303 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. envelope are the same. To simplify the explanations, the size of the unrelated actual mating envelope and the produced hole size are assumed to be equal. A hole specifi cation and position tolerance are shown in the given fi gure. Limits of size for the hole are .500″ and .510″ diameter. The position tolerance is specifi ed as .025″ diameter at MMC. Any hole produced at the exact MMC size of .500″ diameter is permitted a position tolerance of .025″ diameter. Any hole produced at a larger diame- ter is permitted a larger amount of position tolerance. For every .001″ diameter increase in size of the hole, there is an allowable .001″ increase in the diameter of the position tolerance zone. Two examples of produced holes are shown. One hole is produced at .500″ diameter, the maxi- mum material condition for the hole. The position tolerance for this hole is .025″ diameter. The other hole is produced at .510″ diameter, the least material condition for the hole. The position tolerance for this hole is increased to .035″ diameter. The MMC modifi er does not permit violation of the limits of size. The example hole must be produced within the limits of size of .500″ and .510″ diameter. The MMC modifi er is typically used for clear- ance hole applications. An example is a clearance hole through which a bolt is passed. It makes sense to allow the position tolerance to increase as hole size increases on applications where the hole is not used to locate the part. If a clearance hole is made larger, it may be off true position by a greater amount and still let the bolt pass through. It is logical that the larger a clearance hole is, the more position variation that can be allowed, while still allowing the fastener to pass through. Two boundaries are created by a position toler- ance that is applicable at MMC on a hole. There is a virtual condition (an inner boundary for a hole) that is equal to the MMC hole size minus the specifi ed position tolerance. A resultant condition (an outer boundary for a hole) is created by the LMC hole size plus the allowable position tolerance. Figure 8-11 shows four circles at the MMC size of .500″ diameter at four different allowable loca- tions at the extremes permitted by the .025″ diameter position tolerance zone located on the true position. An inscribing circle is drawn representing the virtual condition. Any hole produced within the allowable size and position tolerance will not violate the vir- tual condition. The virtual condition boundary is the acceptance criteria for a position tolerance applied at MMC. The given fi gure also shows four circles at the LMC size of .510″ diameter at four different loca- tions at the extremes permitted by the allowable .035″ diameter position tolerance zone located on the true position. The position tolerance is increased by .010″ to the .035″ diameter as a result of the hole size increasing by .010″. It can be seen that this condition does not violate the virtual condition. Additionally, a circumscribing circle is drawn representing an outer boundary known as the resultant condition. The surface of any hole produced within the allow- able size and position tolerance will not violate the virtual condition or the resultant condition. The virtual condition and resultant condition boundaries are not size limits for the hole. They are not the minimum size or maximum size used for cal- culating the fi t of a mating part. They are worst case boundaries that the surface of the hole is not to cross. The virtual condition is an acceptance boundary that can be used to check for hole position. The resultant Produced hole sizes and allowable position tolerances Virtual and resultant condition boundaries Hole specification and position tolerance Ø.500 Hole produced at MMC size at LMC size Ø.510 Hole produced MMC specified Ø.035 Allowable Ø.510 LMC size position tolerance Ø.500 MMC size Ø.475 Virtual condition Ø.025 Allowable position tolerance Ø.025 Allowable position tolerance Ø.035 Allowable position tolerance Ø.545 Resultant condition Goodheart-Willcox Publisher Figure 8-11. The position tolerance zone increases in size when the MMC modifi er is applied to the tolerance specifi cation and the unrelated actual mating envelope size of the feature departs from MMC.