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GD&T: Minding Mechanical Specifications Pays Off in Production

OEM Mechanicals

When you specify electrical components on a drawing, you use electrical specs.  Mechanical components need their own spec system that is repeatable, clear and not overly restrictive. The recent revision of the American Geometric Dimensioning and Tolerancing (GD&T) Standard, ASME Y14.5M-1994, is fact becoming the spec system of choice for mechanical components by today's OEMs.

Two reasons for this preference are:

  • Geometric tolerancing is the key to specifying mechanical parts that can be measured repeatedly.
  • The latest revision of Y14.5 is ninety percent compatible with the GD&T sections of the ISO standards, bringing OEMs more in step with the global manufacturing scene.
An International Language
GD&T is an international language used on drawings to accurately describe a part.  The language consists of a well defined set of symbols, rules, definitions, and conventions that can be used to describe the size, form, orientation, and location of part features.  GD&T is an exact language that enables designers to “say what they mean” on a drawing, thus improving product designs.  Production uses the language to interpret the design intent, and inspection looks to the language to determine set up.  By providing uniformity in drawing specifications and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the manufacturing process.

Cost Benefits
Because of the lack of studies demonstrating the cost benefits of GD&T, many upper management people, especially those without engineering backgrounds, lack the understanding needed to estimate actual savings. Consequently, they may be reluctant to fund GD&T training programs. The benefits of using GD&T to specify mechanical parts can readily be demonstrated when we compare GD&T to coordinate dimensioning, which has been in use for over 150 years. 

Here are seven advantages that add up to significant savings in production:

  1. The design philosophy of GD&T is that of functional dimensioning, which means that a part is defined by how it functions in the final product.  Instead of copying a tolerance from an existing drawing, the designer bases the tolerance on part function, this allows the maximum amount of tolerance to produce the part.  When properly applied, functional dimensioning can often double or triple the amount of tolerance on many component dimensions, which reduces manufacturing costs.  With coordinate dimensioning, tolerance zones are not related to functional requirements.  Problems can result when designers assign tight tolerances because they are not focused on determining a functional tolerance.
  2. GD&T allows round tolerance zones. In Figure 1, Arrow A points to a GD&T symbol which specifies a round tolerance zone (for the mount holes).  The zones specified by coordinate dimensioning (see Figure 2, Arrow A).  Round tolerance zones allow for 57% more tolerance than square zones, resulting in more usable parts.  By allowing more tolerance on parts, the process will be more capable, reducing manufacturing costs.
  3. In addition to the tolerance gained from using round zones, GD&T allows a “bonus” tolerance under certain conditions.  This bonus tolerance is gained by using the MMC (Maximum Material Condition) modifier, as indicated by Arrow B in Figure 1.  The MMC modifier allows a hole to have additional tolerance when it is produced larger than its minimum size.  This is a win-win situation for the OEM because engineering can be assured that the part will assemble when the holes are the smallest, and manufacturing can have additional tolerance when the holes are larger than their minimum size.  In coordinate tolerancing, the tolerance zone is always fixed in size (Figure 2, Arrow A), at all hole conditions.  This results in a number of functional parts being scrapped and a more stringent condition for manufacturing.  With bonus tolerance, parts that are more functional are used, and more tolerance is allowed for production, resulting in lower operating costs.
  4. GD&T's datum system clearly communicates one set up for inspection. Datums are theoretical planes, points, or axes, and are simulated by the inspection equipment.  The symbol used to specify a datum feature is shown on Figure 1, Arrows C.  These symbols denoted which part surfaces touch the gaging equipment during inspection.  Datum features are selected on the basis of part function and assembly requirements; they are often the features that mount and locate the part in its assembly.  Datum reference letters are specified (see Figure 1, Arrows D) inside the geometric controls and denote the sequence in which the part surfaces contact the gaging equipment.  This sequence is needed in order to have multiple inspectors set up the part in an identical manner.  In coordinate dimensioning datums are implied, allowing choices for set up when inspecting the part.  Different inspectors may get different results; some good parts may be scrapped and some bad parts may be accepted.
  5. GD&T reduces assembly problems. Since the inspection process with GD&T ensures the at parts will assembly properly, assembly methods no longer need to be addressed by the guy on the assembly line with a two-by-four and a hammer.  The inspection process with GD&T ensures that OEMs can use competitive souring or obtain multiple sources for the same part, resulting in increased profitability.
  6. In the area of inspection, GD&T supports the use of SPC. GD&T's Datum system provides the repeatable part measurements that are necessary for making a meaningful SPC chart. With coordinate tolerancing, SPC data may include assumptions, which reduce the accuracy of the data. The use of the profile control is another example of how GD&T supports SPC. The profile control helps in two ways:
    • It establishes a mathematically defined tolerance zone, and it relates the measurement to datums.  When coordinate tolerancing is used, the precise tolerance zone definition simply doesn't exist.  For example, try to define the size and location of the radius (Arrow B) in Figure 2.
    • Because the datum system and profile control allows SPC data to be more accurate, needless changes in the manufacturing process are avoided, regarding the OEM with time and cost savings.
  7. GD&T is supported by national and international standards. ASME Y14.5M-1994 and a series of ISO standards rigorously document the interpretation of each GD&T symbol and concept.  On the other hand, coordinate tolerancing is like folklore; it's not well documented, even though it has been around for 150 years.  Producing parts to GD&T's documented standards assures the OEM that parts will be accepted by the customer. Fewer replacement parts will be needed and recalls can be avoided, saving time and money.

