In the process of machining and manufacturing industry and precision mechanical parts processing, the most contact is the mutual cooperation of mechanical equipment parts. So what is the cooperation of precision machining industry? What aspects should be paid attention to in dimensional tolerance fit in the machining process?
Overview of interchangeability of precision part fitting: the interchangeability of machining fitting usually includes the interchangeability of geometric parameters (such as size) and mechanical properties (such as hardness and strength). Only the interchangeability of geometric parameters is discussed here. The allowable variation of dimensions and geometric parameters of mechanical parts is called “tolerance”. The interchangeability of mechanical equipment spare parts in terms of geometric parameters is reflected in the tolerance standard. The tolerance standard is the basic standard in machining and manufacturing. It provides a theoretical basis for the standardization, serialization and generalization of machines, so as to shorten the time of mechanical design and promote the rapid development of new products.
Definition of “size” of mechanical parts: size is a number representing the standard value in specific units. Basic size of mechanical parts: the size given by the design. Actual size: the size obtained by measurement. Limit dimension: refers to the two limit values of allowable dimension change. Maximum solid state (MMC) and maximum solid size: the maximum solid state refers to the state when the hole or shaft has the largest amount of material within the dimensional tolerance. The size in this state is called the maximum entity size, which is a general designation of the minimum limit size of the hole and the maximum limit size of the shaft. Minimum solid state (LMC) and minimum solid size: the minimum solid state refers to the state when the hole or shaft has the least material within the dimensional tolerance. The size in this state is called the minimum entity size, which is a general designation of the maximum limit size of the hole and the minimum limit size of the shaft. Action size: the maximum ideal shaft size connected with the actual hole on the full length of the mating surface is called the action size of the hole. The size of the smallest ideal hole external to the actual shaft is called the action size of the shaft.
Definition of “tolerance and deviation” of mechanical parts. Dimension deviation: refers to the algebraic difference obtained by subtracting a certain dimension from its basic dimension; Dimensional tolerance: refers to the variation of allowable dimensions; Zero line: in the tolerance and fit diagram (tolerance zone diagram for short), a datum line for determining the deviation, i.e. zero deviation line; Tolerance zone: an area defined by two straight lines representing the upper and lower deviations in the tolerance zone diagram; Basic deviation: it is used to determine the upper deviation or lower deviation of the tolerance zone relative to the zero line. Generally, it refers to the deviation close to the zero line. When the tolerance zone is above the zero line, its basic deviation is the lower deviation; When it is below the zero line, its basic deviation is the upper deviation; Standard tolerance: any tolerance specified in the national standard to determine the size of the tolerance zone.
Definition of “fit” of mechanical parts: it refers to the relationship between combined holes and shaft tolerance zones with the same basic dimensions. Basic hole system: it is a system in which the tolerance zone of a hole with a certain basic deviation forms various fits with the tolerance zone of an axis with different basic deviations; Base axis system: it is a system in which the tolerance zone of an axis with a certain basic deviation forms various fits with the tolerance zone of holes with different basic deviations; Fit tolerance (or clearance tolerance): it is the variation of the allowable clearance, which is equal to the absolute value of the algebraic difference between the maximum clearance and the minimum clearance, and also equal to the sum of the matched hole tolerance zone and the shaft tolerance zone; Clearance fit: the tolerance zone of the hole is completely above the tolerance zone of the shaft, that is, the fit with clearance (including the fit with minimum clearance equal to zero); interference fit: the tolerance zone of the hole is completely below the tolerance zone of the shaft, that is, the fit with interference (including the fit with minimum interference equal to zero) ; transition fit: in the fit between hole and shaft, the tolerance zones of hole and shaft overlap each other. Any pair of holes and shaft match, which may have clearance or interference fit.
The machining tolerance fit of mechanical equipment parts can also be matched according to the actual machining process effect on the customer’s drawings to meet the best requirements in the process of equipment assembly. The above are the basic concepts of machining tolerance fit of precision mechanical parts. Thank you for your attention.
ken. tang@chengcg.com
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