A knowledge-navigation system for dimensional metrology

Journal of Research of the National Institute of Standards and Technology, March-April, 2002 by Howard T. Moncarz

Geometric dimensioning and tolerancing (GD&T) is a method to specify the dimensions and form of a part so that it will meet its design intent. GD&T is difficult to master for two main reasons. First, it is based on complex 3D geometric entities and relationships. Second, the geometry is associated with a large, diverse knowledge base of dimensional metrology with many interconnections. This paper describes an approach to create a dimensional metrology knowledge base that is organized around a set of key concepts and to represent those concepts as virtual objects that can be navigated with interactive, computer visualization techniques to access the associated knowledge. The approach can enable several applications. First is the application to convey the definition and meaning of GD&T over a broad range of tolerance types. Second is the application to provide a visualization of dimensional metrology knowledge within a control hierarchy of the inspection process. Third is the application to show the coverage of interoperability standards to enable industry to make decisions on standards development and harmonization efforts. A prototype system has been implemented to demonstrate the principles involved in the approach.

Key words: dimensioning and tolerancing; dimensional metrology; knowledge navigation; manufacturing training; VRML.

1. Problem Statement

Dimensional metrology is the science of measurement based on length. To fully understand the subject, a broad knowledge base that includes the measurement process, the language of measurement, devices, standards, traceability, and statistics is necessary (1). Dimensional metrology is important because it is the basis for making parts correctly. Unfortunately, confusion in the correct application of dimensional metrology is common (2).

Among components of the knowledge base, two parts include (1) geometric dimensioning and tolerancing (GD&T) and (2) the overall inspection process. These represent two different perspectives; GD&T is the basis for some of the specific processes within the overall inspection process.

1.1 Geometric Dimensioning and Tolerancing

GD&T is a method to specify the dimensions and tolerances of a part so that it will meet its design intent, often to mate with other parts. Tolerances need to be specified tightly enough so that the part will "work" (i.e., meet the design intent); they need to be specified loosely enough so that the part can be manufactured at a reasonable cost.

The information required for GD&T and a symbology to communicate it on a part drawing have been standardized by the American Society of Mechanical Engineers (ASME) in ASME Y14.5M-1994 (3) (and referred to in this paper as Y14.5 for short). A similar system for GD&T has been developed by the International Organization of Standardization (ISO) as a set of standards [4]. However, we will focus on the use of Y14.5 here.

A large store of information is contained in the Y14.5 standard to guide the user on how to specify different types of tolerances and how to use the proper symbology. The subject is difficult to master because it is based on 3D geometric features and relationships that are difficult to visualize from textual descriptions, even when supplemented with 2D static figures. Also, when trying to interpret a particular tolerance and symbology, supplementary information is often useful but is not readily available without further page flipping and searching through the standard and other references. To fully convey the definition of the standard is difficult; to convey a deeper, intuitive understanding of it is much more difficult. However, that is the level of understanding necessary for a practitioner of GD&T.

1.2 Interoperability Standards

Analyzing the accuracy of a part based on tolerances is only a portion of the inspection process. That process includes inspection planning, data preparation, inspection execution, data acquisition, results analysis, and, finally, either acceptance of the part or feedback of the results to adjust an errant manufacturing process. These processes are supported by many software applications, including those that are incorporated into machine tools, e.g., numerical code execution systems. The entire system is most effective if the software applications are seamlessly integrated together at the information interfaces. Interoperability standards defined at the interfaces provide that capability.

Interoperability standards enable a manufacturing company to create a "best-of-breed" system, comprised of applications individually selected to best meet its needs and that can be integrated together within the system. The standards specify information exchanges among the applications to meet particular requirements. The challenge for standards' developers is to specify a minimum set of standards to provide coverage for the information exchanges required that will also enable integration for the full range of software applications presently available and likely to be available in the future.