With a drawing as the only link between them, designers and inspectors can use GD&T to keep from rejecting good parts and accepting bad parts. By Nate Rose

In the days before globalization of manufacturing, parts were designed, made and inspected in the same facility. Any disputes about interpretation of design intent during inspection were easily resolved by face-to-face discussion with the designer.

Today, with products being designed in one country and manufactured in others, such communications are dif- ficult and unlikely to take place. With designers and inspectors in different time zones or unable to communicate in a timely fashion, the drawing is the only link between them.

This open-loop process can have cost- ly implications with good parts being rejected or bad parts being accepted. It helps if the inspector has an under- standing of the way the part is expected to function. A good drawing should provide this information. Geometric Dimensioning and Tolerancing (GD&T) makes it easier for designers to convey design intent so inspectors know how to do their measurements.

What is GD&t?

GD&T is a communications protocol that is particularly helpful in a global economy where parts may be madein one part of the world and used in another. GD&T is increasingly being accepted as the best way of specifying 3-D design dimensions and tolerances on engineering design drawings. The currently recognized GD&T standard in the United States is ASME Y 14.5M (1994). The ISO parallel of ASME Y14.5 is ISO 1101. Although both stan- dards have the same goal, they are not exactly equivalent, and it is important to know to which standard the drawing complies when measuring a part. As design software and machine tools have become more capable,increasingly complex parts are being designed. Designers use computer- aided design (CAD) software to design parts with specific dimensions that must be verified during the inspection phase of manufacturing those parts.

An example of a part with critical dimensions and tolerances is a plastic part designed to eliminate fasteners. By making a part with features that mate to another part, assembling those parts can eliminate the need for screws, reducing costs in parts and in assembly operations. However, sizes and positions of those mating features can be critical, with extremely tight tolerances. An inspector measuring those features, but referencing dimensions from the wrong positions may make the wrong decision. Properly specifying and using datums in GD&T can help avoid such errors.

the GD&t evaluation environment

Inspection equipment measures parts for two main reasons—production acceptance, which is compliance to specified tolerances, and troubleshoot- ing or problem solving of the manufac- turing process.

GD&T drawings typically reflect the first reason: they specify conditions, or tolerances, for part acceptance from a functional point of view. A drawing will have one or more datum reference frames (DRF) with tolerances located