11 Chapter 1 Introduction to Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. the principles with other knowledgeable people. Care should be taken to discuss the principles and methods that are defi ned by the standard, and not to create new methods that confl ict with those already standardized. This requires knowledge of the princi- ples and methods, and it also requires knowledge of specifi c terminology that is applicable to dimension- ing and tolerancing. There is a wide range of dimensioning and tolerancing capabilities throughout industry. Many companies have utilized the principles covered in ASME Y14.5 for decades. Others have less expe- rience, and some may have not yet started. Even within companies that have extensively used the methods, there can be a wide range of capabilities. Wherever there is an inadequate level of knowledge, there is risk of excessive fabrication or assembly costs and possible confusion regarding requirements. Not everyone will have the same opportuni- ties to work with the dimensioning and tolerancing principles, and not everyone will learn them at the same rate. Each person will encounter different appli- cations, may understand how to apply the principles to some applications, and may not fully understand others. Regardless of the reasons, various levels of capability do exist. It is likely that anyone using the ASME Y14.5 principles will need to be able to explain what he or she has done. This is especially true when it comes to working with complex requirements. It is also nec- essary to be able to understand the explanations of others as they describe new situations. The discussion of geometric dimensioning and tolerancing can be kept its clearest by using the correct technical terminology. Calculations Properly calculated tolerances take advantage of the maximum amount of part variation that can exist, yet still ensure proper function of the part. Many tolerance calculations are simple to complete others require a signifi cant amount of thought and may require the utilization of variation analysis soft- ware. An explanation of various tolerance calculation methods is given in the following chapters of this book. Properly calculating tolerances is an import- ant part of ensuring product quality and minimum cost. If tolerances are properly calculated, parts made within specifi ed values will fi t together correctly and function as intended. If tolerances are calculated, the values are known to be the permissible amount of variation that will meet the functional and quality requirements. A benefi t of maximizing tolerances is that larger tolerances typically reduce part cost. When design schedules are rushed, there may be a tendency to assign tolerance values without completing calculations. This is a risky situation. Parts made to assumed tolerances will not function correctly if the tolerances are larger than those that should result from calculations. If the assumed toler- ances are smaller than necessary, the parts will cost more than necessary. Product liability and the competitive industrial market should deter anyone from guessing at toler- ance values. Records of calculations are maintained by some companies to provide evidence of a good design. Calculated tolerance values give the designer confi dence the design will work. Anyone that has ever assigned tolerance values by “feel” or “guess- ing” knows the type of anxiety that is involved. A designer’s career can be directly impacted by how well that person’s designs work. If tolerances are guessed, the designer must wait until parts are fab- ricated to determine if they will function properly. If the guesses are incorrect, then the chances of the design working correctly are signifi cantly reduced. The wait and the uncertainty can be avoided by properly completing the tolerance calculations. Designs do not always function as intended by the designer. When the fi rst units are built, the designer must fi gure out what to do if the design fails to assemble or operate correctly. If tolerances are calculated, there is one less possible cause for the problem. Technical Terminology This book uses the exacting terminology estab- lished by national standards, and additional terms are defi ned where the standards do not yet provide guidance. The exacting terminology prevents using one term with multiple defi nitions established through context. This enables clear and concise explanation of requirements. Colloquial terms are often used in industry and can lead to confusion. As an example, the word “datum” is incorrectly used in some companies to mean a feature on a part, an axis in the coordi- nate system, or a tooling feature that locates a part. In context, people generally understand the intent, but in teaching the subject it would be confusing to use the terminology without exactness in meaning. The exacting terms of the applicable standards are used throughout this book to avoid the risk of mis- communicating requirements that need to be accu- rately explained. Colloquial terms are mentioned in a few chapters to provide awareness of widespread usage.