323 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. distance follows the symbol. On the drawing view where the leader connects to one of the toleranced holes, a heavy chain line is drawn adjacent to the controlled axis and a dimension applied to specify the projection distance. The dimension and chain line clearly indicate to which side of the part the zone is to project. Bidirectional Position Tolerance It is sometimes desirable to permit more posi- tion tolerance in one direction than the other. This is referred to as bidirectional position tolerance. Bidirectional tolerances may be applied on round holes and slotted holes. The application methods are applicable to both models and orthographic views. When applied in a solid model, the feature con- trol frames are placed in the same plane as the size dimensions. A common application of bidirectional position tolerance is on a slotted hole. See Figure 8-36. Two feature control frames are used to show the amount of tolerance on the hole, with each of the feature control frames specifying allowable variation along one axis. The given fi gure shows a position tolerance of .014″ applied to the .188″ slot width dimension. A position tolerance of .060″ is applied to the .500″ slot length dimension. Neither of the two position tolerances includes a diameter symbol. The tolerance zone created by the bidirectional specifi cation is rectangular. This tolerance zone can- not take full advantage of the functionally acceptable zone, but it does provide advantages over coordinate tolerances. The bidirectional zone is specifi ed relative to a datum reference frame. This gives a well-defi ned true position. The fi gure shows an interpretation with the tolerance zone included. An acceptable loca- tion of an MMC slot is shown with its center within the allowed tolerance zone. The bidirectional zone may be specifi ed with the MMC or LMC modifi er. This provides the advan- tage of having additional tolerances when the fea- tures depart from the specifi ed material condition. The given fi gure includes the MMC modifi er on each tolerance. This allows the rectangular zone to increase in size as the slot departs from MMC. With the tolerances specifi ed MMC, a virtual condition is created, and the surface of the slot is not permitted to violate the virtual condition boundary. There are actually two virtual conditions, one for the Orthographic view Model Projected tolerance zone symbol Projection distance Projection direction Goodheart-Willcox Publisher Figure 8-35. In orthographic views, the direction a tolerance zone projects may be shown with a heavy chain line drawn adjacent to the centerline of the hole. .060 .014 When slot length is .490 (MMC) When slot width is .178 (MMC) .430 .164 True position Permitted position variation Virtual condition Virtual condition Goodheart-Willcox Publisher Figure 8-36. Two feature control frames are used to show a bidirectional position tolerance.