Machinery's Handbook, 31st Edition
Positional Tolerance 633 has been common practice for many years. Several new modifiers introduced with the 1994 US standard include free state, tangent plane and statistical tolerancing, see Fig. 12. New modifiers introduced in the 2009 standard include MMB, LMB, and RMB. ST P T L F M
CF Continuous Feature
U Unequally Disposed
Statistical Tolerance
MMC and MMB
LMC and LMB
Free State
Tangent Plane
Projected Tolerance Zone
Fig. 12. Tolerance Modifiers Projected Tolerance Zone: Application of this concept is recommended where any vari ation in perpendicularity of the threaded or press-fit holes could cause fasteners such as screws, studs, or pins to interfere with mating parts. An interference with subsequent parts can occur even though the hole axes are inclined within allowable limits. This interference occurs because, without a projected tolerance zone, a positional tolerance is applied solely to the depth of threaded or press-fit holes. Unlike the floating fastener application involving clearance holes only, the attitude of a fixed fastener is restrained by the inclination of the produced hole into which it assembles. With a projected tolerance zone equal to the thickness of the mating part, the inclinational error is accounted for in both parts. In other words, the projected zone is an extension of the tolerance zone begun inside the hole and projected out, in this case 14 mm, on the same axis within the specified tolerance of location. The minimum extent and direction of the projected tolerance zone is shown as a value in the feature control frame. The zone may be shown in a drawing view as a dimensioned value with a heavy chain line drawn closely adjacent to an extension of the center line of the hole, see Fig. 13. It has been noted that the projected tolerance zone illustration shown here differs slightly from that shown in ASME Y14.5-2018. However, it is believed that the illustration shown here is a more realistic portrayal of the intent of the text in the standard. Statistical Tolerance: The statistical tolerancing symbol is a modifier that may be used to indicate that a tolerance is controlled statistically as opposed to being controlled arithmetically. With arithmetic control, assembly tolerances are typically divided arithmetically among the individual components of the assembly. This division results in the assumption that assemblies based on “worst case” conditions would be guaranteed to fit because the worst case set of parts fits — so that anything better would fit as well. When this technique is restrictive, statistical tolerancing, via the symbol, may be specified in the feature control frame as a method of increasing tolerances for individual parts. This procedure may reduce manufacturing costs because its use changes the assumption that statistical process control may make a statistically significant quantity of parts fit, but not absolutely all. The technique should only be used when sound statistical methods are employed. Tangent Plane: When it is desirable to control the surface of a feature by the contacting or high points of the surface, a tangent plane symbol is added as a modifier to the tolerance in the feature control frame, see Fig. 14. Free State: The free state modifier symbol is used when the geometric tolerance applies to the feature in its “free state,” or after removal of any forces used in the manufacturing process. With removal of forces, the part may distort due to gravity, flexibility, springback, or other release of internal stresses developed during fabrication. Typical applications include parts with extremely thin walls and non-rigid parts made of rubber or plastics. The modifier is placed in the tolerance portion of the feature control frame and follows any other modifier.
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