GD&T – Datum Features Have Axes

This video, presented by Don Day from Tec-Ease Inc., shows very clearly the importance of referencing correctly the plans of parts according to their future function. This case shows a common mistake, a design mistake that meant huge costs for the company that made it. Customers often make the same type of mistakes for the production of checking fixtures, because they tend to underestimate their usefulness. Choosing as a basic principle to invest in staff training for the use of checking means is not a cost, it is an INVESTMENT !!! Here is the written transcription of what Don Day says in this video.

You know that some companies continue to work with some house standards that refer to the part underline. And I continue to see the datum features simply placed on underlines under the drawings.
The standard allows only the datum targets used to indicate the datum features.
That was not the case for a company designing lense systems to be used in aerospace.
Here is a simplified drawing of one of their lenses bearers. They applied a total runnout to a critical diameter within the face of a lense.
This means that when checked with an indicator, the full indicator could not be greater than 0.0006 when the part was rotated around axis A.
A customer placed the datum features symbol on the underline. The supplier used it to continue features, such as the largest tool to check the runnout. Unfortunately, the customer held it in the way it was assembled in the lense mark, and the parts failed.
The result cost the customer nearly $8,000,000.
The problem is that each feature has a different axis. Holding the part differently gets different results. Hence, if parts can be made perfect, there would be no need for tolerances underlines.
So, it must be made clear on the drawing which features to use to establish a datum.

This way, the drawing makes clear the design intent. This problem is easily avoided by following the standard. The datum feature symbol must identify a feature to use to establish a datum.
Hereby associating the feature symbol with a size dimension, everyone knows what the drawing means. The supplier knows how the part will be inspected and has a better chance of making the part right the first time. Or, your company can live with their old addage that engineers do not make mistakes, they make revisions. This one could have been avoided.