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Digital Tolerence PUBLIC ACCESS

Model-based Definition Could Spell the end for Traditional Drwaings, But First, There's and Issue of Trust to Address.

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Mechanical Engineering 132(07), 32-34 (Jul 01, 2010) (3 pages) doi:10.1115/1.2010-Jul-3

This article demonstrates the use of model-based definition (MBD), which can lead to improve productivity and reduce time-to-market. MBD is a method of annotating 3D computer-aided design models with geometric and tolerancing information. With Boeing moving to model-based definition, members of industry have approached the team in Montreal about investigating MBD’s usefulness. Experts believe that the MBD format, if widely adopted, would have implications for inspection. The part would be inspected against the tolerances contained within the CAD model rather than against the 2D engineering drawings, and that would speed the process and potentially make it even more accurate. The cultural change that will have to take place at most engineering companies will be difficult to tackle, and engineering and manufacturing managers know this. A model-based inspection software is being developed to read geometric, dimension, and tolerance information attached to a solid model. This will minimize operator input during development of coordinate measuring machine programs and improve the accuracy and integrity of the inspection process.

About five years ago, Boeing began using what the company's executives term model-based definition to see if it could improve productivity and reduce time to market.

Model-based definition is a method of annotating 3-D computer-aided design models with geometric and tolerancing information. Some wonder if it can replace the traditional two-dimensional engineering drawings used throughout industry today.

While the jury is still out on whether model-based definition, or MBD, can indeed improve and accelerate the design, manufacturing, and inspection process, a group of researchers from École Polytechnique Montréal and the University of Quebec École de Technologie Supérieure said it shows promise, though some barriers to common use still need to be worked out.

They base their research on more than 34 interviews with representatives from two major Canadian aerospace companies that wanted to see what the process could do for them.

MBD stands in for traditional engineering drawings, which still play an essential part in the capture and distribution of non-geometric engineering information at most engineering companies today, both big and small, said Louis Rivest, a professor of automated production engineering at the École de Technologie Supérieure. He and fellow researchers, including Virgilio Quintana, a Ph.D. student at the same institution, set out to look at the feasibility of MBD's one day taking the place of engineering drawings.

The Canadian aerospace manufacturers had asked the researchers to investigate the feasibility of a large-scale move across the engineering profession to model-based definition and also to look at how MBD might benefit industry. In addition to potentially speeding product development and manufacturing, might it also, for example, ensure a more accurate inspection process?

CAD tools—now widely adopted by industry—have changed the way engineers work, Rivest said.

“We’re trying to answer two questions: Is it possible in the near future to get rid of traditional 2-D engineering drawings, and if the answer is yes, then how much benefit could that bring?” he said.

Rivest stressed that the researchers are looking at the feasibility of eliminating traditional 2-D engineering drawings, not eliminating paper itself.

A definition of MBD is in order. It allows engineers working on CAD models to make drawings directly onto a 3-D model, thus doing away with the need for the traditional engineering drawings. The method has its roots in ASME Y14.41-2003 Digital Product Definition Data Practices, which sets the requirements for technology users to follow when annotating model tolerancing on 3-D solid models.

The standard, published by ASME in 2003, sets the requirements for tolerances, dimensional data, and other digital design annotations. Prior to ASME Y14.41, nothing in the industry dealt with the display of tolerancing information on 3-D models, said Alex Krulikowski, president of Effective Training Inc., a geometric dimensioning and tolerancing consultancy of Westland, Mich.

It was as chairman of the ASME Y14.41 Committee on Solid Model Tolerancing that Krulikowski helped spearhead the standard.

The standard is also meant to guide CAD software developers as they improve their products’ modeling and annotation capabilities, he said.

“For a long time in the industry, we’ve been moving toward tolerancing solid modeling rather than two-dimensional modeling,” Krulikowski said at the time of the standard's adoption. “Tolerancing means you show the dimensions and tolerances right on a model.”

The standard addresses the depiction of tolerances in model-viewing mode. When a user rotates the model in model view, the tolerances rotate with the model.

Engineers and manufacturers can use the standard to communicate model tolerancing in an accepted way, Krulikowski said. “If one company shows model tolerances one way and another company another way, the user doesn’t know how to find them on the drawing, or how to interpret them and read them,” he said.

With the tolerancing standard in effect, and with Boeing moving to model-based definition, members of industry approached the team in Montreal about investigating MBD's usefulness. They wanted to find out whether the method was ready for prime time and whether it could benefit industry by speeding product development and time to market.

The research, led by Rivest, began three years ago by interviewing 34 people in the engineering, drafting, configuration management, airworthiness and certification, manufacturing, inspection, and knowledge management departments at the two large Canadian aerospace companies.

“We identified two technical issues that had to be addressed,” Rivest said. “If we want to get rid of 2-D, we’ll have to move tolerancing into the 3-D mockup. But are the tools ready for this? Do they work well enough so we can get rid of the role traditionally supported by 2-D drawings?”

