CHAPTER 3 GEOMETRICAL TOLERANCES Why Are Geometrical Tolerances
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CHAPTER 3 GEOMETRICAL TOLERANCES
Why Are Geometrical Tolerances Needed? A shaft has the same diameter measurement in all possible positions but is not circular; A shaft is circular in all cross-sections but is not straight. A part has the same thickness throughout but is not flat; The form of these components can be controlled by means of geometrical tolerances.
What is Geometrical Tolerance ? Geometrical tolerance : the maximum permissible overall variation of a feature，e. g. form tolerance or position tolerance. There are 14 geometrical tolerance characteristics in ISO 1101 standard which is named GPS (Geometrical product specifications). There also Chinese standard GB/T , which is similar to ISO standard. There also ASME Y 14. 5 standard which is named GD&T (Geometry Tolerancing and Dimensioning).
Geometrical Tolerance There are 4 kinds of geometrical tolerance : Form tolerance Orientation tolerance Location tolerance Run-out tolerance
Toleranced Feature Toleranced features can contain no more than a tolerance symbol and a tolerance value, but also modifiers, and datum references. The tolerance feature is labelled in the following ways: (1) Placed on the outline of a feature (2) Placed on an extended dimension line
Datum Feature Datum features are the surfaces and features of size used to establish the locations for datum planes and axes. The datum feature symbol and triangle may be used to identify a surface or feature of size as a datum feature. No two datums on a single part should have the same datum letter.
Single Feature and Related Feature Single feature: The geometrical tolerance is unrelated with other features, such as the straightness of the element line of the shaft. No datum is needed in single feature. Related feature: The geometrical tolerance is related with other features, such as a line parallel with another line. Datum is needed in related feature.
Tolerance Frame The geometrical tolerance requirements are specified in frames, symbols, numbers, and datum reference letter are generally adequate for specification of standardized tolerance controls. Tolerance symbol Tolerance Value Datum letter Leader line Arrow
