Geometrical Tolerancing

Geometrical Tolerancing is a system that allows tolerances to be applied to features rather than dimensions on engineering drawings and CAD models. It allows different aspects of form, orientation and location to be toleranced independently of each other. This allows tolerances to reflect the functions and relationships that the designer intends. Advantages include:-

  • productivity can be improved without relaxing tolerance requirements
  • tolerance zones can often be enlarged
  • form requirements like roundness and flatness can be toleranced
  • relationships like coaxiality can be toleranced
  • complex surfaces can be toleranced far more effectively
  • drawings are less ambiguous and can even be simplified

Geometrical tolerancing is no longer a special tool to be dusted off for awkward geometry and high precision requirements. It is a better way of applying tolerances to all features, and is central to the Geometrical Product Specification methodology.

The greater flexibility and precision of this system allows engineers and designers to document design intent more accurately and more completely. An understanding of this system is increasingly a contractual requirements, and manufacturing organisations cannot afford to remain in ignorance of it.

Five common misconceptions about Geometrical Tolerancing:

1.  We’ve always managed fine without it.

You may have got away without using it, but sooner or later you won’t. It is not technically possible to fully define component geometry without it, unless an extraordinary number of plain language notes are used.

2.  Geometrical tolerancing just makes drawings more complicated.

Drawings can look more complicated just because of a lack of familiarity with, and understanding of, the symbology.

Drawings can also look more complicated because, properly used, geometrical tolerancing will be controlling aspects of work-piece geometry which were previously not controlled at all. Drawings contain more annotation simply because they are more complete specifications.

Used correctly, geometrical tolerancing can make specifications simpler.

3.  Geometrical tolerances are more difficult to inspect.

Geometrical tolerancing, properly used, makes inspection easier.

It also removes ambiguity from the specification, so it becomes possible to give a more definite pass/fail result for a component.

4.  Use of geometrical tolerancing makes parts more expensive.

No it doesn’t. Suppliers who don’t understand geometrical tolerancing sometimes use it as an excuse to increase prices, but there is no justification for this.

Properly used, geometrical tolerancing enlarges tolerance zones and improves productivity. Its use should lead to a reduction in cost.

5.  Our suppliers don’t understand it, so there is no point in using it.

It is a fact that many suppliers do not understand geometrical tolerancing properly, just as in many cases their customers do not.

However, a properly defined workpiece is essential for practical and legal reasons.  No effective design analysis is possible if the component geometry is not correctly defined, and the risk of component and product failure is greatly increased.  No engineer would dream of replacing a material specification with a generic term like ‘metal’ or ‘plastic’ because a supplier didn’t understand the technical definition of a particular grade of alloy or polymer.  In the same way, no engineer should be willing to reduce the definition of a workpiece geometry to an oversimplified and highly ambiguous set of requirements just to make it easier for a supplier to understand.

Avoiding the use of geometrical tolerancing, and missing out on all its potential benefits, is not a long term solution for anyone.

Increasingly, companies are requiring their suppliers to demonstrate a degree of competence in the interpretation of mechanical specifications.  Suppliers are being assessed not just on their manufacture and quality control capabilities, but also on their ability to interpret geometrical tolerancing (training in geometrical tolerancing is now compulsory for all Jaguar Land Rover suppliers). In this way, the level of competence is slowly improving across the board, to the benefit of everyone.