Manufacturing variability and tolerances

In the example shown in Figure 4.16, it was assumed that the holes and the bolts were all perfectly cylindrical and perfectly round. As has been explained above, this is not the case. The bolts and holes will all deviate from true circles due to manufacturing variabilities. An example of this is shown in Figure 4.17. This is a cross-section through the lower-right example in Figure 4.16. Here it can be seen that both bolts and holes deviate from circular. The deviation has been exaggerated for convenience of presentation and to make the point. The hole and bolt deviations are enclosed by maximum and minimum circles. The difference between the outer and inner circles gives the manufacturing variability. The contact position of the bolt in the hole will be given by the point at which the maximum enclosing diameter of the bolt touches the minimum enclosing diameter of the hole. The eccentricity created by this is shown by the equations of the diagram in Figure 4.17. Thus, the maximum permitted centre-line spacing of the holes (comparable to Figure 4.16 bottom-left diagram) will be the centre distance plus the two eccentricities. This is shown in the equation attached to Figure 4.17 and is the difference between the values of C(a) and C(b).

Figure 4.17 The influence of bolt and hole owt-of-roundness on hole centre position

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