Section 1 of the ASME Y14.5 standard is entitled "Scope, Definitions, and General Dimensioning".  It doesn’t say tolerancing.  Fifty years ago in Y14.5 there was minimal support for applying ± tolerances to locating dimensions.  About 30 years ago this minimal support in Y14.5 was removed in favor of basic dimensions and geometric tolerancing -- especially for controlling the location of features.  Yet, some people insist that Section 1 somehow legitimizes locating features using ± toleranced dimensions.  It doesn’t!

There is a figure in the Standard that is very similar to this first drawing. If these dimensions were all ± toleranced the drawing would have many different meanings depending on who is doing the reading.  There is no support in the ASME Y14.5 standard for using ± tolerancing on locating dimensions such as these.

However, if there is a note stating that UNTOLERANCED DIMENSIONS ARE BASIC and geometric tolerancing is applied as shown on the second drawing, there is only one meaning, no matter who does the reading.  So, let’s all move up to today’s print reading in accordance with the ASME Y14.5 standard.

Before you email me that the drawing is incomplete, read my Tip on Simultaneous Requirements.    

The video on the Premium Site will show you how each one of these symbols may be addressed using the ASME Y14.5-2009 standard.

Please email us any suggestions or topics that you would like to see covered in our Tip-of-the-Month series.