316 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. One possible produced part is shown below the drawing. The edges of the part have form variations that are exaggerated to make them visible. The edges do not form a perfect 90° angle. Based on Rule #1, as previously explained, variation of form is per- mitted when features of size (such as the length and width of this part) depart from MMC. Variation of the angles is acceptable because the shown drawing note permits a ±.5° tolerance on angles. Immediate questions start to arise as it becomes necessary to locate hole #1. Because the edges are not perpendicular, from where should measurements be made and from which edge should measurements be oriented? These are diffi cult questions to answer because no datums are identifi ed or referenced. If some assumptions are made, it is possible to make measurements for holes #1, #2, and #3. Locations for these holes could be made relative to the edges nearest the hole. Of course, surface varia- tions will cause some location difference for the ori- gin used to measure each of the holes. The direction of measurements is left somewhat undefi ned because the two edges of the part are not perpendicular. Because of the surfaces not being fl at and not being perpendicular, someone may suggest creating a coor- dinate system and measuring from that. So there are multiple options for setting the origin, and measure- ments will differ depending on the method selected. Each person may make different assumptions and obtain different measurement results. The given fi gure shows one possible position for the fi rst three holes if we assume that measurements may be made from the imperfect part edges. Hole #1 is located .510″ from the left edge and .385″ from the bottom edge. Hole #2 is located .490″ from the left edge and hole #3 is .365″ from the bottom edge. The hole #1, #2, and #3 locations result in centerlines between them that form an angle greater than 90°. A considerable problem now exists for locat- ing hole #4. Should measurements continue from the edges of the part, or should measurements now be made from one or more of the other three holes? If they are made from the holes, at what angles should measurements be made? With the centerlines forming an angle greater than 90°, the direction of measurement is unclear. There are no defi ned answers for the questions that arise. There are no answers because there are no standards defi ning what the dimensions mean. The ambiguity in coordinate location tolerances is caused by part variations that can exist and have been shown in the given fi gure. Each person who attempts to determine how the part is to be measured must assume a set of guidelines. Each person who assumes different guidelines can obtain results that are different from anyone else’s. These types of possible differences in interpre- tation are not acceptable if parts are expected to meet functional or assembly requirements. The confusion can be eliminated through the application of position tolerances. Position tolerances have not always been used, so how is it that good parts were ever produced to coordinate location tolerances? In many situations, manufacturing processes were set up to produce good parts based on the obvious function of the part. This was done regardless of the incomplete require- ments defi ned by the drawings. The good intentions of the manufacturing com- panies are not always suffi cient. Not all parts pro- duced to coordinate tolerances will work as expected. Situations have existed where parts were produced ?? ?? ?? ?? Coordinate measurements Orthographic view Measure perpendicular to the centerline between holes #1 and #2 Measure parallel to the centerline between holes #1 and #3 Measure from hole #1 Measure from hole #2 Goodheart-Willcox Publisher Figure 8-24. Plus and minus tolerances on location dimensions are ambiguous. The origin and direction for measurements are not defi ned.