Projected Tolerance Zone

The application of this concept is recommended where the variation in perpendicularity of threaded or press-fit holes could cause fasteners, such as screws, studs, or pins, to interfere with mating parts. See Fig. 5-31. An interference can occur where a tolerance is specified for the location of a threaded or press-fit hole, and the hole is inclined within the positional limits. Unlike the floating fastener application involving clearance holes only, the attitude of a fixed fastener is governed by the inclination of the produced hole into which it assembles. Figure 5-32 illustrates how the projected tolerance zone concept realistically treats the condition shown in Fig. 5-31. Note that it is the variation in perpendicularity of the portion of the fastener passing through the mating part that is significant. The location and perpendicularity of the threaded hole are only of importance insofar as they affect the extended portion of the engaging fastener. Where design considerations require a closer control in the perpendicularity of a threaded hole than that allowed by the positional tolerance, a perpendicularity tolerance applied as a projected tolerance zone may be specified. See Fig. 6-38.

5.5.1 Clearance Holes in Mating Parts. Specifying a projected tolerance zone will ensure that fixed fasteners do not interfere with mating parts having clearance hole sizes determined by the formulas recommended in Appendix B. Further enlarge-

Perpendicularity Tolerance
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