Geometric Dimensioning & Tolerancing

Cost to fix an error during product development cycle increases exponentially WRT time! It is minimum at Idea stage and maximum at manufacturing stage! To reduce the errors, it is necessary that the drawings should contain precise and correct information. The communication through the drawing should be correctly understood by all across the organization. Hence a common method to convey the design intent is needed. Geometric Dimensioning and Tolerancing known as “GD & T” is a compilation of symbols and rules that efficiently describe & control dimensioning. It also helps in tolerancing of all drawings (castings, machined components, etc.). The concept of “GD&T” was introduced by Stanley Parker from Scotland in the late 1930’s. However it was not used till late since vast majority of products were made in house. The designer could discuss with the manufacturing personnel (die designer, foundry, foreman, machinist & inspectors) what features were to be controlled. Also when two or more features were shown coaxial or symmetrical around these “centrelines”, the questions that needed to be answered by the designer was, “how concentric or symmetrical do these features have to be to each other”? GD & T principles answer these questions. GD&T gives a uniform meaning to the drawing across the organization that conveys the correct design intent. During the assembly of machine the individual tolerances add up. This can result in various unforeseen problems. If the overall tolerances are not estimated and controlled, it may not be possible to assemble the machine as intended. Hence there is a need to calculate tolerance stack up. This also helps in controlling overall dimensions.