Effective Training Inc., an SAE International company Livonia,  MI. (800) 886-0909  
Volume 03: Issue 1

ETImail is an online publication devoted to geometric dimensioning & tolerancing featuring a host of GD&T resources and links, as well as dimensioning tips by noted GD&T author and ETI founder, Alex Krulikowski.


Feature Article
Minding Mechanical Specs Pays Off in Production Savings


Alex's Tech Tip
How to Select Datum Features on a Part

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Recognizing the need is the primary condition for design.

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SAE World Congress 2016

Minding Mechanical Specs Pays Off in Production Savings 

Alex Krulikowski 

When you specify electrical components on a drawing, you use electrical specifications. Mechanical components need their own specifications (or specs) using a system that's repeatable, clear, and not overly restrictive.

ASME Y14.5-2009 is the preferred dimensioning system for mechanical components used by most OEMs. Two reasons for this preference are:

  • Geometric dimensioning and tolerancing (GD&T) is the key to specifying mechanical parts that can be measured repeatedly;
  • The 2009 revision of Y14.5 is 70-80% compatible with the ISO geometrical tolerancing standards, bringing OEMs more in step with the global manufacturing scene.

An International Language
For those still unfamiliar with GD&T, geometric dimensioning and tolerancing is an international language used on drawings to accurately describe a part.

GD&T 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 setup.

By providing uniformity in drawing specification 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 professionals — 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 used for well over a century.

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

#1. Design Philosophy

The design philosophy of GD&T is that of functional dimensioning -- a dimensioning approach that defines a part based on its function in the final product.

Instead of copying a tolerance from an existing drawing, the designer bases the tolerance on what is needed for proper function. Assignment of dimensions and tolerances based on part function 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.

Click on the figure below to see an enlarged view.

#2. GD&T Allows Round Tolerance Zones

Arrow A shown in Figure 1 points to a GD&T symbol that specifies a round tolerance zone (for the mounting holes). The zones specified by coordinate dimensioning result in a square tolerance. See Figure 2, Arrows A.

Round tolerance zones allow for 57% more tolerance than square zones, resulting in more usable parts. The added part tolerances allow the process to become more capable, reducing manufacturing costs.

#3. "Bonus" Tolerances = Hidden Savings

In addition to the tolerance gained from using round zones, GD&T allows a "bonus" tolerance under certain conditions.

This bonus is gained by using the maximum material condition (MMC) 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 at all hole conditions. See Figure 2, Arrow A. This results in a number of functional parts being scrapped and leads to a more stringent condition for manufacturing.

With bonus tolerance, more functional parts are used, and more tolerance is allowed for production. Both benefits result in lower operating costs.

#4. The Datum System

GD&T's datum system communicates a concise set up for inspectors.

Datums are theoretical planes, points, or axes simulated by the inspection equipment. Figure 1, Arrow C shows the symbol used to specify a datum feature. This symbol denotes the requirements that dictate which part surfaces touch the gaging equipment assembly; they are often the features that mount and locate the part in its assembly.

Datum reference letters are specified inside the geometric controls and denote the sequence in which the part surfaces contact the gaging equipment. See Figure 1, Arrows D. This sequence is necessary in order to have multiple inspectors set up the part in an identical manner.

#5. GD&T Reduces Assembly Problems

With GD&T, the inspection process ensures that parts will assemble properly, which means that assembly methods no longer need to be addressed on the assembly line. OEMs can use competitive sourcing for the same part, which results in increased profitability.

#6. Statistical Process Controls

In the area of inspection, GD&T supports the use of SPC through the use of the datum system, which provides the repeatable part measurements necessary for making a meaningful SPC chart. With coordinate tolerancing, SPC data may include assumptions that reduce the accuracy of the data.

GD&T also supports SPC though the use of the profile tolerance. The profile tolerance helps in two ways: it establishes a mathematically-defined tolerance zone, and it relates the measurement to datums.

When using coordinate tolerancing, the precise tolerance zone definition simply doesn't exist. For example, try to define the size and location of the radius of Arrow B in Figure 2.

Because the datum system and profile control allow SPC data to be more accurate, needless changes in the manufacturing process are avoided, rewarding the OEM with time and cost savings.

#7. GD&T is Supported by International Standards

The ASME Y14.5-2009 Standard 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 over a century.

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.

Send comments about this article to: ETImail
Your opinions will be posted in the next issue.



Do you know if ASME plan to incorporate target holes in the new version of ASME Y14.5M-200x? We have target area, line & point. Some companies use target hole. 

I have not heard any discussion on target holes at the Y14.5 meetings. Typically, targets are considered gage elements that are used as datum simulators to establish datums. Datum targets are often located with basic dimensions to ensure gage to gage repeatability. The point lines and areas can all be used on cylindrical shapes (including holes). 

If you feel there is a need for a new type of datum target, I encourage you to write a proposal and send it to ASME for consideration by the Y14.5 Committee. When submitting a proposal to ASME, it may take 3 to 10 years for the new concept to appear in the standard. 

Also, if you submit your proposal to ETI's Linkedin User Group, you could get comments from knowledgeable people from other companies, resulting in quicker feedback and potentially gain additional support from industry pros. You may want to use this method to refine your proposal before submitting it to the ASME Committee. 

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Send your GD&T questions to: ETImailbag.

Other than the simplest parts, most parts will have more than one datum reference frame. This often means the part will have many datum features, and deciding which surfaces to use as datum features is always controversial.

There are two common — but conflicting — approaches used in industry: the manufacturing/inspection approach; and, the functional approach.

In the manufacturing/inspection approach, engineers believe the largest part surface should be the primary datum, the second largest surface should be the second datum feature, and the third largest surface should be the tertiary datum feature.

In the functional approach, engineers believe that datum features should be functional. Here are my thoughts on how to use the functional approach to select datum features on a part.

Using the functional approach, I categorize all datum features into two types: mounting datum features; and, performing datum features. Mounting datum features are the datum features that orient and locate the part in an assembly. Performing datum features are the part features that are used to define functional relationships.

Using mounting datum features to establish the principle datum reference frame ensures that the part is measured the way it is held in the assembly. As a result, the dimensional measurements are related to the way the part functions in the assembly.

The Mounting Datum Feature Rule:
The features that orient and locate the part in the assembly will be the principle set of datum features on the part.

From teaching ideas to new products that will assist you in training or on the job, the ETImail Tech Tip will keep you informed about new technology and ideas.

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Share your comments about current or archived articles in ETImail. All comments will be posted as a forum for more discussion about GD&T topics.

Reader response to "The 9 Rules of Composite Position Tolerancing"
I found your condensed rules for Composite Position Tolerancing helpful and interesting. It has prompted me to open up my [Fundamentals of GD&T '94] book and read the section. You obviously have spent quite a bit of time with [the subject] to be able to boil down the contents to your Nine Rules. Thanks so much -- David C. Lemke

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