Biggest design mistakes for CNC

Computer numerical control (CNC) is very popular with manufacturers today, and it’s easy to see why. Not only is CNC machining compatible with a wide range of plastics and metals, it’s also a reliable manufacturing process that can produce precise, durable parts. Computer-programmed cutting tools remove material from a solid block to produce a final product that meets your exact specifications, every time. However, if you don’t use best practices for CNC design, production times and costs may continue to skyrocket.

Design sharp inside corners

Because CNC bits are round, creating sharp inside angles is expensive and time-consuming. Machining a 90° angle with a round bit is impossible because a round tool always creates a corner radius when milling a vertical inside edge, which prevents parts from fitting together properly or end mills from stopping mid-process.

Designing rounded corners eliminates the problem of sharp inside corners altogether, but if your design calls for inside angles, design them with radii and mitigate them by increasing the corner radii. Corner radii of the same diameter as the cutter can lead to chatter and excessive tool wear, while corner radii that are too small will cause your machinist to use several smaller cutters at lower speeds. By increasing your corner radii by as little as 0.005″, you can save money and avoid mistakes.

If you have a square male part, you can use dogbone or T-bone fillets to make sure it fits into a female cavity with slightly rounded inside corners. Just make sure the entry point for the fillet is 15-20% larger than the diameter of your router bit.

Including thin-walled parts

Parts are sometimes designed with thin walls to minimize the amount of material used, but this “solution” causes more problems than it solves. Walls that are too thin can cause the part to break or warp, reducing the surface finish of the metal and the accuracy of the machining process. Thin walls can also break, bend, or chip during machining due to the machining forces and excessive vibration behind the CNC cutting tool.

The ideal wall thickness for your part depends on what material is being CNC machined. For example, when CNC machining aluminum, a wall thickness of 0.8 mm is fine. For plastics, however, a wall thickness of 1.5 mm is ideal. Also keep in mind that the higher the wall thickness, the greater it needs to be to increase rigidity. Consult an experienced CNC fabrication partner to ensure your dimensions are correct.

If your design calls for tall, thin walls, try to maintain a width-to-height ratio of 3:1. You can also add a slight distortion to speed up machining and reduce the amount of scrap material.

Machined text

CNC routers can engrave or emboss text and symbols onto parts with high precision, but machining text can cost you. First, your machinist will need to use a separate cutting tool for the text. You also need to consider how much time – and money – text editing will add to your project, since the small end mills that cut the text are relatively slow.

The good news is that you have several options if you need text on your part. If you need to machine text, opt for recessed text instead of raised text so the machine doesn’t have to remove material over the entire surface of the part. You can also have the marking applied after machining. Laser marking parts after CNC machining, for example, can save you time and money.

Including deep cavities, holes and threads

Milling tools have a finite length, and their length determines how deep you should make cavities. In most cases, milling tools are most efficient and accurate when they mill cavities to twice or three times their diameter depth. Even deeper milling cavities can increase cycle times, cause tool deformation or breakage, or lead to chatter and chip evacuation difficulties. They may even require more expensive special tooling.

It’s best to avoid deep-cavity parts altogether, but if your part requires a deep cavity, hole or thread, reduce the depth of the cavity as much as possible. Also consider the length of the milling tool.