Top 5 secrets of GD&T every design engineer should know 


Apply these 5 tips for using GD&T on your design drawings to help you get better parts faster and with less hassle. 


Hello again and welcome. I’m Gordon Styles of Star Rapid, proud to launch season two of Serious Engineering for serious engineers. We had so much fun in the first season, and we thank you all for your helpful and insightful comments. We hope to continue to improve so you get useful information from a manufacturer’s point of view. 


Today we’re going to talk about the 5 key points that product developers need to keep in mind when you’re specifying the dimensions and tolerances on your 2D design drawings using GD&T. 

To begin with, let’s make sure we know what GD&T is. Geometric Dimensioning and Tolerancing is a logical system of numbers, symbols and conventions that are applied to 2D prints. It’s used to specify the size, location, form and tolerance for every important feature of a part. For this reason, it’s an invaluable tool that engineers use to communicate their design intent with their manufacturing partners. And because it contains such valuable information, it’s essential that GD&T is used correctly to avoid mistakes. 


Therefore, to help you improve your designs while saving time and money in the process, we’d like to share the top 5 tips for using GD&T the right way. 


Number One:  

Be sure to always indicate which GD&T system you’re using! The most common ones used around the world are metric ISO and Imperial ANSI. In addition, make sure you clearly indicate the revision, because every time a new revision comes along, they change some of the definitions of the geometric symbols and that can cause all kinds of misunderstandings that you really want to avoid. 


Number Two 

Specify the data. That’s plural for “datum”. A datum is a fixed reference point chosen by you, the product design engineer. What data are important to you? That depends entirely on your design intent, or what you want the part to do. 


Normally you will nominate one primary reference point, let’s call it Datum A. A single datum can be an axis, a plane or a point. 

This represents the specification that is most important, either for cosmetic or performance reasons. Then you nominate a secondary Datum B, and sometimes even Datum C. A Datum C is most often found on cylindrical parts to control the rotation on one axis. 


Once you have your data, all of the measurable features of your part will be based on their relationship to these points. 


Number Three: 

Control the degrees of freedom. This is very important to think about when you prepare your drawings, as it directly affects how – or even if – your part can be made. Consider for a moment a part floating in 3D space. 


In the Cartesian coordinate system, an object is said to have six degrees of freedom. It can move forward and back on each of the X, Y and Z axes, and it can rotate on any of those three. Datums, therefore, are essential not only for providing a reference point for measurement but also for controlling degrees of freedom. That’s why we consider these two concepts, datums and degrees of freedom, to be linked. 


It works like this. The purpose of the primary datum, Datum A, is to lock as many degrees of freedom to that point as possible, at least 3 and preferably 4. Then datum B, the secondary datum, also controls another one, or preferably 2 more, degrees of freedom – but these must be different than those of datum A! If there is a conflict, the part cannot be made as specified or within the given tolerance. 


When all 6 degrees are locked, that means that the manufacturer knows where every dimension needs to be and in reference to what. If a degree of freedom is not properly specified, then one or more dimensions are free to move around and that might adversely affect the part’s function or how it assembles with some other part.  


Now, do you always have to constrain 6 degrees? Not necessarily always. For example, if you want to drill a hole on the circumference of a perfectly symmetrical cylinder, maybe you don’t care where that hole is on the circumference. But if that hole needs to line up with a key on some other part, then the location must be specified. Again, it’s up to your design intent, but it’s always best practice to control all 6. 


Number Four:  

Limit the CTQ, or Critical to Quality, dimensions. In our experience, ten is usually enough for even the most complex part. This is because too many tight tolerances will adversely affect what is called “tolerance stack up”. 


This simply means that the cumulative total of all tolerances in a drawing are combined to yield an overall part tolerance. Consider that, if a series of dimensions are all very tight compared to one another, then the result is that it eventually becomes impossible to make the next one without affecting the others. The dimensions fight against each other.  


That’s why we recommend specifying as few CTQ as are absolutely necessary. The remaining, un-dimensioned features will otherwise be manufactured in accordance with overall quality standards. In modern CNC machine shops using equipment in good working order, this is more than accurate enough for most jobs. 


Number Five:  

Take a course. After all, even experienced design engineers may not be aware of how GD&T on a drawing affects manufacturability. And, in addition, standards are constantly being updated to reflect the latest advances in materials and manufacturing technology, so it’s always a good idea to keep up to date.  


We hope you have found this useful and informative. If so, be kind enough to ding the bell, like us and subscribe. We’ll see you next time with another episode of Serious Engineering.