4 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. examples of simple measurement instruments: an out- side caliper and a dial caliper. Although both can be used to make accurate measurements, the dial caliper enables considerably greater accuracy. This tool can be used to make accurate external, internal, and depth measurements. However, other instruments available today can provide far more accurate measurements than possible with outside calipers or dial calipers. Development of Documentation Standards The ability to accurately specify and verify dimensions and tolerances for a product required not only distance measurement standards, but also dimensioning and tolerancing standards. Dimen- sioning and tolerancing standards were developed and are now maintained by the American Society of Mechanical Engineers (ASME). The primary standard for dimensioning and tolerancing is ASME Y14.5, but there are additional standards of importance. Before the development of standards and pre- cision tools, machine parts were fi tted together and assembled by hand. Without standards, it was not possible to specify the allowable variation in a clear and precise manner, nor was it possible to verify by measurement that separately produced parts would fi t together. The development of accurate fabrication and measurement equipment made it possible to produce and verify relatively small variations in part size. See Figure 1-2. This capability made it reasonable to specify relatively small limits of acceptable variation for a dimension. Once this became reasonable, a stan- dard practice to specify limits in a consistent, clear, and understandable manner was needed. If limits of variation are correctly specifi ed in a manner that is commonly understood, and all parts made within the specifi ed limits, mass production of interchangeable parts is possible. Effi cient assem- bly of mass produced parts is only possible when all parts are made within the tolerances that assure assembly. This is not to say that each part manufac- tured within specifi cations is exactly alike, because variations will be present. The important point is that allowable variations can be specifi ed in a com- monly understood manner, and parts produced to those requirements will assemble and correctly function. The repeatable measurement accuracy that resulted from establishing a fi xed distance measure- ment standard helped to make interchangeable parts possible. Eli Whitney is often credited with making the fi rst interchangeable parts (1798). Repeatable accuracies that were achieved in the 18th century seem crude by today’s capabilities, but they were adequate for making some interchangeable parts. Today, where needed, accuracies smaller than .001 inch are possible for some part feature types. Present industrial capabilities make very small tolerances feasible, although there is a cost associated with small tolerances. Success in producing the needed accuracy for a specifi c design depends on the clear specifi cation of allowable variation. To achieve clarity in the speci- fi cation of allowable variation requires the applica- tion of dimensions and tolerances in a manner that is commonly understood. This requires dimensioning and tolerancing standards. Prior to the existence of a standard that included geometric tolerancing, tolerance requirements were communicated in notes. Notes were subject to the language skills of the individuals who created the product defi nition, fabricated the part, and inspected the part. The number of methods used to defi ne a Built to crude distance standards to which tolerances would not be logical Wooden yoke Drill bushing Built to accurate distance standards to which tolerances are logical Goodheart-Willcox Publisher Figure 1-2. Tolerances only make sense when there is an accurate unit of measurement.