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GD&T: Working Through the Misconceptions

By Doug Keller | Published March 5, 2014


This article is a continuation of my previous post, “Variation in Production and Why GD&T is Superior to Coordinate Tolerancing”.

 

Geometric Dimensioning and Tolerancing (GD&T) eliminates the trial and error approach to product design.  It should be viewed as a design methodology ensuring dimensions and tolerances are developed to suit feature function and mating relationships.  The following is a short list outlining the benefits of using GD&T:

  • Eliminate guessing at product tolerancing, resulting in fewer prototype iterations and drawing revisions.
  • Specify the maximum available tolerance for manufacturing while protecting feature function.
  • Provide clear instructions for quality control by using established rules governing part setup and tolerance zone interpretation, ensuring repeatable measurement results.
  • Be assured that functional parts pass inspection and non-functional parts don’t.
  • Identify the exact rework requirements when manufacturing produces an out of specification feature.
  • Take advantage of additional (or bonus) tolerances for increased part acceptance.
  • Use functional gages to determine part acceptance, which can in many cases reduce inspection time and reliance on highly skilled inspectors.
  • Control machine allowances, wall thickness and interface dimensions on castings and fabrications.
  • Install large and heavy components without rework and with reduced reliance on adjustment.
  • Document design information and enable tolerance analysis accounting for all geometric variation.
  • Develop multiple sources of supply by providing a complete product definition with a single dimensional interpretation.

 

Given all these benefits, why are some engineers still reluctant to adopt GD&T?

Unfortunately many mistakenly see GD&T as making tolerances tighter, parts more expensive, or that GD&T is not applicable to their product and generally fail to see value in its implementation.  Often used excuses include:

  • GD&T is Complex,
  • My vendors don’t understand GD&T,
  • GD&T is only applicable to high volume product,
  • We’ve managed without GD&T so far,
  • GD&T is too difficult to learn.

 

I’ve heard many comments both for and against GD&T.  Those who are “for” GD&T have obviously seen benefits from proper application of tolerancing in their organizations.  The negative comments however also have some merit.  I believe it’s important to pay attention to, and understand why some are “against” GD&T.  Their opinions are based on negative experiences that are far too common in manufacturing.  Those against find it easier to blame GD&T than to consider the reasons why they have experienced negative results.  Misconceptions about GD&T come from:

  • Inexperience,
  • Lack of adequate training,
  • Improper or incomplete implementation.

 

GD&T itself is not complex.  Many product designs however are, and this gets reflected in the tolerancing specifications.  If your parts are simple, the GD&T is simple.  On the contrary, parts with many complex interrelated features require a precise language capable of describing allowable variation in size, form orientation and location relationships. It takes practice and experience to apply GD&T to complex parts.  Engineers must accept that as they begin to apply the concepts they will make some mistakes.  They will learn from these experiences and through analysis gain a better understanding how GD&T improves product design.

Lack of, or inadequate training compounds the problem.  Sometimes engineers guess at what the symbols mean rather than consult the standard.  Providing a solid grounding in the fundamental concepts through training helps engineers avoid errors improving the chances of successful GD&T implementation.  Just as with learning a foreign language; some learn enough to ask simple directions and can find their way to the hotel, say please and thank you, or order a beer; while others, with practice, learn to communicate more fluently and discover wonderful insights from the people they meet enhancing their touring experience.  Those that fail to study the basic rules or language structure speak unintelligible gibberish.  They may eventually find their way to the hotel, but only after a frustrating, round-a-bout and likely expensive taxi ride.

Sadly many engineers graduate with just a few hours of GD&T instruction and then enter the workforce to be mentored by those, who themselves, have little understanding of tolerancing.  It is left to individual companies to recognize the importance of tolerancing, and ensure appropriate training is provided.  Because of the aforementioned misconceptions, many don’t.

Tolerancing in general, let alone GD&T, is arguably the most misunderstood and underappreciated quality initiative in engineering and manufacturing today.  I use the term “tolerancing” because GD&T is only one component of a Dimensional Management (DM) strategy that is needed to understand the effects of manufacturing variation on the function, assembly, inspection and cost of producing a product.  It takes knowledge from many individuals, from all parts of the organization to develop a complete understanding of the effects of manufacturing variation.  The designer should not have to develop the tolerancing scheme in isolation.  A systematic approach is need to document requirements, analyse the design, establish part tolerances, monitor manufacturing processes and verify conformance.  Most manufacturers have some or all of these elements in place, but often missing is a DM strategy to coordinate and make effective use of the data collected.  Perhaps this is another reason some GD&T efforts fail?

A DM team is made up of representatives knowledgeable in the application of GD&T, from design engineering, manufacturing and quality control.  The DM team provides direction for the development of the product tolerancing scheme, focuses analysis efforts on the characteristics most likely to impact product performance, and coordinates the collection of measurement data.  This ensures effective implementation, maximizes the benefits of tolerancing efforts and creates a spirit of interdepartmental cooperation.

A company just beginning to use GD&T should start by providing training in the fundamentals of GD&T to all employees that need to interpret the engineering drawing.  Some employees such as design engineers will require advanced application skills and tolerance analysis training.  As well, the company should strive to develop expert representatives from design, manufacturing and quality control.  These are the individuals that will form the DM team, provide mentoring and resolve tolerancing issues.

