The relationship between surface roughness and tolerance level in CNC machining industry

Surface roughness is an important technical index, which reflects the microscopic geometric error of the surface of the part, is the primary basis for inspecting the surface quality of the part, and is directly related to the quality, service life and manufacturing cost of the product.

There are three main methods to determine the surface roughness of mechanical parts: calculation method, experimental method and analogy method. The most common method is the analogy method, which is simple, fast and effective. Using analogies requires enough references, and of course there are enough references out there. In general, for parts with dimensional tolerance requirements, the tolerance level has a certain corresponding relationship with the surface roughness value. The smaller the dimensional tolerance requirements of mechanical parts, the smaller the surface roughness value is, but the difference between There is no fixed functional relationship. For example, some machines, instrument handles, handwheels, sanitary equipment, food machinery, and some surface-modified mechanical parts require very smooth surfaces, that is, high surface roughness requirements, but very low dimensional tolerance requirements.


At present, there are many experiences and formulas for the relationship between surface roughness of mechanical parts and dimensional tolerances of mechanical parts in some manufacturing guidelines, and some are still very different, even if the same empirical formula is used.


In actual work, different machines have different requirements for the surface roughness of parts under the same dimensional tolerance, which needs to be fully considered when designing and manufacturing machine parts. Generally, there are three types.


The first type is mainly used in precision machinery, which requires higher coordination stability. The wear limit of parts during use or after repeated assembly should not exceed 10% of the dimensional tolerance of the part. Mainly used for the surface of precision instruments, meters, precision measuring tools, and 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.


The second is mainly used for ordinary precision machinery. This requires high fit stability, the wear limit of the part does not exceed 25% of the dimensional tolerance of the part, and the contact surface is required to be tight. Mainly used for relatively high-speed contact surfaces such as machines, tools, surfaces that work with rolling bearings, tapered pin holes, mating surfaces of sliding bearings, and machined surfaces of gear teeth.


The third type is mainly used for general machinery where the wear limit of mechanical parts does not exceed 50% of the dimensional tolerance value, and there is no contact surface for relatively moving parts such as box covers and sleeves. Tight surfaces, keys and keyway work surfaces. Relatively slow contact surfaces, such as bracket holes, bushings, working surfaces with pulley shaft holes, reducers, etc.


In the design work, in order to make a reasonable choice, when choosing the surface roughness and comprehensively measuring the surface function of the part and the economics of the process, everything needs to be based on reality.

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