The ISO Geometrical Product Specifications Handbook - Find your way in GPS

The following is an article I wrote for the April 2012 issue of ISO Focus+. The article starts on page 39, if you follow the link and download the magazine.

Geometrical Product Specifications (GPS) is the international symbol language used to express tolerances in technical drawings. It defines the nominal geometry of the work pieces that make up the product, and the tolerances that quantify the allowable deviations from this nominal geometry.

Any time a design has to be captured and communicated, it is done using GPS. This global specification language enables a drawing of a component developed in one country to be sent to another country, where it can be understood and the component manufactured. GPS is an effective link, even when the designer and supplier do not have a common language. It helps products to be made more easily and efficiently, using fewer resources in the process.

ISO has just published a new handbook giving trainees and other industry novices an overview of the fundamental rules and principles in GPS. It will improve the common understanding of this established global language, which is the foundation for manufacturing and international trade.

Clearly defined specifications

Globalization has greatly increased the need for GPS standards that can express complex geometrical requirements and relationships unambiguously. Tribal knowledge within a company, and lunchtime discussions between design, manufacturing and quality staff, can no longer be used to fill in the product information that is not captured in technical drawings.

Modern manufacturing requires a robust specification language in which tolerances are well defined and not open to interpretation. Unambiguous specifications allow the tolerance values to be larger, while still ensuring product function. This reduces the cost of the product and gives technically sophisticated companies a competitive advantage.

In response to these market factors, the GPS language, as defined in various ISO International Standards, has developed over the past couple of decades. It has evolved from a few self-contained documents written for craftsmen, to a large set of complex and interrelated documents written for, and by, engineers and mathematicians.

Rule-based standards

The format has changed from example-based standards to rule-based standards. Example-based standards are appealing and appear easy to read and understand, because they often omit or hide complexities. The shortcomings of the examples only become evident when they have to be applied to specify real products that are typically much more complex.

At first glance, modern GPS standards may therefore appear unnecessarily complex and difficult to read. However, this complexity is necessary to provide a sufficiently rigorous theoretical basis for the specifications to be unambiguous, even when work pieces have significant geometrical deviations.

Making GPS user-friendly

It is quite difficult to start learning GPS because the information and rules are spread out in many standards. This means there may be fundamental rules written in one standard that have to be known in order to apply another standard correctly.

In many ways we can compare GPS standards to laws. Both are written in a language and a style that are very precise and unambiguous to the trained reader, but which novices may find difficult to understand.

New ISO Handbook on GPS

The ISO Handbook is the result of collaboration between: Danish Standards; the secretariat of ISO technical committee ISO/TC 213; Dimensional and geometrical product specifications and verification, the technical committee that develops GPS standards, ISO, and the Chair of ISO/TC 213.

Presenting the basic rules clearly and using colour illustrations, the handbook provides an introduction to GPS, the background required to start using GPS, and guidelines on reading and applying GPS standards.

The handbook is partly intended to serve as a text book in technical schools and universities. However, it can also be used for self-study and as a post-study reference.
It aims to give the reader sufficient knowledge to read and interpret GPS drawings. The handbook will also enable readers to have enough "vocabulary" and knowledge of the grammar to express geometrical requirements to a component as correctly formulated GPS requirements.

Easy to understand

Continuing the law analogy, when a person learns to drive, he or she is not required or expected to learn the rules of the road by reading the traffic laws directly. Instead, the student driver is given a driver's manual that presents the important points in the traffic laws in an easily understandable format.

In some ways, the ISO Handbook is like a driver's manual for GPS. It provides an overview of the basic rules of GPS in one place and in a structured format. It goes on to explain how tolerances are indicated and specifications interpreted using simple, straightforward language, enabling the reader to start using GPS.

Armed with the overview and understanding provided by the ISO Handbook, the user can then read individual GPS standards and appreciate how they fit into the overall GPS rules complex, without getting lost in the process.

The ISO Handbook also provides a possibly even more important benefit. In another analogy, we can think of the GPS standards as a combined dictionary and grammar guide. Those two alone do not explain to an aspiring author how to write a good story. Similarly, the designer tolerancing his or her design will find little guidance in the ISO standards on how to apply GPS tolerancing in a systematic manner.

The handbook presents the IfGPS eight-point design procedure, which is a structured approach to applying GPS tolerances to a design. This provides an easy to follow step-by-step process which guides the designer's thought process and allows him or her to create drawings in a distinct GPS dialect. Such drawings are easily recognizable for readers in manufacturing and quality who are familiar with the procedure.

Making the designer's thought process obvious to those who have to make the product a reality helps avoid misunderstandings and improve the chances of manufacturing it cost effectively.

I hope universities and technical schools will adopt the ISO Handbook to teach engineers and technicians GPS. I also hope manufacturing companies will use it for internal training to improve the common understanding of GPS among their technical staff.

The ISO GPS Handbook is published as a cooperation between Danish Standards and ISO, the International Organization for Standardization. It is about 370 pages in length and can be ordered from ISO, Danish Standards or your national standardizations organization.