306 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. The only functional requirement known for the given parts is that the plate with clearance holes must fi t over the two pins. To keep the explanation simple, it is assumed that the plate with the pins is produced as a perfect part. The effects of varying the clearance holes in the mating plate are shown in the given fi gure. One of the clearance holes has a snug fi t on its pin (there is no visible clearance in the fi gure). The second clearance hole is larger, and has some clearance between it and the pin. There are two detail views of the second hole. The fi rst detail view shows an extreme possible position for the clearance hole when the hole is at its smallest limit of size (the MMC size). Only a small amount of clearance exists between the hole and the pin and therefore only a small amount of position tol- erance is possible. The acceptable position tolerance for the hole is directly related to the amount of clear- ance between the pin and the hole. The second detail view shows an extreme possi- ble position for the clearance hole when the hole has departed from MMC and is at the maximum limit of size (LMC). The amount of clearance between the hole and the pin has increased because of the increase in hole size. The increased clearance permits a greater amount of position tolerance and the parts will fi t together. Increased hole size in the above explanation resulted in an increase in the amount of permitted position tolerance. The characteristics described show that departure from MMC does permit increased position tolerance when clearance fi ts are used in an assembly. This provides proof that the MMC concept is valid. Application of an MMC modifi er on a tolerance value has a well-defi ned impact. When the MMC modifi er is applied to a tolerance value, the permitted position tolerance increases by exactly the same amount as the departure of the unrelated actual mating envelope size from MMC. Tolerance Calculation Tolerance values should always be calculated. Calculation is the only means of making sure the tol- erances will always result in produced parts that will assemble properly. There are multiple approaches to calculating tolerances. The limit stack method (worst case calculation) ensures that all parts will assemble even if all parts are made at the very worst limit of the tolerance values. Another method is to fi nd the square root of the sum of the squares for all tolerance values. This method assumes that variation is never at the worst case on all parts. Other statistical meth- ods may be used, including computer simulation of fabrication variation combined with assembly sequences and locating constraints. The scope of the explanations in this text primarily addresses the limit stack approach. Two simple formulas are all that must be remembered to complete the calculations for many common tolerancing applications where a limit stack analysis is appropriate. These two formulas and their proper use are defi ned in the following sections. These formulas are used to calculate tolerances for the assembly of parts in either of two fastener condi- tions. These conditions are the fl oating fastener con- dition and the fi xed fastener condition. Floating Fastener Condition A fl oating fastener condition exists when a fastener passes through multiple holes and none of those holes fi x the location of the fastener. A fl oating fastener condition exists when all the holes have a diameter larger than the fastener. See Figure 8-14. Fastener Clearance holes Goodheart-Willcox Publisher Figure 8-14. A fl oating fastener condition exists when a bolt or shaft passes through clearance holes on mating parts.