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Basic concepts of tolerance fit – Overview of interchangeability. Interchangeability in the machinery and instrumentation manufacturing industry generally involves the interchangeability of geometric parameters (such as dimensions) and mechanical properties (such as hardness and strength); here, only the interchangeability of geometric parameters is discussed. The allowable variation in part dimensions and geometric parameters is called “tolerance”. The interchangeability of parts and components in terms of geometric parameters is expressed through tolerance standards. Tolerance standards are fundamental standards in the machinery and instrumentation manufacturing industry; they provide a theoretical basis for the standardization, serialization, and generalization of machines, thereby reducing machine design time and facilitating the rapid development of new products. “Term and definition of “dimension”: Dimension: A numerical value representing length in specific units. Basic dimensions: The dimensions specified in the design. Actual size: It is the size obtained through measurement. Limit dimensions: refer to the two extreme values within which dimensional variations are permitted. Maximum material condition (abbreviated as MMC) and maximum material size: The maximum material condition refers to the state in which a hole or shaft has the greatest amount of material within its dimensional tolerance range. The dimension in this state is called the maximum material dimension; it represents the minimum allowable dimension for holes and the maximum allowable dimension for shafts. Minimum entity condition (abbreviated as LMC) and minimum entity size: The minimum entity condition refers to the state in which a hole or shaft has the least amount of material, while still remaining within the dimensional tolerance range. The dimension in this state is called the minimum feature size; it represents the maximum allowable dimension for holes and the minimum allowable dimension for shafts. Effective dimension: Along the entire length of the mating surface, the maximum ideal shaft dimension that fits within the actual hole is referred to as the effective dimension of the hole. The size of the smallest ideal hole that is tangent to the actual shaft is called the effective dimension of the shaft. “Terms and definitions of \"tolerances and deviations\": Dimensional deviation refers to the algebraic difference obtained by subtracting the basic dimension from a particular dimension. Dimensional tolerance: refers to the allowable variation in size. Zero line: In tolerance and fit diagrams (commonly referred to as tolerance zone diagrams), it is a reference line that determines the deviation, that is, the line of zero deviation. Tolerance zone: In a tolerance zone diagram, an area defined by two lines that represent the upper and lower tolerances. Basic deviation: It is used to determine whether the tolerance zone is above or below the zero line, and generally refers to the deviation that is closer 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 an upper deviation. Standard tolerance: Any tolerance specified by national standards, used to determine the size of the tolerance zone. “Terms and definitions of “fit”. Fit refers to the relationship between the tolerance zones of holes and shafts that have identical basic dimensions and are fitted together. Hole system: It is a system in which the tolerance zone of holes has a fixed basic deviation, and various fits are formed by combining these with the tolerance zones of shafts having different basic deviations. Base shaft system: It is a system in which the tolerance zone of a shaft with a fixed basic deviation forms various fits with the tolerance zones of holes having different basic deviations. Fit tolerance (or clearance tolerance): It is the allowable variation in the clearance; it equals the absolute value of the algebraic difference between the maximum and minimum clearances, and it is also equal to the sum of the tolerance zones of the hole and the shaft that are fitted together. Clearance fit: The tolerance zone of the hole lies entirely above the tolerance zone of the shaft, meaning it is a clearance fit (including those with a minimum clearance of zero). Interference fit: The tolerance zone of the hole lies entirely within the tolerance zone of the shaft, meaning it is an interference fit (including those cases where the minimum interference is zero). Intermediate fit: In the fit between a hole and a shaft, the tolerance zones of the hole and the shaft overlap each other; when any pair of hole and shaft is combined, the fit may be either loose or tight.