First is the requirement of material hardness, for some occasions, the higher the hardness of the material is the better, just limited to the hardness requirements of the processed machine parts, the processed material can not be too hard, if the hardness than the machine parts is also impossible to process.
Second, the material is soft and hard moderate, at least one grade lower than the hardness of the machine, but also depends on the role of the processed device is to do what with the machine reasonable selection of materials.
In short, precision machining of the material requirements or some, not what material is suitable for processing, such as too soft or too hard material, the former is not necessary for processing, and the latter is not processed.
Therefore, the most basic one is in the mechanical parts processing plant before processing must pay attention to the density of the material, if the density is too large, equivalent to the hardness is also very large, and the hardness if more than the machine parts (lathe turning tool) hardness, is unable to process, not only will damage the parts, but also cause danger, such as turning tool flying out of the crash injury. Therefore, generally speaking, for mechanical processing, the material material should be lower than the hardness of the machine tool, so that it can be processed.