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.Actual mating size is the dimensional value of the actual mating envelope (defined in section5.6.4.2), and represents the worst-case mating potential for a feature of size.See Fig.5-38.Thus, actual mating size is the most suitable measure of actual size in clearance-fit applications or formost features having a boundary of perfect form at MMC.For a hole having an actual mating size ".001larger than its MMC, ".001 of bonus tolerance is added to the specified geometric tolerance.Likewise, fora tab.002 smaller than its MMC,.002 is added to the specified tolerance value. 5-44 Chapter FiveFigure 5-38 Actual mating envelope ofan imperfect holeAt LMC Bonus tolerance equals the arithmetic difference between the feature s actual minimummaterial size and its specified LMC size limit.Actual minimum material size is the dimension of the actual minimum material envelope.Actual minimum material envelope is defined according to the type of feature, as follows:(a) For an External Feature.A similar perfect feature counterpart of largest size, which can beinscribed within the feature so that it just contacts the surface(s).(b) For an Internal Feature.A similar perfect feature counterpart of smallest size, which canbe circumscribed about the feature so that it just contacts the surface(s).In certain cases, the orientation, or the orientation and location of an actual minimum materialenvelope shall be restrained to one or two datums.Notice from Fig.5-39 that the actual minimum material envelope is the inverse of the actual matingenvelope.While the actual mating envelope resides in the air at the surface of a feature, the actualminimum material envelope is embedded in part material.That makes it impossible to simulate with tangiblegages.The actual minimum material envelope can only be approximated by scanning point data into acomputer and modeling the surface a process called virtual gaging or softgaging.Let s consider a cast boss that must have an adequate shell of part material all around for cleanupin a machining operation.If its LMC size limit is ".387 and its actual minimum material size is ".390, a bonus of ".003 shall be added to the specified geometric tolerance.In section 5.6.3.1, we described some rare features having boundaries of perfect form at both MMCand LMC.Those features have an actual mating envelope and actual mating size that s used in the contextof the geometric tolerance and/or datum reference at MMC.For the LMC context, the same featureadditionally has an actual minimum material envelope and actual minimum material size.As might beapparent from Fig.5-39, the greater the feature s form deviation (and orientation deviation, as applicable),the greater is the difference between the two envelopes and sizes. Geometric Dimensioning and Tolerancing 5-45Figure 5-39 Actual minimum materialenvelope of an imperfect hole5.6.5.2 Level 2 Adjustment Actual Local SizesSince Level 3 and 4 tolerances impose no additional form controls, the center method permits use of auniform tolerance zone and an all-encompassing envelope size.Level 2 tolerances, however, are intendedto control feature form.Thus, the tolerance zone must interact with actual feature size independently ateach cross section of the feature.Though the effective control is reduced from 3-D down to 2-D, inspec-tion is paradoxically more complicated.Perhaps because there s rarely any reason to use the alternative center method for Level 2 tolerances, neither Y14.5 nor the Math Standard defines it thoroughly.In ourown following explanations, we ve extended actual mating/minimum material envelope principles to emu-late accurately the controls imposed by Level 2 virtual condition boundaries.Straightness of a Cylindrical Feature at MMC The central tolerance zone is bounded by a revo-lute, within which the derived median line shall be contained.At each cross-sectional slice, the diameter ofthe tolerance zone varies according to the actual mating local size.Within any plane perpendicular to theaxis of the actual mating envelope, actual mating local size is the diameter of the largest perfect circle thatcan be inscribed within an internal feature, or the smallest that can be circumscribed about an externalfeature, so that it just contacts the feature surface.The straightness tolerance zone local diameter equalsthe stated straightness tolerance value plus the diametral difference between the actual mating local sizeand the feature s MMC limit size.At any cross section of the pin shown in Fig.5-26, as the pin s actual mating local size approachesMMC (".063), the straightness tolerance zone shrinks to the specified diameter (".010)
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