319 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. holes in Figure 8-27, the holes could purposefully be drilled larger than MMC to increase the allowable position tolerance. If holes are manufactured at a diameter of .285″, the resulting additional tolerance of .004″ can be added to the specifi ed position tol- erance to obtain an allowable position tolerance of .035″ diameter. This additional tolerance may permit fabrication on a less expensive process, thus meeting requirements for a lower cost. Additional tolerance may also be used when reworking holes that are slightly outside the speci- fi ed position tolerance. All that needs to be done to correct the out-of-tolerance condition is to open the hole diameter within size limits and without drifting (pulling) the hole further off center. This increased hole size will result in additional position toler- ance that may be used to accept the produced hole location. Using the hole specifi cation in Figure 8-27, consider a part in which one hole is produced too far out-of-position. Consider a hole that has a .281″ diameter and a position of .032″ diameter. For this hole, an additional tolerance of only .001″ diame- ter is needed. This means the hole only needs to be increased to a diameter of .282″. Special Applications Position tolerances may be applied to features of size other than simple clearance holes. Some common applications include counterbored holes, threaded holes, and noncylindrical features of size. Some application requirements affect how the tolerance is specifi ed and where the tolerance zone is located. Although the default location of a tolerance zone is along the length of the controlled feature, the zone is sometimes specifi ed to lie outside the feature. This can be done with a projected tolerance zone. Position tolerances must include datum feature references to establish requirements for the position between the toleranced features and datum features on the part. There are some special applications when the toleranced features are not related to datums, but in those cases the toleranced features must serve as the primary datum feature for other location tol- erances applied to other features on the part. This is explained in a later section of this text. Counterbored Holes A common use for a counterbored hole is to permit the installation of a fl ush-head screw or bolt. It is only possible to install the screw or bolt if the clearance hole and the counterbore are the correct size and located with suffi cient accuracy. Counterbores are often produced in a separate machining operation from the through hole, so the two features may not lie on the same axis. Positional tolerance specifi cations may be applied in a manner that ensures assembly of a screw or bolt into the counterbored hole. There are three methods of achieving the desired results. Counterbore diameter calculations are com- pleted using the same formulas as used for calculating the clearance hole diameter. The maximum material condition for the screw head is used in calculations to determine the counterbore diameter. If there is a fi llet radius at the bottom of the counterbore, the diameter of the fl at at the bottom of the counterbore must be considered to ensure the fastener head does not ride up on the fi llet. Counterbored Holes—One Position Tolerance A single position tolerance feature control frame may be placed next to the hole and counterbore callout to indicate the same position tolerance applies to both the hole and counterbore. See Figure 8-28. A leader is extended from the counterbore and the number of places noted. The hole size and size tolerance may be on the same line as the number of places. The coun- terbore size and tolerances are placed immediately below the hole size. The position tolerance is placed immediately below the counterbore size. The posi- tion tolerance applies to both features. The two features act independently and may vary in opposite directions within the specifi ed zone. When the MMC modifi er is included in the position tolerance, there are virtual Ø.022 Tolerance zone centered on true position Counterbore Hole Goodheart-Willcox Publisher Figure 8-28. One feature control frame may be applied to specify the same tolerance for the location of a counterbore and hole.