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7 Deadly Sins of Dimensioning and Tolerancing


by Alex Krulikowski

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Engineers use many on- and off-line quality control tools in their zeal to make it right the first time. These tools have their place, but the quest for quality must begin at the drawing board. Engineering drawings communicate part makeup and function to suppliers, manufacturing, and customers. For clear, unambiguous drawings, geometric dimensioning and tolerancing (GD&T) rules govern size, location, orientation, and form expressions for each part surface. GD&T leaves no room for ambiguity; it assumes the person reading the drawing has no knowledge of part function.

Unfortunately, dimensioning rules are violated frequently. These errors have appeared so often, and for so long, they are accepted without question by many drawing makers and users. Violations, however, are dangerous and expensive because they introduce ambiguity, multiple interpretations, and guesswork into the manufacturing process. Before a company can eliminate errors, it must be able to recognize them. These errors or deadly sins generally fall into seven categories.

 1. Improper use of the word "thru"
This is a dangerous word, unless it is used with absolute precision. It means one thing, and one thing only; namely, "all the way through." If through is not the intent, it is best to specify a depth.

2. Improper use of the word "central"
The question to ask before using this word is: Central to what? It may relate to any of several diameters. When using the word central, include a frame of reference or the dimension will just float on the drawing.

3. Unnecessarily tight titleblock tolerances 
To produce prints quickly, some designers use tight title block tolerances so fewer exceptions have to be thought through and noted elsewhere on the print. Manufacturing eventually bears the brunt of these false goals. People work harder than necessary, time and effort are wasted, and more deviations are asked for. Engineering time is consumed processing print-change requests.

4. Use of esoteric notes 
Many situations handled using notes would be better handled using geometric tolerances to eliminate any ambiguity.

5. Imaginary dimensions 
A dimension—for example, from a hole center line to an undefined vertical line—cannot be measured during part inspection. With GD&T, the dimension would be from the hole center line to a defined plane.

6. Dimensions without tolerances
These usually occur when there are many basic dimensions. A tolerance—for instance, for location—should be included with every basic dimension.

7. Missing dimensions 
This seldom happens with principal or obvious dimensions, but can when components are not thoroughly documented. Missing dimensions are particularly dangerous because inspectors are trained to inspect what is on the drawing, not what is missing. Missing dimensions may go undetected for years until there is a warranty problem or lawsuit.

GD&T violations cause many problems. For instance, if a company uses drawings to decide what machines will be needed and what production rates will be, they may calculate product costs incorrectly. Violations can also extend product cycle time by causing manufacturing to tool up with the incorrect equipment. Errors may force manufacturing to guess at the designer's intent, and a finished product may function poorly.

Incorrect costs, delays, and sub-optimal products are all expensive. Eliminating tolerancing errors can help a company decrease scrap, rework, changes, confusion, and downtime. To eliminate GD&Ts seven deadly sins, a company needs a combination of training, feedback, and quality audits. Everyone involved with prints should be trained to be conscious of common errors. Each part feature should be defined in terms of size, location, orientation, and form. Vigilance by trained people will help companies catch dimensioning errors before they cause real damage.

A regular feedback system, whether a report, memo, or meeting, will allow drawings and problems to be discussed and solved. Dimensioning errors often are not caught because companies do not communicate effectively. Designers send drawings out, and no one reports back about detected errors.

For errors on drawings in use, independent quality audits, similar to those used in manufacturing, could find and reject bad drawings. Engineering should retrieve for zero defects on all drawings, the way manufacturing does for parts.

Try the GD&T Potential Savings Calculator
The calculator is a tool that helps companies understand the amount of unnecessary expenditures each year due to employees not knowing how to correctly apply and interpret GD&T.

Where to find out more about GD&T

Effective Training Inc. is a world leader in the field of geometric tolerancing. ETI founder, Alex Krulikowski is an expert on geometric tolerancing, with a degree in industrial vocational education and over 30 years of industry experience. He has taught GD&T to thousands of students through classroom seminars, and to countless others through his books, self-study workbooks, videos, and CD-ROMs. 

ETI provides expert GD&T training with an emphasis on practical, on-the-job application. Onsite workshops include GD&T fundamentals and advanced concepts; tolerance stacks; statistical tolerance stacks; an ISO/ASME comparison; a GD&T overview; and solid model tolerancing.

Online training is also available at their ETI Learning Center. ETI's GD&T Trainer is a complete course in GD&T fundamentals available in single-user, multi-user or LAN software. 

 

With proper training and implementation, GD&T will help your company reduce scrap, increase the percentage of usable parts, simplify inspection and assembly, replace fewer parts, avoid recalls, and increase efficiency. Geometric dimensioning and tolerancing can give your company the edge over the competition in today's cost competitive marketplace, and Effective Training can provide the training and materials you need to reap those benefits.

If you’d like to discuss how geometric tolerancing will benefit your company, call 800-886-0909 or email info@etinews.com today.

 



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This file last modified 04/09/14