【正文】 here the allowance may be either positive or negative (clearance or interference) , an interference fit where the allowance is always negative. Type of Limits and Fits The ISO system of Limits and Fits, widely used in a number of leading metric countries, is considerably more plex than the ANSI system. In this system, each part has a basic size. Each limit of part, high and sign being obtained by subtracting the basic size form the limit in question. The difference between the two limits of size of a part is called the tolerance, an absolute without sign. There are three classes of fits: 1) clearance fits, 2) transition fits ( the assembly may have either clearance or interference ), and 3) interference fits . Either a shaftbasis system or a holebasis system may be used. For any given basic size, a range of tolerance and deviations may be specified with respect to be line of zero deviation, called the zero line. The tolerance is a function of the basic size and is designated by a number symbol, called the gradethus the tolerance grade. The position of the tolerance with respect to the zero line also a function of the basic sizeis indicated by a letter symbol(or two letter), a capital letter for holes and a lowercase letter for shafts. Thus the specification for a hole and shaft having a basic size of 45mm might be45H8/g7. Twenty standard grades of tolerance are provided, called IT 01,IT 0 ,IT 118, providing numerical values for each nominal diameter, in arbitrary steps up to 500mm (for example 03,36,610…, 400 500mm). The value of the tolerance unit, I, for grades 516 is 1i D D?? Where i is in microns and D in millimeters. Standard shaft and hole deviations similarly are provided by sets of formulas, However, for practical, both tolerances and deviations are provided in three sets of rather plex tables. Additional tables gives the values for basic sizes above 500mm and for “Commonly Used Shafts and Holes” in two categories “General Purpose” and “Fine Mecbanisms and Horology”. 中文翻譯 極限與誤差 機械零件的破壞，一般總是從表層開始的。因此，接觸面的粗糙度有一個最佳值，其值與零件的工作情況有關(guān)，工作載荷加大時，初期磨損量增大，表面粗糙最佳也加大。表面粗糙度值愈大，則凹谷中聚積腐蝕性物質(zhì)就愈多。非常明顯圖紙應在有一種且只有一種解釋的方式下標注。 工藝制造流 程發(fā)展大批量制造低成本依靠零部件的互換性。公差是在某一零件上被允許的變化量或在尺寸上總的變化量。如果一個零件有幾個或多個面要生產(chǎn)，當要求小的偏差生產(chǎn)成本將急劇上升。如果尺寸標注為 177。 除絕對需要的尺寸之外，不應該再有更多的尺寸，而在任一方向上，只能在一個尺寸上注上特殊要求。一般，如一個公差被標注的尺寸數(shù)字的旁邊。如名義尺寸 30mm公差帶 ?? 極限尺寸 。任一給定基本尺寸，公差范圍和偏差被標準化根據(jù)零偏差，這個零偏差稱為零線。軸表給出基本尺寸在 500mm 以上和“通用的軸和孔”分成兩個表 “一般用途”和“精密 機械和鐘表”。 這里有三種配合： 1）間隙配合 2）過渡配合（此配合可組成過渡或過盈）和 3）過盈配合。雙邊公差的公差帶會以名義尺寸上下移動，單邊公差的 公差帶只能在名義尺寸的一邊變動，在這種情況下名義尺寸可能是一個極限尺寸。整體尺寸應該能表現(xiàn)出現(xiàn)。每一個尺寸必須表達不同視圖里尺寸至少且只能為一次。單邊公差意味以前任何偏差只在名義尺寸的一個方向。配合的零件都必須保持渴望的允差值，制造過程有效，生產(chǎn)裝配中實現(xiàn)最小成本和最 大利率。 尺寸和公差 在圖樣標注時，除非設計者有意標明，注在尺寸線上的數(shù)字表明的尺寸僅是近似的，并不代表任何精確的等級。在單向尺寸標注中一個公差是零，所有的偏差由另一個公差給出。要熟悉生產(chǎn)圖樣的所有情況，需要對制造過程非常熟悉并具有很多經(jīng)驗。表面層殘余拉應力將使疲勞裂紋擴大，加速疲勞破壞；而表面層殘余應力能夠阻止疲勞裂紋的擴展，延緩疲勞破壞的產(chǎn)生。一般說表面粗糙度值越小，其磨損性越好。 ). Dimensions given close tolerances mean that the part must fit properly with some other part. Both must be given tolerances in keeping with the allowance desired, the manufacturing processes available, and the minimum cost of production and assembly that will maximize profit. Generally speaking, the cost of a part goes up as the tolerance is decreased. If a part has several or more surfaces to be machined, the cost can be excessive when little deviation is allowed from the nominal size. Allowance, which is sometimes confused with tolerance, has an altogether different