I don't think all of the tools are readily available yet.
Based on my experiences (dealing with MBD in CATIA format from our
customers)
1) Non-geometric information (attribute info) is added as notes to the
model tree. To be sure we have been alerted to all of the attribute
data we have a small program that will 'slurp' all of the attribute data
from the model, and 'belch' the results into an Excel spreadsheet.
2) Design intent is based on extensive use of GD&T definitions. You
have to be very comfortable with using GD&T for everything. Having the
GD&T symbol highlight the surface, line, etc... where it is applied is
essential to understand the scope of the requirement. I don't
understand the question about construction features, although range of
motion, etc... issues can be shown as 2D sketches, with the added notes
in the tree. Testing requirements, tensile test locations, etc are
shown in zones, like keep in / keep out definition in Pro-E
3) For production workers, we create in-house documentation that
distills the needed info in a compact format. 2D drawings (in PDF
format) are used. For tooling and fixturing work, we create 3D STEP
files for use by outside vendors, and then create a 2D .pdf files for
illustration of critical features and GD&T requirements. The drawing is
not a complete definition, and requires the 3D model for many details of
the geometry. As a nice benefit, all of our documentation is reduced to
'A' sized sheets. Easier to print, and easier to store.
4) No models used are allowed to be released that deviate from
requirements. The model has to be constructed to meet the requirements,
or the requirements are modified to meet the model. The exception to
this may be very small fastener features on large parts like rivets
holes, etc... In this case the features can be shown schematically as a
point location, with a note relating back to a specification that
describes the condition of the feature.
Now 'design intent' with regard to hole locations, etc... where the
construction may have been mirrored from a central plane, but the
drawing may show dimensions coming from a edge is eliminated. No
location dimensions are shown for holes, only the true position location
to the appropriate datums in the model. Datum callouts, whether point,
zone, or surface must be visible on the model.
I agree that 2D drawings were not just a mechanical document, but also
told a 'story' of the part, what was important, and how to manufacture
the component. This is the toughest thing I see to implement. A CMM
working off of the MBD may close this loop and be a check and balance to
manufacturing although the designer and inspector may still disagree,
their conversation is driven higher up the 'food chain'.(which is a good
thing) I don't see how manual inspection is feasible in most cases
without very deep inspection of the MBD, and/or supplemental 2D
documents.
In my opinion, for MBD to work your downstream users of design
information must have very good tools to interpret the MBD, and these
people need the most training and collaboration. It is also essential
that the machine operators and the part inspectors be two different and
uninfluenced groups. I don't see how the part can be inspected in the
machine it was made on, but this is a topic for a different discussion.
Can MBD reduce cost? I believe so. Much like Philip Crosby's mantra of
'Quality if Free' is true, if you are strong enough and clever enough to
use this tool to best advantage.
Theoretically, MBD will reduce errors, but like all things, requires the
organization to be transformed. Some type of document control and
access system seems to be mandatory, and the review and markup tools,
(in my opinion) still need work. For one-off tooling and fixturing, I
would hope that STEP-NC (or similar) would take off allowing the design
to be reviewed and programmed at the machine tool, if the proper GD&T
information can be embedded in the STEP file.
Christopher F. Gosnell
FPD Company
124 Hidden Valley Road
McMurray, PA 15317
