In the previous examples, the geometrical tolerance has been related to a feature at its maximum material condition, and, provided the design function permits, the tolerance has increased when the feature has been finished away from the maximum material condition. Now the geometrical tolerance can also be specified in relation to a datum feature, and Fig. 22.13 shows a typical application and drawing instruction of a shoulder on a shaft. The shoulder is required to be coaxial with the shaft, which acts as the datum. Again, provided the design function permits, further relaxation of the quoted geometrical control can be achieved by applying the maximum material condition to the datum itself.
Various extreme combinations of size for the shoulder and shaft can arise, and these are given in the drawings below. Note that the increase in coaxiality error which could be permitted in these circumstances is equal to the total amount that the part is finished away from its maximum material condition, i.e. the shoulder tolerance plus the shaft tolerance.
Condition A (Fig. 22.14). Shoulder and shaft at maximum material condition; shoulder at maximum permissible eccentricity to the shaft datum axis X.
Datum axis X
Datum axis X
0.1 Radial eccentricity
Condition B (Fig. 22.15). Shoulder at minimum material condition and shaft at maximum material condition. Total coaxiality tolerance = specified coaxiality tolerance + limit of size tolerance of shoulder = 0.2 + 0.2 = 0.4 diameter. This gives a maximum eccentricity of 0.2.
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