This information provided by Bryan R. Fischer, of TDP360 LLC.

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There are two major systems of geometric tolerancing in the world today:

• ASME Geometric Dimensioning and Tolerancing (GD&T)

• ISO Geometrical Product Specifications (GPS)

Both systems provide rules, tools, and methods to define perfect geometry of parts and assemblies and the allowable geometric deviation on actual imperfect manufactured parts and assemblies. ASME GD&T and ISO GPS are both symbolic languages allowing users to define specifications using numbers, text, and symbols. ASME GD&T and ISO GPS look very similar. To an untrained observer, the specifications likely look the same. There are many subtle differences in drafting techniques and how information is presented, and there are prominent differences such as explicit statements on drawings that indicate that the ASME Y14.5 standard applies instead of ISO GPS standards.

ASME GD&T

ASME stands for The American Society of Mechanical Engineers, which is based in the U.S. ASME volunteers have been developing GD&T and the ASME Y14.5 standard since the middle of the 20^th century. GD&T is based on the ASME Y14.5 standard, thus, GD&T is backed by the rules and requirements defined in the standard.

ASME GD&T was developed in the U.S. predominantly for use by U.S. industry and their suppliers. However, ASME GD&T is used throughout the world, because many U.S. companies have global supply chains and because some companies outside the U.S. prefer ASME GD&T to ISO GPS. ASME GD&T is defined mostly in a single standard, ASME Y14.5 Dimensioning and Tolerancing. There are other standards that affect dimensioning and tolerancing, but ASME Y14.5 is the official GD&T standard – the other standards either present new methods or reinforce ASME Y14.5. Refer to the section on GD&T and Related Standards below.

ISO GPS

ISO stands for the International Organization for Standardization, which is based in Switzerland. ISO volunteers have been developing GPS and its standards since the middle of the 20^th century.

ISO GPS was developed by people from many countries, including the U.S. ISO standards have always been intended to be international standards, the default in most cases. ISO GPS is defined in more than twenty standards. Each ISO GPS standard is specialized. Whereas ASME GD&T is mainly defined in one standard, which covers all dimensioning and tolerancing topics, each ISO GPS standard specializes in one aspect of GPS. For example, ISO 1101 covers geometrical tolerancing, ISO 1660 covers profile tolerancing, ISO 2692 covers MMR and LMR, ISO 5459 covers datums and datum systems, ISO 8015 covers rules, ISO 14405-1 covers linear sizes, and many other specialized topics are covered by other ISO GPS standards. By contrast, most of these topics are covered in the ASME Y14.5 standard.

Units in ASME GD&T and ISO GPS

ASME GD&T

ASME GD&T clearly supports metric units (millimeters) and inch units (inches). Specifications using metric and inch units are clearly supported by the standard.

In ASME GD&T, different rules apply for leading and trailing zeroes and the number of decimal places in dimension and tolerance values for metric and inch units. See the section Format of directly-toleranced dimensions for deeper explanation and examples.

ISO GPS

ISO GPS clearly supports metric units but does not directly include information about specifications based on inch units. Specifications using metric units are clearly supported by ISO GPS standards, but it is unclear if ISO GPS standards support specifications using inch units. This is a major shortcoming for many U.S. based organizations, as inches are still used in many U.S. industries that use GD&T.

Pros and Cons of ASME GD&T and ISO GPS

There are pros and cons for GD&T and GPS, and much of it depends on your point of view and where you are in the world. ISO is accepted as the worldwide system of standards by many governmental bodies and business organizations. ASME standards are also accepted internationally, and their use, prevalence, acceptance, and relevance apply around the world.

ASME GD&T and ISO GPS are complex systems, as they are symbolic languages to define perfect geometry and the level of imperfection allowed for manufactured part geometry. Describing perfect and imperfect geometry using numbers, text, and symbols is abstract and thus seems complicated. However, once ASME GD&T and ISO GPS are understood, the requirements they define are crystal clear, rigorous, and unambiguous. The alternative to using ASME GD&T and ISO GPS is chaos. Part and assembly geometry defined by directly toleranced dimensions (dimensions with plus and minus tolerances) is not rigorous – they requirements it imposes on the product geometry are unclear and unambiguous.

ASME GD&T and ISO GPS are critically important for Model-Based Definition (MBD) and Model-Based Enterprise (MBE). ASME GD&T or ISO GPS must be used in MBD. In fact, ASME GD&T or ISO GPS must be used on model-based product definition data sets. If only plus and minus tolerances are used, manufacturing and inspection personnel must guess what the specifications mean and how they apply to the model geometry.

Recommendation: Use ASME GD&T or ISO GPS. Using plus and minus tolerancing alone may seem simpler and less expensive, but the lifecycle cost of the product, including scrap and rework, problems in assembly and installation, and replacement parts, far exceeds the additional training needed to understand ASME GD&T or ISO GPS.

Invoking Standards – Do ASME GD&T or ISO GPS Standards Apply?

To make sure that everyone using a drawing or annotated model knows whether ASME GD&T or ISO GPS standards apply, it is best practice to indicate which standard applies to the product specifications by explicitly invoking the applicable standard(s). Invoking a standard means that the standard and its rules and requirements apply to the product specifications, which in turn, means the specifications must be interpreted based on the rules and requirements in the standard. However, this best practice is not always followed.

