Interpretation of Modifier Ⓜ “Circle M” in ASME Y 14.5 GD&T: Intuitive or Deceptive?

Geometric Dimensioning and Tolerancing (GD&T) is a system for defining and communicating engineering tolerances by using a symbolic language on engineering drawings that describe nominal (theoretically perfect) geometry of controlled features, as well as their allowable variation in size, other geometrical characteristics (form, orientation and location) and variation between features. Per this language, dimensions and tolerances are selected to suit function and mating relationship of a part and are subject to a unique interpretation. It allows design engineers, manufacturing personnel, and quality inspectors to describe geometry and allowable variation of parts and assemblies in an efficient and effective manner. When compared to coordinate dimensioning, GD&T has the benefits of reducing the manufacturing cost and number of drawing revisions, describing an important functional relationship on a part, saving inspection time by using functional gages, and improving measurement repeatability. However, GD&T has a fairly complex rule-based system, and as a result can be difficult to teach and learn. One such concept relates to the use of modifier circle M. In GD&T, a feature control frame is required to describe the conditions and tolerances of a geometric control on a part’s feature. The feature control frame may consist of up to four pieces of information, (1) GD&T symbol or control symbol for the feature, (2) Tolerance zone type and its size, (3) Tolerance zone modifiers and (4) Datum references (if required by the GD&T symbol). When circle M is used as a feature tolerance zone modifier, it is relatively easy to understand that there is a possibility of getting bonus tolerance, and in turn, a higher total tolerance. However, what is not very intuitive is the size of the feature counterpart on the functional gage to inspect the given feature control frame. Apparently, it is not the Maximum Material Condition (MMC) size of the feature. Rather, the size is what is called a virtual condition (VC) of the feature, which is defined as the theoretical extreme boundary condition of a feature of size (FOS) generated by the collective effects of MMC and applicable geometric tolerance. When circle M is used as a datum feature/reference modifier, it is even more strenuous to calculate the datum boundary or the size of the datum feature counterpart on the functional gage. In this case, it is the Maximum Material Boundary (MMB); a virtual condition of the datum feature governed by a specific rule of GD&T that establishes this VC with respect to the preceding datum in the feature control frame. This would necessitate one to look for a specific applicable geometric tolerance that is an exclusive relationship between the datum feature and its preceding datum in the feature control frame. Even worse, in case of position tolerance (which, often times, is a lumped sum tolerance controlling orientation and location geometric characteristics of the datum feature simultaneously), it is even trickier to find an exclusive relationship between the datum feature and its preceding datum. In this article, authors have made an attempt to clarify the above-mentioned situations through numerous examples. Hopefully, this can be successfully implemented in undergraduate and graduate education reinforcing the premise that a better educated workforce would be able to contribute significantly higher to advanced manufacturing, design, quality tools and advanced metrology.