CNC machining is an efficient, cost-effective way to have metal or plastic parts produced that require cutting or drilling, and this manufacturing technology is made an even more attractive for engineers by the fact that they can be manufactured to your own design direct from CAD software.

The development of CNC machining has a fascinating history, with the earliest CNC machine tool developed in the 19670s, using code to control the movement of the production equipment. In today’s world, parts delivered right first time might mean a competitive edge over competitors, meeting delivery targets or hitting budget, and therefore it is more important than ever that the design used for production is not only suitable for CNC machining, but is also critical to ensuring the most cost-effective, accurate manufacturing processes are used. Getting your CAD right will save you time in bringing your products to market whilst helping to avoid the cycle of revision and re-work that can become a problem when your model isn’t optimised for the manufacturing process selected; CNC machines are extremely versatile, but every tool has its limitations.

The global CNC machine market is expected to reach $115 billion USD by 2026, giving an idea of just how popular this manufacturing method is. According to a report by The Manufacturing Technologies Association (MTA), the turnover for the manufacturing technology sector in the UK in 2018 was around £2.5 billion, with a large proportion of those manufactured goods set for export. The MTA report also shows that there is little data available on the use of manufacturing technology such as CAD software systems, so it is difficult to say how many of us are now designing and engineering our own components for manufacture, but our experience at Geomiq is that more of our customers than ever are designing and developing their CNC machined parts in house, and the more we can do to help in getting those products right first time, the happier we – and you – will be. To give you the best chance of ensuring your design is both cost-effective and suitable for CNC machining, we’ve put together the following tips for improving your CAD design ready for manufacture:

#1 Design cavities with a suitable width to depth ratio

End CNC milling tools are limited in the length that they can cut, usually restricted to around 3-4 times their diameter. If you limit the depth of your cavity to 4 times the width, your design will be machinable, therefore if the cavity of your CNC machined part is 20mm wide, you should limit the depth to no more than 80mm.

#2 Keep walls of CNC machined parts to a minimum machining width

Thin walls can reduce the stiffness of the component and therefore create vibrations during the CNC machining process, lowering the surface finish quality and reducing accuracy. Keep wall thicknesses within your design above 0.8mm for metals and 1.5mm for plastics to avoid manufacturing process issues.

#3 Consider manufacturing tolerances carefully

Tight tolerances increase CNC machining time and therefore cost. For example, a hole with a tight tolerance applied will require a boring tool or a reamer rather than a standard drill bit. CNC machines vary in their standard tolerances and if you apply none to your model then the machine will default to its standard tolerance. Where you have a specific need for a tight tolerance on a CNC machined part, apply it only to that dimension – and maintain a consistent tolerancing method across the remainder of the CAD design to save time and cost.

#4 Apply a radius to internal edges and corners

Most CNC machine cutting tools are cylindrical in shape and therefore sharp corners are not achievable. If your design incorporates corners which are at a 90-degree angle, rather than use a radius you can feature an undercut instead.

#5 Use text sparingly

If the process of machining text can be avoided it will save cost. Text features are often undertaken post-machining as a painted solution within the finishing process. However, if you have text which must be CNC machined, ensure that it is recessed or engraved, as this required less machining and material removal than raised text. Also opt for a san serif font at a size of 20 point or larger with spacing of at least 0.5 mm between characters.

#6 Keep threaded hole lengths to 3 times their diameter

Most of the strength of a threaded connection happens within the first few turns, and given that the longer the hole, the greater the time and cost for CNC machining, we recommend that you keep your threads to a length that is necessary rather than excessive, except for blind holes that require an additional unthreaded length at the bottom.

#7 Consider each feature from a manufacturing perspective

Features that exist for aesthetics only will inevitably add cost to a CNC machined part; there is always a balance to be had between the creative and the practical element of a design, but features which are extremely small, for example, will require a specialist tool, and many features which are purely aesthetic can be achieved through a finishing process rather than as part of the CNC manufacturing process. If a feature is necessary, consider whether it is actually feasible for it to be manufactured using CNC machining techniques – for example, curved holes are not achievable through CNC machining but could be produced using EDM as a separate process. Finally, consider whether your unnecessary feature takes your CNC machined part from a 3-axis machining method to a 5-axis machining method, or even if it simply introduces additional manual intervention; the latter two solutions will prove a more expensive manufacture method.

Geomiq offer the full range of CNC machining facilities, including CNC milling, CNC turning and both 3-axis and 5-axis machining options. With more than 180 fully vetted CNC manufacturers available through our partnership network and a range of over 100 metal and plastic materials to choose from, we will be able to support you in whatever manufacturing process you require for your CNC machined parts. However, if you would like to know more about the various types of CNC machining methods available and which one might best suit your CAD design, take a look at our CNC design guide.