The Invocation Principle in the ISO 8015 standard states that if product specifications include a GPS symbol, such as a geometrical tolerance symbol like flatness ▱, MMR Ⓜ, the ± symbol, etc., then ISO GPS standards apply to the product definition data set. This means that the rules and requirements defined in ISO GPS standards apply to the product specifications.

The way standards are invoked has evolved differently in different countries. For example, in the U.S., we require that the specific version of the ASME Y14.5 standard is explicitly invoked in the product definition data set. The specific version is based on the release date of the standard. We do this because the rules and requirements in the standard have evolved and changed over time. Invoking a specific version of the standard ensures that the product specifications are interpreted correctly based on the appropriate standard. We also do this to make it clear that the ASME Y14.5 standard applies to the product specifications. Countries that predominantly use ISO GPS standards, such as countries in the European Union, recognize ISO standards as the default international standards. They have used these standards for dimensioning and tolerancing for many years, and many have not explicitly stated that ISO GPS standards apply to their product specifications. This makes sense, as in many cases, many ISO GPS standards were initially based on standards that were developed in these countries. Which led to the Invocation Principle.

In response to the ISO GPS Invocation Principle, ASME developed and released ASME PDS 1.1. This standard states that if product definition data contains symbols from a US dimensioning and tolerancing standard, then the applicable US dimensioning and tolerancing applies to the product definition data set.

Given that ISO GPS and ASME GD&T use many of the same symbols, these default invocation rules are problematic, as it might not be clear which set of rules and requirements apply. Both sets of standards are well meaning and trying to ensure that the correct rules and requirements apply to product definition data, but without explicit invocation, confusion may occur. Hence, best practice, as recommended above, is to explicitly invoke the applicable standards.

Explicitly Invoking ASME Y14.5 or ISO GPS Standards

The ASME Y14.5 standard is usually explicitly invoked in a product definition data set. ISO GPS standards may also be explicitly invoked in a product definition data set. Often, a note is used to explicitly invoke the standard(s). The note may be placed in the general notes, in or near the title block, or in another location on an engineering drawing or annotated model, it may be defined in the management data or metadata for a product definition data set, or it may be other documentation or data related to the product requisition, such as a purchase order or company standard or specification.

Explicitly Invoking the ASME Y14.5 Standard

One of the first sections in the ASME Y14.5 standard states that if a design organization wants the ASME Y14.5 standard to apply to a product definition data set, a note or other method must be used to explicitly invoke the standard. The information invoking the standard must also indicate which version of the standard applies by including the year of the standard, such as ASME Y14.5M-1994, ASME Y14.5-2009, or ASME Y14.5-2018.

See below for examples of notes invoking the ASME Y14.5 standard.

• Dimensioning and tolerancing per ASME Y14.5-2018

• Interpret dimensioning and tolerancing per ASME Y14.5-2018

• ASME Y14.5-2018 applies to this product definition data set

• Geometric requirements defined in this product definition data set are in accordance with ASME Y14.5-2018

• The following standards are invoked: ASME Y14.5-2018

• Dimensioning_and_Tolerancing_Standard = “ASME_Y14.5-2018”
  Dim_and_Tol_Std = “ASME Y14.5-2018”
  Dim_and_Tol_Std = “ASME Y14.5-2009”
  (These are examples of a field in an information model that invokes the standard)

Explicitly Invoking ISO GPS Standards

The longstanding tradition of the Invocation Principle in ISO 8015 means that ISO GPS standards have historically not required explicit invocation of ISO GPS standards. However, with today’s global supply chains and multinational corporations, it is becoming more important to explicitly state that ISO GPS standards apply to product definition data sets based on ISO GPS standards. Additionally, it is also becoming important to indicate which version (date) of ISO GPS standards apply. Unlike ASME Y14.5, it is not a universal requirement to indicate which version of ISO standards apply to a product definition data set. There are several reasons for this: the invocation principle means that many countries that use ISO GPS do not explicitly invoke ISO GPS standards, and some countries expect that the most current version of standards apply to their product definition data sets, regardless of when the drawing or annotated model was released. For example, for a drawing released 25 years ago that includes ISO GPS symbology, the latest current versions of ISO GPS standards apply. In countries that use this approach, they do not explicitly invoke ISO GPS standards in their product definition data, and they do not explicitly invoke a specific version of the applicable ISO GPS standards. There are benefits and problems that come with this practice. I strongly recommend explicitly invoking the applicable dimensioning and tolerancing standards, whether ISO or ASME, and explicitly invoking the specific version of the standard. In this way, any confusion about which standards and which set of rules apply is eliminated.

See below for examples of notes invoking ISO GPS standards.

• Tolerancing ISO 8015

• Tolerancing per ISO 8015

• Tolerancing per ISO GPS standards

• Geometrical product specifications per ISO 8015

• ISO GPS standards apply to this product definition data set

• Geometric requirements defined in this product definition data set are in accordance with ISO GPS standards

• The following standards are invoked: all ISO GPS standards

• The notes above may be indicated in or near the title block.

• Dimensioning_and_Tolerancing_Standard = “ISO_8015”
  Dim_and_Tol_Std = “ISO 8015”
  Dim_and_Tol_Std = “ISO GPS”
  (These are examples of a field in an information model that invokes the standard)