1. The first is the requirement of material hardness. For some occasions, the higher the material hardness, the better. It is only limited to the hardness requirements of processing machine parts. The processed materials cannot be too hard. If they are harder than the machine parts, they cannot be processed.
2. Secondly, the material should be soft and hard, which should be at least one grade lower than the hardness of the machine parts. At the same time, it also depends on the function of the processed devices, so as to select the reasonable materials for the parts.
In a word, the material of the former is not suitable for machining, and the latter is not suitable for machining.