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

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.

Tolerance Notes

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

Plus/Minus (Limit) Tolerancing

Plus/Minus (Limit) Tolerancing

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”. max allowed error for hole center +.005 .007 perfect location for hole center

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.

Limit Tolerancing

Limit Tolerancing

Limit Tolerancing

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

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 The first dimensioning and tolerancing standards in the USA were military standards in the 50’s. There were revisions A,B and C. The standards have undergone name changes in the past, American Standards Association (ASA). The current standard is the American Society of Mechanical Engineers (ASME Y14.5-2009) The M in the title, which meant “metric compatible” was dropped because it was deemed no longer necessary. The standard can be used with both inch and metric values. It is a ASME standard with American National Standards Institute approval. Standards are always improved and work is taking place on the next revision. Additional information can be found at www.ASME.ORG.

Geometric Dimensioning and Tolerancing (GDT) establish a reference coordinate system by defining datums provide basic dimensions (perfect dimensions) relative to the datums specify allowable tolerances

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