possible
To machine parts with thin walls the speed of the CNC machining action must be slowed right down, increasing machining time. Thin walls can also mean that the usual tight tolerances that CNC machining is renowned for cannot be held. Walls should have a minimum thickness of around 0.794mm for them to be feasible for CNC machining, and if the product design requires a thinner wall then it may be better to look at sheet metal fabrication as an alternative manufacturing method. You can find more details about sheet metal fabrication in our sheet metal design guide.
#4 Keep parts simple
Added complexity means added cost. If your product design is particularly complex, it may be worth breaking down the component into several pieces for CNC machining. Although this will add to the cost of assembly, it may be more cost-effective overall than trying to machine a single component.
Tolerances
Standard CNC machining tolerances are usually around +/-0.127mm, but +/-0.005mm can be achieved in many cases, with even tighter tolerances on critical areas. However, to avoid adding unnecessary cost to your parts, it is recommended that you specify only the critical surfaces or features with numerical tolerances and leave the rest of the model within a standard tolerance range. This will keep your costs to a minimum.
Machining of text
Where possible, avoid using text on a CNC machined part as the detailed nature of the work involved will push costs up. If text is absolutely necessary then opt for engraved rather than embossed text as this requires less material to be removed and is therefore a better cost option.
CNC machining is a great solution for mid-range manufacture volumes of plastic parts, or for mid to large-scale production of metal components. The rule of thumb when looking at part design should be to keep it simple and ask whether each feature is necessary; something seemingly innocuous might add more cost than you anticipate. For an online quote from a network of CNC machinists with delivery in days, upload your CAD file to our platform and find out within 24 hours what your CNC machined part design will cost to produce.
