What is GD&T?
In a grossly-over simplified definition, GD&T is a language. It is a precise mathematical language the describes the form, orientation, and location of various features on a part. Like learning any other language, knowing when, where, and how to correctly apply GD&T will take time. In any given manufacturing or engineering corporation there will be people that have varying levels of understanding GD&T. The checker may be fluent in GD&T while the drafter may only have a basic understanding of the language.
GD&T arose from the idea to produce products that had interchangeable parts. Products that could be quickly disassembled and re-assembled with the parts from a separate, identical product. The very first form of dimensional control was the plus/minus dimensioning that many in the engineering and manufacturing world are familiar with today. Early parts were often checked using a Go/No-Go gage to confirm whether or not the part was made within the plus/minus dimensions.
The beginnings of the GD&T system that we have today has been credited to Stanely Parker from the Royal Torpedo Factory. Like many advances in technology, GD&T came from the military's need to produce products and parts with interchangeability being an essential requirement. The main difference between the plus/minus tolerances and the new GD&T tolerances was the shape of the allowed tolerance zone (more on this in a future post). Mil Std 8 was the first standard in the 1950s and the most current standard is ASME Y14.5-2009.
Why Use It? & Who Uses It?
GD&T allows engineers/designers to convey their design intent. When applied correctly, GD&T can only be interpreted one way. This eliminates the confusion often associated with plus/minus toleranced parts.
GD&T is used by engineers, designers, drafters, machinists, checkers, inspectors, quality control, vendors, purchasing agents, customers, and occasionally legal personnel.
Form Tolerances: Control the shape of a feature or features
Orientation Tolerances: Control the tilt of a feature or features
Location Tolerances: Control location of a feature or features (includes position and profile tolerances)
This is just a brief overview/introduction of the GD&T system. Future posts will cover GD&T in more detail.