Geometric Dimensioning and Tolerancing or GD&T
Do you know what these symbols mean? I didn’t. Frankly, these symbols made me nervous as there was such a large depth of knowledge needed to understand them. The ASME course is a weeklong! Looking back, there are a few major items that you need to know in order to get a foothold in this very beneficial tool, so take 3 minutes to read.
What I often tell people, is that even if the machine shop or fabrication shop hasn’t used much GD&T, they will pay attention to the fancy symbols. So, use the symbols in line with your design intent. Put fancy symbols where the dimensions matter, so the machinist or fabricator pays attention to it. Also use it when it’s difficult to use standard dimensions to explain what you want.
It’s not as hard as it seems. As with many things, you can apply the Pareto principle. 80% of the time, you only need 20% of the symbols. Disclaimer – there are several industries that require a lot more GD&T than most such as the guts of some medical devices, firearms and aerospace.
Which symbols are most used? I have marked a standard list below with RED ARROWS for the ones that are often most applicable:
The Position symbol is one of the most used callout, as you can use it with the Datums and a tolerance to define the locations clearly.
For Shafts, the Total Runout callout works great for critical surfaces. I said critical – not all surfaces. This is an “expensive” callout.
The ones I’ve marked are the ones that you can use 80% of the time. When you start to apply the symbols, you’ll see why. Here is a simple example that is quite common:
Next blog I’ll cover when you should use GD&T and when you shouldn’t.
About the Author: Willem Mast is the owner and operator of WMD Squared. He loves mechanical design of all types, optimizing engineering systems, and bicycles.
