We can begin by asking the question: Do you see any problem with the GD&T definition in Figure 1? In the figure, datum symbol A is attached to a centerline and then is referenced in a total runout tolerance.

Figure 1. A problematic GD&T definition. (Image courtesy of a Tec-Ease GD&T tip video.)

This actually turned out to be a million-dollar problem. The original part by a customer was a lens barrel in a space telescope on which the opening at the left interfaced with a lens, which is why the total runout tolerance was controlled tightly at 0.0006 inch. Figure 1 is a simplified illustration of the part.

A tight tolerance is fine as long as the product function justifies it. The real problem with this part is the datum label attached to the centerline on the customer drawing, because it didn’t specify which tangible feature would serve as the datum feature to inspect the tight tolerance. A centerline is theoretical and intangible. In the actual production, the supplier inspector didn’t have definitive instructions on how to hold the part. Figure 2 shows an exaggerated example of a machined part by the supplier. Clearly, the smaller cylinder on the right of the figure was misaligned.

Figure 2. An exaggerated example of a machined part.

If the supplier grabs a convenient feature such as the larger outer cylinder on the left, spins the part, and then inspects the runout, the part is good as what is shown in Figure 3.

Figure 3. Inspecting the total runout by holding the convenient larger outer cylinder.

Unfortunately, the customer held the barrel in the way it would assemble in the lens mount. As a result, the smaller cylinder on the right should be spun to inspect the part based on the intent of the design. Now, as shown in Figure 4, the total runout is violated and the part should be rejected. This ambiguity led to a lawsuit of nearly$8 million.

Figure 4. Inspecting the total runout by holding the smaller outer cylinder.

This problem could have been easily avoided if the symbol was specifically defined to an intended tangible feature, rather than an ambiguous geometry. Figure 5 shows the recommended definition using SOLIDWORKS MBD. In this approach, you can select the smaller outer cylinder to define the feature. The software then highlights the actual face once a label is selected and automatically aligns the datum symbol to the size diameter and tolerance callout.

Figure 5. A recommended datum feature definition.

This lens barrel case demonstrates the costly downside of ambiguous GD&T definitions. Although this issue can occur in both 2D drawings and 3D annotations, some MBD software guides the definitions with built-in GD&T rules to ensure solid practices. For example, Johnson Controls estimated significant value benefits with improved GD&T practices in the CATIA MBD environment.

Similarly, SOLIDWORKS MBD follows the ASME Y14.5-2009 GD&T standard closely. For instance, according to this standard, the datum feature symbol B in the two figures when compared to Figure 6 conveys two completely different design requirements. The one shown on the left indicates that datum feature B is the width feature because the label B is aligned with the width dimension line, while the one shown on the right indicates that datum feature B is only a single face because the label is not aligned with the width callout.

Figure 6. A drawing comparison between a width feature as a datum feature (left) and a single face as a datum feature (right).

In order to avoid this common confusion, SOLIDWORKS MBD automatically aligns the label to the width feature size dimension line as shown on the left of Figure 7. If the design requires only a face as the datum feature, then you can define a face, rather than a width.

Figure 7. An MBD comparison between a width as a datum feature (left) and a single face as a datum feature (right).

By the way, a width datum feature gives the middle plane between the two opposing faces as the theoretical datum. I hope this answers the frequent question posed at the beginning of this article. You can find more about the differences between datum feature and datum here.

Let’s expand to several other examples. Figure 8 shows the datum features A, B and C on a shifter stick. A is the width size feature, B is the two coplanar shoulders as highlighted in green, and C is the pattern of two mounting holes that is supported by a new enhancement in SOLIDWORKSM MBD 2018.

Figure 8. Datum features A, B and C on a shifter stick.

In this ABC datum framework, I added the Maximum Material Boundary (MMB) modifiers to A and C, which are size features. This allows datum shifts to accept more good parts. However, if I were to add MMB to datum feature B as shown in Figure 9, the software would flag it because B is not a feature of size and maximum material boundary doesn’t apply in this case.

Figure 9. A warning message against using an incorrect MMB modifier.

You may also notice that when a feature control frame is selected, the coordinate system as defined by the ABC datum references is automatically created and highlighted in green in the graphics area. This provides an instant visual confirmation that makes interpretation easier. It also helps automate the coordinate system alignment for other downstream manufacturing software.

On this GD&T editing dialog, if a user forgets to type in a primary or secondary datum letter before a tertiary one in a feature control frame, the dialog automatically displays a warning message to alert the user as shown in Figure 10.

