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【正文】 h limits or deviations are calculated. Deviation: the difference between the hole or shaft size and the basic size. Upper deviation: the difference between the maximum permissible size of a part and its basic size. Lower deviation: the difference between the minimum permissible size of a part and its basic size. Actual size: the measured size of the finished part. Fit: the tightness between two assembled parts. The three types of fit are: clearance, interference and transition. Clearance fit: the clearance between two assembled mating parts. Interference fit: results in an interference between the two assembled partsthe shaft is larger than the hole, requiring a force or press fit, an effect similar to welding the two parts. Transition fits: may result in either an interference or a clearance between the assembled partsthe shaft may be either smaller or larger than the hole and still be within the prescribed tolerances. Selective assembly: a method of selecting and assembling parts by trial and error and by hand, allowing parts to be made with greater tolerances at less cost as a promise between a high manufacturing accuracy and ease of assembly. The basic hole system utilizes the smallest hole size as the basic diameter for calculating tolerances and allowances. The basic hole system is efficient when standard drills, reamers, and machine tools are available to give precise hole sizes. The smallest hole size is the basic diameter bemuse a hole can be enlarged by machining but not reduced in size. The basic shaft system is applicable .when shafts are available in highly precise standard sizes. The largest diameter of the shaft is the basic diameter for applying tolerances and allowances. The largest shaft size is used as the basic diameter because shafts can be machined to smaller size but not enlarged. International tolerance (IT) grade: a series of tolerances that vary with basic size to provide a uniform level of accuracy within a given grade. There are I8 IT grades: IT01,IT0, IT1 ..... IT16. Tolerance symbols: notes giving the specifications of tolerances and fits。Dimensional Tolerances and Surface Roughness The manufacture of machine parts is founded on the engineering drawing. Everyone engaged in manufacturing has a direct or indirect interest in understanding the meaning of the drawings on which the entire production process is established. The engineer in industry is constantly fated with the fact that no two machine parts can ever be made exactly the same. He learns that the small variations that occur in repetitive production must be considered in the design so that the tolerances placed on the dimensions will restrict the variations to acceptable limits. The tolerances provide zones in which the outline of finished part must lie. Proper tolerancing practice ensures that the fi
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