Since threaded fasteners generally center themselves in the threaded hole, RFS (Regardless of Feature Size) can be argued to be the correct choice when applying a position tolerance to a threaded hole. The position tolerance then applies to the axis of the thread pitch cylinder. To measure the location of the pitch cylinder diameter, thread gages can be purchased that normally have a short cylindrical surface that is coaxial with a short threaded portion that centers on the pitch cylinder of the threads. The gage location can provide good quality feature location information, but the typically short length of both the threaded portion and cylindrical portion of the gage yields little or no feature orientation data.
This poor orientation measurement capability is a data quality issue, and it creates an associated issue. Without good feature orientation data, applying a projected tolerance zone will not be effective, even though a projected tolerance zone is necessary to optimally tolerance a feature that locates and orients a fixed fastener.
ASME Y14.5 standards for over 50 years have supported tolerancing threaded holes by applying position tolerances at MMC (Maximum Material Condition), with a projected tolerance zone. Since Y14.5 is not a measurement standard, the method for evaluating such a tolerance has not been in the standard. Position at MMC applied to features of size can be evaluated using a surface method, or resolved geometry (also know as “axis”) method. For the resolved geometry method, a bonus tolerance may result if the feature’s size departs from MMC. These normal methods do not work for threaded features though, since a measurement of the pitch cylinder diameter is not practical, and we don’t want a position tolerance that adds a bonus tolerance when threads fit more loosely anyway. For the surface method, a functional gage is normally made that verifies the feature’s surface does not violate its virtual condition boundary. This also does not work for a threaded feature, since there is no simple feature to base a virtual condition boundary on within a threaded hole.
What does work though, is to make a thread gage that also includes the largest specified size of the shank of the threaded fastener, with the length of the shank protruding from the hole, complying with the specified projected tolerance zone height. A functional gage can be used to evaluate the surface boundary that is applied to the shank portion of the threaded fastener. The functional gage will have cylindrical holes, made to tooling tolerance per ASME Y14.43, at the virtual condition for the shank. Please see the part drawing and gaging method shown below for an example of this method.
MMC is needed on threaded features to legitimize this hard gaging of the shank portion of the fasteners, as well as the projected tolerance zone modifier, needed to specify that the tolerance applies over the thickness of the mating part. This method of tolerancing a threaded hole, with the evaluation method shown below, addresses the functional needs of the bolted assembly. The evaluation method shown is an extension of the principles provided in Y14.5. This extension of Y14.5 principles is supported by Figure B-20 in ASME Y14.43-2011 “Dimensioning and Tolerancing Principles for Gages and Fixtures”.
