315 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. tolerance is not affected by surface variations on the datum features. Avoidance of the surface variation effects on position is a benefi t of positional tolerances and is not achievable with the old system of coordi- nate location tolerances. Clarity of the position tolerance requirement is further enhanced by using basic dimensions to defi ne the nominal feature locations. In Figure 8-23, the basic dimensions create a tolerance zone frame- work that is theoretically perfect. The tolerance zone framework includes true locations and true orien- tations for the features based on the dimensions locating the holes. The tolerance zone framework is, to the extent required by the datum feature refer- ences, constrained to the datum reference frame by the dimensions shown in the drawing or the data in the solid model. There is no tolerance accumulation on basic dimensions. Because there is no tolerance accumula- tion, the position tolerance zones are always centered on the true positions as located by the tolerance zone framework. This does not require that a produced part be perfect. Only the tolerance zones must be located at the true positions. The toleranced features may be anywhere within the defi ned tolerance zones that are part of the framework. Freedom exists to express the required level of control, ranging from very small to very large toler- ances, when using position tolerances. One miscon- ception about position tolerancing is that it should only be used to achieve a high degree of accuracy. Of course it may be used to express small tolerances, but it is just as useful for expressing large tolerances. The only limits on the tolerance values placed in a feature control frame are the functional require- ments of the part and the available manufacturing capabilities. Flexibility to meet a wide range of design appli- cations is built into the position tolerancing methods. The methods are not restricted to complex parts, or limited to simple ones. Only the level of control required by the function of the part needs to be spec- ifi ed when using position tolerances. Position tolerancing methods are well-defi ned by the current dimensioning and tolerancing stan- dard. They are widely accepted throughout the United States and internationally. Military and defense contractors use these methods extensively. The automotive industry uses these methods. Large and small companies recognize the benefi ts of this system and adopt it for utilization in their designs. It is necessary to understand and properly utilize positional tolerancing methods to be compatible and competitive in today’s industry. Ambiguity of Coordinate Location Tolerances A very long time ago, before coordinate toler- ances were used, some products were made with success. Few if any had interchangeable parts, and the clarity of requirements was not all that well-communicated. Coordinate location tolerances were fi rst used perhaps 200 years ago, and a great number of parts have been successfully produced. At times, the success was to some extent a result of manufacturers assuming requirements that were not specifi ed. Within the past 50 years, there were many times when requirements were further explained in notes, in separate documents, and even verbally. Regardless of the challenges, there were many successes. Now, there is a system of position tolerances that is far superior to using plus and minus location tolerances. The risks of using a system that is poorly defi ned can be eliminated. The past successful fabri- cation of poorly defi ned parts is not an adequate rea- son for continued use of a system that is ambiguous. Industry has moved forward. It is important to be aware that there is no stan- dard that completely defi nes the meaning of directly applied coordinate location tolerances. Because the practice is not standardized, coordinate location tolerances have the disadvantage of creating unclear requirements. Where they are used, there is a pos- sible accumulation of tolerances that is subject to multiple interpretations. A simple plate may be used to show the ambiguity of coordinate tolerances. See Figure 8-24. There are four holes in the given plate. The drawing shows hole #1 dimensioned from the lower-left corner of the plate. Perhaps it is not really dimensioned from the lower-left corner, but from the left side and the lower side. So, which is it: the corner or the sides? There is no standard that defi nes this. Whether the dimensions come from the corner or the sides can make a difference. Holes #2 and #3 are dimensioned from hole #1. At least it appears that way. But are they really? It appears that one dimension for each hole extends from hole #1, and one dimension extends from one of the sides (or the corner). It just keeps getting more ambiguous. It is not clear whether hole #4 is dimen- sioned from holes #2 and #3 or from hole #1. Sides of the plate are drawn straight and at a 90° angle to one another. Dimensions to the holes are drawn parallel and perpendicular to the edges. Coordinate tolerances of ±.010 are directly applied on all the hole location dimensions.