TABLE OF CONTENTS GTs apply variability constraints to a particular feature having a geometrical form. A GT can be applied to any feature that can be defined by a theoretically exact shape, e.g. a plane, cylinder, cone, square, circle, sphere or a hexagon. GTs are needed because in the real world, it is impossible to produce an exact theoretical form. GTs define the geometric deviation permitted such that the part can meet the requirements of correct functioning and fit.
Note it is always assumed that if GTs or indeed tolerances in general are not given on a drawing, it is with the assumption that, regardless of the actual situation, a part will normally fit and function satisfactorily.
The chart in Figure 5.13 shows the various geometrical tolerance classes and their symbols given in ISO 1102:2002.
The tolerance box is connected to the feature by a leader line. It touches the box at one end and has an arrow at the other. The arrow touches either the outline of the feature or an extension to the feature being referred to. A tolerance box has at least two compartments. The left compartment contains the GT symbol and the right the tolerance value (see Figure 5.16). If datum information is needed, additional compartments are added to the right. Figure 5.15 shows a three compartment box (one datum) and Figure 5.14 shows a four compartment box (two datums). The method of identifying the datum feature is by a...
