# Introduction to GD&T.pptx - Introduction to GD&T Paul...

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Introduction to GD&TPaul McPhersonPurdue UniversitySchool of Engineering Technology
Purpose and Benefits of GD&TTo help better explain the design intent of the parts being manufacturedManufacture both passable and functional partsMust consider the relationship/assembly of parts to ensure tolerances are not stacked.
What is a feature control frame
Basic TerminologyMaximum Material condition (MMC) Largest Shaft, Smallest HoleLeast Material Condition (LMC) Smallest Shaft, Largest HoleRegardless of Feature Size (RFS) Inferred unless MMC or LMC is stated Geometric tolerance applies at all possible dimensions for feature
Types of Geometric Controls - FormSymbolFlatnessCircularityCylindricityStraightnessMeaningSimple DefinitionControls surface or axis of an object to be in a straight line. Most commonly applied to shafts.Controls the waviness of a surface between two parallel planesControls how circular a 2-dimensional cross section of a part is between two concentric circlesControls both the straightness and circularity of a part, throughout the entire length of the part, not just a cross section
StraightnessDrawingMeaningNote: Requires perfect form at MMCNote: Does NOT require perfect form at MMC
Consider:The size of the hole for which the shaft is to go through. Calculations must be completed to ensure that the allowable tolerance will still allow assembly of parts. Utilizing unit increments to define straightness. Straightness
When applied to rectangular part/feature it controls the straightness of the median plane throughout the part or feature. Straightness
FlatnessDrawingMeaningApplied to a surfaceApplied to a size dimension
CircularityDrawingMeaningNote: The tolerance Mustbe less than the size toleranceApplies at cross-sectional areas
CylindricityDrawingMeaningNote: Cylindricity controls both circularity and longitudinally Imagine two concentric tubes that define the geometric tolerance zone
Introduction to DatumsWhat is a datumTheoretically perfect plane, point, axis, or lineSelecting datumsMust consider manufacturing methods, inspection methods, relationship to features, function of the part, and how the part interacts/mates with other partsConsider largest mating surfaces, machined surfaces, critical axisPrimary datums should control degrees of freedom: 3,2,1 ruleShould be consistent between mating partsAlways consider actual surfaces of a part rather than center planes
How to represent Datums on