317 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. and the manufacturer believed that its parts met the drawing requirements. However, the parts would not fi t into the required assembly. Whenever a manufac- turer makes parts that it believes are correct to the drawing, the manufacturer wants to be paid for the parts. When a customer pays to have parts produced, the customer expects them to fi t in the required assem- blies. When the fabricator and the customer disagree about the requirements on a drawing, the disagree- ment can lead to production delays, lost profi ts, and broken business relationships. In extreme cases, disagreements may lead to litigation. It is much better to completely and clearly defi ne requirements using position tolerances than to argue about product requirements after fabrication is com- pleted. Directly applied coordinate tolerances should never be used for the location of features of size and should be avoided when specifying location require- ments for surfaces. Increased Tolerance Zone Area Round tolerance zones specifi ed by positional tolerances provide a signifi cantly increased amount of tolerance zone area when compared to the square tolerance zone created by a coordinate tolerance. See Figure 8-25. The top half of the given fi gure shows a square tolerance zone. The square is .015″ across the fl ats. When a square zone is properly sized, the cor- ners of the zone will produce a good part. The center point (the center cross) represents the true position for two holes. The X and Y coordinate variations of two produced holes are plotted relative to the true position in the fi gure. Locations for the centers of the two holes are indicated by the small fi lled circles. Both of the center points are exactly the same distance (.0106″) from the center of the toler- ance zone. One center point is in the corner of the tolerance zone, and is therefore acceptable. The other center point is on the vertical centerline. It is located outside the square tolerance zone therefore, it is not acceptable. The illustrated square zone forces the rejection of one of the two holes, although the magni- tude of variation in the two holes is identical. Fortunately, position tolerances may be used to avoid rejecting functionally good parts. The bot- tom half of the fi gure shows a circular tolerance zone circumscribing the square. The tolerance zone has a .0212″ diameter. This size of round tolerance zone permits the same .0106″ location variation in any direction therefore, the two shown center point loca- tions are acceptable. Initial impressions of the improvement from a square to a circular zone may not seem signifi cant, but calculations of the areas for the two shapes show there is actually a 57% increase in area. The area of a .015″ square is .000225 in2. The area of a .0212″ diame- ter circle is .000353 in2. Dividing the difference in area (.000128 in2) by the original area (.000225 in2) and multiplying by 100 calculates the percentage increase of 57%. The increased tolerance area realized with the round zones may be used to increase the producibil- ity of parts. An increase in producibility can often reduce part cost. Choosing to ignore the advantages of the larger zone, and continuing to use square tolerance zones, unnecessarily reduces the available tolerance and may cause rejection or rework of func- tionally good parts. It is a bad practice to specify part requirements that force the rejection or rework of functionally acceptable parts. Sometimes the transition from using plus and minus tolerances to position tolerances results in attempts to convert plus and minus tolerances into position tolerances. At times, complaints are heard that the tolerances are being reduced. See Figure 8-26. The given fi gure shows what may happen if an attempt is made to convert, rather than calculate, Round tolerance zone Square tolerance zone Reject Accept Accept Accept Area of square = L2 Area of circle = πR2 π .01062 = .353 × 10–3 in2 .0152 = .225 × 10–3 in2 Goodheart-Willcox Publisher Figure 8-25. A round tolerance zone permits the same amount of variation in all directions. A square tolerance zone does not.