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Leveraging GD&T for Inline Quality Control

Leveraging GD&T for Inline Quality Control

Geometric Dimensioning and Tolerancing (GD&T) is the language used by mechanical engineers to define and communicate tolerances on manufactured parts and assemblies.

How it Works

GD&T is a symbolic language used in engineering drawings to explicitly describe nominal (i.e., ideal) geometry and its allowable variations. This language enables manufacturing staff to program factory machinery to deliver the accuracy and precision required for each controlled feature of a part. 

Geometric Dimensioning and Tolerancing Symbols
Table with key GD&T tolerances, characteristics, and symbols.

GD&T is used to define:

  1. Nominal geometry of parts and assemblies

  2. Allowable variation in form and possible size of individual features, and;

  3. Allowable variation between features

The different GD&T geometry requirements can be expressed in five main tolerance categories: form, profile, orientation, location, and runout tolerances.
 

Metrology Applications of GD&T – Verifying Design

At present, GD&T is used primarily by metrologists to carry out 3D measurements on first articles of a part in order to verify features are meeting design intent.

This is usually done offline, in a clean, controlled laboratory environment that relies on contact-based solutions such as CMMs to acquire data of the part. Time is not critical here, with the emphasis instead placed on measurement precision and accuracy.
 

Bringing GD&T Inline – 100% Part Inspection

While GD&T has traditionally been reserved for offline applications, we have identified a strong market need for the use of GD&T in the inline production environment in order to provide 100% quality control (i.e., where every part on the line is inspected, not just the first article or a random sample).

Leveraging GD&T for inline inspection is a potentially game-changing solution to simplify the setup for quality control of parts by providing manufacturers with an international design language that meets ISO standards. GD&T is a natural quality standard for inline QC applications where specific geometric elements on parts or features must be verified to meet production specifications.

GD&T drawing
Example of a design drawing using explicit GD&T callouts.

The Challenge

The challenge of supporting GD&T in an inline environment is in acquiring a metrology-accurate 3D model of the part, in addition to meeting the fast cycle times required for inline production. Traditional offline solutions such as CMMs are unable to meet these requirements, which prevents GD&T from migrating to an inline setting.
 

The Solution –3D Smart Sensors with Built-In GD&T

LMI believes that building a user experience based on GD&T callouts using a 3D smart sensor would simplify quality control setup. 3D smart sensors are designed for metrology grade digitization and “smart” algorithm processing to deliver GD&T results.

As a result, the future would see process control engineers (not metrologists) set up a 3D smart sensor to carry out quality control on new parts by translating the GD&T tolerance data from drawings into digital callouts applied to scanned 3D surfaces. Dashboards could monitor and report on tolerance variation during production in a language that directly relates to the design drawings and support process engineers to tune and maintain 100% quality control.
 

Conclusion

As a solution, we believe GD&T analysis could be brought inline and used to examine the conformity of manufactured parts. LMI sees this as the future—a non-contact, automated inspection system that can easily apply GD&T for inline applications to help manufacturers monitor for part tolerances.


We invite you to download our Inline Metrology white paper. This paper explores the differences between lab-based metrology and industrial inline inspection solutions—and how 3D smart sensors effectively combine these approaches to deliver 100% quality control through inline metrology.

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