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 partIt is a design toolIt communicates design intent

ORIGIN OF GD & T

The world war two story

USASI Y14.5 – 1966

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

ENGINERINGDESIGNDRAFTINGMANUFACTURINGASSEMBLYQUALITY

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 dimensioning2. Arrowless tabular dimensioning3. Chart dimensioning

CHAIN DIMENSIONINGDATUM DIMENSIONING

TYPES OF DIMENSIONING

ARROWLESS ARROWLESS TABULAR

ALTERNATE DIMENSIONING PRACTICES

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 fit3. Transition fit

MMCLMC

SYMBOLS AND TERMS

GD & T symbols are divided into 5 types

1. Dimensioning symbols2. Datum feature and Datum target symbols3. Geometric characteristic symbols4. Material condition symbols5. 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 INDICATORPROFILE PROJECTORCOORDINATE MEASURING MACHINE

(CMM)VERNIER HIEGHT GAUGESLIPSVERNIER SCALE AND SCREW GAUGESSURFACE PLATEANGULAR PLATE…etc…,

FORM

PROFILE

ORIENTATION

RUNOUT

LOCATION

PROJECTED TOLERANCE ZONE

INTERNAL FEATURE EXTERNAL FEATURE

VIRTUAL CONDITION