What are the material considerations for precision parts machining?

Precision mechanical parts processing, not any material can be precision processing, some materials hardness is too large, across the hardness of the machine parts, may break the machine parts, so these materials are not suitable for precision machining, unless it is made of special materials, or laser cutting. So what are the material requirements for precision parts processing?
　　There are two main categories of materials for precision mechanical parts processing, metal and non-metallic.
　　For metal materials, the hardness is stainless steel is the largest, followed by cast iron, followed by copper, and finally aluminum.
　　The processing of ceramics, plastics, etc. is attributed to the processing of non-metallic materials.
　　1, first of all, the requirements of material hardness, for some occasions, the higher the hardness of the material, the better, but limited to the hardness requirements of the machine parts, processing materials can not be too hard, assuming that harder than the machine parts can not be processed.
　　2, secondly, the material soft and hard moderate, at least lower than the hardness of the machine parts a level, together to see what the role of the processing device is to do, the machine parts reasonable material selection.
　　In short, the requirements of precision machining materials are still some, not all materials are suitable for processing, such as too soft or too hard materials, the former is not necessary for processing, the latter is not able to process.
Therefore, before processing must pay attention to the density of the material, assuming that the density is too large, equivalent to the hardness is also very large, and the hardness if cross the hardness of the machine parts (carbed snapors), is not processable, not only will damage parts, but also pose a danger, such as the car knife crash out to hurt people. Therefore, in general, about mechanical processing, the material material is lower than the hardness of the machine knife, so that it can be processed.