Fourteen tolerance representations for mechanical drawings

The allowable variation of the actual parameter value.

Parameters, including geometric parameters in machining, as well as parameters in physical, chemical, electrical and other disciplines. So tolerance is a widely used concept. For mechanical manufacturing, the purpose of setting tolerances is to determine the geometric parameters of the product so that the variation is within a certain range in order to meet the requirements of interchange or fit.

Geometric parameter tolerances include dimensional tolerances, shape tolerances, position tolerances, and the like.

1 dimensional tolerance. Refers to the allowable size variation, which is equal to the absolute [size=6][/size] value of the difference between the maximum limit size and the minimum limit size.

2 shape tolerances. Refers to the total amount of variation allowed by the shape of a single actual element, including straightness, flatness, roundness, cylindricity, line profile, and face profile.

3 position tolerance. Refers to the total amount of change allowed by the position of the associated actual element to the reference. It limits the mutual positional relationship between two or more points, lines and faces of the part, including parallelism, perpendicularity, inclination, and coaxiality. 8, symmetry, position, round beat and full jump 8 items.

The tolerance indicates the manufacturing accuracy requirements of the part, reflecting the ease of processing.

The tolerance grade is divided into 20 levels of IT01, IT0, IT1, ..., IT18, the grades are sequentially reduced, and the tolerance values ​​are sequentially increased. IT represents international tolerances.

The basic principle of tolerance class or tolerance value selection is: the comprehensive economic effect of machine parts manufacturing cost and use value should be the best, generally use IT5～IT13 for the size, IT2～IT5 for special precision parts, IT12 for non-fit size ~IT18, the raw materials are used with IT8~IT14.