Geometric Dimensioning and Tolerancing

Unit 1 – Introduction, Symbols, and Terms

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Everyone should have a workbook to follow along.

It contains necessary reference information along with class exercises.

Page numbers in yellow on the slides match pages in the workbook.

Geometric Tolerancing Workbook

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Eli Whitney, is the inventor of the cotton gin and a pioneer in the use of mass production methods.

Around 1798, he won a contract to supply muskets to the United States government. The firearms manufacture were based on the concept of interchangeable parts.

He made a presentation to congress by building 10 guns and assembling and disassembling them claiming the same exact parts and mechanisms.

Eli Whitney 1765 - 1825

History and Background

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Tolerances

• All dimensions require a tolerance.

• A tolerance should be as large as possible without interfering with the function of the part to minimize production costs.

• Consider how your part will be checked to see if it meets the tolerances.

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Tolerance Notes

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Limit Tolerancing

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Limit Tolerancing

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Limit Tolerancing

• Is the .620-.630 hole horizontal position measured from a true vertical plane or from the as built face?

• A .005” tolerance on the horizontal and vertical position of the hole means that the position could be off by as much as .007”.

+.005

+.005

.007

perfect locationfor hole center

max allowed errorfor hole center

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Limit Tolerancing

• Limit tolerances don’t have an origin or any orientation or location relative to datums.

• The datums are usually implied.

• The drawings are subject to different interpretations.

• Plus/minus tolerancing works well for individual features of size (ex. diameter of a shaft), but does not control the relationship between individual features very well.

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Mil Std 8 1950’sMil Std 8AMil Std 8B

Mil Std 8C-1963ASA-Y14.5-1957

USASI Y14.5-1966ANSI Y14.5-1973

ANSI Y14.5M-1982ASME Y14.5M-1994ASME Y14.5-2009

1.3

History of Dimensioning and Tolerancing Standards in the USA

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Introduction

• GDT is a language of symbols

• We will see an overview of the symbols associated with:1. Dimensioning Symbols2. Geometric Characteristics3. Modifying Symbols

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Dimensioning Symbols

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Dimensioning Symbols

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Diameter Radius R

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Dimensioning Symbols

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Spherical Diameter S Spherical Radius SR

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Dimensioning Symbols

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Controlled Radius Tolerance CR Number of Places

A controlled radius has no ‘reversals’, but is a smooth curve within the tolerance zone.

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Dimensioning Symbols

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Counterbore/Spotface Countersink

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Dimensioning Symbols

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Depth/Deep Dimensions Not to Scale

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Dimensioning Symbols

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Conical taper Slope

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Dimensioning Symbols

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Square Reference Dimension

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Dimensioning Symbols

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Dimension Origin Arc Length

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Dimensioning Symbols

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All Around All Over

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Dimensioning Symbols

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Chain Line

Used to identify a limited region to be treated differently from the rest of the part.

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Introduction

• We will see an overview of the symbols associated with:1. Dimensioning Symbols2. Geometric Characteristics3. Modifying Symbols

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Geometric Characteristics

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Explained in detail in Chapters 6 and 8.

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Introduction

• We will see an overview of the symbols associated with:1. Dimensioning Symbols2. Geometric Characteristics3. Modifying Symbols

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Modifying Symbols

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Modifying Symbols

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Maximum Material ConditionMMC

MMC

Refers to the size of the feature associated with the most material.

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Modifying Symbols

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Least Material ConditionLMC

LMC

Refers to the size of the feature associated with the least material.

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Modifying Symbols

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Continuous Feature Basic Dimension

Used to describe a theoretically exact size.

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Modifying Symbols

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Between

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Geometric Dimensioning and Tolerancing (GDT)

1. establish a reference coordinate system by defining datums

2. provide basic dimensions (perfect dimensions) relative to the datums

3. specify allowable tolerances

Common Symbols applied to a Drawing

1.8

±1°

18

25

10

units are mm

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Indexer Assembly

1.17

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Indexer Plate

1.23

Workshop Exercise 1.1

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