307 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. Two plates are shown each has a clearance hole that is larger than the bolt that passes through them. Because the holes are larger than the bolt, the bolt is free to fl oat within them. When designing parts that assemble in a fl oat- ing fastener condition, it is common to use the same size clearance hole in each of the parts. When using the same size holes, the following simple formula may be used to calculate a position tolerance: T = H – F T = Tolerance (position tolerance applied to all holes) H = Hole (MMC size for all holes) F = Fastener (MMC size) Care must be taken to only use this formula for the holes in a fl oating fastener application. It must not be used for holes in a fi xed fastener application or where mating holes have different diameters. The calculated tolerance value (T) is the maximum position tolerance that may be specifi ed as applicable at MMC for the holes on each of the parts. The hole and fastener sizes used in the calcula- tion must be the MMC sizes of each feature, because it is the MMC sizes that result in the worst condi- tion fi t (the least clearance) between the two features. Because the position tolerance value is determined using the MMC sizes for the parts having a clearance fi t, the tolerance may be specifi ed with the MMC modifi er if the relative positions of the mating parts are not a primary concern. Figure 8-15 shows information that validates the fl oating fastener formula. The fi gure shows two plates. The edges of the plates are aligned (assuming they are the referenced datum features) and held in a fi xed location relative to one another. A hole is shown in each of the plates. The two holes are equal in size. A shaft is passed through the two holes. The two holes and the shaft are shown at their MMC sizes. The shaft is shown centered on the true positions for the holes. The holes are offset to the maximum extent possible without moving or interfering with the shaft. Offsetting the holes in opposite directions locates the axes of the holes on opposite sides of the true posi- tion axis. The distance between the axes of the two offset holes is equal to T, which is equal to H – F. The condition shown is the worst case. Any other position of either hole will be better than the condition shown. Because the tolerance formula T = H – F works for the worst case condition, it will work for all others. If the aligned upper edges of the parts in this fi gure are datum features referenced in the position tolerance for the holes, it will be possible to align the datums when the parts are assembled. Although the hole sizes and positions allow the datums to align, the holes and fastener do not force alignment of the edges. Observing the clearances in the given fi g- ure, it can be seen that until fasteners are torqued, the parts can move to locations where datums do not align. If alignment is essential at assembly level and position tolerances are applied with the MMC modi- fi er, additional requirements must be documented at the assembly level. Example Calculation Problem: Required: Calculate the position tolerance for a fl oating fastener application. Given information: Specifi ed hole diameter: .328″ ±.004″ Fastener diameter: .312″ MMC Solution: T = H – F T = .324″ – .312″ T = .012″ The solution of a fl oating fastener problem requires that at least two parameters be known. If there is only one known parameter, such as the size of the fastener, then either a tolerance or a hole size must be selected. Usually, a standard hole size may be selected that results in a position tolerance that is large enough to be practical for fabrication. T = H – F T is the position tolerance at MMC for both parts Basic dimension Tolerance zone (T) Axis of one hole Axis of one hole True position Datum features can remain aligned H – F Goodheart-Willcox Publisher Figure 8-15. The fl oating fastener formula calculates the allowable position tolerance based on the clearance between the holes and fastener.