In previous articles we looked at how geometric tolerance specifications always define a controlled feature component, a tolerance zone, and constraints. The next series of articles will focus on how to apply these concepts at the inspection stage, when determining actual values of geometric characteristics on real parts.
ASME Dimensioning and Tolerancing Standards
Engineering drawings must reference a specific standard for Dimensioning and Tolerancing. At present, the most common in North America is ASME Y14.5M-1994 with the recent ASME Y14.5-2009 revision gradually being adopted. These Y14.5 standards define the geometric characteristics, tolerance zones, acceptance boundaries, and datum reference concepts needed to assess whether or not an as produced feature conforms to the specified geometric tolerance. Many of these definitions enable the use of a simple functional gage as a straightforward means to get a pass/fail attribute result. This ability to use simple gaging techniques is often promoted as a major advantage of the geometric tolerancing system.
However, inspection personnel are commonly called upon to generate variables data. In other words, a pass/fail result is usually not sufficient and numerical values are required. But the Y14.5 Dimensioning and Tolerancing standard only defines the tolerance zones and acceptance boundaries – it does not provide definitions for these numerical values. For geometric characteristics, there is no guidance given on how to extract an actual value from a given as-produced feature. The only actual values defined in the Y14.5 standard relate to size (Actual Mating Size and Actual Local Size).
Industry Practices for Actual Values and Reporting
With no standardized definition for actual values, different approaches were developed that suited the particular requirements of certain industries or measurement devices.
For certain simple geometric characteristics (e.g. Flatness, Cylindricity, Position RFS), a reasonable actual value can be derived from the tolerance in a straightforward way. This is because there is a simple one-to-one correspondence between the size (thickness or diameter) of the tolerance zone and the specified tolerance value. Some other characteristics (e.g. Position at MMC, unilateral Surface Profile) are not as simple, and deriving a unique actual value is not nearly as straightforward. In addition, single actual values for Position and Profile provide little or information about the process and their usefulness is often questioned. This has led to the widespread practice of reporting deviation values, which do not directly correspond to the specified geometric tolerance but are useful for process analysis and correction. In short, a variety of actual value approaches for these characteristics have been developed and adopted in industry.
Standardization of Actual Values
In the late 1980s the need for mathematical rigor in dimensioning and tolerancing was acknowledged, and a separate standard was published in 1994 called ASME Y14.5.1M-1994 - Mathematical Definition of Dimensioning and Tolerancing Principles. This standard contains a lot of useful information, but the most important for dimensional inspectors is that it provides definitions for actual values of geometric characteristics. But the Y14.5.1 standard (sometimes referred to as the “math standard”) is not nearly as well known and widely used as the Y14.5 standard, partly because it was written for readers with extensive mathematical backgrounds. The actual value definitions, which are of great value to many inspectors and CMM programmers, are mostly obscured by heavy mathematical language and vector algebra. As a result, many of the actual value definitions in Y14.5.1M-1994 are not well understood or followed in industry (even by experienced GD&T professionals).
The ASME Y14.5.1M-1994 standard provided much-needed clarification of datum reference frame construction and has been very valuable in that respect. Some of the actual value definitions for certain characteristics have not been as well received, as they either conflict with Y14.5 tolerance definitions (Size) or are based on +/- deviations (Profile). As a result, the reporting of Size and Profile inspection results in industry typically does not follow the definitions specified in ASME Y14.5M-1994. There were also several Y14.5 geometric characteristics for which no actual value definition was provided (e.g. boundary Position, Straightness at MMC).
Current Status and Future Developments
When the Y14.5-2009 standard was released, there was no accompanying revision of the Y14.5.1 Mathematical Definitions standard. So there are currently gaps in the coverage of actual value definitions for some geometric tolerances. A new revision of Y14.5.1 is currently in development, that will accompany the next release of Y14.5 and address these gaps. The Y14.5.1 subcommittee has an opportunity to reach a wider audience with this new revision, by presenting actual value concepts separately from the mathematical jargon. Major improvements are expected.
Another related ASME standard is also currently in development, that will fill in the picture even more. This standard has been named ASME Y14.45 - Measurement Data Reporting Practices. It will provide guidelines for reporting measurement results for geometric characteristics, including single measured values for conformance assessment as well as other related data for process-related considerations. This new standard, in conjunction with an improved Y14.5.1 Mathematical Definitions standard, promises to further standardize and streamline the use of geometric tolerancing in inspection.
Future articles will focus on the main concepts of actual value definition for geometric characteristics, and the challenges that they present in CMM inspection of real parts.