There are three methods to select the surface roughness of mechanical parts: calculation method, test method and analogy method. The most common use in machine part design is simple, fast and effective analogy. The application of analogy requires sufficient references. The existing types of mechanical design manuals provide more comprehensive information and literature. The most commonly used is surface roughness compatible with acceptable levels. In general, the smaller the dimensional tolerance requirements of mechanical parts, the smaller the surface roughness value of mechanical parts, but there is no fixed functional relationship between them. For example, the surface of some machines, instrument handles, hand wheels, sanitary equipment, food machinery, and some surface modified mechanical parts is required to be treated very smoothly. That is, the surface roughness is required to be high, but there is dimensional tolerance. The requirements are very low. Generally, for parts with dimensional tolerance requirements, the tolerance grade has a certain corresponding relationship with the surface roughness value.
Some mechanical parts design manuals and mechanical manufacturing monographs have many experiences and formulas on the relationship between surface roughness of mechanical parts and dimensional tolerance of mechanical parts. The list is for readers to choose. You can see that the values in the list are different, and some of them are quite different, even if the same empirical formula is used. This confused people who were not familiar with the situation in the region. At the same time, it also increases the difficulty of selecting surface roughness when machining mechanical parts.
In practice, different machines have different requirements for the surface roughness of parts under the same dimensional tolerance. This is the stability of cooperation. When designing and manufacturing machine parts, different types of machines have different requirements for component stability and compatibility.
The existing machine parts design manual reflects three types:
1. It is mainly used in precision machinery and requires high matching stability.
The wear limit of parts during use or after repeated assembly shall not exceed 10% of the dimensional tolerance of parts. It is mainly used for the surface of precision instruments, meters and precision measuring tools, as well as the friction surface of very important parts such as the inner surface of the cylinder, the main journal of precision machine tools and the main journal of coordinate boring machines.
2. It is mainly used for general precision machinery.
This requires a high fit stability, the wear limit of the parts does not exceed 25% of the dimensional tolerance of the parts, and the contact surface is required to be very tight. It is mainly used for relatively high-speed contact surfaces such as machines, tools, surfaces working with rolling bearings, tapered pin holes, mating surfaces of sliding bearings, machining surfaces of gear teeth, etc.
3. It is mainly used in general machinery where the wear limit of mechanical parts does not exceed 50% of the dimensional tolerance value, and the relative moving parts such as box cover and sleeve have no contact surface.
Tight surfaces, key and keyway working surfaces. Contact surface with relatively slow speed, such as support hole, bushing, working surface with axle hole, reducer, etc.