321 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. always specify position tolerances RFS on threads. Some may use MMC. Some prefer RFS because threaded features should align on the axes of the mating pitch cylin- ders. Some prefer MMC because the two threaded features have a fi xed diameter and there is some clearance between mating threads. Those who pre- fer RFS sometimes believe that a thread that is held off center will “load up” unevenly when tightened. Those who prefer MMC are content to accept some risk that threads may sometimes load up unevenly or that one of the features may deform a few ten thousandths of an inch. Depending on the company, decisions regarding RFS and MMC on threads may be driven by such factors as product quality, safety, or reliability, or decisions may be driven by cost and inspection ease or other factors not listed here. Projected Tolerance Zones A projected tolerance zone lies outside the tol- eranced feature. This is appropriate for features such as a threaded hole and a removable fastener or a hole and a press fi t or permanently installed fastener. See Figure 8-32. In the upper portion of the fi gure, a fastener passes through a clearance hole in one part and is threaded into another part. In this application, the effect of the threaded hole on the position of the fastener where it passes through the clearance hole is important. A position tolerance that projects through the clearance hole is needed on the threaded hole. In the lower portion of the fi gure, an interfer- ence fi t pin is passed through a clearance hole in one part and pressed into another part. In a case such as this where fasteners are not removable, the posi- tion tolerance on the press fi t hole must project far enough to ensure the clearance hole allows the parts to assemble. As the parts are moved together, the hole must allow the pin to clear the entire path. The pro- jected tolerance zone must extend the full distance the fastener extends above the surface. Tolerance applied to the pitch diameter Special application noted Application noted No notation shown Goodheart-Willcox Publisher Figure 8-31. A position tolerance applied to a thread is understood to control the location of the pitch diameter unless noted otherwise. Threaded hole establishes the axis of the screw True position Axis of hole and fastener Projected tolerance zone for the threaded hole Press fit hole establishes the axis of the pin True position Axis of hole and pin Projected distance to clear by pin during assembly Projected tolerance zone for the press fit hole Goodheart-Willcox Publisher Figure 8-32. A projected tolerance zone is used in applications where the tolerance zone must project to the outside of the controlled feature. In a removable fastener application, the tolerance zone must project through the clearance hole to ensure the parts assemble. In a press fi t or permanent installation fastener application, the tolerance zone must project far enough from the mating surface to ensure the parts assemble.