MECHANICAL DESIGN ENGINEERING – Why you MUST apply Geometrical Dimensioning and Tolerancing in your Product Design?

Mechanical Design is a subset of Material Science. Without understanding how materials behave you can never be a great mechanical design engineer. In the past (I would say before the year 1980) mechanical designers didn´t have the modern computer software (CAD platforms) like we do have today. Still these designers managed to create remarkable products because they knew how to create assembly from parts that fit together. The international norms and standards for that were also not very clearly defined like they are today, so it was a bit of hard word to be a mechanical engineer to create products drawings that can effectively communicate internationally. Luckily due to the computer engineering development in this area, things became much easy today and if you know where to look and how to use these modern tools, mechanical design engineering work can become really pleasant. You can successfully communicate your design intention in any part of the world, but you must learn how.


My journey in mechanical design engineering started in 2005, by that time CAD software were already available, so I started to learn how to use these modern tools too. Also international mechanical design engineering norms were clearly defined so that every mechanical engineer worldwide can understand how a product drawing must be made and what it must communicate. These norms for mechanical engineering drawings all together are exactly the international language of mechanical engineers. Is it called: THE GEometric Dimensioning and ToleranciNg.

This language is mandatory for every mechanical engineer worldwide. Before to learn any design engineering in-depth – such as using the CAD software- if you aspire to become a professional mechanical engineer then you must fluently speak the GD & T language. Only after you understand how GD & T works , you can successfully develop new products using any of the CAD software available.

If you only know how to use a CAD that´s means exactly nothing. You are not mechanical engineer only by knowing that. Catia, Solidworks, Creo, Siemens NX, Inventor, Fusion, Autocad, Solid Edge just to name the most widely used CAD softwares used today for mechanical engineering are actually just computes games. They are customized to assist you in your mechanical development work for new products, but they don´t do any mechanical engineering by themselves, all what these software do is to apply the algorithms based on inputs received from humans. That´s why they are called Computer Aided Design Software a.k.a. CAD.

If your product failS to work as expected is nEVER the CAD software failure, it´s always the human failure.

So , I say it again, if you wish to become a great mechanical design engineers learn to speak fluently GD &T language. Keep reading this post and in the next ones I will share in a lot more in detail how GD &T works. Absolutelly apply this on your future products.

So here is how it works.

When a hobbyist needs a simple part for a project, he might go straight to the little lathe or milling machine in his garage and produce it in a matter of minutes. Since he is designer, manufacturer, and inspector all in one, he doesn’t need a drawing. In most commercial manufacturing, however, the designer(s), manufacturer(s), and inspector(s) are rarely the same person, and may even work at different companies, performing their respective tasks weeks or even years apart.

A designer often starts by creating an ideal assembly, where all the parts fit together with optimal tightnesses and clearances. He will have to convey to each part’s manufacturer the ideal sizes and shapes, or nominal dimensions of all the part’s surfaces. If multiple copies of a part will be made, the designer must recognize it’s impossible to make them all identical. Every manufacturing process has unavoidable variations that impart corresponding variations to the manufactured parts. The designer must analyze his entire assembly and assess for each surface of each part how much variation can be allowed in size, form, orientation, and location. Then, in addition to the ideal part geometry, he must communicate to the manufacturer the calculated magnitude of variation or tolerance each characteristic can have and still contribute to a workable assembly.

For all this needed communication, words are usually inadequate. For example, a note on the drawing saying, “Make this surface real flat,” only has meaning where all concerned parties can do the following:

  • Understand English
  • Understand to which surface the note applies, and the extent of the surface
  • Agree on what “flat” means
  • Agree on exactly how flat is “real flat”

Throughout the 20th century, a specialized language based on graphical representations and
math has evolved to improve communication. In its current form, the language is recognized throughout the world as Geometric Dimensioning and Tolerancing (GD&T).

What is GD & T?

Geometric Dimensioning and Tolerancing (GD&T) is a language for communicating engineering design specifications. GD&T includes all the symbols, definitions, mathematical formulae, and application rules necessary to embody a viable engineering language. As its name implies, it conveys both the nominal dimensions (ideal geometry), and the tolerances for a part. Since GD&T is expressed using line drawings, symbols, and Arabic numerals, people everywhere can read, write, and understand it regardless of their native tongues. It’s now the predominant language used worldwide as well as the standard language approved by the American Society of Mechanical Engineers (ASME), the American National Standards Institute (ANSI), and the United States Department of Defense (DoD).

