10 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. Interpretation Parts are usually manufactured by someone other than the person who completed the product defi nition. In large companies, many people are involved in making a part. Each of them completes some portion of the part requirements. This requires that each person understand the meaning of the specifi ed requirements. See Figure 1-5. People need to develop enough knowledge of dimensioning and tolerancing principles to be able to effi ciently inter- pret and understand the product defi nition. Correct interpretation of dimensions and toler- ances is required to produce a good-quality part at the minimum possible cost. Through correct under- standing of the requirements, it is possible to match the machine processes to the specifi ed accuracies. The correct processes ensure that excessive errors will not be made, nor will excessive control be used. Competent designers produce the product defi - nition to accurately show the design requirements for their parts. Any person working with the product defi nition must interpret it. Interpretation is not a matter of attempting to fi gure out what may have been meant by unclear specifi cation. GD&T interpretation is a matter of looking at the applied symbology and notes and reaching the one correct understanding of what the product defi nition says must be achieved. Because some designs are complex, it may take extensive knowledge of dimensioning and tolerancing princi- ples to properly interpret the design requirements. Every specifi cation shown on a complex part may not be directly supported by a specifi c fi gure or paragraph in the standard or in any book. A person must be able to interpret dimensions and tolerances when they are applied to parts that have greater complexity than the fi gures shown in the standard or in reference books. This means that the principles must be understood well enough to extend them to complex situations. In some instances, a product defi nition may be found to contain dimensioning and tolerancing errors, and, therefore, is not in complete compliance with the applicable standards. One reason for incom- plete compliance is that someone in the design pro- cess may have recently started to learn the subject, and that person is still working to master it. When product defi nition errors are identifi ed, a decision must be made—work can be completed while ignoring the errors, or corrections can be requested. Sometimes, it may appear to be neces- sary to work with an incomplete or incorrect product defi nition. This is a risky practice, however, requiring that a person attempt to guess what requirements the designer meant to specify. Before attempting to guess what was intended, it is wise to consider the risks being taken, and the possibility of other errors that are not obvious. If a portion of the product defi ni- tion is known to be inadequate or unclear, then parts made to the assumed requirements may not fi t or function correctly. Working with an incorrect product defi nition puts at risk the full cost of parts that may not be acceptable due to someone else’s assumption of what the requirements were meant to state. The amount of risk taken when working on the basis of an incorrect document is dependent on the type of errors. When errors are known to exist, a judgment must be made as to the severity of errors and the potential cost of incorrectly interpreting what is intended. It is usually better to ask for the require- ments to be corrected than it is to risk manufacturing a large quantity of bad parts. When there is doubt about the meaning of a design requirement, it is a good idea to discuss the product defi nition with the person who made it. If the verbal requirements and the product defi nition requirements do not correspond, the product defi ni- tion should be corrected to refl ect the actual design needs. The ability to interpret dimensions and tol- erances is only an ability to understand what is meant when the dimensions and tolerances are com- pleted in compliance with the applicable standards. The ability to interpret requirements is not an ability to guess what is meant by an incomplete or inaccurate product defi nition. Discussion of Principles Understanding the dimensioning and toleranc- ing methods well enough to apply and interpret them is important. It is also important to be able to discuss Ø.005 Perp tol at MMC Ø.258 Virtual condition Ø.253 MMC Feature surface within virtual condition boundary Axis within tolerance zone Datum Plane A Goodheart-Willcox Publisher Figure 1-5. Permissible geometry variations can be determined if tolerances are properly understood.