shaping the future of metrology - Brown & Sharpe

probing speed, probing method (singlepoint
or scanning), and the computation
power of the CMM's software.Yet, throughput
rather than speed is essential for reducing
reverse engineering time cycles.There
are CMMs which get more throughput
via a little intelligence. Computers help
them to follow an optimal path, avoid obstacles,
and speed up between
probing points.
In the case of reverse
engineering applications,
throughput includes the
time taken to probe the
original sample part, write
the inspection program for
the reverse engineered part,
measure this part, and compute
the results accurately.
Off-line part programming
can enable an average-speed
CMM to have an acceptable
total throughput time.
Speeding Up
The Process
Companies can often save weeks of
development time by scanning a sample
on a CMM and then “reverse engineering”
surfaces from the point data instead
of modeling surfaces directly on a CAD
system.The geometric data from the
CMM will be generated in industry-standard
formats such as IGES,VDA-FS, ISO
G-Code, DXF and delimited ASCII and
CAD, CAM; and analysis packages support
at least one of these formats.
Once the digitized image is captured,
users still need the right CAD software to
speed up the reverse engineering process.
Ideally, CAD software should be equipped to:
• Import geometric data of virtually
any format.
• Work with point data, often on the
order of several million data points.
• Work with contoured surfaces from
creation through modification and
analysis.
• Output geometry to downstream
processes.
• Analyze geometry to evaluate form
integrity with the sample.
34 mfg. Shaping the Future of Metrology
Most important, the software should
allow the user to visualize the part in a
3D perspective. A 3D model fully defines
the shape of the part, eliminating the
need for multiple view projection. Designers
can rework surface contours, and
toolmakers can then machine parts from
the electronic mockups.
Data point cloud from an automotive air bag cover.
The cloud was converted into a curve network and a
surface model.
The idea is that the software should
accelerate the reverse engineering time
cycle by:
• Improving the quality of surfaces by
creating smooth, continuous curve
networks.
• Cutting the time needed to prepare
engineering documentation.
• Eliminating the need for prototypes.
• Increasing product quality with a
variety of analysis tools.
Rapid Prototyping
Of the several CAD packages available,
mold makers may prefer Imageware’s
Surfacer which generates high quality
curve and surface geometry from digitized
3D point data. “Point cloud” data
collected by probing can be visualized on
the computer screen in various forms in
order to construct surfaces.
The process of building and verifying
production tooling is time-consuming
and expensive. Surfacer helps streamline
this critical process by providing quick
and complete verification of the most
complex, free-form shapes. Users can
precisely align scan data with CAD
geometry to evaluate the differences between
sample and engineered
parts.Variations can
be calculated and displayed
as color-coded plots, clearly
illustrating 100 percent
geometric inspection.
Surfacer’s Rapid Prototyping
Module (RPM) can
quickly produce prototypes
from digitized data or surface
geometry from other
systems.This shortens the
production cycle between
digitizing physical prototypes,
creating CAD models,
then finally generating
prototypes. A new "Rapid
Tooling" option to RPM
dramatically improves the use of rapid
prototyping technology for creating prototype
tooling.
Working In 3D
Other CAD packages may be more
suited for product re-engineering. One
CAD/CAM solution which might become
the industry standard is CATIA ® .
Developed by Dassault Systèmes of
France, CATIA enables designers and engineers
to work simultaneously on design
problems and manufacturing details
without actually building prototypes. Designers
are able to create “virtual prototypes”
to determine which one would
best meet the needs of the real world.
The idea is to do more work on simulating
manufacturing techniques before actually
making the part on the machine.
The CATIA/CADAM system provides
products particularly suited for reverse
engineering:
• A bi-directional bridge for moving
data back and forth between CATIA
and CADAM environments.
• An interface can upload or download
a model to another CAD/CAM
system.
• An interface product which translates
data between the CATIA system
and other CAD/CAM/CAE systems
using the Initial Graphics Exchange
Specification (IGES) neutral format.
• A 3D functional dimensioning and
tolerancing product which facilitates
the specification of tolerances
for exact solid models and adheres
to international standards for tolerance
rules and syntaxes.
• Downstream applications which can
use the resulting model
for further design, analysis,
manufacturing, and
other operations.
• A feature-based design
product that enhances the
designer’s ability to create
prismatic parts by incorporating
features such as
a single process for creating
a family of parts or a
design for manufacturing.
• A 2D/3D integration
product that fully integrates
the 2D and 3D
environments.This product
enables designers and
drafters to create projections
and planar sections from 3D
geometry for 2D drawings.
Making Quality
Part Of The Process
Once the part is made, it has to be inspected.
If different brands of CMMs are
being used, an inspection package that
works to an open standard is a must.
One such program is PC-DMIS ,a
PC-based software package that works
within the WINDOWS operating system.
The software provides a bi-directional
link between the CMM and the CAD software
that enables the CAD model to be
used to scan the workpiece. In effect, the
CMM’s direct computer control (DCC) is
driven by PC-DMIS and the measured re-
sults are compared with the CAD model.
Another package is VW-Gedas’ audimess
which creates programs in the
DMIS (Dimensional Measuring Interface
Standard) format, a standard for exchanging
data between CMM software solutions
and CMMs. Such a standard makes the
program “CMM-independent”.
The audimess system operates within
CATIA and uses CATIA data to create,
modify and simulate the inspection program.
Since the programming is also
done off-line, it can be modified on the
screen at any time.This reduces the costs
of inspection programming as well as
enabling CMMs to be used for actual inspection
tasks only.
Working on the CAD model, the user
aligns the part, determines the probe
path and selects the features to be inspected.Then
audimess generates the
DMIS program which is interpreted by a
post-processor to convert the program
for the required CMM.
The whole program created by the
audimess user can be simulated and
graphically shown on the monitor
screen.The starting point of simulation
has to be selected and the probe will
move along the programmed path on the
screen. Simultaneously, a small window
shows the actual DMIS code.
Currently audimess runs on Silicon
Graphic workstations, IBM RS/6000 or
mainframes running MVS or VM.
Transition From
2D To 3D
Dimensional data and the model constructed from
it are compared. Combining CMMs with modeling
software offers a fast, accurate means of reverse
engineering complex shapes.
The availability of all the above software
should be enough to convince any
user about the advantages to be obtained
through a CAD/CMM systems interface.
The key lies in making the transition
from old-fashioned 2D CAD to the latest
3D techniques. Here,
engineering management
has to focus on
the correct application
of 3D design to
speed up reverse engineering
processes.
Apart from training,
the change from
2D to 3D requires the
creation of new standards
for product and
part engineering documentation.This
can
be easily justified,
given the strategic importance
of building
reverse engineering
skills in today’s competitive
environment.
Finally, keep reverse
engineering in mind
when planning the
next CAD or CMM investment. o
Nitin Shankar is an electronics engineer and
Brown & Sharpe’s Area Manager in India. He
is a frequent contributor to a number of trade
and technical magazines.
Circle 711 on the READER SERVICE CARD
mfg. The Brown & Sharpe Publication of Precision Manufacturing
35