Mastering Geometric Tolerance: An Essential Guide

Table of Contents

What is Geometric Tolerance

Geometric tolerance refers to the permissible deviation specified for the shape, size, orientation, and positional relationship of an object, as defined in ISO standards on geometrical product specifications (GPS). Geometric characteristics represent attributes such as shape, size, and orientation, tolerance indicates the allowable error. Geometric tolerances encompass not only size but also define permissible errors in shape and position.

Difference between dimensional tolerance and geometric tolerance

Design drawings typically employ two description methods: dimensional tolerances and geometric tolerances. Dimensional tolerances specify the lengths of various parts, while geometric tolerances regulate shape, parallelism, tilt, position, and deflection.
geometric tolerance vs. dimensional tolerance

dimensional tolerance

dimensional tolerance

geometric tolerance

geometric tolerance

geometric tolerance: The parallelism of the indicated surface (A) during machining should not exceed 0.02.

Advantages of geometric tolerance

Geometric tolerances are essential for accurately communicating the designer’s intentions, which dimensional tolerances alone cannot achieve.

For instance, simply using dimensional tolerances in ordering plate parts may result in receiving unsuitable or defective parts due to oversight in specifying parameters like parallelism.

geometric tolerances types

In contrast, drawings featuring geometric tolerances, along with size, include geometric specifications such as parallelism and flatness. This comprehensive approach helps prevent issues arising from relying solely on dimensional tolerances.

geometric tolerances

a: parallelism tolerance, b: flatness tolerance, c: primary datum

Principle of independence

Dimensional and geometric tolerances specify different parameters: dimensional tolerances regulate length, while geometric tolerances govern shape and positional relationships. Both are measured using different instruments and methods, necessitating a combined approach for efficient tolerance indication.

Dimensional and geometric tolerances-benchmark
Dimensional and geometric tolerances in parts

The principle of independence, as defined by ISO standards, underscores that tolerance specifications indicated in drawings are independent of other dimensions, tolerances, and characteristics unless specified otherwise. While ISO adheres to the independence principle, some organizations, like the American Society of Mechanical Engineers (ASME), may follow different guidelines. Hence, when dealing with international partners, clear specification through prior discussions is advisable.

Picture of Chris Lee
Chris Lee

This article was written by Chris Lee, who has over 10 years of experience in the manufacturing industry. Before joining SogaWorks, he was responsible for process management of CNC machining, sheet metal fabrication, injection molding and other processes.

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