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Dimensioning & Tolerancingfor Gages & Fixtures – Part 2

(to ASME Y14.43-2003)

Bruce A. Harding

Mechanical Engineering Technology

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Gaging Terminology• Actual local size: the value of any individual distance at

any cross section of a feature (see ASME Y14.5M-1994).

•  

collect attributes data; for example, GO and functionalgages.

• Attributes data: information obtained from an inspectionprocess that indicates only whether a part is acceptableor not acceptable.

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• Gages that check envelopes or boundaries are all designed onsimilar principles, whether they inspect maximum material condition(MMC) or virtual condition (MMC concept). GO gages determinecompliance with the MMC envelope that is defined by ASMEY14.5M-1994. Functional a es are used to ins ect for com liance

Gage Design Principles

with the virtual condition boundary created by use of the MMCconcept defined by ASME Y14.5M-1994.

• On the other hand, larger-toleranced gages will less reliablydistinguish in-tolerance parts from out-of-tolerance parts and mayreject more in-tolerance parts. Thus the gage cost must be weighed

against the cost of the workpiece accept/reject rate. Therefore, thedesigner must consider the break-even point, and decide on thecorrect balance between gages with prohibitive up-front costs andprohibitive long-range costs caused by rejection of good parts (i.e.,parts meeting drawing specification) compared to the acceptance of

bad parts.

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• While the goal of gaging is to accept all good parts andreject all bad parts, the actual production of gagingequipment introduces variability, making this impossible.Depending upon the tolerancing policy chosen, the size

Goal of Gaging

range of gage elements may be larger, smaller, orstraddle the boundaries they are inspecting. Thetolerance policy chosen will determine whetherborderline part features are accepted or rejected. The

practice of gage tolerancing requires a gage designedwith size tolerances and/or geometric tolerances assmall as economically feasible.

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• The design and production of gages and fixtures takesplace within a specific economic context. The smaller theallowed tolerances for the gage, the more expensive it isto manufacture and the greater the number of parts

Economic Context

within specification the gage will accept when usedproperly. However, smaller gage tolerance allows lessroom for gage wear, therefore shortening the life of thegage. As the gage wears beyond acceptable limits, it

begins to accept technically bad parts. Gages must beinspected periodically and replaced or repaired beforethis happens.

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• Fixed limit gages, in theory, accept all workpiecesdimensionally conforming to specification and reject allworkpieces which do not conform. The GO gage and the

Type and Use of Gages

 

inspected. The NOGO gage does not receive theworkpiece in any position.

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• GO Plug Gages. A GO plug gage enters the hole over its

full length when applied by hand without using excessiveforce. If it is not possible to use a full form plug gage or if therule concerning perfect form at MMC is not in effect, GOsegmental gages, if used, are applied to the hole in axial

Type and Use of Gages

p anes un orm y str ute aroun t e c rcum erence.Unless otherwise specified, perfect form is required at MMCfor rigid features, necessitating full-form MMC sizedcylindrical plug gages for holes and full-form MMC sizedcylindrical ring gages for shafts. When non-rigid workpiecessuch as thin-walled parts are gaged, considerable care isrequired to use zero force as this may distort the hole andgive false results. For non-rigid features, perfect form atMMC is not required.

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• NOGO Gages. The LMC limit of the workpiece is checked

with a gage designed to contact the workpiece, if a cylinder,at two diametrically opposed points separated by a distanceexactly equal to the least material condition size limit. ThisNOGO gage shall not pass into or over the workpiece at any

Type and Use of Gages

pos t on. t s two-po nt oppos ng po nt type o measurementcannot be used, a NOGO cylindrical or spherical plug gage isused without applying excessive force. Excessive force is thatwhich would be sufficient to damage or deform either theworkpiece or the gage. The hole is checked from both ends, if

possible. A NOGO gage with segmental spherical gagingsurfaces is introduced into the hole by tilting it, it can be donewithout using excessive force. The inspector is responsible forall sets of opposing points within the hole.

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• GO Cylindrical Ring Gage. This gage encompass the

complete length of the shaft, and when applied by handusing zero measuring force (or any corrected valuespecified). If a cylindrical ring gage cannot be usedbecause the perfect form at MMC rule has been eliminated

Type and Use of Gages

 

for a specific workpiece and a GO snap gage is to be used,the GO snap gage shall,(a) pass over a dimensionally conforming shaft, the axis ofwhich is horizontal, under its own weight or to the specific

force marked on the gage;(b) pass over a dimensionally conforming shaft, the axis ofwhich is vertical, when applied by hand without usingexcessive force.

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• NOGO Snap Gages. A NOGO snap gage,

(a) does not pass over a dimensionally conforming shaft,the axis of which is horizontal, under its own weight or theforce marked on the gage(b) does not pass over a dimensionally conforming shaft,

Type and Use of Gages

 

the axis of which is vertical, when applied by hand withoutusing excessive force.

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• Functional Gages. A functional gage pin enters the hole

over the entire depth of the hole without excessive force. Afunctional gage hole (ring) receives the shaft over theentire length of the shaft without excessive force.

Type and Use of Gages

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• Functional Gages (Continued). If planar datum features

are simulated on the gage, the datum features on theworkpiece contacts the datum feature simulators on thegage as appropriate. (For example, a minimum of threepoints of high point contact on a primary planar datum

Type and Use of Gages

 

feature, a minimum of two points of high point contact on asecondary planar datum feature, and a minimum of onepoint of high point contact on a tertiary planar datumfeature. If restraint is to be applied to the datum features, it

is specified on the workpiece drawing, or the workpiece isrestrained so as not to alter the measurement readings ofthe same part measured in the free state.

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• Functional Gages (Continued). When using functional

gaging principles, it is recommended that,1) gages, production tooling, and parts (to include tolerancesand allowances) should be designed using concurrentengineering principles,

Type and Use of Gages

 

2) gages be defined using the same geometric characteristicsthat define the part being gaged.

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• Functional Gages (Continued). When using functional

gaging principles, it is required that,1) gages simulate datum features as defined by part datumfeatures or datum targets,2) functional gages that verify positional requirements, have

Type and Use of Gages

 

gaging elements located at basic dimensions conforming tofeature locations dimensioned on the product drawings,3) gages simulate the MMC concept of the controlledfeatures virtual condition or MMC, as applicable,

4) all functional gage elements go into or over the partfeatures simultaneously where simultaneous requirementsare invoked by the product specification.

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• Functional Gages (Continued). When using functional

gaging principles, note that specifying one datum referenceframe per part requires one gage to be used for acceptance.Any increase in the number of datum reference frames willincrease the number of gages and inspection setups.

Type and Use of Gages

 

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Normative References

• ASME B4.2, Preferred Metric Limits and Fits

• ASME B46.1, Surface Texture (Surface Roughness,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.”