plm documentation dim tolerancing gages fixtures tol policies

8/6/2019 PLM Documentation Dim Tolerancing Gages Fixtures Tol Policies

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Dimensioning & TolerancingPrinciples for Gages & Fixtures

(to ASME Y14.43-2003)

Bruce A. Harding

Mechanical Engineering Technology


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Gaging Tolerance Policies The following slides explain

the four gage tolerancingpolicies, 1) absolute(pessimistic), 2) optimistic, 3)

absolute policies (see PLMDocumentation Dimensioning& tolerancing gages & fixtures-Terms, for definitions). A gageor fixture designer selects one

for specific implementation.Examples are based on thissample workpiece.


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Absolute Tolerancing (Pessimistic) Gage tolerances add material to the

gaging element, beginning at thelimit [e.g., MMC or virtual condition(MMC concept)] of the feature beinggaged. Gages produced in

accept most part features that arewithin tolerance, reject all partfeatures not within tolerance, andreject a small percentage ofborderline part features that are

technically within tolerance.


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Optimistic Tolerancing Applies gagemakers tolerances, wear

allowances, measurement un-certainties,and form controls all outside of theworkpiece limits of size and geometriccontrol. Gage tolerances subtractmaterial from the gage, beginning at them e.g., or v r ua con on

concept)] of the feature being gaged.Gages produced to this policy will acceptpart features that are within tolerance,reject most features not within tolerance,and accept a small percent of borderline

part features that are technically notwithin tolerance. See next slide too.


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Tolerant Tolerancing is accomplished by applying

gagemakers tolerances, wearallowances, measurementuncertainties, and form controls insuch a manner that some of the

workpiece limits of size andgeometric control, and some of thetolerance on the gage is outside theworkpiece limits of size andgeometric control.


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Tolerant Tolerancing (Continued) Gage tolerances both add and

subtract material from the gage,beginning at the limit [e.g., MMC orvirtual condition (MMC concept)] ofthe feature being gaged. Gages

policy will accept most part featuresthat are within tolerance, reject mostpart features not within tolerance,accept a small percentage ofborderline out-of-tolerance features,

and reject a small percentage ofborderline within-tolerance features.


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Practical Absolute Tolerancing This is accomplished by applying gagemakers tolerances, wear

allowances, measurement uncertainties, and form controls in sucha manner that all of the tolerance on the size of the gage is insidethe workpiece limits of size, but allows geometric tolerance a smallinfrin ement on the acce table virtual condition boundar of the

workpiece.


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Statistics Statistical tolerancing can be used to calculate tolerances on parts that

are to be gaged. This can increase part yield. However, gages that usethe absolute tolerancing method will reject some borderline parts thatare technically within drawing tolerances. This is to ensure randominterchangeability of mating parts. Statistically toleranced parts com-

mon y use o erances a a ow v r ua con on oun ar esconcept) to be generated on mating parts that reduce interchange-ability. GO and functional gages would then accept some parts withstatistically based tolerances that would not assemble in worst-casesituations. Actual value distributionis the term associated with the

amount a feature has deviated from its perfect geometry. As with theparts being toleranced, it is recommended that gage deviation bestudied with respect to a gage features manufacturing process capa-bility and that this be used for analysis and setting of gage tolerances.


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Normative References

ASME B4.2, Preferred Metric Limits and Fits

ASME B46.1, Surface Texture (SurfaceRoughness,Waviness, and Lay)

ASME B89.6.2, Temperature and Humidity Environmentfor Dimensional Measurement

ASME B89.7.2, Dimensional Measurement Planning

ASME Y14.5M-1994, Dimensioning and Tolerancing

ASME Y14.5.1M-1994, Mathematical Definition ofDimensioning and Tolerancing Principles


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Acknowledgments

The author wishes to acknowledge the support from the Society forManufacturing Engineers - Education Foundation, SME-EF Grant #5004for Curriculum Modules in Product Lifecycle Management.

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