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1、 Dimensional Tolerance The manufacture of machine parts is founded on the engineering drawingEveryone engaged in manufacturing has a direct or indirect interest in understanding the meaning of the drawings on which the entire production process is establishedThe engineer in industry is constantly fa
2、ced with the fact that no two machine Darts can ever be made exactly the same.He learns that the small variations that occur in repetitive production must he considered in the design 80 that the tolerances placed on the dimensions will restrict the variations to acceptable limits .The tolerances pro
3、vide zones in which the outline of finished part must lie. Proper tolerancing practice ensures that the finished product functions in its intended manner and operates for its expected life. A designer is well aware that the cost of a finished product can increase rapidly as the tolerances on the com
4、ponents are made smallerDesigners are constantly admonished to us the widest tolerances possible. Situations may arise，however，in which the relationship between the various tolerances required for proper functioning has not been fully exploredUnder such conditions the designer is tempted to specify
5、part tolerances that ale unduly tight in the hope that no difficulty will arise at the time of assemblyThis is obviously an expensive substitute for a more thorough analysis of the tolerancing situation The allocation of proper production tolerances is therefore a most important task if the finished
6、 product is to achieve its intended purpose and yet be economical t0 produceThe size of the tolerances，as specified by the designer，depends on the many conditions pertaining to the design as well as on past experience with similar products if such experience is available.A knowledge of shop processe
7、s and machine capabilities is of great assistance in helping to determine the tolerances in the most effective maimer. A revision of the design may be called for if the tolerances are too small to be maintained by the equipment available for producing the dimension. Ambiguities in engineering drawin
8、g can be cause of much confusion and expense. When specifying the tolerances，the designer must keep in mind that the drawing must contain a11 requisite information if the designers intent is to be fully realizedThe drawing must therefore give complete information and at the same time be as simpe as
9、possibleThe detail of drawing must be capable of being universally understoodThe drawing must have one and only one meaning to everyone who will use itthe design，purchasing，tool design,production，inspection，assembly，and servicing departments. Tolerances may be placed on the drawing in a number of di
10、fferent ways. In theunilateral system one tolerance is zero and all the variation of the dimension is given by the other tolerance. In bilateral dimensioning a mean dimension is used with plus and minus variations extending each way from the mean dimension. Unilateral tolerancing has the advantage t
11、hat a tolerance revision can be made with the least disturbance to the remaining dimensions. In the bilateral system a change in the tolerances also involves a change in at least one of the mean dimensions .Tolerances can be easily changed back and forth between unilateral and bilateral for the purp
12、ose of making calculations. A part is said to be at the maximum material condition (MMC) when the dimensions are all at the limits that will give a part containing the maximum amount of materialFor a shaft or external dimension，the fundamental dimension is the largest value permitted,and all the var
13、iation，as permitted by the tolerance，serves to reduce the dimension. For a hole or internal dimension，the fundamental dimension is the smallest value permitted,and the variation as given by the tolerance serves to make the dimension larger. A part is said to be at the least material condition (LMC)
14、when the dimensions are all at the limits that give a part with the smallest amount of material .For LMC the fundamental value is the smallest for&it external dimension and the largest for an internal dimension .The tolerances thus provide parts containing larger amounts of material. Maximum materia
15、l tolerances have a production advantage. For an external dimension，should the worker aim at the fundamental or largest value but form something smallThe parts may be rework to bring them within acceptable limitsA worker keeping the mean dimension in mind would have smaller margins for any errorsThe
16、se terms do,however, provide convenient expressions for denoting the different methods for specifying the tolerances on drawings Dimensional variations in manufacturing are unavoidable despite a11 efforts to keep production conditions as constant as possibleThe reasons for the variation in a chosen dimension on parts all made by the same process are of interest .The reasons can usually be grouped into two general cla
