Increase Solid Model Value with 3D GD&T

Combining the right technology with a Dimensional Management Lifecycle will increase the value of 3D CAD Models through greater tolerance analysis, design collaboration, and improved product.

Time to create 2D drawings can be reduced significantly and if suppliers are fully integrated, drawings can be eliminated in favor of 3D visualization of design and manufacturing information. In addition, the 3D GD&T can be easily used for tolerance stack-up analysis from feature to feature in a part, or part to part in an assembly. For inspection, the same 3D GD&T can be leveraged for inspection planning and reporting. Finally, the information is reliably up to date as it is associated with the CAD model eliminating discrepancies between CAD models and drawings.

Communicate, Understand, Decide, Act
Figure 1: 3D GD&T in a Lifecycle Management Process Using Products from UGS Corp.

Readers should be, if not already, aware of the 3D Digital Product Definition Practices standard ASME Y14.41-2003. This standard demonstrates the cultural move to leveraging 3D CAD technology, specifically 3D GD&T annotation, to lean out product definition documentation.

The use of 3D GD&T and product manufacturing information (PMI) results in significant reduction in time by eliminating the non-value added replication of information on drawings and provides greater clarity of design. For example, identifying datum features on drawings normally requires several duplications of a datum feature symbol in numerous views.

The reason for the numerous duplications is for clarity and with the common minimum of three datum features per part; the clarification effort can lead to as many as fourteen and sometimes more duplications of a single symbol. This of course is time consuming for the draftsperson and often still does not clarify the datum as it is related to whatever feature shown in whatever view. Another example is basic dimensioning. If specifying GD&T specifications like tolerance of position, the drawing standard specifies that the basic dimensions must be shown. This can be a considerable number of added dimensions to a drawing requiring an exceptional amount of time to produce a drawing. Also, populating the drawing with all the basic dimensions adds to the clutter of dimensioning on the drawing. The interpretation of drawings with the various 2D views (details & sections) and the shear clutter of dimensions, notes, and symbols can be very time consuming and mistakes are made.

Figure 2: An example of embedded 3D GD&T in UGS NX4 System. The same GD&T is used in 3D Tolerance Analysis with TSV and VisVSA. Note the highlight of Datum F per user query.

However, with 3D GD&T, basic dimensions are not required nor are the other features that can fall under the realm of a title block tolerance specification. Instead, only the tolerance specifications are required since the CAD model is the "basic" definition. A single datum symbol clearly defines the datum feature that is visually highlighted on the 3D model. As for detail and section views, normally shown in wireframe on a drawing, any decent 3D viewer of a model can easily zoom in on details and dynamically section a part or assembly model. Thus, providing an unlimited number of views instead of the limited details and sections on a drawing that a draftsperson assumes will be enough for someone else to fully interpret the design. As for clutter, a viewer can filter out the PMI to display only what a consumer wants to see. For example, during a review with a supplier, it is easy to turn off all dimensions except for the Datum features so that understanding and agreement on the datum features can be easily reached.

View of 3D GD&T
Figure 3: View of 3D GD&T using UGS Corp. VisMockup Viewer and JT Direct Model Format. Separate model views can be saved in the CAD system to simplify filtering. Again note the highlighted feature

What are the metrics? Each industry has varying levels of complexity. So, to find the metrics related to 3D GD&T versus drawings, look for the number of changes; drawing changes and tooling changes. Often changes to a drawing require added dimensions left off and/or adding additional detail and section views to more fully document or understand the design. These changes occur not only while still in the development stage but during the production of a product. Depending on the regulatory authority, a drawing change, besides the expense of the change request documentation, can delay an assembly line adding very significant expense to the change. Then there is the common issue where the drawing has a change, but if the change is a dimension value (product design) the CAD models are often not kept up to date. However, in many manufacturing processes today, the tooling or parts are manufactured using the 3D solid model. There in lies another key metric; tooling changes. During development tooling changes will happen, but many tooling changes occur because of drawings that do not accurately reflect the 3D model. The cost in pure tooling dollars is significant enough but delays in producing the product can be much higher. When using 3D GD&T, a change occurs to the model assuring the model is up to date. Another key efficiency is that solid models will have the details, fillets and draft, with less time spent on producing a drawing. Adding section or detail views is all but eliminated since the any section or detail is made available using 3D model viewers, and virtually no dimension is missing since any measurement can be made off the "basic" 3D model. Is efficiency in dimensioning drawings being recognized? The hole concept of "limited dimensioned" drawings is about efficiency to produce a drawing, be able to read through the clutter to perhaps understand the design and what features are important. In 3D GD&T, there is no clutter and 3D visual collaboration of parts and assemblies is already accepted as better communication tool to understand a design. As for determining the "important features", a good tolerance analysis is in order.

Using the right tools, the 3D GD&T can be used down stream in 3D tolerance analysis early on in concept development through production release. For example, if embedded 3D GD&T is applied using the UGS NX4 cad system, early and fast tolerance analysis can be used within the system using the Tolerance Stack-up Validation module (TSV). TSV has the added benefit of checking the GD&T for validity according to the ASME Y14.5M- 1994 standard. How many drawing changes occur, or worse should occur, because of illegal specifications used by associates not truly versed in GD&T? Using a good GD&T checker will improve associates skills in applying valid tolerance specifications. Why care about quick and easy tolerance analysis in concept design? Normally concept design includes multiple concepts and tolerance sensitivity to function and manufacturing can help with determining which concept is really best for full production development. Tolerance analysis of part features and assemblies can be related to CTQs and manufacturability of a design. Choosing the right concept early is critical to the cost and success of the product. Once a concept is chosen and a detail design phase is started, the use of 3D GD&T, again in a tool like the UGS NX4 system, can be directly imported into the more powerful tolerance analysis tool, VisVSA.

