Designing parts for CNC machining may seem straightforward—raw materials is removed from a solid block to create the desired shape. However, there are many ways your designs can encounter issues may arise from parts that are not designed with manufacturability in mind. We conclude some of the most critical issues, that our engineering experts frequently meet in quote requests. Avoiding the issues will help you improve your part design, shorten lead time, and even perhaps save some costs along the way.

Not all features are achievable, a typical example is an internal angle of 90°. See Figure-1. This is because all current CNC milling tools have a cylindrical shape, which creates a rounded edge instead of a straight or sharp angle when cutting along the edges of the machined cavity. If a straight angle must be retained, the usual approach is to use EDM (electrical discharge machining), which is a more expensive manufacturing process than CNC machining.

When designing for CNC machining, it is important to avoid designing excessively thin or tall walls. While thin walls may be necessary for certain projects, it is important to note that studies have shown that wall thickness is proportional to the stiffness of the material, which can impact the achievable accuracy due to vibrations during machining. The standard minimum thickness for walls is 0.8 mm for metals and 1.5 mm for plastics, see Figure-2. Meanwhile, tall walls may result in issues such as undesirable surfaces, difficulty in meeting part tolerances, and the risk of chipping, bending, or breaking. A good rule of thumb for walls is a width-to-height ratio of 3:1.

Although CNC machining can tolerate some of the tightest tolerances in manufacturing process, it doesn't mean that every part needs to have the strictest tolerances. In fact, using tight tolerances only when absolutely necessary can greatly reduce costs. As precision requirements increase, more precision manufacturing processes are needed, leading to lower production efficiency and higher costs. But sometimes, the flawless assembly can be achieved through creating reasonable gaps, simplifying assembly relationships, and using positioning features. In addition, it is also important to maintain same tolerancing as this would reduce machining time. See an example in Figure-3.