Three MBD Advantages over 2D Drawings in GD&T Compliances

Oboe Wu | Comments | December 28, 2017

Geometric dimensioning and tolerancing (GD&T) is an engineering language widely used in both 2D drawings and MBD. ASME Y14.5-2009 and ISO 1101-2017 are the latest and most popular GD&T standards that specify the required and recommended practices. In model-based definition (MBD) implementations, it’s becoming increasingly important to discern the GD&T differences between 2D drawings and MBD because the standards were mostly based on 2D drawings. This article summarizes three key advantages of MBD GD&T definitions.

  1. Define features directly and unambiguously.

One of the requirements in GD&T practices is to define features directly rather than geometries because, ultimately, it’s the features that deliver the product function. Also in the actual production, features are what get machined or inspected. On the other hand, geometries, such as edges, centerlines or middle planes, are only derivatives of features. Some are even intangible and difficult to control.

Figure 1 illustrates one difference. The drawing on the leftattached a datum feature symbol B to a bottom line. We may be able to assume that the bottom line represents a bottom mounting face. However, we are not sure if this part should be mounted to this leg alone or to the bottom faces of all the legs. The model on the right removed this ambiguity by clearly specifying all the bottom mounting faces as datum feature B.

Figure 1. A comparison between a datum feature definition in 2D drawing and MBD.

Using SOLIDWORKS MBD, you can create a compound plane by selecting multiple coplanar faces as shown in Figure 2.

Figure 2. Create a compound plane by selecting multiple coplanar facesto define datum feature B.

An even more confusing case is all-around profile tolerances. Figure 3 shows a comparison.

Figure 3. A comparison between 2D drawings and MBD on an all-around profile tolerance.

The drawing on the left defines an all-around profile tolerance. However, it doesn’t clarify whether the side faces on the thinner fin are subjective to this control. Two possibilities are clarified in the models. One controls 26 faces and the other controls 36 faces. The model-based approach directly specifies the exact features to be controlled.

  1. Ensure compliances with built-in GD&T intelligences.

Based on the solid foundation of feature definitions, SOLIDWORKS MBD built in GD&T intelligences to provide instant feedback. A previous article, “Check Your Grammar: Verifications for GD&T in MBD,” shared several checks in the software. Let’s examine more examples here.

Figure 4 quotes a method from the ASME Y14.5-2009 standard about bidirectional positional tolerancing. It defines a rectangular tolerance zone in which the tolerances are different from one direction to the other.

Figure 4. Bidirectional positional tolerancing in ASME Y14.5-2009.

Figure 5 shows a simplified model with the bidirectional positional tolerances in accordance to the standard. The tolerance status is in green, indicating the definitions are in good condition.

Figure 5. A simplified model with bidirectional positional tolerances.

However, if I were to tweak the feature control frames as shown in Figure 6, the software would flag several issues, and the tolerance status would display violations.

Figure 6. GD&T violations flagged according to ASME Y14.5-2009.

First, the diameter symbols in the feature control frames specified cylindrical tolerance zones, which conflicted with the bidirectional rectangular zone intention. Additionally, please note that the original lower compartment.010-inch tolerance, as shown in Figure 5,didn’t include any datum references. It only refined the relationship between multiple instances in a pattern. It didn’t need datum references. In Figure 6, the positional tolerances doesn’t show 3x, so only one hole instance is defined rather than the pattern. Now the .010-inch tolerance has become independent, so it would need its own datum references. That’s why the software displayed a warning message against a missing primary datum feature, as shown in Figure 7.

Figure 7. A warning message against a missing primary datum feature.

It’s interesting to see that seemingly tiny changes matter significantly in the definitions. As mentioned at the beginning of this article, GD&T is a language. Similar to any language, small details can convey different meanings. Therefore, the challenge is to detect and avoid violations in small details besides the complicated rules.The good news is that we can leverage the strengths of software. SOLIDWORKS MBD can help automatically monitor significant details that could be easily overlooked by human eyes.

For instance, just by looking at the model on the right without reading the warning message on the dialog, as shown in Figure 8, we may not be able to catch the violation in this simplified case.

Figure 8. A warning message against a straightness on a hole feature.

The original design intent was to control the straightness of the derived median line from the hole feature. However, in the actual definition, an important modifier—a diameter symbol—was missed in the tolerance value box. Fortunately the software caught it because straightness of a derived median line from a cylindrical feature needs a cylindrical tolerance zone. A simple fix is to add the diameter modifier, as shown in Figure 9.

Figure 9. Resolve the issue by adding a diameter tolerance zone modifier to the straightness tolerance.

Just a side note, another way to address the warning message in Figure 8 would be to delete the Maximum Material Condition modifier. But that would mean to control the straightness of the line elements on the hole surface, rather than the derived median line.

  1. Facilitate design interpretations with visual aids.

As explained on the previous two points, GD&T rules are complicated and can be hard to interpret, especially when a manufacturer is working with a spectrum of suppliers at different skill levels. SOLIDWORKS MBD provides visual aids to facilitate interpretations.

For example, Figure 2 showed that when you select a feature control frame, such as the profile tolerance, all the controlled faces are highlighted. This makes the understanding much more accurate and easier. Figure 5 showed the tool to check the tolerance status, which presents a direct color scheme of how well each feature is toleranced. Then in Figure 7, when you selected a feature control frame, the coordinate system XYZ determined by the datum references A, B and C was created and highlighted automatically.

In the 3D PDF published by SOLIDWORKS MBD, you may use the context menu commands to highlight associated datums or basic dimensions as shown in Figures 10, 11 and 12. Unfortunately these commands are not yet available in the SOLIDWORKS environment. I hope they will come in future SOLIDWORKS releases.

Figure 10. Context menu commands to highlight associated datum features and basic dimensions.

Figure 11. Highlighted associated datum features in red.

Figure 12. Highlighted associated basic dimensions in red.

To summarize this article, I listed three advantages of applying GD&T to 3D models:

  1. Define features directly and unambiguously.
  2. Ensure compliances with built-in GD&T intelligences.
  3. Facilitate design interpretations with visual aids.

Please notice that these advantages need a digital environment to take place. None of these would be possible on a static paper document. Although an MBD implementation doesn’t have to exclude hard copy printouts, a digital environment with appropriate software can certainly help realize these advantages. On the other hand, an MBD implementation doesn’t mean that you have to digitize everything either. You may identify several pilot workflows with digital software and hardware to test the water. Based on the accumulated experiences, you can gradually expand the pilot.

I hope that this article is helpful. If you have any comments or questions, please feel free to leave them in the comments area below. To learn more about how SOLIDWORKS MBD can help implement your model-based enterprises, please visit its product page.


About the Author

Oboe Wu is a SOLIDWORKS product manager with 20 years of experience in engineering and software. He is an advocate of model-based enterprise and smart manufacturing.

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