Geometric Dimensioning and Tolerancing Chapter 1, Introduction

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 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.

• specifying tolerances became very important

Eli Whitney 1765 - 1825 EML 2023 Department of Mechanical and Aerospace Engineering

2

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 2023 Department of Mechanical and Aerospace Engineering

3

Tolerance Notes

EML 2023 Department of Mechanical and Aerospace Engineering

4

Plus/Minus (Limit) Tolerancing • Plus/minus tolerancing seemed to work well for many applications.

EML 2023 Department of Mechanical and Aerospace Engineering

5

Plus/Minus (Limit) Tolerancing

EML 2023 Department of Mechanical and Aerospace Engineering

6

Plus/Minus (Limit) Tolerancing

EML 2023 Department of Mechanical and Aerospace Engineering

7

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”. perfect location for hole center .007

max allowed error for hole center +.005

+.005

EML 2023 Department of Mechanical and Aerospace Engineering

8

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 2023 Department of Mechanical and Aerospace Engineering

9

Limit Tolerancing

EML 2023 Department of Mechanical and Aerospace Engineering

10

Limit Tolerancing

EML 2023 Department of Mechanical and Aerospace Engineering

11

Limit Tolerancing

EML 2023 Department of Mechanical and Aerospace Engineering

12

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 2023 Department of Mechanical and Aerospace Engineering

13

History of Dimensioning and Tolerancing Standards in the USA Mil Std 8 1950’s Mil Std 8A Mil 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

EML 2023 Department of Mechanical and Aerospace Engineering

14

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 2023 Department of Mechanical and Aerospace Engineering

15

replace form total symbols ASME clarity communication size & location function & relationship tolerance tolerances & interchageability size plus/minus

EML 2023 Department of Mechanical and Aerospace Engineering

16