Geometric Dimensioning and Tolerancing (GD&T): Introduction

Last updated on 17th June, 2020

Whilst I was at university I remember the lecture in which we were introduced to Geometric Dimensioning and Tolerancing (GD&T). I distinctly remember thinking what on earth is the point in that! Well, fast-forward a few years and having learnt the hard way, I can tell you that it actually has some fantastic uses.

So wait, what is GD&T and how does it differ from what is normal dimensioning?

So what is GD&T?

Typically, people would default to linear tolerancing and then use GD&T on particular features. Sometimes this is the correct way of working and sometimes its not. Take the simple example below: a 20mmx40mm block. The left hand example is what most people would do. The right hand example is an alternative method to dimension this part using GD&T. Its worth noting that these are not the same requirements – but we’ll get into that later.

GD&T is a method of dimensioning a part or component both in terms of its size and form. It is more heavily featured in ISO drawings then other standards such as ASME since ISO is an “independent system” – but we will cover that later. GD&T lends itself to being a common technical drawing language which allows parts to be manufactured all over the world regardless of where it was designed.

Is a 50p coin round in GD&T?

Lets take another example that most people can relate to. Image you specified a diameter of Ø27.3mm+/-0.15 on an ISO drawing. You would expect a fairly exact round part which you could put your callipers across and measure. Well, you would be wrong. I can tell you that if you were to measure a 50p coin, it would measure a diameter of Ø27.3mm across any of its opposing edges! That’s right, it would happily fall within your drawing tolerance and be accepted as a part despite not being round. Now yes, generally you would specify something like ISO 2768 or an envelope principle on your drawing to avoid this. But you hopefully this gets the point across!

The key piece of information to get across is that that ISO standards adopt an “independent system” which means that a the form and size of an object are independent unless otherwise stated. By contrast, the ASME system is a “dependant system” which means that an objects form is controlled by the size requirement. In our 50p coin example, an ASME drawing would reject the 50p coin since it was not round. Whereas an ISO system would accept this part since no requirement of roundness had been specified.

So, why would you ever want to adopt an independent system? Well, the simple answer is flexibility. It maybe slightly more complex to correctly dimension a drawing, but an independent system gives you incredible power to specify exactly what is needed. It gives you the ability to describe the function of the part to both the manufacturer and the inspection department. In turn, this should highlight critical areas of the design and help to make parts cheaper.

Features of size (FOS) and non-features of size (NFOS)

In ISO drawings, an important concept is that of a “feature of size (FOS)” and therefore a “non-feature of size (NFOS)”. A FOS is a feature on a part which can be defined by a linear size. This breaks down into 3 types of feature:

• 2 Flat parallel surfaces with the same extent: facing each other or face away from each other
• Cylindrical – both internal and external
• A sphere

A simple way to remember the difference between a FOS and NFOS is can it be measured with a simple set of hand callipers*? If the answer is yes, then generally it is a FOS. If the answer is no, then generally it is a NFOS.

*Not including the depth gauge 😉

These types of feature can be defined using only a size requirement – i.e. the left hand method of the diagram above. All other features (NFOS) should be defined using GD&T.

All “Non-Features Of Size”should be defined using GD&T

Check out the below image for both correct ways to dimension some FOS and some incorrect ways to dimension NFOS. How many times have you seen something similar on drawings?

Those with a keen eye might notice that I originally highlighted the 20+/-0.1mm width as a FOS and therefore could be dimensioned with a size tolerance. In fact, it is not a true FOS since the surfaces are not parallel over the full extent. This is because the diameter on the left hand side (60mm+/-0.1) is larger than the one on the right hand side of the width.

The ‘Envelope Principle

Although a little tricky to initially get your head around, the Envelope Principle (or envelope requirement) is designed to ensure assembly of components. The simplest example to imagine is that of a dowel (shaft) aligning on a plate with a hole.

The envelope requirement means that the feature shall not interfere with a perfect mating part at the maximum material condition. In the case of our dowel, it should fit into a perfectly straight hole with a diameter of the upper tolerance of the dowel. In the case of the hole, it should always be able to accept a perfect straight dowel with a diameter of the lower tolerance of the hole.

The geometric tolerance symbols and structure

Geometric tolerancing has a set method for referencing the tolerance type, size, modifiers and datums. Here is an example with each component explained:

1. The tolerance symbol [required]. In this example it is a positional tolerance. Check out the table below for a complete list of tolerance types.
2. The tolerance zone size [required]. Here we have a diameter symbol which indicates that the tolerance is cylindrical with a diameter of 0.5mm. We also have a maximum material modifier here.
3. The primary (1st) datum. Again, we have the maximum material modifier which has been applied to datum A.
4. The secondary (2nd) datum. Here we have an example of a common (or compound) datum which is made up of datums B and C.
5. The tertiary (3rd) datum. In this case, datum D has been used.

Note, that items 3 through 5 aren’t always required. You do not need any datums if using flatness, straightness, roundness or cylindricity. In a similar note, modifiers (like the maximum material modifier used above) are not always required.

There are 14 types of geometric tolerance that can be used. I’ve tried to summarise the table symbols below and I will try to add more information and examples to each in time.

Symbol	Name	Datum Required
	Position	Yes*
	Surface Profile	Yes**
	Line Profile	Yes**
	Concentricity	Yes
	Symmetry	Yes
	Parallelism	Yes
	Perpendicularity	Yes
	Angularity	Yes
	Flatness	No
	Cylindricity	No
	Straightness	No
	Roundness	No
	Circular Run Out	No
	Total Run Out	No
*Except where the positional feature is being used as the primary datum **Optional but must be defined with TEDs.

Final Thoughts

Hopefully I’ve helped you get a taste for what GD&T is about and some of the fundamental concepts which are involved. If you want more information then check out some of my other posts on #TechnicalDrawing or here’s some of my favorite references:

• S., H., 2012. The Iso Geometrical Product Specifications Handbook – Find Your Way In Gps. Iso.
• GD&T Basics Website

Please share, like, comment or contact me if you have any questions on anything in this article.