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

China cnc machining services
China cnc machining services

Surface roughness is an important technical indicator, which reflects the microscopic geometric error of the surface of the part, and is the primary basis for inspecting the surface quality of the part. Whether it is reasonable or not is directly related to the quality, service life and manufacturing cost of the product.

There are three methods for selecting the surface roughness of mechanical parts: calculation method, test method and analogy method. The most common usages in machine part design are simple, fast, and effective analogies. Applying analogies requires adequate references. Existing types of mechanical design manuals provide more comprehensive information and literature. The most commonly used is a surface roughness compatible with an acceptable level. 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, some machines, instrument handles, handwheels, sanitary equipment, food machinery, and some surface-modified mechanical parts require very smooth surface treatment. That is, the surface roughness requirements are high, but there are dimensional tolerances. The requirements are very low. In general, for parts with dimensional tolerance requirements, the tolerance level 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 the surface roughness of mechanical parts and the dimensional tolerance of mechanical parts, and the list is for readers to choose. See that the values ​​in the list are different, some are quite different, even using the same empirical formula. This confuses people unfamiliar with the situation in the area. At the same time, it also increases the difficulty of selecting surface roughness when machining mechanical parts.
In actual work, 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. Existing machine part design manuals reflect 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 a 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.
When designing a machine, the surface roughness needs to be selected according to the surface roughness of the machine.
In the design work, in order to make a reasonable choice, when selecting the surface roughness and comprehensively measuring the surface function of the part and the economy of the process, everything must ultimately be based on reality. I have. The tolerance classes and surface roughness values ​​shown in the table can be used as a design reference.

Surface roughness is an important technical indicator, which reflects the microscopic geometric error of the surface of the part, and is the primary basis for inspecting the surface quality of the part. Whether it is reasonable or not is directly related to the quality, service life and manufacturing cost of the product.

There are three methods for selecting the surface roughness of mechanical parts: calculation method, test method and analogy method. The most common usages in machine part design are simple, fast, and effective analogies. Applying analogies requires adequate references. Existing types of mechanical design manuals provide more comprehensive information and literature. The most commonly used is a surface roughness compatible with an acceptable level. 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, some machines, instrument handles, handwheels, sanitary equipment, food machinery, and some surface-modified mechanical parts require very smooth surface treatment. That is, the surface roughness requirements are high, but there are dimensional tolerances. The requirements are very low. In general, for parts with dimensional tolerance requirements, the tolerance level 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 the surface roughness of mechanical parts and the dimensional tolerance of mechanical parts, and the list is for readers to choose. See that the values ​​in the list are different, some are quite different, even using the same empirical formula. This confuses people unfamiliar with the situation in the area. At the same time, it also increases the difficulty of selecting surface roughness when machining mechanical parts.
In actual work, 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. Existing machine part design manuals reflect 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 a 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.
When designing a machine, the surface roughness needs to be selected according to the surface roughness of the machine.
In the design work, in order to make a reasonable choice, when selecting the surface roughness and comprehensively measuring the surface function of the part and the economy of the process, everything must ultimately be based on reality. I have. The tolerance classes and surface roughness values ​​shown in the table can be used as a design reference.

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