GD&T [PDF]

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GEOMETRIC DIMENSIONING AND TOLERANCING



WHAT IS GD & T ?



 GD & T is symbolic language  It is used to specify the size, shape, form, orientation



and location of features on a part  It is a design tool  It communicates design intent



ORIGIN OF GD & T  The world war two story  USASI Y14.5 – 1966



 ANSI Y14.5 – 1973  ANSI Y14.5 – 1982  ASME Y14.5 – 1994



 ASME Y14.5 - 2009 (currently used)



WHEN TO USE GD & T  Drawing delineation and interpretation need to be 



     



the same Features are critical to function or interchangeability It is important to stop scrapping perfectly good parts It is important to reduce drawing changes Automated equipment is used Functional gagging is required It is important to increase productivity Companies want across-the-board savings



WHERE TO USE GD&T  ENGINERING  DESIGN  DRAFTING  MANUFACTURING  ASSEMBLY  QUALITY



DIMENSIONING STANDARDS



 ISO - - millimeters (mm)  US - - inches (inch)



 SCALE FACTOR : 1 inch = 25 mm



GEOMETRIC DIMENSIONING  Dimension – numerical value indicated in a drawing



to define size, shape, location and surface texture.



 TYPES  1. Chain dimensioning



2. Datum dimensioning  Alternate dimensioning practices:



1. Arrowless dimensioning 2. Arrowless tabular dimensioning 3. Chart dimensioning



TYPES OF DIMENSIONING CHAIN DIMENSIONING



DATUM DIMENSIONING



ALTERNATE DIMENSIONING PRACTICES ARROWLESS



ARROWLESS TABULAR



GEOMETRIC TOLERANCING  Tolerance – The total amount that a specific dimension is  



 



permitted to vary. Limits – largest and smallest numerical value possible for the feature within the tolerance specified. Fits - A coupling of two parts creates a fit 3 types: 1. Clearance fit 2. Interference fit 3. Transition fit MMC LMC



SYMBOLS AND TERMS GD & T symbols are divided into 5 types 1. 2.



3. 4. 5.



Dimensioning symbols Datum feature and Datum target symbols Geometric characteristic symbols Material condition symbols Feature control frame



DIMENSIONING SYMBOLS



DATUM A datum is theoretically exact points, lines, or planes . It is the true geometric counter part of the datum feature.



DATUM FEATURE & DATUM TARGET SYMBOLS



FEATURE CONTROL FRAME



GEOMETRIC CHARACTERISTIC SYMBOLS



INSTRUMENTS USED



 DIAL GAUGE INDICATOR  PROFILE PROJECTOR  COORDINATE MEASURING MACHINE (CMM)



 VERNIER HIEGHT GAUGE  SLIPS  VERNIER SCALE AND SCREW GAUGES



 SURFACE PLATE  ANGULAR PLATE…etc…,



FORM



PROFILE



ORIENTATION



RUNOUT



LOCATION



PROJECTED TOLERANCE ZONE



VIRTUAL CONDITION INTERNAL FEATURE



EXTERNAL FEATURE