The extent of my exposure to geometric dimensioning & tolerancing in college can be summed up with a quote from my teacher on the first day of drafting class, “GD&T is important and many of you will need to learn it eventually, but it will not be discussed here.”

Apparently my experience isn’t an uncommon one. According to a survey shared in this video clip on 100 technical schools, only 17% of ABET accredited mechanical engineering programs had it as part of their core curriculum. (Whereas 92% of technical schools with machining associates degrees did offer it.)

So for far too long I never learned how to use GD&T nor why I should care. Which is a shame because it is in fact a very useful tool for many mechanical engineers and because the ‘why’ is a fascinating lesson in itself.

Ask 2010 me what GD&T is and I’d probably say ‘it sounds a bit like pseudo-scientific junk invented so the in-crowd can sell expensive DVDs to suckers like me. What could GD&T possibly do that just adding more +/- tolerances won’t solve?  There’s like a million different symbols for no reason. Who came up with this junk anyway?’

(2010 Me:) So who came up with this junk anyway?:

Legend says a craftsman named Stanley Parker (who we know surprisingly little about) created the first GD&T concepts while working at a torpedo factory in Scotland during WWII.

He started questioning traditional tolerancing techniques after noticing a situation where torpedo parts failed to meet inspection criteria, but were actually functional parts.

Stanley traced the discrepancy to the fact that 2 point tolerances result in a square shaped zone of acceptability, but in reality parts can still function if the tolerance zone is expanded into a circle that encompasses the square’s corners. This additional area allows for 57% more area of acceptability.

By 1957 his idea had developed into a complete organized system of controls and a global standard. GD&T now has four core elements (size, form, location, and orientation) that are the basis for 14 primary feature control symbols such as parallelism, circularity, circular runout, and perpendicularity. So not a ‘million’ symbols after all.

(Image Source: www.gdandtbasics.com)

Forget Stanley & his torpedoes, what does MODERN GD&T do for ME?

Let me tell you a little engineering secret: The world is even more imperfect than you think.

Your assembly may fit together on the computer screen but as soon as you bring those parts to life all bets are off. The naked eye is a bad judge of subtle imperfections, but zoom in close enough and you’ll find that your flat surfaces are expanses of mountains and valleys. Your square cross sections are actually parallograms and your perfectly round hole is anything but.

Zoom out a bit and get your calipers ready. Better double check that 5mm pin you ordered, those jerks in the machine shop probably made it 4.99mm again. Will it still work I wonder? ‘Maybe’ is not an acceptable answer.

Above all else, GD&T quantitatively communicates an answer to, ‘how far off the CAD/perfect model can a part be and still function?’  GD&T also:

• Provides an unambiguous reference coordinate system.
• Makes clear how a part is to be inspected and tells how to constrain the part when making a measurement.
• Reduces the need for special notes & the use of qualitative requirements.
• Reduces time/cost spent achieving tight tolerances on unimportant part features.
• Enables the use of pass/fail inspection blocks (see maximum material condition)

OK OK, but why can’t I do the same thing by adding a lot more +/- tolerances on my drawing?

A series of 2 point tolerances doesn’t fully control the shape or location of part features. Take this simple drawing of a block with a hole, nothing ambiguous about there…right?

Well I’ll just machine that block for you real quick…Here ya go, now let’s inspect it to make sure it meets the spec. (Drawing shown below with distortions exaggerated for clarity…I’m not that bad a machinist!)

Oh no! I get different measurements depending on which side I square up first. The order of constraint seems to affect the measurement.

While the drawing clearly calls for a distance, it is not clear from where on the sides the dimension should begin. Even in a typical (2-point) fully toleranced drawing, you lack the ability to control the form, relative location, & orientation of features. That’s where things get interesting.

To solve this issue, GD&T calls for the use of datums. A Datum is a theoretical perfect point, line, plane, point-on-line, line-on-plane, or a point-on-line-on-plane that part features can be referenced to on a drawing. Just like the A & B faces on the block example above, datums are used to constrain degrees of freedom of a coordinate system.

When designing a part, imagine it slowly floating through outer space. Which way is it moving? Well there are only 6 ways it could possibly be moving. Three translational directions and three rotational directions. All 6 of these degrees of freedom must be constrained to mathematically apply or measure geometric tolerances.

Tip: Thinking about parts & assemblies in this rigorous way can help you design better parts.

As the designer, you get to pick which features you call the datums based on your knowledge of the parts function, and what’s reasonable from an inspector’s point of view. The trick here is understanding that constraining different feature types have different effects on the parts remaining degree of freedom. This chart helps explain it.

Those are the basics anyway, neat stuff I think.

Further Reading:

So am I saying you should cough up $1,000 for a set of training DVDs or something? No way!

Engineers by nature are great at self-teaching, and while GD&T looks intimidating, it is not so complicated that you couldn’t grasp the functional essentials after some YouTube videos and good a book.

If learning GD&T is essential for your job then your employer should pay to train you. And if a company cares that much about getting the GD&T totally perfect they’ll have a devoted guru. Nonetheless, if you design parts for manufacturing then you will still benefit from knowing the basics.

So for any readers interested in self-directing their GD&T education, I’ve collected these resources below:

*3D model Datum feature simulator:

This is a 3d printable tool for helping you learn/teach GD&T! (geometric dimensioning & tolerancing)
It comes with 3 lessons in pdf form.
GD&T_lesson1: What is a Datum Feature Simulator
GD&T_lesson2: How gd&t helps Engineers improve designs
GD&T_lesson3: Why Engineers Should Care to Learn GD&T

*Video 1: Importance of GD&T Training for New Engineers  (The other videos on this channel are great too.)

*Video 2: Beginning Engineers GD&T

*Two Wall Chart PDF references to keep on hand: this one here and this one here.

*A great free online read-through tutorial: CNC Cookbook Beginner’s Guide to GD&T Tutorial

*Test your knowledge Symbol interpretation quiz.

*An online PDF of the 1994 version of the GD&T ASME  Y14.5M standard. (Free because 2009 is the current revision)

*Recommended Book: GD&T Pocket Guide (the ~$25 price is right, ignore the expensive listings.)

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There you go. One article you can read in a single sitting to get you started on your GD&T education. That’s all they had to do. But not to knock my school too much for lacking this degree of an introduction. Like I said, it’s common. ( I just checked its current curriculum and GD&T is now a high level elective course. Good on them.) Also there are some branches of mechanical engineering that do not involve part design (such as HVAC) that don’t need GD&T at all to my knowledge.