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Technisches Magazin Ausgabe 1 | 2021

Gili Omri, GD&T and Tolerance Analysis Expert | Founder @ TES-RnD

ISO GPS / ASME GD&T Your Tools to success

Geometric Product Specifications with ISO 14638:2015 (GPS) or ASME Y14.5 (GD&T), are the tools to mitigate fit & function risks of mechanical parts. The following article shows a working method how you can reduce costs and risks in design and manufacturing.

ISO GPS or ASME GD&T (Geometric Dimensioning and Tolerancing) are international, concise and precise symbolic languages developed for the express purpose of documenting design intent, to detail the functional requirements of a manufactured assembly part and to ensure the compatibility and interchangeability of those parts.

GPS/GD&T are the tools to define functional specifications for size, shape, orientation precisely and clearly. They provide the links between theoretical 3D models from CAD/CAM (Computer Aided Design and Manufacturing) and CAI (Computer Aided Inspection), which defines the position of the elements of a manufactured part in the real world.

Why adopt GPS or GD&T as a Work method?

The ISO GPS / ASME GD&T usage:

  • Improves the quality of the design, production and control processes and preserves the organization’s knowledge base.
  • Reduces costs by shortening the production cycle time as well as hidden costs.

  • Improves communication between company departments (design, production, control) through a uniform and common language. A uniform understanding among all affected organizational functions also reduces the necessary coordination between suppliers and manufacturers to explain the design intent by approx. 50 percent.

  • Allows the manufacturer to focus on functional constraints in the design and eliminate tolerance specifications where they are not really needed. As a result, the production and control processes are optimized.

  • Allows the functional inspection of a single item (geometric requirements reflect the functional interfaces and constraints).

  • The number of changes in drawings can be reduced by performing the correct tolerance analyses, creating compatible drawings and setting the correct tolerance values.

"TES R2GS" method

The "Requirements to Geometric Specification method", referred in short R2GS, developed by Gili Omri, is a straightforward, practical workflow. Aligning product level functional requirements to clear unambiguous geometric specifications, utilizing either ISO-GPS or ASME GD&T Geometric Dimensioning and Tolerancing Standards.

How to Document and Interpret geometric product specification

The geometry of the mechanical design solution (CAD model) and overall variation limits (tolerance budget) are derived from the primary product requirements. To ensure the functionality of a mechanical system, each part should meet the designated geometric specifications.

RQMTFigure 1: Design intent with GPS differs for significantly influenced functions


R2GS outlines the path from product requirements to geometric specification of parts in a three step process:

  1. Datum System:
    - It defines the functional reference frame of the parts and simulates the physical assembly conditions in which the parts are arranged and aligned.
     - It relates the reference elements in the order of the datum features to satisfy the functional aspect and specifies the orientation and location of the tolerance zones.
  2. Key Features:
    Located to Datum System for establishing geometric variation limits to all features that significantly affect product function, assembly, safety or the part manufacturing process.
  3. Non-Key Features:
    Located to the reference frame, set default limits for geometric variations for all other features (may be driven by manufacturing process).

Let's see how it works: “TES-R2GS” Example


Figure 2

  • The screw base part (yellow part in Figure 1)
  • Part thread advances the clamp drive screw

Primary Datum System

Datum System depicts functional constraints, arresting rotation & translation degrees of freedom as in a real-world assembly.

Datum Features are controlled relative to each other in the functional order of precedence!


Figure 2

  • Primary Datum Feature A: mounting face, has only form deviation controlled by flatness

  • Secondary Datum Feature B: should be flush to table, controlled by perpendicularity orientation to A & form

  • Tertiary Datum Feature C: part symmetry plane, perpendicular to A & B

Key Features

Key Features are critical features to the parts function.

In this case, the M8 driving thread and the two socket head counter boreholes.

F3Figure 3

Driving threads location variation is controlled by position relative to relevant datum system. The Socket Head Holes are controlled in the same manner.

Non-Key Features

All features that are needed (otherwise they would not be modelled or fabricated), have a secondary effect on part function. Non-Key Features are most likely to be controlled by a general profile tolerance related to the main datum system.


Figure 4

CENIT and TES-RnD joined together to promote the proper use of ISO-GPS/GD&T as a cross organizational common language and to enhance focus on functional requirements and constraints.
CENIT and TES-RnD deliver the best solutions and training programs to establish in-depth understanding of ISO-GPS/GD&T principles & proper utilization of geometric tolerances & symbols. The trainings and consulting will help organizations to capture knowledge (design intent), optimize design, improve communication, reduce product development cycle time, and decrease product costs.