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.
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