It’s equally important to understand what GD&T is not.

  • It is not a creative design tool;
  • it cannot suggest how certain part surfaces should be controlled.
  • It cannot communicate design intent or any information about a part’s intended function. For example, a designer may intend that a particular bore function as a hydraulic cylinder bore. He may intend for a piston to be inserted, sealed with two Buna-N O-rings having .010″ squeeze. He may be worried that his cylinder wall is too thin for the 15,000-psi pressure. GD&T conveys none of this. Instead, it’s the designer’s responsibility to translate his hopes and fears for his bore—his intentions—into unambiguous and measurable specifications. Such specifications may address the size, form, orientation, location, and/or smoothness of this cylindrical part surface as he deems necessary, based on stress and fit calculations and his experience.

It’s these objective specifications that GD&T codifies. Far from revealing what the designer has in mind, GD&T cannot even convey that the bore is a hydraulic cylinder, which gives rise to the Machinist’s Motto.

“Mine is not to reason why; Mine is but to tool and die.”

Finally, GD&T can only express what a surface shall be. It’s incapable of specifying manufacturing processes for making it so. Likewise, there is no vocabulary in GD&T for specifying inspection or gaging methods. To summarize, GD&T is the language that designers use to translate design requirements into measurable specifications.

Where Does GD&T Come From?—References

The following American National Standards define GD&T’s vocabulary and provide its grammatical rules.

  • ASME Y14.5M-1994, Dimensioning and Tolerancing
  • ASME Y14.5.1M-1994, Mathematical Definition of Dimensioning and Tolerancing Principles

The supplemental Math Standard expresses most of GD&T’s principles in more precise math terminology and algebraic notation—a tough read for most laymen. Internationally, the multiple equivalent ISO standards (DIN EN ISO 1101 & ISO 14660) for GD&T reveal only slight differences between ISO GD&T and the US dialect. Unlike computer software, the American National and ISO Standards are strictly rulebooks. Thus, in many cases, for ASME to issue an interpretation would be to arbitrate a dispute. This could have far-reaching legal consequences.

Why Do We Use GD&T?

When several people work with a part, it’s important they all reckon part dimensions the same. In the figure 1 shown below, the following situation happens:

  • 1st the designer specifies the distance to a hole’s ideal location;
  • 2nd the manufacturer measures off this distance and (“X marks the spot”) drills a hole; then
  • 3rd an inspector measures the actual distance to that hole.

All 3 parties must be in perfect agreement about 3 things:

  1. from where to start the measurement,
  2. what direction to go, and
  3. where the measurement ends.
Figure 1. Drawing showing distance to ideal hole location

When measurements must be precise to the thousandth of an millimeter, the slightest difference in the origin or direction can spell the difference between a usable part and an expensive paperweight. Moreover, even if all parties agree to measure to the hole’s center, a crooked, bowed, or egg-shaped hole presents a variety of “centers.” Each center is defensible based on a different design consideration. GD&T provides the tools and rules to assure that all users will reckon each dimension the same, with perfect agreement as to origin, direction, and destination. It’s customary for GD&T textbooks to spin long-winded yarns explaining how GD&T affords more tolerance for manufacturing. By itself, it doesn’t. GD&T affords however much or little tolerance the designer specifies. Just as ubiquitous is the claim that using GD&T saves money, but these claims are never accompanied by cost or Return on Investment (ROI) analyses.

A much more fundamental reason for using GD&T is revealed in the following study of how two very different builders approach constructing a house.

A primitive builder might start by walking around the perimeter of the house, dragging a stick in the dirt to mark where walls will be. Next, he’ll lay some long boards along the lines on the uneven ground. Then, he’ll attach some vertical boards of varying lengths to the foundation. Before long, he’ll have a framework erected, but it will be uneven, crooked, and wavy. Next, he’ll start tying or tacking palm branches, pieces of corrugated aluminum, or discarded pieces of plywood to the crude frame. He’ll overlap the edges of these flexible sidings 1-6 inches (25,4 to 152,4mm) and everything will fit just fine. Before long, he’ll have the serviceable shanty shown in Figure 2, but with some definite limitations: no amenities such as large windows, plumbing, electricity, heating, or air conditioning.

