324 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. length and one for the width of the slot. The slot must meet the size requirements and both position toler- ance requirements. Noncylindrical Features of Size Location requirements for slots, tabs, rails, and other noncylindrical features of size may be specifi ed using position tolerances. See Figure 8-37. The tolerance specifi cation is placed adjacent to the size dimension of the feature to be controlled, or it may be attached to the dimension line. The feature control frame must not be placed on one of the exten- sion lines. Placement on an extension line would incorrectly indicate a tolerance on one surface, and position tolerances are not applied to surfaces. The position tolerances in the given examples are placed adjacent to the slot width dimensions, and therefore only defi ne a tolerance for the position of the slot width. Neither of the position tolerances applied to the shown slots has any effect on the bot- toms of the slots. The depth of the slot is controlled by either a directly toleranced dimension or a profi le tolerance. The fi rst example in the given fi gure shows a position tolerance of .016″ at MMC on a slot. The tolerance is referenced to datum feature A at RMB, which is the width of the part. Datum plane A is established at the center of the unrelated actual mating envelope that contacts the two outside sur- faces of the part. The unrelated actual mating enve- lope of the referenced datum feature is used in this application because datum feature A is the primary datum feature. Because the controlled slot is centered on the referenced datum, the slot is understood to be symmetrically located. For this reason, no dimension is needed to locate the slot. The second example in the given fi gure shows a keyseat in a shaft. A position tolerance of .010″ at MMC is applied to the width of the keyseat. It is referenced to datum axis B at RMB. Datum axis B is established by the axis of the unrelated actual mating envelope that contacts the outside diameter of the shaft. The .010″ tolerance zone and the virtual condi- tion are centered on the datum axis. Position Tolerance at MMC on a Slot When the position tolerance applied to a slot (a rectangular feature of size) includes the MMC modifi er, the position tolerance requirement may be explained in terms of the surface method or the axis (center plane) method. Refer to Figure 8-37. The two methods are both illustrated in the given fi gure. When surface variations are evaluated for compliance with the position tolerance at MMC and that evaluation gives a different result than the axis method, the surface method takes precedence. Datum plane A Center plane of slot .016 Tolerance zone when slot is .740 (MMC) Plane through datum axis B Datum axis B .010 Tolerance zone when keyseat is .240 (MMC) Virtual condition .724 .230 Virtual condition Goodheart-Willcox Publisher Figure 8-37. A position tolerance applied at MMC to a width feature of size creates a rectangular virtual condition that controls the feature’s location.