GEOMETRIC DIMENSIONING TOLERANCING [GD & T]
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Prepared By Jignesh patel
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HISTORY OF GD&T Ø Ø Ø Ø Ø Ø Ø Ø Ø
Drawings existed as far back as 6000 B.C. Unit of measurement at that time is royal cubit. During 4000 B.C standardized to 18.26 inches Manufacturing started in 1800s. The total process was conducted under one roof; Communication among craftsmen was immediate and constant. There were variation, but back then measuring instruments were not precise enough to identify them. Now Engineers understand the variation is unavoidable. The variation acceptable without impairing the function of assembly is identified as tolerance. This led to the development of Co-ordinate dimensioning.
ENGINEERING DRAWING STANDARDS 1935
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American Standards Association (ASA) published “American Drawing and Drafting Room Practices. ” World War II Stanley Parker of Royal Torpedo Factory in Alexandria, Scotland created positional tolerancing with cylindrical tolerance zone rather than square tolerance zone. 1940 Draftsmen’s handbook by Chevrolet, U.S. 1944 &1948 - British published “Dimensional Analysis of Engineering Design”. 1946 - ASA published II edition of “American Standard Drawing And Drafting Room Practice.” 1946 - SAE published “Aeronautical Drafting Manual” 1949 - U.S Military published MIL-STD-8 1952 - “SAE Automotive Drafting Manual ” 1953 MIL_STD revised. 1982 - American National Standards Institute (ANSI) published ANSI Y14.5 1994 - ANSI Y14.5 Revised.
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ASME Y14.5 – 1994 ASME Y14.5 M 1994 Ø Ø
- American Society of Mechanical Engineers - Standard Number - Metric (Standard) - Date from which standard was officially approved. The 1994 revision is the current, authoritative reference document that specifies the proper application of GD & T. In this document we follow ASME Y14.5 – 1994.
GD&T STANDARDS GD&T STANDARDS FOR TECHNICAL DRAWINGS (2D) Ø Ø Ø
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ASME Y14.5M-1994 Dimensioning and Tolerancing ASME Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing Principles ISO 286-1:1988 ISO system of limits and fits — Part 1: Bases of tolerances, deviations and fits ISO 286-2:1988 ISO system of limits and fits — Part 2: Tables of standard tolerance grades and limit deviations for holes and shafts ISO 1101:2005 Geometrical Product Specifications (GPS) — Geometrical tolerancing — Tolerancing of form, orientation, location and run-out ISO 5458:1998 Geometric Product Specifications (GPS) — Geometrical tolerancing — Positional tolerancing ISO 5459:1981 Technical drawings — Geometrical tolerancing — Datums and datum-systems for geometrical tolerances
GD&T STANDARDS FOR CAD SYSTEMS (3D) ASME Y14.41-2003 Digital Product Definition Data Practices Ø ISO 16792:2006 Technical product documentation - Digital product definition data practices (Note: ISO 16792:2006 was derived from ASME Y14.41-2003 by permission of ASME.) Ø
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GD&T STANDARDS FOR DATA EXCHANGE AND INTEGRATION Ø
ISO 10303 Industrial automation systems and integration — Product data representation and exchange ·
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ISO 10303-47:1997 Integrated generic resource: Shape variation tolerances ISO/TS 10303-1130:2006 Application module: Derived shape element ISO/TS 10303-1050:2006 Application module: Dimension tolerance ISO/TS 10303-1051:2006 Application module: Geometric tolerance ISO/TS 10303-1052:2005 Application module: Default tolerance ISO/TS 10303-1666:2006 Application module: Extended geometric tolerance ISO 10303-203:2007/8 Application protocol: Configuration controlled 3D design of mechanical parts and assemblies ISO 10303-210:2001 Application protocol: Electronic assembly, interconnection, and packaging design ISO 10303-214:2003 Application protocol: Core data for automotive mechanical design processes ISO 10303-224:2006 Application protocol: Mechanical product definition for process planning using machining features ISO 10303-238:2007 Application protocol: Application interpreted model for computerized numerical controllers (STEP-NC)
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CO-ORDINATE DIMENSIONING OR PLUS-MINUS TOLERANCING OR TRADITIONAL WAY OF TOLERANCING
Tolerance zone
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THREE MAJOR SHOTCOMES OF CO-ORDINATE DIMENSIONING I. Square/rectangular tolerance zone.
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Diagonally more tolerance (0.28) than vertical and horizontal direction (0.2) More logical and functional approach is to allow same tolerance on all sides, creating cylindrical tolerance zone. Cylindrical zone allows 57% more tolerance zone, which used in geometric dimensioning and tolerancing.
