305 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. The given fi gure also shows four circles at the MMC size of .500″ 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 being reduced by .010″. It can be seen that this condition does not violate the virtual condition. Additionally, an inscribing circle is drawn representing an inner boundary known as the resultant condition. The sur- face of any hole produced within the allowable size and position tolerance will not violate the virtual condition or the resultant condition. Proof of the MMC Concept The MMC modifi er may be used in a majority of clearance hole applications where the main concern is free assembly of the parts. Application of MMC permits greater freedom in how the part is produced. However, if location accuracy for mating parts is a concern, the effects of larger holes and larger posi- tion tolerances may make the application of MMC undesirable. Where mating part location is import- ant, tolerances applicable RFS may be appropriate. Application of an MMC modifi er on a position tolerance for a hole permits an increase in location tolerance as the size of the unrelated actual mat- ing envelope increases. Generally, the hole size is approximately equal to the unrelated actual mating envelope size, but they are different, and the differ- ence depends on the amount of form variation in the hole. Manufacturing fl exibility during the pro- duction of the part can benefi t from the allowable increase in position tolerance that occurs as hole size increases. When the MMC modifi er is on the tolerance, the machinist or fabrication planner may determine how to best utilize the total permitted tolerance if there are no other specifi ed requirements such as statistical process control. It may be determined by manufacturing that it is best to produce a hole near its least material con- dition. This allows a maximum amount of position tolerance. In some situations, it may be determined that it is best to work near the MMC size, and try to achieve the specifi ed position tolerance. Working near the MMC limit permits more latitude for rework of the part if an error is made. For some very expen- sive parts, allowing some remaining material for rework can be important to avoid scrapping a part because of small discrepancies. Understanding how feature size affects position tolerance requirements supports making decisions regarding the utilization of the MMC modifi er on posi- tion tolerances. The following explanation shows how feature size can affect the allowable position tolerance. Figure 8-13 shows two simple plates. One has two pins, and the other has two clearance holes. It is unnecessary to know the dimensional information for the two parts. The geometry and relative sizes can be used to see how size affects location requirements. increases Larger hole (departed from MMC) Pin at true position Large clearance hole, off true position, loose fit on pin Small clearance hole, snug fit on pin Plate with clearance holes Plate with pins Center of pin on true position Available tolerance for hole position Clearance Center of hole Small hole (MMC) Large pin (MMC) Center of pin on true position Available tolerance for hole position Large pin (MMC) Center of hole Greater clearance as produced hole size Small hole Large hole Goodheart-Willcox Publisher Figure 8-13. A hole with a large amount of clearance can tolerate a larger position variation than can a close-fi tting hole.