【正文】 parts. Thus the designer must determine both the proper tolerances for the individual parts, The manner of placing tolerances on drawings depends somewhat on the kind of product or type of manufacturing process. If the tolerance on a dimension is not specifically stated, the drawing should contain a blanket note which gives the value of the tolerance for such dimensions. However, some panies do not use blanket notes on the supposition that if each dimension is considered individually, wider tolerance than those called for in the note could probably be specified. In any event it is very important that a drawing be free from ambiguities and be subject only to a single interpretation. Dimension and Tolerance In dimensioning a drawing, the numbers placed in the dimension lines represent dimension that are only approximate and do not represent any degree of accuracy unless so stated by the designer. To specify a degree of accuracy, it is necessary to add tolerance figures to the dimension. Tolerance is the amount of variation permitted in the part or the total variation allowed in a given dimension. A shaft might have a nominal size of in. (), but for practical reasons this figure could not be maintained in manufacturing without great cost. Hence, a certain tolerance would be added and , if a variation of 177。 ). 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 meaning. It is the minimum clearance space intended between mating parts and represents the condition of tightest permissible fit. If a shaft, size ?? , is to fit a hole of size ?? , the minimum size hole is and the maximum size shaft is . Thus the allowance is and the maximum clearance is as based on the minimum shaft size and maximum hole dimension. Tolerances may be either unilateral or bilateral. Unilateral tolerance means that any variation is made in only one direction from the nominal or basic dimension. Referring to the previous example, the hole is dimensioned ?? , which represents a unilateral tolerance. If the dimensions were given as 177。研究機械加工表面質量的目的就是為了掌握機械加工中各種工藝因素對加工表面質量影響的規(guī)律，以便運用這些規(guī)律來控制加工過程，最終達到改善表面質量、提高產品使用性能 的目的。一般說表面粗糙度值越小，其磨損性越好。但也不是冷作硬化程度愈高，耐磨性久愈好，這是因為過分的冷作硬化將引起金屬組織過度疏松，甚至出現裂紋和表面金屬的剝落，使耐磨性下降。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，延緩疲勞破壞的產生。 表面層的殘余拉應力會產生應力腐蝕開裂，降低零件的耐磨性，而殘余應力則能防止應力腐蝕開裂。要熟悉生產圖樣的所有情況，需要對制造過程非常熟悉并具有很多經驗。 尺寸標注是一項復雜的工作，要熟悉它需要長期的實踐經驗。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。生產者把公差標注在圖紙上依靠產品類型和制造工藝類型給出注釋。 尺寸和公差 在圖樣標注時，除非設計者有意標明，注在尺寸線上的數字表明的尺寸僅是近似的，并不代表任何精確的等級。因此一個確定的公差將被加上，如果177。配合的零件都必須保持渴望的允差值，制造過程有效，生產裝配中實現最小成本和最 大利率。允差是設計最小的公差空間在配合零件和代表最緊的配合條件下。單邊公差意味以前任何偏差只在名義尺寸的一個方向。單邊系統(tǒng)允許公差值發(fā)生變化但依然能保持相同的允差或配合類型，對雙邊的公差系統(tǒng)，它是不可以的，如果配合零件中一個或兩個名義標注尺寸發(fā)生變化。每一個尺寸必須表達不同視圖里尺寸至少且只能為一次。在這種情況下，尺寸應該用括號括起來，以便參考。整體尺寸應該能表現出現。 公差系統(tǒng)是必須允許這樣一種變化在制造過程中必須出現的精度變化，仍然提供互換性和功能真確的零件。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。 極限和配合的種類 世界標準組織的極限和配合，廣泛用在許多先進米制國家，它比英制標準體系更相當復雜。 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。公差帶位置相對零線的位置也是基本尺寸一個功能 — 用字母符號來表示，用大寫字母表示孔和用小寫字母表