Which of the following materials is NOT commonly processed by CNC cutting machines?

CNC cutting machines are widely utilized for a variety of materials due to their versatility and precision. However, glass is generally not processed by CNC cutting machines in the same way as metals, plastics, and wood. While it is possible to cut glass using CNC technology, specialized methods and tools are required due to the material's unique properties. Glass is brittle and can shatter easily, necessitating careful handling and often limiting the types of cuts that can be made.

In contrast, metals are commonly cut using CNC techniques for applications ranging from manufacturing parts to assembling machinery. Plastics are also frequently processed with CNC equipment as they allow for efficient machining and intricate designs. Wood is another standard material for CNC cutting due to its ease of machining, availability, and wide range of applications in furniture making and carpentry. Thus, while CNC machines can be adapted to cut glass, it is not as prevalent as the other materials mentioned, making it the least common material processed by CNC cutting machines in standard practices.