The Y14.5 standard is just that, a standard. Although it provides the rules, terms and definitions for dimensioning and tolerancing, there is almost no guidance for application. It mentions design intent over and over without providing a roadmap on how to accomplish this. Over the past few decades of applying GD&T, I have developed an approach to application I call The GD&T Hierarchy which is explained in my book by the same name. Many past Tips have provided flowcharts and examples of this approach. In this Tip I thought I would give all of our readers "The GD&T Hierarchy Pyramid" to fold up and keep as a handy reference. I suggest you print it on glossy paper with a weight of 24 to 100 lbs. Feel free to share this Tip with others, but we ask that you do not change it in any way or profit from our efforts. This will be especially beneficial to those who have taken our training.
The first panel shown supports the three steps to applying GD&T:
- Establish datums
- Control datum features
- Locate all other features to established datums
This panel deals with features of size. It shows how the limits of size control the form of the feature, that they can be located with position and that cylinders (lathe type parts) may also be located using the runout or profile controls.
The lowest panel shows how to properly refine various features when necessary.
Here is a flow chart that usually works when following step 2—control the datum features.
This panel is a general reference for the common symbols found on drawings. The table also indicates where datum referencing is appropriate and puts the geometric symbols into the categories of location, orientation and form.