308 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. The fl oating fastener formula can be easily used to solve for a hole size when the fastener and a pre- selected tolerance are known. Simple mathematics is used to solve for the hole size. T = H – F T + F = H – F + F T + F = H H = T + F Fixed Fastener Condition A fi xed fastener condition exists when a fastener passes through multiple holes and one of those holes fi xes the location of the fastener. The fastener location can be fi xed by threads, a press fi t, a taper, or any other feature that prevents movement of the fastener. Only one part is permitted to fi x the fastener location. The other parts include clearance holes. See Figure 8-16. Two plates are shown. One has a threaded hole. The other has a clearance hole. The fastener has a location that is fi xed by the threaded hole. If the threaded hole is produced somewhere off true position, then the fastener will be pulled off true position with the hole. When designing parts that assemble in a fi xed fastener condition, the following simple formula may be used to calculate a position tolerance: T = (H – F)/2 T = Tolerance (position tolerance applied to all holes) H = Hole (MMC size of clearance holes) F = Fastener (MMC size) NOTE This given fi xed fastener formula assumes the toler- ance on the fi xed fastener feature is applied using a projected tolerance zone. (Further explanation is given later in this chapter.)ter.)p Care must be taken to correctly use this formula for the holes in a fi xed fastener application. The cal- culated tolerance value is the value that is applied to the holes on each of the parts. This formula and the resulting values should only be used if the tolerance applied to the fi xed fastener feature is specifi ed as a projected tolerance zone or the clearance hole goes through a very thin part. Figure 8-17 shows information that validates the fi xed fastener formula. The fi gure is simplifi ed for illustration purposes and shows a tolerance zone within the threaded hole. In practice, the tolerance zone for the threaded hole should be specifi ed to project into the clearance hole. Projected tolerance zones are explained later in this chapter. The fi gure shows two parts. The edges of the two parts (noted as datum features in the fi gure) are aligned and held in a fi xed location relative to one another. A threaded hole is shown in one part a clearance hole is shown in the other. A fastener is passed through the clearance hole and threaded into the second part. The fastener and the threaded hole are shown on the same axis because screw threads are generally self-centering. The clearance hole and the fastener are shown at their MMC sizes. The threaded hole and the clearance hole are offset in opposite directions from the true position. Because moving the threaded hole to one side also moved the fastener, the clearance hole could not be moved very far before one side of the clearance hole made contact with the fastener. In effect, the total amount of the position tolerance applied to the T g a applied projected tol (Further explanation cha Clearance hole Threaded hole establishes the location of the screw Goodheart-Willcox Publisher Figure 8-16. A fi xed fastener condition exists whenever the location of the fastener is determined by one of the parts.