Figure 10. A warning message about missing primary or secondary datum letters.

Besides the manual annotations, the software follows the GD&T rules in the automatic dimension creation as well. Figure 11 illustrates an error in which the two datum features in the red box share the same axis. The features are defining the same theoretical datum, so the tool catches their unnecessary duplication.

Figure 11. An unnecessary datum feature duplication caught in the automatic dimension scheme.

As mentioned at the beginning of this post, when interpreting a GD&T definition, a user first needs to remember the datum references. So, a handy command is to automatically highlight the associated datum symbols and features. The 3D PDF generated by SOLIDWORKS MBD provides this command shown in Figure 12. You can right-click on a feature control frame and click on the context menu command “Highlight associated datums.” I hope that a similar handy capability can be added to the SOLIDWORKS environment in the future.

Figure 12. Highlight associated datum symbols and features for a feature control frame.

With that, let’s conclude this article with several key points:

1. Datum features are the foundation of GD&T definitions.
2. You should define datum features on tangible faces, rather than intangible ambiguous geometries.
3. SOLIDWORKS MBD builds GD&T rules into the software to help detect violations.

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.

Figure 1. An energy storage product by Johnson Controls. (Image courtesy of Johnson Controls.)

Johnson Controls first started the MBD implementation in 2011, with a mission to enable seamless sharing of product information within the extended enterprise and automotive industries. The company clearly saw the advantages of 3D models integrated with product and manufacturing information (PMI) over 2D drawings.

For example, Ram Pentakota, the engineering director with the Johnson Controls automotive seating division, shared several reasons why 3D geometric dimensioning and tolerancing (GD&T) are better than 2D drawings. Figure 2 shows a typical 2D drawing.

Figure 2. A typical 2D drawing. (Image courtesy of Johnson Controls.)

In order to make sense of a single GD&T feature control frame, a reader of this drawing must find the related datum symbols by manually scanning through the entire sheet and possibly many other sheets. Moreover, multiple datum references require multiple rounds of this type of manual searching. After locating a datum symbol, an engineer needs to interpret the datum feature identified by this symbol and remember the relationship between them to understand a feature control frame.

As you can see, the experience feels similar to looking for several needles in a haystack and weaving a thread through these needles. A simple question—such as, “Where is datum feature A?”—may be fairly challenging in the haystack.

As a comparison, with 3D GD&T defined in CATIA V5, you can easily select the datum symbol A in the feature tree list. Then the datum feature A is automatically highlighted in the orange color in the viewport, as shown in Figure 3. This case is especially tricky in 2D drawings because the datum feature A includes multiple surfaces identified by multiple datum target symbols: A1, A2 and A3. Now with 3D GD&T, the presentation is straightforward and intuitive.

Figure 3. Multiple datum feature surfaces are automatically highlighted with 3D GD&T. (Image courtesy of Johnson Controls.)

Another common question to ask is how many datum features there are in total. The answer can affect the machining and inspection fixtures and setups. With the 2D drawing as shown in Figure 2, you must manually scan and count all the datum symbols and features. But with the 3D GD&T, the datum symbols are well organized in the feature tree, as shown in Figure 4. In addition, all the views, datum reference frames and geometric tolerances are also listed. Of course, selecting them in the tree on the left will highlight the corresponding callouts and features in the viewport on the right. The cross highlighting provides an instant visual confirmation to reduce miscommunications.

Figure 4. Datum symbols, reference frames, geometric tolerances and views are well organized in the feature tree. (Image courtesy of Johnson Controls.)

One step further, with the 3D GD&T definitions (as shown in Figure 5), the software can intelligently help you flag GD&T errors. For example, the highlighted Simple Datum 6 in the tree is a conflicting duplicate to the previously created Simple Datum 1 constructed by three datum targets. Therefore, these datum callout icons along with the Datum Reference Frame 1 are all attached with yellow warning signs. Similarly, SOLIDWORKS MBD has built in hundreds of rules to verify your GD&T, as discussed in a previous article, “Check Your Grammar: Verification for GD&T and MBD.” This type of automatic GD&T verification can greatly improve the annotation quality and consequently, the manufacturing quality, but it isn’t available in 2D drawings.

Figure 5. The problematic datum callouts and reference frame are automatically flagged with warning signs. (Image courtesy of Johnson Controls.)

By the way, please notice that the three separate datum targets, A1, A2 and A3, are cohesively constructing one datum feature A. It’s nice to see that the software can automatically recognize separate targets collectively as one datum feature. This automatic construction isn’t available in 2D drawings either.

