The concept of attaching data to CAD models, such as GD&T, seems to be growing. Using model-based definition, the industry is beginning to take advantage of the technique.
We are used to thinking that programming metrology equipment is more art than science. While electronic CAD models are now imported by most metrology software, to create a measurement program quality professionals often still glean tolerances and GD&T by poring over 2D paper drawings, decorated with call-outs and notes. Through an alchemy of critical judgment and experience, inspectors who create measurement programs, “wizards” if you will, craft a measurement routine. Another programming wizard looking at the same data might craft a different program. Even though both create valid routines, the results are inconsistent, sowing doubt. The process is time consuming to boot. Wizards must be skilled.
Is there a better way? Maybe, with model-based design (MBD).
MBD is the technology of attaching GD&T to a CAD model, typically with full “smart” associativity to create a semantic model. A semantic model—theoretically at least—allows for smart software to automatically create a measurement program from that semantic CAD model.
That is what the software program Verisurf does, according to Ernie Husted, president and CEO of Verisurf Software Inc. (Anaheim, CA). “We started developing this about 20 years ago when an aerospace customer needed profile tolerances reported on different surfaces of an airplane.”
In those days, manufacturers used a map with colors showing different tolerances as part of the model. Husted admits that it was an antiquated way of showing tolerances with a CAD model, although state-of-the-art for its time.
In the late 1990s, the idea piqued the interest of one of the giants in the aerospace industry. “Boeing partnered with us to develop an associated tolerance to a CAD model and its surfaces, and that is where it started,” explained Husted. “The goal is always to eliminate drawings, or blueprints—to go paperless.”
He now sees MBD starting to catch on. “This is especially true now that [some of] the CAD systems can associate [tolerancing to their CAD models],” he said. He cited four CAD models as most prominent in providing attached MBD to their CAD models—CATIA and SolidWorks from Dassault Systemes, NX from Siemens PLM Software, and Creo from PTC.
Just as important as creating CAD models with semantic MBD attached is the ability of downstream programs to read MBD models and create the programs. Verisurf is composed of four modules: CAD, Measure, Automate, and Analysis. “Once Verisurf receives a model with intelligence built into it, we can automatically pick up those tolerances for our inspection in real time and create an inspection plan,” using the Automate module within Verisurf, he explained.
It becomes a matter of a user simply choosing what CMM device to program and clicking on the surfaces of the CAD model in the software. The semantic MBD/CAD model then tells Automate what GD&T is associated with it. Verisurf/Automate uses a predefined set of rules to create a program for the desired device.
Verisurf’s optional Universal CMM software means it will program and drive any common brand of CNC-driven CMM. “You eliminate interpretation and common errors, like picking the wrong tolerance because you couldn’t read the print or simply typing in the wrong numbers. There is always room for error in how it is done now,” he said.
The right combination of valid semantic model and software that can use it creates consistent, easy-to-create measuring programs. Engineers can have confidence in accuracy. Programming wizards can now be apprentices, not masters, and still create good results.
Besides speed and consistency, another benefit Husted sees to creating inspection programs from smart MBD/CAD models is that it now becomes a soft gage or inspection gage. “With a smart CAD model, I can take a portable measuring device and measure a point anywhere and it can tell me if it is within or outside tolerance,” he said.
Acceptance and Hurdles
Has the practice gained wide acceptance? “As far as industry as a whole, we are seeing aerospace
and defense customers as leaders in attaching MBD to their CAD models, with a slower adoption in other industries,” said Scott Lowen, software product manager for Carl Zeiss Industrial Metrology LLC (Brighton, MI).
For its use to grow, CAD models with MBD must available. A knowledgeable engineer must attach the original GD&T (or tolerances) to the CAD model. A downstream manufacturer or job shop that will make the part should be receiving a smart model, not trying to make one.
“They rely on their OEM customer to do that. If it doesn’t have the PMI attached to the CAD when it gets to the job shop, there is a question if they would find any advantage to them adding it themselves,” Lowen said.
Outside of accepted communities that use such smart models, such as aerospace and defense, MBD is a new concept for most manufacturers. With anything new, it takes a while to get it right.
“People we talk to don’t complain about the process being too expensive, but we have seen struggles in how to properly apply it,” said Lowen. Calypso is the Zeiss software for metrology programming, capable of creating measuring routines both for Zeiss equipment and wide array of other machines as well. “We take in the ‘big four’ CAD model definitions and those four have their own proprietary way of attaching MBD,” he said. For example, CATIA models use their own FTA format.
There is a STEP format, AP 242, for attaching MBD to a generic STEP CAD model. Its implementation points to some of the difficulties in the field today. A CAD model with MBD attached with associativity is a fully semantic, smart model. However, there is another way of attaching it, with no associativity, according to Lowen.
“Non-semantic PMI is lines and circles that are human readable, but with no IT intelligence as to what it actually is meant to represent. You cannot automatically program a measurement routine using that kind of MBD,” he said. “This has created some misunderstandings of MBD, because the AP 242 standard allows for both. A CAD model with attached MBD to the AP 242 standard may mean the model is not intelligent—and cannot be used for automatic programming. There are still benefits, because all the information is in one place. But it is not as useful as a smart model.”
This and other issues remind him of the adoption cycle for CAD. “The whole industry went through the exact same thing when CAD models were first introduced to the metrology industry 20 years ago,” he said. Not all CAD models were built correctly. They could be inconsistent—or just plain wrong. Then, everyone downstream from manufacturing to service had problems. “It was all part of implementing the new technology of CAD. Now we are seeing the same behaviors in the same companies with the same problems with introducing MBD attached to the CAD models,” he said.
