Recently, Boeing began using model-based definition (MBD) to see if it could improve productivity and reduce time to market.
MBD is a method of annotating computer-aided design models with
geometric and tolerancing information that allows engineers to draw
directly onto a 3-D model. It shows promise for improving and
accelerating design, manufacturing, and inspection processes.
A group of researchers from École Polytechnique Montréal and the
University of Quebec École de Technologie Supérieure set out to
investigate the feasibility of MBD's one day taking the place of 2-D
engineering drawings used throughout industry today.
Traditional engineering drawings are still essential for capturing
and distributing non-geometric information at most companies today,
notes Louis Rivest, research leader. He stressed they are looking at the
feasibility of eliminating the 2-D drawings, not eliminating paper
itself.
From ASME Y14.41: figures illustrate (from top) a solid
modelwith all annotation displayed, with one type of annotation
displayed, and with selected annotation displayed.
View larger
Members of industry approached the team to find out whether MBD was
ready for prime time, whther it could benefit industry by speeding
product development and time to market, and the feasibility of a
large-scale move across the engineering profession to model-based
definition.
Based on Standards
For a long time, the industry has been moving toward tolerancing
solid models rather than two-dimensional models. MBD has been rooted in
the 2003 ASME Standard Y14.41-2003 Digital Product Definition Data
Practices, which sets requirements for CAD software developers to follow
for tolerances, dimensional data, and other digital design annotations
on 3-D solid models.
Tolerancing is showing dimensions and tolerances on a model. Prior to
this standard, nothing in the industry dealt with displaying
tolerancing on 3-D models, mentions Alex Krulikowski, chairman of the
Committee on Solid Model Tolerancing that helped spearhead the standard.
The standard addresses the depiction of tolerances in model-viewing
mode: rotating the model rotates the tolerances with it. It also ensures
that engineers and manufacturers are communicating tolerancing in an
accepted way, so a user knows how to find them on a drawing, how to
interpret them and read them, Krulikowski explained.
Unique Implementation
With the tolerancing standard in effect, the researchers interviewed
34 representatives from two major Canadian aerospace manufacturing
companies and 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 actual engineering, manufacturing, and inspection processes would
need to be redesigned if MBD, rather than 2-D drawings, were to be used
depends on the adopting industry. "Releasing the engineering drawing is
a process; each company has thousands of processes defined. We couldn't
address them all. We concentrated on what would need to be done to make
the best use of MBD data and get rid of engineering drawings." Rivest
said.
Digital Product Definition Data Practices specifies the form for
entering queries ... as in these examples, for a question of size
tolerance (above) and geometric tolerance (below).
View larger
Ensuring long-term storage and appropriate archiving of 3-D models that
include dimensioning and tolerancing information, given the speed at
which CAD systems change and the consistent incompatibility among
systems, is being investigated separately.
Simplified Viewing
Industry-wide, when 3-D models replace engineering drawings,
downstream users who currently rely on these drawings would need another
method for communicating product dimensioning and tolerancing data.
Many non-engineer users have no access to or familiarity with CAD; a
lightweight format is needed to communicate this information, said
researcher Virgilio Quintana.
Viewer programs convert MBD data from different CAD applications to
lightweight format files, containing the model along with dimensions,
tolerances, annotations, management information, and revision history.
These files could be readily opened, read, and understood by users
without CAD experience, he explained.
Inspection Aid
According to Rivest, inspection processes would be quicker and
potentially even more accurate by adopting the MBD format. Time savings
would occur by minimizing operator inputs required during programming
and running coordinate measurement machine inspections since model-based
software would compare parts against tolerances contained within the
CAD model itself rather than from engineering drawings whose tolerances
traditionally had to be derived from 3-D data.
The accuracy and integrity of the inspection process would be
improved by the software's ability to check contour and hold positions
as well as constraints, such as flatness, concentricity, and angularity.
Model-based software also enables automated inspection routines, which
ensures parts are inspected in the same way, in the same places, and
with the correct tolerances every time, wrote Rivest, Quintana, and
others in Computers in Industry (March 2010).
A Matter of Trust
Overall, the Canadian researchers found evidence that moving away
from 2-D drawings is now feasible for most industries, and does offer
real benefits. Some barriers could be addressed with simple technology,
but some are cultural and difficult to change.
People still aren't fully convinced about MBD, Quintana said. It's
hard for them to trust an electronic file versus a printed 2-D drawing
kept in a safe place. Still, they may have to justify making the changes
sooner rather than later to supply or compete with bigger companies who
are early adopters.
[Adapted from "Digital Tolerance," by Jean Thilmany, Associate Editor, Mechanical Engineering, July 2010.]
Recent Comments