Besides flagging existing errors, 3D GD&T in CATIA V5 can also help you prevent errors. Depending on the feature you select, the software will provide you only the valid GD&T callout options. For instance, Figure 6 shows that a flat surface has been selected.

Figure 6. A flat surface is selected. (Image courtesy of Johnson Controls.)

Please notice that on the dialog box, as shown in Figure 7, only the valid callout types are provided for the selected flat surface feature, such as datum symbols, straightness, flatness, line profile and surface profile tolerances. Other irrelevant callouts are hidden automatically.

Figure 7. Only relevant callout types are provided for the selected flat surface feature. (Image courtesy of Johnson Controls.)

If you select a cylindrical surface, as shown in Figure 8, then the callout options are adjusted accordingly, as shown in Figure 9. For instance, the flatness tolerance doesn’t apply to a cylinder, so this option is hidden. Instead, the circularity and cylindricity tolerances are now presented.

Figure 8. A cylindrical surface is selected. (Image courtesy of Johnson Controls.)

Figure 9. The callout options are adjusted automatically according to the selected cylindrical surface. (Image courtesy of Johnson Controls.)

Figure 10 shows a more complex case in which a torus surface is selected.

Figure 10. A torus surface is selected. (Image courtesy of Johnson Controls.)

Now you can see that several dimensioning and tolerancing options specific to torus surfaces appear on the dialog box (as shown in Figure 11), including the torus body diameter and radius along with the revolving ring diameter and radius.

Figure 11. The callout options are adjusted automatically according to the selected torus surface. (Image courtesy of Johnson Controls.)

In summary, Johnson Controls is convinced of the advantages of 3D GD&T over 2D drawings, including:

• straightforward identification and organization of datum symbols and datum features,
• intelligent GD&T error flagging and
• automatic prevention of GD&T errors by providing only valid callout options according to the selected feature.

I hope that you are convinced of these advantages as well. Most importantly, I hope that you can enjoy similar benefits in your own MBD practices. To learn more about how SOLIDWORKS MBD can help you with your MBD implementations, please visit its product page.

About the Author

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

For example, let’s look at a manufacturer who practically lives by the ASME Y14.5-2009 GD&T standard. At this company, every new engineer must go through one week of Y14.5 training and then apply GD&T to all designs. However, with that level of commitment, the team members, including seasoned engineers, still make mistakes frequently due to lack of knowledge, oversights or fatigue—similar to those typos in an email that we’ve all made.

Fortunately, Microsoft Office provides a spelling and grammar check to flag typos for us. Wouldn’t it be nice if an engineering design tool could help flag GD&T mistakes too? After all, just like other languages, GD&T is structured and has a well-defined set of rules and best practices.

Well, this is where a real-time grammar verification such as the one in SOLIDWORKS MBD can assist.

To start, note that this checking function only gives warnings. It doesn’t stop a workflow, force us to correct an error or automatically fix the error. We still have the flexibility to ignore it after a careful review. Therefore, engineers still need to take the control and responsibility.

Figures 1 and 3 show where a flag can be raised:

1. In the graphics area, a feature control frame in error is turned in yellow.
2. On the DimXpert tree, the top node of this entire PMI scheme is prefixed with a warning sign and presented in red.
3. The questionable feature node is prefixed with a warning sign.
4. The questionable GD&T node is prefixed with a warning sign and presented in red.
5. A warning message explaining the root cause is displayed in a pop-up bubble when the mouse cursor is over the GD&T node.
6. The same warning message is displayed at the bottom of a GD&T definition dialog in Figure 3.

Figure 1. A GD&T error is flagged in both the graphic area and the tree nodes.

Now, you may wonder: What’s wrong with this position tolerance? The warning message says, “No size tolerances defined for feature Boss5.” Aha! Because this position tolerance contains a maximum material condition (MMC) modifier   (M in a circle) to the diameter tolerance zone Φ.020in, it needs to know the boss feature’s overall size tolerances to calculate its MMC.

We can fix it easily either by adding the boss size tolerances as shown in Figure 2 or removing this MMC modifier. The former approach could save cost because it only requires the Φ.020in position tolerance at the MMC, which is Φ.810in, or the biggest boss. At the least material condition, Φ.790in, or the smallest boss, the position tolerance could be as loose as Φ.040in or Φ.010in + Φ.010in + Φ.020in. Therefore, if it meets the functional requirements, Figure 2 would be a more cost-effective recommendation.

Figure 2. The added size tolerances for the boss feature corrected the feature control frame.