Pathways to Adoption
Cyle Caplinger, senior sales manager, software, for 3D Systems Inc. (Rock Hill, SC) agreed that while most of the CAD suppliers have adopted MBD in their CAD programs, uptake by engineers is slow. “Best practices in MBD for engineers tend to be slowly adopted and time consuming to establish within each engineering group,” he said.
When it does happen, MBD implementation tends to begin with senior engineering management. “The investment in both time and resources to fundamentally change long-held and proven workflows rarely occurs from the user level up,” said Caplinger. “[Adopting] MBD requires users to approach the design process a bit differently, with more consideration to the full lifecycle of the design which includes the manufacturing and inspection processes,” he said. Change is hard for anyone and without senior management encouraging—even requiring—the change, it is easy to stay in one’s comfort zone.
“The biggest issue is with training,” he said. Not training of the quality professionals, but the originating design engineers. There are many engineers who are so far removed from the actual manufacturing process that they do not really understand GD&T and the impact it has downstream, Caplinger noted. In response, Geomagic Control X software makes it easy to implement GD&T tools.
“Over the past several years, an increasing number of manufacturers [have begun] implementing quality control processes throughout the manufacturing chain, not just at the end of the process in the metrology lab,” he said. There are numerous benefits to doing that—reduced waste, increased efficiency, or accelerated time to market.
But it also poses several challenges: It requires metrology tools to be simple enough for the shop floor while maintaining the level of sophistication required by the metrology team. It also adds complexity to the challenge of file management. “We have spent considerable time making Geomagic Control X both powerful and easy-to-use. When Geomagic Control X is combined with MBD and a good PLM system, the opportunity for errors is drastically reduced,” he said.
Pathways to Communication
Another opportunity for improvement is breaking down “information silos” that tend to grow and isolate functions within a manufacturing organization, according to Ray Stahl, business development director for the EVOLVE suite of products from Kotem (Budapest, Hungary), a QVI company. “Manufacturing is doing what the information technology world did a decade ago, breaking down the barriers of proprietary architectures that form around metrology companies, CAM companies, and CAD companies and the functions they serve,” he explained.
Kotem’s EVOLVE Design, part of the EVOLVE Suite, includes in its functions a computer-aided engineering (CAE) tolerancing software, which helps engineers apply correct GD&T to their CAD models. “What our company does is make GD&T available from design through inspection and on to customer service,” he said. “The state of the art in moving GD&T around is very fragmented, and still highly proprietary with CAD companies providing GD&T in their own formats,” he said.
Stahl is a proponent of using either semantically correct STEP AP 242 models or the emerging Quality Information Framework (QIF) formats—both of which EVOLVE can process. In partnership with another company, Capvidia, they provide a solution that can read many popular CAD formats, extract GD&T in a semantic model and define and proof the tolerances. In the future Kotem will also calculate tolerance stack-up of assemblies and generate reports.
Using a product like EVOLVE can help with the lack of training among design engineers who will be tasked with properly specifying GD&T. The American Society of Mechanical Engineers (ASME) has found that only 17% of graduating engineers have been trained in GD&T, he said.
Information silos can exist within sight of each other, say, between a metrology machine and nearby machine tools that need to use its data. Quality Vision International Inc. (Rochester, NY) a provider of vision measuring machines and CMMs, also provides its ZONE3 software to both generate measurement routines and provide analysis. It imports CAD models with MBD attached to auto- mate creating routines, a good thing.
But the software also exports the data that was measured so it can be used to automatically adjust machine tool parameters in a closed loop system, an even better thing.
Closing the loop between data collection and data use is a sure way to start breaking down information walls and silos. “There is a database behind the scenes where all of this information about the part and its product lifecycle is stored,” said Tom Groff, vice president of sales for QVI. “That database holds information about the part, its tolerances, its design features, and its intended measurement path.”
Consistency is key. “MBD has within it information on how it should be reported,” explained Groff. “So, when customers and vendors and suppliers look at the same part, they are looking at the same report. They are not confused.”
Digital Thread, Changing Roles
There is a wider arena for which MBD is vital—establishing an appropriate digital thread that can feed data to digital twins. There are a variety of motivations pushing for a digital thread. Attaching MBD to CAD models for use in expediting metrology measurements both feeds data to build measurement programs, but also helps companies adapt to the changing roles of their workers, according to Ken Woodbine, president, metrology software product line for Hexagon Manufacturing Intelligence (North Kingstown, RI).
“What we are seeing is a tendency in the industry to rely more on differently skilled people to operate metrology equipment,” he said. He noted that today’s shop workers are absorbing a number of responsibilities and should not be required to focus solely on creating and interpreting measurement routines. “We have to make sure we tailor the experience to their skill set. That means making it really visual and really easy to create and execute the routines that are going to produce the information they need,” he said.
For the metrology side, that means using all available information used in production to streamline the creation of measurements. “That means embedded GD&T with a CAD model,” he said. According to Woodbine, it tells you what you need to measure and informs on how to do it. “[That means] building a database or knowledge base through machine learning that indicates best practices for that part of the process,” he explained. “So, if you use all the available manufacturing information and leverage the MBD, you can create measurements automatically and rapidly achieve the goal of metrology, which is collaboration based on actionable intelligence.”