Which of the following materials is commonly used in CNC milling?

Aluminum is commonly used in CNC milling due to its favorable properties for machining. It is lightweight, has excellent strength-to-weight ratio, and is easy to cut, making it ideal for precision applications. Its good thermal conductivity helps to dissipate heat generated during the milling process, which reduces the risk of warping or altering the dimensions of the part being machined. Additionally, aluminum can be easily finished and is available in a variety of alloys that can be selected based on specific engineering requirements.

In contrast, while copper can also be machined, it is less commonly used as it can be challenging to machine due to its tendency to stick to cutting tools and its high thermal conductivity, which can lead to excessive tool wear. Glass and concrete are not typically associated with CNC milling processes; glass requires specialized techniques for cutting and shaping, and concrete is not suitable for precision milling applications due to its hardness and brittleness.