# geometric dimensioning and tolerancing chapter 1, introduction

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Geometric Dimensioning and Tolerancing Chapter 1, Introduction

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Geometric Dimensioning and Tolerancing

Chapter 1, Introduction

EML 2023Department of Mechanical and Aerospace Engineering 2

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

EML 2023Department of Mechanical and Aerospace Engineering 3

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.

EML 2023Department of Mechanical and Aerospace Engineering 4

Tolerance Notes

EML 2023Department of Mechanical and Aerospace Engineering 5

Plus/Minus (Limit) Tolerancing

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

EML 2023Department of Mechanical and Aerospace Engineering 6

Plus/Minus (Limit) Tolerancing

EML 2023Department of Mechanical and Aerospace Engineering 7

Plus/Minus (Limit) Tolerancing

EML 2023Department of Mechanical and Aerospace Engineering 8

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

EML 2023Department of Mechanical and Aerospace Engineering 9

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.

EML 2023Department of Mechanical and Aerospace Engineering 10

Limit Tolerancing

EML 2023Department of Mechanical and Aerospace Engineering 11

Limit Tolerancing

EML 2023Department of Mechanical and Aerospace Engineering 12

Limit Tolerancing

EML 2023Department of Mechanical and Aerospace Engineering 13

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

EML 2023Department of Mechanical and Aerospace Engineering 14

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

EML 2023Department of Mechanical and Aerospace Engineering 15

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

EML 2023Department of Mechanical and Aerospace Engineering 16

communicationsymbols

ASMEclarity

replace

totalform

size & location

function & relationship

tolerances & interchageabilitytolerance

plus/minussize