Machinery's Handbook, 31st Edition
676 British Standard Metric ISO Limits and Fits be associated, and with each of these deviations any one of the tolerance grades IT01 to IT16 could be used. All the limits of tolerance that the system is capable of providing for sizes up to and including 500 mm can be calculated from the standard tolerances given in Table 4, and the fundamental deviations given in Table 5a, Table 5b, Table 6a and Table 6b. The range includes limits of tolerance for shafts and holes used in small high-precision work and horology. The system provides for the use of either hole-basis or shaft-basis fits, and the Standard includes details of procedures for converting from one type of fit to the other. The limits of tolerance for a shaft or hole are designated by the appropriate letter indicat ing the fundamental deviation, followed by a suffix number denoting the tolerance grade. This suffix number is the numerical part of the tolerance grade designation. Thus, a hole tolerance with deviation H and tolerance grade IT7 is designated H7. Likewise, a shaft with deviation p and tolerance grade IT6 is designated p6. The limits of size of a component feature are defined by the basic size, say, 45 mm, followed by the appropriate tolerance designation, for example, 45 H7 or 45 p6. A fit is indicated by combining the basic size common to both features with the designation appropriate to each of them, for example, 45 H7-p6 or 45 H7/p6. When calculating the limits of size for a shaft, the upper deviation es, or the lower deviation ei, is first obtained from Table 5a or Table 5b, depending on the particular letter designation, and nominal dimension. If an upper deviation has been determined, the lower deviation ei = es – IT. The IT value is obtained from Table 4 for the particular tolerance grade being applied. If a lower deviation has been obtained from Table 5a or Table 5b, the upper deviation es = ei + IT. When the upper deviation ES has been determined for a hole from Table 6aor Table 6b, the lower deviation EI = ES – IT. If a lower deviation EI has been obtained from Table 6a, then the upper deviation ES = EI + IT. The upper deviations for holes K, M, and N with tolerance grades up to and including IT8, and for holes P to ZC with tolerance grades up to and including IT7 must be calculated by adding the delta ( D ) values given in Table 6b as indicated. Example 1 : The limits of size for a part of 133 mm basic size with a tolerance designation g9 are derived as follows: From Table 5a, the upper deviation (es) is – 0.014 mm. From Table 4, the tolerance grade (IT9) is 0.100 mm. The lower deviation (ei) = es – IT = 0.114 mm, and the limits of size are thus 132.986 and 132.886 mm. Example 2 : The limits of size for a part 20 mm in size, with tolerance designation D3, are derived as follows: From Table 6a, the lower deviation (EI) is + 0.065 mm. From Table 4, the tolerance grade (IT3) is 0.004 mm. The upper deviation (ES) = EI + IT = 0.069 mm, and thus the limits of size for the part are 20.069 and 20.065 mm. Example 3 : The limits of size for a part 32 mm in size, with tolerance designation M5, which involves a delta value, are obtained as follows: From Table 6a, the upper deviation ES is – 0.009 mm + D = –0.005 mm. (The delta value given at the end of Table 6b for this size and grade IT5 is 0.004 mm.) From Table 4, the tolerance grade (IT5) is 0.011 mm. The lower deviation (EI) = ES – IT = – 0.016 mm, and thus the limits of size for the part are 31.995 and 31.984 mm. Where the designations h and H or j s and J s are used, it is only necessary to refer to Table 4. For h and H, the fundamental deviation is always zero, and the disposition of the tolerance is always negative ( – ) for a shaft and positive ( + ) for a hole. Example 4 : The limits for a part 40 mm in size, designated h8 are derived as follows: From Table 4, the tolerance grade (IT8) is 0.039 mm, and the limits are therefore 40.000 and 39.961 mm. Example 5 : The limits for a part 60 mm in size, designated j s 7 or J s 7 are derived as follows: From Table 4, the tolerance grade (IT7) is 0.030 mm, and this value is divided equally about the basic size to give limits of 60.015 and 59.985 mm.
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