机械加工是一种用加工机械对工件的外形尺寸或性能进行改变的过程。按被加工的工件处于的温度状态﹐分为冷加工和热加工。一般在常温下加工,并且不引起工件的化学或物相变化﹐称冷加工。一般在高于或低于常温状态的加工﹐会引起工件的化学或物相变化﹐称热加工。冷加工按加工方式的差别可分为切削加工和压力加工。热加工常见有热处理﹐煅造﹐铸造和焊接。
Machining is a process of changing the shape, dimension or performance of workpiece with machining machine. According to the temperature state of the workpiece to be processed, it can be divided into cold processing and hot processing. Generally, it is processed under normal temperature, and does not cause chemical or phase change of workpiece, which is called cold working. Generally, the processing above or below normal temperature will cause chemical or phase change of the workpiece, which is called hot processing. Cold working can be divided into cutting and pressure machining according to the difference of machining methods. Heat treatment, forging, casting and welding are common in hot working.
另外装配时常常要用到冷热处理。例如：轴承在装配时往往将内圈放入液氮里冷却使其尺寸收缩，将外圈适当加热使其尺寸放大，然后再将其装配在一起。火车的车轮外圈也是用加热的方法将其套在基体上，冷却时即可保证其结合的牢固性（此种方法现在依旧应用于某些零部件的转配过程中）。
In addition, cold and heat treatment is often used in assembly. For example, when the bearing is assembled, the inner ring is often put into liquid nitrogen to cool and shrink its size, the outer ring is properly heated to enlarge its size, and then assembled together. The outer ring of the wheel of the train is also sheathed on the substrate by heating method, which can ensure the firmness of its combination when it is cooled (this method is still used in the transfer process of some parts).