Y14.5 Versions
Magnhild Mongolo
The rise of Geometric Dimensioning & Tolerancing has been incredibly phenomenal. A primitive concept in the 1940s, GD&T is now an internationally acclaimed system that has become the industry standard for mechanical part production.
Rather than being marked by radical changes, GD&T has experienced stable growth. Each update to existing rules is made rationally, after collecting feedback from industry members and evaluating the exact pathways to further refinement. In fact, GD&T is perhaps one of the few industries where changes are first reviewed and debated by professionals for years before being fully adopted as common practice.
Like its predecessors, recent ones of which came out in 2009 and 1994, the much-awaited 2018 publication has attracted constructive debate from people curious about the changes made to it and how they contribute to the ever-improving flow of GD&T.
In this article, we identify the major changes from 2009 to 2018 and present our opinion on how they will make GD&T better. In the end, there is a short section on how concerned you should be about integrating it into your procedures.
We would like to start with the change that is most obvious. Those who have purchased the digital version of ASME Y14.5 2018 must have received an instant shock upon seeing the number of pages, which have dramatically gone up by more than 100 to 326: the 2009 version was 214 pages long.
There is nothing to worry about though, as this is mainly attributed to structural changes in the document. In previous versions, the information in the standard was organized in a way that the text and the images related to it were presented side by side, on the same page or adjacent pages.
ASME decided to change this by moving all graphical figures and examples to the end of each section. This gives them more freedom in terms of the size of the pictures, which are now bigger and clearer. Moreover, the inclusion of more 3D images (designed using the Model-Based Definition method) makes certain topics more comprehensible than they used to be.
The digital document has direct links to the images wherever they are referenced in the text. This means that if you have the correct PDF reader, you can very easily switch back and forth between text and images.
This is perhaps the most noticeable change in 2018. Up till 2009, the ASME standard had 14 basic symbols for defining geometric features. Two of these symbols: concentricity and symmetry, have been withdrawn from the toolset.
This change is largely due to the hassles related to using these symbols. To start with, it is always possible to define central features using other, more commonly used symbols. For example, concentricity may have its own symbol, but it is often better defined using controls such as position tolerance and runout.
Speaking in terms of part functionality, only a small proportion of cases actually require the use of these special symbols. It can be argued in these cases that the designer is better off using concentricity/symmetry instead of alternate options. However, this simplification for the designer does not continue down the line on the production side and creates more problems than it solves. Measuring concentricity and symmetry are typically incredibly daunting tasks. This is due to the fact that they require a huge amount of measurements to be taken to be accurate; a median value is determined at multiple points, all of which must fall within the established tolerance zone for the product to pass. A CMM or digital scanner is critical for this measurement, which adds to the cost and time of the inspection process. The digital data file from the CMM takes time to construct and inspect and binds a skilled person who could be used elsewhere. In addition, most machine shops and smaller companies cannot afford such expensive equipment to accurately inspect these features.
Overall, this change is highly welcomed. By removing these two symbols, ASME has ended a lot of confusion and pushed users towards adapting dedicated usage of Position controls, which is exactly what standards aim for.
Up till now, the datum stabilization guidelines set out by ASME allowed for several ways to do it. As shown in figure 1 if a planar surface rocks from side to side, there could be a number of acceptable positions that would pass as a datum. This caused great ambiguity as not everyone had the same datum; one person could stabilize it one way and other people some other way.
This minimum separation orientation can conveniently be determined using a CMM, if that is what you are using for inspection. This is another positive aspect of this change of rules. Its compatibility with modern technology makes it more likely to be universally accepted and lead to greater standardization of GD&T practices.
We are happy to see this as we recommend having a loose, general profile on the drawing to avoid disputes with customers over things such as dents or casting errors (or, god forbid missing dimension).
This will cover your entire part with a loose tolerance that will allow you to reject parts that are out for size, location orientation or form. This is not new, however, it is great that the standard highlights this as a recommended practice.
The dynamic profile symbol is a very specific addition to the GD&T toolbox. Previously, whenever any feature of size was toleranced, its size and form both shared the same tolerance zone. Although this is often a workable scenario, there was room for improvement.
The dynamic profile symbol basically overrides the default control and targets only the form or shape of the feature. By doing so, the designer is conveying the intention that regardless of the size of the feature, its form has to be confined in its own, separate tolerance zone. This gives a tighter control on the shape, without tightening the size tolerance given to the feature.
The Continuous Feature symbol was introduced in 2009. This symbol comes in handy when two or more features sharing the exact same size but have a discontinuity or gap that separates them into two separate features. Figure 5 shows a plate with a slot cut into one of its sides. Now faces A and B are practically coplanar but appear as distinct features on an engineering drawing, prompting professionals to treat them as different from one another. To indicate this continuity and save time, the CF symbol is added to their description.
As in each successive version, the ASME Y14.5 2018 has some minor changes to names and definitions. A summary of some notable ones is given below. It is to be kept in mind that these changes have little to no effect on the interpretation of these concepts. They have been changed though to better clarify when they are to be used.
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The core concepts of Dimensioning and Geometric Tolerancing have not changed. You will find minor tweaks to definitions and clarifications of concepts with new figures. Two new symbols have been added. A couple terms have been renamed. Some outdated or misused concepts have been removed to streamline the document and reduce confusion. Application of tolerancing in 3D models is gaining more traction and a lot of the definition updates in Y14.5-2018 have been to accommodate model-based applications.
The concentricity and symmetry symbols have been removed. These two concepts shown in the 1994 and 1982 versions of Y14.5 have always been controversial and complicated. These symbols controlled the opposing median points of a feature (not the axis or center plane) relative to a datum. This is rarely a functional requirement and often gets confused with axis to axis, center-plane to center-plane requirements set by position tolerancing. The elimination of this confusion and the simplification of the toolset is why concentricity and symmetry are removed.
The 2018 standard has closed the door on the first graphical method. The circle U modifier is now the only way to show unequal profile tolerance. This allows easier readability in the model-based definition and digital inspection world.
All plus/minus tolerances defining the relationship between features have been removed from figures and their use discouraged even more. This type of tolerancing has always been ambiguous and the Y14.5 standard did not give definitions even though some were still displayed on figures. The standard recommends the use of geometric tolerancing symbols of position, profile, orientation, and runout tolerances as the proper way to define relationship between features. This has been a long time coming, with each new version of Y14.5 removing more and more ambiguous plus/minus. I am glad to see the complete removal in 2018 even with an appendix showing the previous examples and ambiguity.
The 2009 standard defined the orientation of multiple line elements around a datum axis with an orientation tolerance and a note underneath: EACH RADIAL ELEMENT. The 2018 standard now shows this with a profile of a line referenced to the datum axis. This is a rarely used concept and a minor change.
True geometric counterpart was the term used in 1994 for the theoretical device used in extracting a datum from a datum feature. This term was switched to datum feature simulator in 2009. Now 2018 has switched it back to the true geometric counterpart.
Restrained condition notes in the datum reference frame section has been expanded and clarified. Explanation of the free state modifier was also moved to this section from the form section to link the concepts. The 2018 standard gives more guidelines for the use of a restrained condition note on a non-rigid part. It lists important parameters that may be included in the note such as: magnitude, location, direction, sequence, and area of restraint. Gravity affecting a non-rigid part is addressed. Examples and rules for a restrained note and datum targets have also been added. The free state symbol may now be used on individual datum references without affecting the restraint of the rest of the part.
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