Dimensions, tolerances, and more right on the model

Douglas Korneffel
Product Manager Drafting/Annotation
Applications
UGS Milford, Ohio

The recent ASME standard Y14.41 establishes requirements, exceptions, and guidelines for documenting digital solid models in 3D. ASME has been developing the new standard for several years with experts in industry, education, and commercial software developers. Although the standard comes from ASME, annotating 3D models is an international movement. Companies can now evaluate what it means and the improvements it can make to their processes.

AN INTRO TO Y14.41

To gain the most advantage from 3D models, designers should learn the rules for documenting and sharing information in this new medium. Y14.41 builds upon the established tolerancing practices of Y14.5 for drawings and sets rules for a 3D environment. ASME literature acknowledges that 3D modeling is the future of design and "ASME Y14.41 sets the rules. Y14.41 extends ASME

Y14.5M into the 3D world. Y14.41 defines the exceptions and additional requirements to existing ASME standards for using product definition data sets or drawings in 3D digital format. This standard supports two methods of application: model only, and model and drawing in digital format."

ASME Y14.41 doesn't stand by itself, nor does it replace Y14.5 or any other standard. It provides rules for using established 2D standards in the context of a 3D CAD model. Tolerancing and dimensioning practices remain nearly unchanged.

The term "product-definition data" is meant to encapsulate all the 3D annotation needed to document a design in 3D. It is mostly the same information found on a drawing, but it can now include more. For example, building blocks for a basic product definition includes model geometry, reference geometry, driving and annotation dimensions, GD&T symbols, tolerances, and finishes. To speak collectively about all this data, many in industry have coined the term Product and Manufacturing Information (PMI). It's the sum of all information needed to define and manufacture a product. The propagation of PMI throughout a product's life cycle is proving to be more beneficial than static drawings.

In the past, for instance, most of this information was defined by the drafter after completing the 3D model. This prolonged the process of completing the design, and even involved rediscovery of information and specifications the model builder had already established. Now engineering information can be captured as the geometry is defined and evolves.

Adding PMI to the 3D model can shorten the overall documentation process. For example, placing information on the model also makes it available to downstream applications such as NC, analysis, inspection, and drafting. More importantly, the information is electronically available, so there is less human interpretation and almost no recreation. Third-party applications can also use the electronic PMI definition through open formats for exchange, such as the UGS PLM XML format. Even simple communication of the design vastly improves with interactive Viewing applications that can rotate and interrogate the model and PMI. This promises to replace static paper plots.

Another benefit: PMI captures more of a design's intended meaning. On a drawing, for example, the user has only a leader and arrowhead to help direct an annotation. In 3D, all attached and associated geometric entities can be included right in the PMI definition. The user can query the PMI for the additional information and learn more about the design intent.

MAJOR CONCEPTS IN Y14.41
First and foremost, drawings are not going away. Drawings and 3D models have a compatible and complementary purpose. Drawings may get some of their PMI directly from the model. Or, drawings may get all of their PMI directly from the model, greatly reducing the time needed to create a drawing. An annotated model in some companies can replace drawings for a few functions — when former drawing recipients agree to the new process.

Y14.41 allows both model-only and model-and-drawing methods of documenting a product. So either, or both, are acceptable ways to document a design. For the model-and-drawing process, annotation on a drawing must be kept in sync with the model PMI.

Many companies are easing their way into 3D PMI by documenting critical information on the model in engineering, using it to get drawings started, and completing the full documentation on the drawing. Y14.41 is flexible this way. To bolster the model and drawing method, Y14.41 accepts PMI in an isometric view. This was forbidden in the previous 2D drawing-only world.

CAD systems may need a few new features or capabilities to deliver a sufficiently documented model. For example:

• Associativity between annotation and model geometry, and even between multiple geometric features. The idea is that when geometry changes, the annotations should update.
• Saved views of specific annotations. The capability calls for tools to organize, capture, and recall annotations.
• Associated groups. These are collections of annotations and geometry for specific purposes.
• Display management. This comes from controls that show only PMI of a certain type, or associate to specific geometry. Annotation planes allow placing PMI on almost any plane in 3D space that rotates with the model.
• Dimension values must be obtained directly from model geometry.
• Query functions allow easy visualization of annotation and all geometry to which it pertains.

Query functions are perhaps the greatest improvement beyond basic annotation. Annotations in 3D may be associated to multiple pieces of geometry. Querying the PMI (usually by picking on a model feature or an annotation) should tell users to what the PMI is associated. For example, pick on a feature-control frame containing datums A and B and the datum symbols and datum geometry should highlight together. Or query geometry to see its associated PMI. This extra information can be captured with every piece of PMI on the model and provide a better understanding of the design intent to a downstream user.

Another major concept in the standard is the use of 3D section views. Of course, sectioned models are not new. But it is powerful to have a 3D section view with crosshatching, cutting plane, and geometry that can be tracked by annotation that update with changes. When annotated section views are only available on a drawing, it quickly reduces the possibility of completing the 3D documentation of a design. By removing this last great obstacle to 3D documentation, nearly any design can be documented in 3D.

FOR A COPY OF Y14.41
A copy of the new Y14.41 standard can be obtained from ASME at their Web site: www.asme.org/codes/pr/y1441.html