Following the same logic to loosen the tolerance requirements and cut cost, I applied the MMC at the datum features A and C in this feature control frame in Figure 2. The warning is now gone.

Can we apply the MMC to the datum feature B? Let’s give it a try. Figure 3 shows the warning, “A material condition modifier applied to a feature that cannot have size tolerances.” Why? Because the datum feature B is a plane, not a feature of size. It can’t have size tolerances or the MMC at all, so the MMC doesn’t apply here and SOLIDWORKS MBD catches this error.

Figure 3. A warning against an incorrect MMC modifier to a datum plane.

As we construct a feature control frame on the Geometric Tolerance dialog in Figure 3, let’s pay attention to the warning messages at the bottom of this dialog as it guides us towards a more robust GD&T creation.

The above are just several quick illustrations of the grammar verifications against the MMC modifier. Now let’s go through the key compartments in a feature control frame to review more examples.

Symbols

There are 14 GD&T symbols, and they are used for specific feature control types. For example, if we change the position tolerance symbol in Figure 2 to a flatness symbol, the grammar verification will give us a warning as shown in Figure 4, “Boss5 is an invalid feature type for flatness,” because, obviously, it doesn’t make sense to define how flat a cylinder is.

Figure 4. An invalid flatness symbol applied to a cylinder.

The similar checking is conducted against other symbols. For instance, defining a circularity control symbol to the datum feature B (a flat plane) would be flagged.

Tolerances

The next compartment is for tolerances. As we move along, we need to keep in mind not only the validity of an individual compartment, but also its relationships with other compartments. That is, whether this compartment definition makes sense in the context of the entire feature control frame.

Here the grammar checking verifies whether a tolerance number or a tolerance zone is valid. For example, the tolerance needs to be a numeric value in the first place. There are several exceptions of letters such as the CZ in the ISO 1101:2012 standards, but for most cases, tolerances are numbers. Furthermore, the tolerance zone needs to be applicable to the corresponding feature control type. Figure 5 shows an invalid zone type specified for a cylindricity tolerance control. We can fix that by removing the diameter modifier Φ from this compartment. As a comparison, if we choose a concentricity control type, then the diameter tolerance zone modifier Φ will be needed in this tolerance compartment. Otherwise, the grammar check will throw out an error.

Figure 5. An invalid tolerance zone type specified for a cylindricity tolerance.

Datum features

Datum features are the foundation of GD&T, so there are very extensive checks in MBD against them. First of all, the datum feature needs to be called out before being referenced in a feature control frame. Otherwise, the GD&T dialog will remind us, “Datum X has not been defined.” Of course, we can still proceed here and define the datum feature X later to fix the reference.

In addition, the control type symbol is checked against a datum feature. In Figure 6, although a runout tolerance is valid on this cylinder, it’s invalid in the context of the datum feature A because a feature for a runout tolerance needs to be coaxial with the datum axis.

Figure 6. An invalid datum feature framework specified for a runout tolerance.

Besides the manual GD&T definition, the datum features are also checked in the auto dimension scheme as shown in Figure 7. The secondary datum feature shouldn’t be collinear with the primary datum feature. Otherwise, they would generate the same theoretical datum, the hole axis, which would be a duplicate.

Figure 7. An invalid secondary datum feature collinear with the primary one.

There are hundreds of rules built into SOLIDWORKS MBD. We can only show very few examples in this article. Please feel free to check out the product and discuss the grammar verifications in detail in the comment area below. To learn more about how the software can help you with your MBD implementations, please watch this video below and visit its product page.

About the Author

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

Lions and mammoths encounter one another in a painting in Chauvet Cave, France. The painting was made between 28,000 and 30,000 years ago.

Drawings have been a staple of planning and recording since prehistory. In caves across the world, humans began drawing by marking their interaction with nature across the walls of their fire-lit homes. As human technology progressed, so did our ability to illustrate ideas.

Fast forwarding through the eons, drawing took on many forms. Perspective was first honed by the Greeks, the Romans brought precision to the drafting table and the Middle Ages brought skewed perspectives with oblique visual descriptions of cities, castles and wars. Though drafting would progress, it wasn’t until the Renaissance that technical drawing came into its own. Working in Florence, Italy, a designer named Filippo Brunelleschi ushered in a new theory of drawing that contemporary people would understand as a blueprint. Using linear perspective, the Italian began constructing some of the most wonderful architectural feats that world had ever seen. To guide their construction, Filippo used his diagrams and, in doing so, introduced the world to technical drawings.