The team also looked at how engineering, manufacturing, and inspection processes would need to be redesigned if model-based definition, rather than 2-D drawings, were to be used.

The second question—now being investigated separately—is how to ensure that 3-D models that include dimensioning and tolerancing information can be stored long term and appropriately archived, given the speed at which CAD systems change and the consistent incompatibility among systems.

The researchers found that the move away from 2-D drawings is feasible for most industries, though it faces some barriers that could be addressed through simple technology changes.

How the actual engineering processes themselves would change with the adoption of model-based definition depends on the industry doing the adopting, Rivest said.

“Releasing the engineering drawing is a process and each company has thousands of processes defined; we couldn’t address all the processes,” he said. “We concentrated on what would need to be done to make the best use of the MBD data set and get rid of engineering drawings.”

From ASME Y14.41: figures illustrate (from top) a solid model with all annotation displayed, with one type of annotation displayed, and with selected annotation displayed.

Grahic Jump LocationFrom ASME Y14.41: figures illustrate (from top) a solid model with all annotation displayed, with one type of annotation displayed, and with selected annotation displayed.

The team soon realized that one change would need to take place across industry with the adoption of model-based definition. If engineering drawings were no longer available to the non-engineers who rely on them, companies would need another method to communicate product data information, including dimensioning and tolerancing. After all, many of these users wouldn’t have access to CAD tools nor would they likely be familiar with those that include tolerancing information.

“When you replace engineering drawings with the 3-D model and its dimensions and tolerances, we realized that this information needed to be available to downstream users and you can’t rely on the CAD format,” Quintana said. “So we really need to rely on a lightweight format to communicate this information with downstream users.”

The system—essentially a viewer that could access many CAD formats—would contain the model along with dimensions, tolerances, annotations, management information, and revision history, all of which could be readily opened, read, and understood by users without CAD experience, he added.

“The MBD model would be in the CAD application, and the lightweight format would be the file exploited by users downstream,” he said. “This viewer is capable of converting MBD data from different CAD applications to a lightweight format.”

Digital Product Definition Data Practices specifies the form for entering queries as in these examples, for a question of size tolerance (left) and geometric tolerance (below).

Grahic Jump LocationDigital Product Definition Data Practices specifies the form for entering queries as in these examples, for a question of size tolerance (left) and geometric tolerance (below).

According to Rivest, the MBD format, if widely adopted, would have implications for inspection. The part would be inspected against the tolerances contained within the CAD model rather than against the 2-D engineering drawings, and that would speed the process and potentially make it even more accurate, he said.

Traditionally, the inspection process has been carried out using 2-D toleranced drawings derived from 3-D data because there was no dimensioning and tolerancing information attached to the solid model, but the MBD concept is changing this approach, the researchers wrote in a paper, published in the March 2010 edition of the journal Computers in Industry. Coauthors with Quintana and Rivest are Robert Pellerin of the Department of Mathematics and Industrial Engineering in the École Polytechnique Montréal, and Frédérick Venne and Fawzi Kheddouci of the automated production engineering program at the École de Technologie Supérieure. The University of Sherbrooke of Sherbrooke, Quebec, also lent expertise to the study.

“Now, model-based inspection software is being developed to read geometric, dimension, and tolerance information attached to a solid model. This will minimize operator input during development of coordinate measuring machine programs and improve the accuracy and integrity of the inspection process,” they wrote.

The software will check contour and hold positions as well as constraints, such as flatness, concentricity, and angularity, Quintana said. It will also allow CMM operators to set up automated inspection routines, ensuring that parts can be inspected in the same way, in the same places, and with the correct tolerances every time, he added.

But will the advantages of model-based definition justify the effort to change engineering, manufacturing, and inspection practices that would need to take place with a move away from 2-D engineering drawings?

“What we found with our interviews is that people still aren’t fully convinced about MBD,” Quintana said. “It's hard for them to trust in an electronic file versus a 2-D drawing. If you can print something and keep it in a safe place—well, it's hard for them to trust in the technology yet.”

The cultural change that will have to take place at most engineering companies will be difficult to tackle, and engineering and manufacturing managers know this, he added. Still, they may have to take on the move sooner rather than later.

“If the bigger companies like Boeing are already moving to MBD, they’ll force their suppliers to move too in order to work together to exchange information,” Quintana said. “Those bigger players will play a great role in introducing MBD to everybody. They have the power to force others over.”

Standard Update

The ASME Y14.41 committee is preparing an update of the standard, which may be published in 2011.

Some of the changes will reflect updates in ASME Y14.5-2010, the standard for stating and interpreting dimensioning, tolerancing, and related requirements for use on engineering (two-dimensional) drawings. Other changes will reflect experience in industry since the original Y14.41 was issued in 2003.

Copyright © 2010 by ASME
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