Machinery's Handbook, 31st Edition
662 TOLERANCES DESIGNATION A fundamental deviation establishes the position of the tolerance zone with respect to the basic size (see Fig. 1). Fundamental deviations are expressed by tolerance position letters. Capital letters are used for internal dimensions and lowercase or small letters for external dimensions. Symbols: By combining the IT grade number and the tolerance position letter, the toler ance symbol is established that identifies the actual maximum and minimum limits of the part. The toleranced size is thus defined by the basic size of the part followed by a symbol composed of a letter and a number, such as 40H7, 40f7, etc. A fit is indicated by the basic size common to both components, followed by a symbol corresponding to each component, the internal part symbol preceding the external part symbol, such as 40H8/f7. Some methods of designating tolerances on drawings are: 40H8 40H8 . . 40039 40000 c m 40H8 The values in parentheses indicate reference only. Preferred Metric Fits.— First-choice tolerance zones are used to establish preferred fits in ANSI/ASME B4.2, Preferred Metric Limits and Fits , as shown in Fig. 2 and Fig. 3 . A complete listing of first-, second-, and third- choice tolerance zones is given in the Standard. Hole basis fits have a fundamental deviation of H on the hole, and shaft basis fits have a fundamental deviation of h on the shaft; they are shown in Fig. 2 for hole basis and Fig. 3 for shaft basis fits. A description of both types of fits, which have the same relative fit condition, is given in Table 1. Normally, the hole basis system is preferred; however, when a common shaft mates with several holes, the shaft basis system should be used. The hole basis and shaft basis fits shown in the table Description of Preferred Fits on page 664 are combined with the first-choice preferred metric sizes from Table 1, page 688, to form Table 2, Table 3, Table 4, and Table 5 , in which specific limits as well as the resultant fits are tabulated. If the required size is not found tabulated in Table 2 through Table 5, then the preferred fit can be calculated from numerical values given in an appendix of ANSI/ASME B4.2- 1978 (R2009). It is anticipated that other fit conditions may be necessary to meet special requirements, and a preferred fit can be loosened or tightened simply by selecting a standard tolerance zone as given in the Standard. Information on how to calculate limit dimensions, clearances, and interferences for nonpreferred fits and sizes can be found in an appendix of this Standard. Conversion of Fits: It may sometimes be necessary or desirable to modify the tolerance zone on one or both of two mating parts yet still keep the total tolerance and fit condition the same. Examples of this appear in Table 1 on page 664 when converting from a hole basis fit to a shaft basis fit. The corresponding fits are identical, yet the individual tolerance zones are different. To convert from one type of fit to another, reverse the fundamental deviations between the shaft and hole, keeping the IT grade the same on each individual part. The examples below represent preferred fits from Table 1 for a 60-mm basic size. These fits have the same maximum clearance (0.520) and the same minimum clearance (0.140). Hole basis, loose running fit, values from Table 2 Hole 60H11 . . 40039 40000 c m Shaft 60c11 Fit 60H11/c11
. . 59860 59670 c m . . 60000 59810 c m
. . 0520 0140 c m . . 0520 0140 c m
. . 60190 60000 c m . . 60330 60140 c m
Hole basis, loose running fit, values from Table 4
Fit 60C11/h11
Hole 60C11
Shaft 60h11
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