Figure 2. House built without all of the appropriate tools

A house having such modern conveniences as glass windows and satisfying safety codes requires more careful planning. Materials will have to be stronger and more rigid. Spaces inside walls will have to be provided to fit structural members, pipes, and ducts. To build a house like the one shown in next figure, a modern contractor begins by leveling the ground where the house will stand. Then a concrete slab or foundation is poured. The contractor will make the slab as level and flat as possible, with straight, parallel sides and square corners. He will select the straightest wooden plates, studs, headers, and joists available for framing and cut them to precisely uniform lengths. Then he’ll use a large carpenter’s square, level, and plumb bob to make each frame member parallel or perpendicular to the slab. As result he will end up building the house as shown in Figure 3.

Figure 3. House built using the correct tools

Why are such precision and squareness so important? Because it allows him to make accurate measurements of his work. Only by making accurate measurements can he assure that such prefabricated items as sheetrock, windows, bathtubs, and air conditioning ducts will fit in the spaces between his frame members. Good fits are important to conserve space and money. It also means that when electrical outlet boxes are nailed to the studs 12″ up from the slab, they will all appear parallel and neatly aligned. Remember that it all derives from the flatness and squareness of the slab.

By now, readers with some prior knowledge of GD&T have made the connection: The house’s concrete slab is its “primary datum.” The slab’s edges complete the “datum reference frame.” The wooden framing corresponds to “tolerance zones” and “boundaries” that must contain “features” such as pipes, ducts, and windows. Clearly, the need for precise form and orientation in the slab and framing of a house is driven by the fixtures to be used and how precisely they must fit into the framing. Likewise, the need for GD&T on a part is driven by the types and functions of its features, and how precisely they must relate to each other and/or fit with mating features of other parts in the assembly. The more complex the assembly and the tighter the fits, the greater are the role and advantages of GD&T.

Another example like the one the Figure 4, shows a non-GD&T drawing of an automobile wheel rotor. Despite its neat and uniform appearance, the drawing leaves many relationships between part features totally out of control. For example:

Figure 4. Manufactured part that conforms to the drawing without GD&T

-what if it were important that the Ø 5.50 bore be perpendicular to the mounting face? Nothing on the drawing addresses that.

-what if it were critical that the Ø5.50 bore and the Ø 11.00 OD be on the same axis? Nothing on the drawing requires that either.

In fact, Fig. 4 shows the “shanty” that could be built. Although all its dimensions are within their tolerances, it seems improbable that any “fixtures” could fit it.

In Fig. 5, I’ve applied GD&T controls to the same design.

Figure 5. Drawing that uses GD & T

The mounting face in now required to be flat within 0,05mm and then labeled it datum feature A. That makes it an excellent “slab” from which we can launch the rest of the part. Another critical face is explicitly required to be parallel to A within 0.03

The perpendicularity of the Ø5.50 bore is directly controlled to our foundation, A. Now the Ø 5.50 bore can be labeled datum feature B and provide an unambiguous origin—a sturdy “center post”—from which the Ø 0,5 bolt holes and other round features are located. Datum features A and B provide a very uniform and well-aligned framework from which a variety of relationships and fits can be precisely controlled.

Just as importantly, GD&T provides unique, unambiguous meanings for each control, precluding each person’s having his own competing interpretation. GD&T, then, is simply a means of controlling surfaces more precisely and unambiguously. And that’s the fundamental reason for using GD&T. It’s the universal language throughout the world for communicating engineering design specifications. Clear communication assures that manufactured parts will function and that functional parts won’t later be rejected due to some misunderstanding. Fewer arguments. Less waste.

As far as that ROI analysis, most of the costs GD&T reduces are hidden, including the following:

  • Programmers wasting time trying to interpret drawings and questioning the designers
  • Rework of manufactured parts due to misunderstandings
  • Inspectors spinning their wheels, deriving meaningless data from parts while failing to check critical relationships
  • Handling and documentation of functional parts that are rejected
  • Sorting, reworking, filing, shimming, etc., of parts in assembly, often in added operations
  • Assemblies failing to operate, failure analysis, quality problems, customer complaints, loss of market share and customer loyalty
  • The meetings, corrective actions, debates, drawing changes, and interdepartmental vendettas that result from each of the above failures

It all adds up to an enormous, yet unaccounted cost. Bottom line:

use GD&T because it’s the right
thing to do, it’s what people all over the world MUST understand, and it saves money.

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