The second category: Change a default condition.
In most cases default conditions are not revised in an addendum; they are addressed on the face of a drawing. What should be in an addendum is a list of which defaults should be overridden on your drawings and instructions on how to override them.
Changing a default can be beneficial because the default may not be the best for your company. For a list of common defaults in the Y14.5 standard, check out the blog article "Do you know which ASME Y14.5-2009 defaults apply to your drawings?") Click here to read the defaults article
If the default conditions are not the same for all types of parts in your company, the addendum should show what parts should have defaults overridden and how to denote the override condition.
The advantage of revising standard defaults on the face of a drawing is that the requirement or condition is visible to the drawing users.
The third category: Clarify a concept, symbol, modifier, or definition for your companies application.
Clarifying concepts from the Y14.5 standard is important because it reduces confusion by providing additional explanation of items that are not fully explained in the standard.
The list below contains a few examples where clarifying a concept from the Y14.5 standard could be in an addendum.
- Where the Independency symbol is applied to a size dimension of an external feature of size, and a form control is applied to one of the surfaces, its outer boundary of the feature of size shall be determined by the MMC of the feature of size plus the effects of the form control. [2.7.3]
- A few terms that you may want to clarify with your addendum are: element, continuous feature, free state, and define what UOS means on a drawing and how to determine when is it overridden. [1.3]
- Clarify whether surface texture is included in size tolerances and geometric tolerances. For a discussion that shows the different interpretations in industry check out the article "Survey results on the interpretation of the ASME Y14.5 standard" Click here to read the article
- How to designate and interpret a complex curve as a datum feature. [4.1]
- Document the standards that apply to your drawings. The standards cited in this paragraph apply at the date shown. Any newer version shall not apply unless explicitly stated in this addendum. If a conflict exists between the text any of the standards cited and this addendum, the text of this addendum shall take precedence. [1.2]
NOTE:
Each ASME standard contains a paragraph citing a list of additional standards it invokes and provides a method to determine which standards are partially invoked.
However, many companies have difficulty understanding exactly which standards, and which versions apply to their drawings.
The fourth category: Discourage/disallow the use of a concept, symbol, or modifier.
Discouraging or disallowing the use of a symbol or concept is important because it reduces the number of tolerancing tools available for use on drawings. There are many tolerancing tools in the Y14.5 toolbox that are not needed for some types of parts. There are also tolerancing tools that are not used often.
There are subtle differences between some of the GD&T symbols. There are often discussions over which symbol is better for an application. The addendum focuses the discussions by disallowing or discouraging certain tolerancing tools leaving the tolerancing tools needed for defining your products.
Tolerancing tools can be disallowed for a variety of reasons. A few tolerance tools are overly restrictive for most applications or are not a good tool for your companies' applications. A few tolerancing tools that are only used in rare applications may be disallowed or discouraged to prevent their widespread use.
The list below contains a few examples of disallowing or discouraging a concept, symbol, or modifier.
- The concentricity symbol shall not be used. [7.6]
- The symmetry symbol shall not be used. [7.7.2]
The Statistical Tolerance modifying symbol shall not be used. (3.3.10)
The "alternative practice" to control parallel and perpendicular relationships shall not be used. [6.6]
Direct tolerancing (often referred to as plus and minus or limit tolerancing) methods shall not be used for location or orientation of features of size. [2.2]
The practice to override the degrees of freedom by reference of an order of precedence shall not be used. [4.22]
- The method of specifying datum features at LMB shall not be used. [4.11.7]
- The method of multiple LMB's shall not be used. [4.11.8]
NOTE:
Another area that I recommend companies to address is to discourage "extension of principle" on their drawings when a tolerance application is not in the Y14.5 standard or in their addendum. Even though the extended principle may make sense to the drawing creator, since its interpretation is not documented, it can be interpreted differently by drawing users.
The fifth category: Document the use of a tolerancing practice that is not in the Y14.5 standard toolbox.
Documenting the use of a symbol or concept that is not covered in the Y14.5 standard is important. It helps drawing users to have a consistent interpretation of the drawing requirements. If you add a new symbol or concept, it should be fully explained from a functional and inspection standpoint.
The list below contains a few examples of tolerancing tools that are not in the Y14.5 standard but may be needed. In this section, only example topics are listed, the entire addendum text and figures to fully describe these items are too lengthy for this article.
- A method to define a uniform Thickness/Gap Tolerance. (e.g. a wall thickness.).
- How to designate a wedge as a datum feature.
- When and how to document restraint conditions.
- The Free-State modifier along with Restraint notes may be used on both non-rigid parts as well as rigid parts when specific assembly conditions must be met. (e.g. the flatness of sealing surfaces.)
- A method to specify how to restrain datum features.
- How to dimension cylindrical parts with bends (e.g tubes).
Caution:
Adding new tolerancing tools in your addendum should be used sparingly. Adding new tolerancing tools can cause problems for CAD, analysis, and inspection software.
Summary and closing thoughts.
In part two, we looked at each of the five content categories of corporate addendums. Each corporate addendum is different based on their unique situation. Every addendum will not address all five of the above categories. An addendum may also include additional information not discussed in this article.
Part three of this article will be posted next week. In part three, you will learn a few tips on creating and implementing an addendum in your organization.
