Geometric Dimensioning and Tolerancing
Walter M. Stites
Walter M. Stites
AccraTronics Seals Corp.
Walter M. Stites is a graduate of California State University, Northridge. His 20-year tenure at
AccraTronics Seals Corp began with six years in the machine shop, where he performed every task from
operating a hand drill press to making tools and fixtures. Trained in coordinate measuring machine
(CMM) programming in 1983, he has since written more than 1,000 CMM programs. He also performs
product design, manufacturing engineering, and drafting. In 12 years of computer-assisted drafting,
he’s generated more than 800 engineering drawings, most employing GD&T. He has written various
manuals, technical reports, and articles for journals. Mr. Stites is currently secretary of the ASME Y14.5
subcommittee and a key player in the ongoing development of national drafting standards.
Introducing Geometric Dimensioning and Tolerancing (GD&T)
When a hobbyist needs a simple part for a project, he might go straight to the little lathe or milling machine
in his garage and produce it in a matter of minutes. Since he is designer, manufacturer, and inspector all in
one, he doesn’t need a drawing. In most commercial manufacturing, however, the designer(s),
manufacturer(s), and inspector(s) are rarely the same person, and may even work at different companies,
performing their respective tasks weeks or even years apart.
A designer often starts by creating an ideal assembly, where all the parts fit together with optimal
tightnesses and clearances. He will have to convey to each part’s manufacturer the ideal sizes and shapes,
or nominal dimensions of all the part’s surfaces. If multiple copies of a part will be made, the designer must
recognize it’s impossible to make them all identical. Every manufacturing process has unavoidable varia-
tions that impart corresponding variations to the manufactured parts. The designer must analyze his
entire assembly and assess for each surface of each part how much v