Another important consideration is to ensure your DM team is qualified.  The only way to prove GD&T proficiency is to pass the American Society of Mechanical Engineers (ASME) test for certification as a Geometric Dimensioning and Tolerancing Professional (GDTP).  But that’s a topic for another day.  For more information on GDTP certification see https://www.asme.org/shop/certification-accreditation.

Let me leave you with a quote that I think sums-up why manufacturers should put more emphasis on using GD&T:

“Tight tolerance do not guarantee a quality part, only an expensive one” – Alex Krulekowski.

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Variation in Production and Why GD&T is Superior to Coordinate Tolerancing

By Doug Keller | Published March 4, 2014

Today’s engineers have at their disposal an impressive array of design tools enabling them to simulate, and design for many of the stresses and conditions that parts are subjected to throughout the life of a product.  Methods used by engineers range from hand calculations to sophisticated computer models.  The design process usually begins by creating models of parts and assemblies using Computer Aided Design (CAD) software.  Designers generate perfect 3D model representations of parts and assemble them into the virtual product using these CAD tools.

Unfortunately manufacturing cannot reproduce this perfection in real parts.  Even modern 3D printing technology cannot produce a dimensionally perfect part.  Manufacturing processes introduce dimensional variation, and this variation can have a significant effect on the performance of the assembled product.  In fact dimensional variation can make the difference between success or failure of a product.  Some factors that can cause dimensional variation include: 

  • Tool wear and machine calibration
  • Differences in material properties
  • Environmental changes such as temperature, humidity and dirt
  • Machine operator skill
  • Damaged or marred parts
  • Measurement uncertainty
  • Shrinkage and warping from heating, cooling and curing

 

Engineers need a means to determine how much the manufactured part geometry can be allowed to deviate from the perfect model geometry while ensuring parts will assemble and perform as intended in the product.  These deviations are expressed on the engineering drawing in terms of minimum and maximum allowable dimensional limits and are known as tolerances.  There are two commonly used methods for expressing tolerances on drawings: Coordinate Tolerancing and Geometric Dimensioning and Tolerancing (GD&T).

Engineers have recognized the effects of dimensional variation for a long time.  Coordinate (sometimes called plus-minus) tolerancing has been used on engineering drawings for over a century as a means of communicating the desired part fit requirements.  Coordinate tolerancing, as the name suggests, is based on an x, y, z coordinate system.  Linear dimensions with plus-minus deviations from nominal are used to express the allowable tolerance limits.

Coordinate tolerancing worked okay when the design engineer and machine operator were in close proximity.

Early manufacturing involved skilled craftsman and often a trial and error approach was used to fine tune processes to ensure components fit together.  As problems arose, the designer and the machinist could get together to work out a solution.  Much of the knowledge necessary to repeatedly build the product was not documented and existed in the memories of the workers.  Workers often stayed with the company for many years and would pass their experience of how to manufacture these parts onto their apprentices.

In the war years, part interchangeability and multiple sources of supply became increasingly important to keep vehicles, aircraft and weapons operating.  Mass manufacturing of consumer goods and outsourcing of parts gained popularity in post war years, highlighting the importance of documenting and communicating part requirements.  In recent decades cheaper production of higher quality products, and six sigma methodologies changed the way manufactures compete globally, and accurate measurement data became a central focus of manufacturing.  More recently complications arising from attempts to offshore manufacturing has forced many in the industry to rethink their supply chain strategies.  Communicating and managing product quality requirements globally has become an enormous challenge. Product geometry has become more intricate and there are a plethora of materials and manufacturing processes to choose from.  Production workers no longer stay in with the same company until retirement.  Coordinate tolerancing lacks the tools to fully define, document and communicate part requirements necessary for producing product using a global supply chain.  Times have changed, but many engineers still insist on creating part drawings using only coordinate tolerancing.

GD&T was developed to address the inadequacies of coordinate tolerancing.  GD&T allows you to specify exactly what you need for parts to function, relaxing tolerances, and controlling variations in size, form, orientation, location and profile.  Unlike coordinate tolerancing, tools are provided to control axial and symmetrical relationships and patterns of features.  There are even tools that allow additional variation or bonus tolerance while protecting feature function and maximizing part acceptance.  GD&T specified in accordance with the American Society of Manufacturing Engineers (ASME) Y14.5 Dimensioning and Tolerancing standard, establishes uniform practices for stating and interpreting dimensioning and tolerancing requirements on engineering drawings.  Using this standard ensures all specifications relay a single meaning, and are measurable.

GD&T ensures part interchangeability, allows the selection of less costly manufacturing and assembly methods, improves communication, and by helping designers account for the effects of manufacturing variation, improves design.  Parts that are properly defined and inspected using GD&T can be produced anywhere in the world and fit their using assemblies without rework.  Coordinate tolerancing on the other hand is based on assumptions and lacks the tools necessary to communicate functional requirements.

 

Check back soon for my next post, “GD&T – Working Through the Misconceptions”.

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The instructor is GDTP certified by ASME in accordance with the qualifications of ASME Y14.5.2 in the Senior level.