Geometric Dimensioning and Tolerancing

Chapter 1, Introduction

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History

• during the early period of manufacturing there were seldom any drawings

• one-off manufacturing• ~1798 – concept of interchange parts

– Eli Whitney won a contract to supply musketsto the United States government. The firearmsmanufacture were based on the concept ofinterchangeable parts.

– He made a presentation to congress by building10 guns and assembling and disassemblingthem claiming the same exact parts andmechanisms.

• specifying tolerances became veryimportant

Eli Whitney 1765 - 1825

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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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Plus/Minus (Limit) Tolerancing

• Plus/minus tolerancing seemed to work well for many applications.

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Plus/Minus (Limit) Tolerancing

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Plus/Minus (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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Limit Tolerancing

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Limit Tolerancing

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Limit Tolerancing

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GDT

• Limit tolerancing can give the size of individual features.• GDT can provide:

– relationships between features– circular tolerance zone for hole placement

• GDT removes any ambiguity from the drawing specification

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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

History of Dimensioning and Tolerancing Standards in the USA

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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

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communicationsymbols

ASMEclarity

replace

totalform

size & location

function & relationship

tolerances & interchageabilitytolerance

plus/minussize