Calculating Positional Tolerance

Figure 5-9 shows a drawing for one of two identical plates to be assembled with four 14 mm maximum diameter fasteners. The 14.25 minimum diameter clearance holes are selected with a size tolerance as shown. Using conventional positional tolerancing, the required tolerance is found by the equation as given in para. B3 of Appendix B.

Cylindrical tolerance zone (equal to positional tolerance) Axis of hole at true position

Extreme positional variation

Extreme attitude variation-

Primary datum

Minimum hole diameter

Minimum hole diameter

True Positional Tolerance

Primary datum

Axis of hole

Axis of hole True position axes

Axis of hole

Axis of hole True position axes

Axis of hole Is Axis of hole Is located Axis of hole Is Inclined coincident with at extreme position to to extreme attitude true position axis, the left of true position within tolerance zone, axis (but within tolerance zone).

Note that the length of the tolerance zone Is equal to the length of the feature, unless otherwise specified on the drawing.

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