The following summarizes the above comparison between GD&T and coordinate tolerancing:

With coordinate dimensioning, tolerance zones are not related to functional requirements. Problems can result when designers assign tight tolerances because they are not focused on determining a functional tolerance. (Click on the figure for larger view.)

Errors can be Costly
In order for OEMs to reap all the benefits of GD&T, it must be used correctly.  Using GD&T incorrectly is likely to result in errors on drawings, and such errors can be costly. For example, if a company uses drawings to decide which machines will be needed and what production rates will be, they may calculate product costs incorrectly. Misuse of GD&T can extend product-cycle time by causing manufacturing to tool up with the incorrect equipment. Errors may also force manufacturing to guess at the designer's intent and the finished product may function poorly. The more proficient a company is at using GD&T, the more cost-efficient it will be.

CAD Systems and GD&T Training
Many of today's CAD systems have GD&T capabilities, but the operator must still understand GD&T in order to use it effectively. While companies routinely send employees for three to six weeks of CAD training, they sometimes only allow two to three days for GD&T training.  Without the proper instruction in GD&T, an OEM will be unable to make full use of their investment in the CAD system.

Training Requirements
Just like learning a foreign language, GD&T requires learning, application and practice. Everyone involved with part drawings should be trained in GD&T so that they can recognize common errors. Many of today's engineering programs only offers a brief exposure to GD&T, with little emphasis on application. (Click on the table for a larger view.)

There are three levels of GD&T training: Fundamentals, Advanced Concepts, and Tolerance Stacks:

  1. The fundamentals of GD&T include the purposes and objectives of GD&T, its symbols, rules, and terminology. 
  2. Geometric tolerancing advanced concepts include how to select datums and how to solve real world problems. 
  3. Tolerance stacks includes using geometric tolerances in stacks to establish part tolerance and analyze product designs.

Design engineers need all three levels of training, consisting of sixty to eighty hours of interactive classroom sessions with testing, and six months of on-the-job training, followed by occasional refresher courses. Machine programmers, ship supervisors, and quality engineers need training in the fundamentals and advanced concepts for a total of forty to sixty hours of instruction, within six month period.  Sales, purchasing, and administration typically need twenty four hours of fundamentals training.

There are different ways that an OEM can get the necessary training. Instructors can come to the site, or an OEM can appoint their own moderator who will schedule training sessions via interactive videos. The third option is team self-study, a concept pioneered by the author in which individual team members use self-study materials to learn GD&T on a prescribed schedule and hold team meetings for discussion and testing.

Some community colleges offer excellent vocational education courses in GD&T, giving local OEMs a fourth training option.

Once training has taken place, the next step is for the designers to use GD&T on mechanical part drawings.  Independent quality audits, similar to those used in manufacturing, can find and reject bad drawings.  Engineering should strive for zero defects on all drawings, the way manufacturing does for parts.  A regular system of feedback, whether by report, memo, or meeting, allows problems to be discussed and solved.

Certification Available
Certification can provide OEMs with a way to verify that their second manufactures have the needed GD&T skills.  A certification program called the Geometric Dimensioning and Tolerancing Professionals (GDTP) [is available] from the ASME. 

Giving Your Company the Edge
With proper training and implementation, OEMs can use GD&T to reduce scrap, increase the percentage of usable parts, simplify inspection and assembly, replace fewer parts, avoid recalls, and increase efficiency. Minding mechanical specs on your drawings can give your company the edge over the competition in today's cost competitive OEM marketplace.

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 today.


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This file last modified 04/09/14