II. Fixed size tolerance zones. Ø
Size features can only be specified at the regardless of feature size condition. Regardless of feature size means that the location tolerance remains the same no matter whatever size of the feature. If a hole, like the Fig.1 increase in size, it has more location tolerance, but there is no way to specify that additional tolerances with plus/minus tolerancing system.
III. Datums are usually not specified. Ø
Datums are usually not specify where the co-ordinate tolerancing is used. Due to this manufacturing, measuring, inspection is difficult. Due to these chances to happen good parts can be rejected and bad parts can be accepted.
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CO-ORDINATE TOLERANCING
GEOMETRIC DIMENSIONING AND TOLERANCING
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WHAT IS GD& T Ø
Definition
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Geometric Dimensioning
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Tolerancing
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GD&T is a symbolic language. GD&T is a design tool. GD&T communicates design intent.
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: Permissible variation of both form and Position using symbols and letters is called Geometric tolerance. : Defining the shape. : Calculating, recording and communicating what is wanted. : Calculating, recording and communicating what is acceptable.
WHEN TO USE GD& T Ø Ø Ø Ø Ø Ø Ø
When drawing and its interpretation must be same. When features are critical to function or interchangeability. When it is important to avoid scrapping of perfectly good parts. When it is important to reduce drawing changes. When functional gauging is required. When it is important to increase productivity. When companies want savings.
ADVANTAGES OF GD& T I. GD & T provides functional dimensioning. Ø
The design philosophy of GD&T is that of functional dimensioning, which means that part is defined in dimensional sets or, by how it functions in the next assembly or final product. Instead of guessing at the tolerance, the designer basis the tolerance on part functions. When applied properly, this allows the maximum or correct amount of tolerance to produce the part. When properly
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applied functional dimensioning can increase the amount of allowable tolerance, which reduces manufacturing costs. With co-ordinate dimensioning, tolerance zones are not related to functional requirements, problems can result when designers assign to tight or loose (arbitrary) tolerances due to the lack of focus on the functional tolerance actually required.
II. Provides maximum production tolerances.
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GD & T allows cylindrical tolerance zones, which allow for up to 57% more tolerance zone then square/rectangular in co-ordinate dimensioning.
III. Bonus tolerance zone often provided. Ø
In addition to tolerance gained from using cylindrical zones, under certain conditions, GD&T allows a “Bonus tolerance” gained by using the MMC modifier.
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This feature control frame specifies the tolerance zone as a circle of 0.25 at MMC, centered according to the basic dimensions given.
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The size of tolerance zone is dependent on the size of hole.
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Hole Dia.
Bonus Tolerance
6.4 (MMC) 6.45 6.5 6.55 6.6 (LMC) Ø Ø
0.00 0.05 0.10 0.15 0.20
Location Tolerance or Tolerance zone Dia. 0.25 0.30 0.35 0.40 0.45
Virtual condition
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The feature control frame can specify the size of tolerance zone at MMC, LMC or RFS. As the actual size of the feature departs from MMC towards LMC, the tolerance of location can be relaxed to an extent that the virtual condition is still maintained. This relaxation leading to additional tolerance is popularly known as “Bonus Tolerance”. Bonus tolerance helps to bring down the manufacturing costs without hampering the functional fit.
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In cases, when the tolerance of location cannot be relaxed, on the basis of tolerance size, the modifier specified on location tolerance is RFS. Omitting of any modifier symbol against the tolerance of location implies RFS. Bonus tolerance can be availed only when a MMC or LMC modifier is specified against the tolerance of location.
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Datums B & C provide reference for the x & y position of hole center. Datum A controls the perpendicularity of the hole axis.
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IV. To improve productivity. Ø
Using functional tolerancing techniques improves productivity by reducing the potential for the rejection of functional parts.
V. Functional Performance. Ø
Properly applied GD&T assures assembly, interchangeability, and functional performance of all mating details.
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VI. Clear Communication Ø
Effective GD&T identifies important dimensional relationships and offers clear communication of functional design requirements.
VII. Uniform Interpretation Ø
Uniform, consistent interpretation of design requirements saves time and money by avoiding errors and controversies resulting from misconceptions and misunderstandings.
VIII. Coordinated Datum Locations Ø
GD&T provides a method of maintaining coordination between functional design features, manufacturing processes & inspection practices.
IX. To Maximize Producibility Ø
Parts designed using GD&T methods have maximized producibility because all available manufacturing tolerance has been included.
ACCEPTANCE Ø
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Unfortunately, GD&T has not become universally known as quickly as its originators feel. Persons related to design, inspection, manufacturing process are willing to understand, without this it ’s not an effective communication tool. But nowadays computer aided design (CAD), complex geometry increases due to this GD&T day by day increases.
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GEOMETRIC TOLERANCES (14)
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