Geometric Dimensioning and Tolerancing

Unit 1 – Introduction, Symbols, and Terms

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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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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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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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Mil Std 8 1950’sMil Std 8AMil Std 8B

Mil Std 8C-1963

ASA-Y14.5-1957

USASI Y14.5-1966

ANSI Y14.5-1973

ANSI Y14.5M-1982

ASME Y14.5M-1994

ASME Y14.5-2009 1.3

History of Dimensioning and Tolerancing Standards in the USA

Common Symbols

1.7Lightning bolts show new symbols

Geometric

Characteristic

Symbols

1.10

1.11

Common Tolerance Zone Shapes

1.12

Geometric Tolerance Categories

“Happy Features”

Features of Size andFeatures without Size

1.9

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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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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

A

B

C

Specify flatness.Document tolerances for the center hole.

Document tolerances for the outer holes.

Document tolerances for the square hole.

Document tolerances for the three slots.

Dimension the thickness.

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Indexer Assembly

1.17

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Indexer Plate

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Indexer Assembly

1.19

ASME Y14.41 - 2003, Digital Product

Definition Data Practices

1.23

Workshop Exercise 1.1

12