Tolerance of Profile and Default Tolerances

Default tolerances play an important role in tolerancing as they require a minimum amount of precision that applies to all part features. Without default tolerances there is always a chance for some features not being controlled at all. Default tolerances assure that a part is free of mistakes or poor workmanship. Profile tolerance statements are used to define the default tolerances. This chapter presents the meaning of profile of a surface tolerance for ensuring all features are controlled at the lowest level of precision expected from the manufacturing processes. The size of smaller features is also controlled by a separate profile of a line tolerance. The meaning of both profile of a surface and profile of a line is presented in this chapter. Profile tolerances often require the use of axis or center plane datums. The meaning of these datums is also explained in this chapter. One of the important tools for defining default tolerances for cast or molded parts is the use of datum targets. Datum targets are briefly explained as the focus of this book is on machine parts.