Measurement Analysis
Figure 4: Example of a measurement analysis using UGS Corp. Tolerance Stack-up Validation module TSV

VisVSA models can be created from sketches on a napkin, much less a well detailed time consuming drawing. However, if 3D GD&T is used in an NX4 assembly, a detailed VisVSA model can be conservatively created in less then half the time. This allows for more time, early on in design, to study where to budget tolerances for greater manufacturing capability while maintaining CTQ requirements and ultimately more robust design. The capabilities of VisVSA are extensive and go beyond the intent of this article, but it is worth mentioning that as successful product is delivered to market using VisVSA analysis, the information is easily reused. VisVSA has a tolerance library capability that can cascade down to the machine producing the part. Therefore, VisVSA enables reuse of real knowledge to help consistently produce quality products while maximizing capability and reducing cost of goods (COGS). In addition to tolerance libraries, VisVSA is CAD neutral, which is why a model can be created from a napkin sketch, so the VisVSA model can be reused on similar part or complete product designs. Reuse and updates are easy because VisVSA elements are easily mapped to new product features so that consistent tolerance values based on known manufacturing and inspection capabilities.

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Review Article
  • Inspection February 21, 2007
    Reviewed by 'Johnny On The Spot'
    Hey there jetero. Your sentiments are echoed by many. However, if your company or those with which you are associated do not embrace model-based definition and eschew the old techniques that include paper drawings, you will be left behind. This is true in aerospace manufacturing even more than any other industry. I am shocked that you are still getting work such as a leading edge rib with the product defined “the old fashioned way.” I am involved in close cooperation with Boeing, particularly for ramp-up on the new 787 aircraft production both for aircraft parts and tooling. They are not providing product definition via any other means besides the “digital thread”, now the norm. I have also just gotten back from visits with Airbus and Lockheed Martin where the plan to move to fully model-based definition is well on the way. One thing you might want to consider when dismissing the importance of MBD for aircraft design and manufacture, is the history of discrepancies between drawings, CAD model, and other defining elements such as mylars and hard-tooled master models. There are many efficiencies to be realized with this approach, the least of which is the cost of paper. Then there are quality and safety issues. If your washing machine breaks down, so what. If your aircraft has a failure because the drawing did not match the CAD model, and the safety of that component depended on the digital definition, you can’t pull the airplane over to the nearest cloud and park it.

      2 of 2 found this review helpful.
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  • Re: Increase Solid Model Value with 3D GD&T December 19, 2007
    Reviewed by 'BrentArmstrong'

    Well I have played with Adobe 3D and now own a liecence of SolidWorks. So when I have more experience with SolidWorks 3D I'll post my thoughts.

    Question. What is the approach used in getting companies on buy into this technology? What does the layout(steps) look like to convince companies to invest?




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  • Inspection February 02, 2007
    Reviewed by 'Johnny On The Spot'
    There are solutions such as Verisurf software that import the GD&T from Unigraphics and Catia into a CAM system such as Mastercam, and provide a metrology interface with digital inspection devices such as CMMs, that utilize the unique GD&T for measurement, analysis and reporting. As the data are available in the CAM system, manufacturing, quality control and vendors have all of the information they need to manufacture the product without needing drawings. I don't know about other software, but Verisurf is fully 14.41 compliant. It is also AIMS (mentioned by one of the respondants) enabled. It is a shame that some continue to live in the past and resist moving towards full model-based definition. I have experienced numerous situations where engineers changed design simply to make the part or assembly dimensionable for paper drawings, when they could have designed, manufactured, and inspected the parts digitally and would have been able to go with the design they actually wanted.

      6 of 9 found this review helpful.
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  • Annotation is not the way to go with GD&T November 15, 2006
    Reviewed by 'Amazed'
    I was rather amazed at the short-sightness of this article. Handling GD&T as an annotation problem is not an effective solution. GD&T must be model based and defined within the CAD model using a structured GD&T database for features, datum reference frames and tolerances. Many CAD systems treat it as simply annotation. The problem is, downstream applications (like inspection) then need to interpret low level annotation and reconstruct the GD&T model. This is poor practice.
    What really surprises me here is UG (the article is all about UG products) does not even mention their best tool to do this job - Tecnomatix products. Tecnomatix has a complete model based solution for GD&T including modeling, analysus, inspection, measurement and reporting.

      One person found this review helpful.

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  • Increase Solid Model Value with 3D GD&T August 08, 2006
    Reviewed by 'CPerhala'
    Article provides a good overview of data needed to manage a product's dimensions over the life cycle. The comments about datum identification and multiple references in numerous drawings is dead on accurate; this has been the root of many headaches.
    The focus on UGS systems only limits the usefulness of the article and can give the impression of an extended advertisement. It would be of interest to know of other packages that can do the same or similar things.
    Some of the grammar should be cleaned up to make it more readable. There are several instances of the wrong word being used (e.g., 'to' instead of 'too') which requires rereading the sentance (sometimes several times) to understand the thought being conveyed.
    On the balance, a useful article. Some additional work would make it more readable (and useful).

      15 of 16 found this review helpful.
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