Since their invention, technical drawings have evolved. In the late 18th century, Gaspard Monge, a French mathematician, developed his ideas of descriptive geometry, codifying the planar views that manufacturers are familiar with today. In the middle of the 20th century, another seismic development occurred in technical drawing. Geometric dimensioning and tolerancing (GD&T) was created to make manufactured goods more consistent with their technical specifications by more precisely defining required precision and allowable variability.

Most recently, the technical drawing’s path has merged with another transcendent technology—the computer. Since the mid-1970s, technical drawing has moved off of the drafting table and onto the computer screen. Today, designers use CAD to create 2D drawings, 3D models and animations that describe how parts should be made. With CAD, technical drawings have become easier to create and share. However, engineers have started to realize that CAD drawings aren’t necessarily the best way to distribute, correct or archive manufacturing documents. Instead of relying on drawings, engineers have started to look for ways to combine the models they’re creating with the technical drawings that were based on those 2D and 3D forms. This new phase of technical drafting has been called model-based definition (MBD).

What Is MBD?

MBD is essentially a term that describes a 3D model that contains all of the annotation data that would be needed for a part to be manufactured. Aside from dimensions, an MBD model will communicate a component’s GD&T requirements, material information, configuration details, and other data that could be useful for anyone that might have input into the manufacturing of a design.

An MBD 3D model. (Image courtesy of Quality Digest.)

Who’s Using MBD?

As of now, MBD is still a nascent technical drawing paradigm. However, industrial catalysts like the U.S. Department of Defense (DoD), select companies in the aerospace industry and the automotive sector have started to insist that any vendors working on affiliated projects have to produce 3D MBD models.

One of the biggest factors driving this trend is the lifespan that some DoD and aerospace products see before they’re eventually retired. For the DoD or Boeing, having a machine run for 20, 30 or even 50 years isn’t out of the ordinary. With service lives that stretch out that long, its easy to see that building design information directly into a model can be invaluable. Who’s to say that the designer of a particular system on a legacy aircraft will be around to impart his or her design intent to the younger engineers deputizing the design work when something goes wrong down the road?

Another benefit that MBD offers engineers is the ability to use their models as a verification datum upon which they can automate inspection of manufactured parts. With all of its dimension and definitions built right into the model, MBD components can be used in conjunction with coordinate measuring machines and 3D scanners to ensure that a part meets documented manufacturing standards without having to reference outside drawings. All of the information that would be needed to compare the two parts would be right there attached to the model, making an often painstaking and time-consuming process much easier and quicker, while also enabling more process automation.

Aside from its communication and validation benefits, MBD has also been able to deliver cost-savings to those who have adopted the emerging documentation trend. Since adopting MBD principles, Toyota, Boeing and BAE have demonstrated up to a 50 percent reduction in costs in the product development processes. What’s more, the Naval Air Warfare Center Aircraft Division based in Lakehurst, N.J., has noted that after implementing MBD into its design workflow, the cost of labor used in component fabrication has dropped 30 percent.

Why Adopt MBD?

In the end, MBD is valuable because it preserves the geometry of a 3D model while still delivering all of its manufacturing information that’s usually associated with a 2D technical drawing. Because MBD relies on 3D geometry, it gives manufacturers or anyone interacting with a 3D model greater insight into the design intent of each feature as well as a more natural way to interact with a virtual part. Though that might seem simplistic in its promise, machinists who have to transform raw stock into goods could really benefit from visualizing a part in 3D before deciding on CAM setups and toolpaths.

Furthermore, design teams working across time zones could use MBD models to better understand their colleagues’ design requirements without having to sift through reams of paper drawings and manually searching for minute bits of information.

Finally, as we’ve seen, drawings have been evolving ever since man wanted to communicate an idea down through the ages. While MBD, with its 3D nature and its intrinsic link to computers, might seem completely disconnected from the cave paintings of old, when you take a step back, you notice that both techniques are trying to do the same thing—communicate an idea in the most sophisticated and enduring way possible so that good ideas find fewer ways to be lost.

If MBD is going to be the next evolution in drawing’s long history, the age-old trade could have done worse. With its ability to condense information into an encyclopedic form, MBD appears to be an excellent way forward for technical drawing.

About the Author

Kyle Maxey is a mechanical designer and writer from Austin, TX. He earned a degree in Film at Bard College and has since studied Mechanical and Architectural drafting at Austin Community College. As a designer Kyle has had vast experience with CAD software and rapid prototyping. One day he dreams of becoming a toy designer.