MCAD Modeling Methods: Scan This15 Dec, 2004 By: Don LaCourse
Reverse engineering starts with 3D scanning
Most new products today are redesigns of older products. Some of those old products weren't modeled in 3D and so pose a reverse engineering problem that MCAD software developers have addressed through increased support for point-cloud data input from CMM (coordinate measuring machines) and 3D laser scanners.
Figure 1: VX CAD/CAM provides built-in tools for importing and working with large point-cloud datasets. This cloud contains more than 30,000 points. VX has special algorithms to efficiently handle datasets with millions of points.
Traditionally, CMM has been used as a quality control device for checking parts to ensure they are within dimension specifications. Eventually, this coordinate point data from CMM started to find its way back into 3D CAD systems. Designers wanted to see the dimensional variations that their parts were experiencing under normal manufacturing conditions to help them come up with better designs in the future.
This process led designers to reverse engineer older designs so that they could use their current 3D CAD systems to further develop the data. The problem is that to accurately define a part, thousands and sometimes millions of coordinate points are required. Early 3D CAD systems struggled to handle such large point datasets.
Today's mechanical CAD applications are developing specialized tools to take dense point-cloud data and turn it into Class A free-form surfaces. Third party applications are also helping to fill the gap.
Figure 2: With VX CAD/CAM, users can analyze the distance between individual points in a point cloud and a given surface fitted through it.
Integrated CMMOnly a few midrange mechanical CAD applications today provide integrated tools that work with dense point-cloud data. VX CAD/CAM from VX Corp. is one. Using its ScanShape tools, users can import point data from CMM, laser scanners, and STL files and fit surfaces through them (figures 1 and 2). The software has manipulation tools so users can work with point blocks to reduce the density of the point cloud.
Other tools divide point clouds into more manageable sections to facilitate work with 360° scan data. Users can also analyze the surface fit to a point cloud. VX CAD/CAM interfaces directly with CMM devices such as those from MicroScribe and Romer Cimcore.
Keycreator, the successor to CADKEY from Kubotek, also offers reverse engineering capabilities. Many industrial design applications come with built-in point-cloud processing tools, including Alias, ICEM Surf, and form*z.
Third-party CMMThird-party CMM tools are available for many mechanical CAD applications—for example, CMMWorks for SolidWorks by AccuMetria (figure 3). CMMWorks uses an intelligent, metrology-based programming environment to create and edit CMM programs. Users don't need to know any CMM languages—they just select the geometry to inspect, the probe to use, and the tolerances. With CMMWorks, it's easy to change the sequence of the program and to update the program when the SolidWorks geometry changes.
Figure 3. When users are ready to download to the CMM, AccuMetria CMMWorks outputs an ANSI-standard DMIS 4.0 program.
AccuMetria CMMWorks also provides CMM motion simulation complete with collision detection. Using simulation, users can verify that there won't be unwanted and expensive collisions between the probe and the part, the fixture, and the CMM.
Stand-Alone Software OptionsSome applications are designed specifically to process large point clouds, then output the data in a neutral format such as STEP or IGES as well as in supported native CAD formats. Raindrop Geomagic, for one, includes three modules in its Geomagic Studio product. Geomagic Capture processes the actual scanned data. Geomagic Wrap converts point clouds to polygons. Geomagic Shape continues the conversion process by turning polygons into NURBS surfaces.
In this article
HardwareIn the manufacturing realm, point-cloud data can be obtained through CMMs, which use various scanning technologies such as laser, touch probe, camera, or combination. Specifications to consider include resolution, measuring length, and whether the unit is fixed or portable.
In recent years, smaller, more portable desktop digitizers have emerged, such as the Faro arm series and the MicroScribe G2. These devices generally accommodate objects within a limited workspace and collect data via a mechanical touch probe.
For larger projects, a 3D laser scanners mounted on a tripod emits a laser beam to transfer 3D coordinates from objects that it encounters. By sweeping the surrounding area, the 3D laser scanner can transmit hundreds of thousands of coordinates to define an entire scene. The reflected signals allow it to calculate distances as well as coordinates.
3D laser scanning is used in the AEC market to capture site information for plant and facilities, civil, and building and architectural applications, as well as existing site modifications and asset management. 3D laser scanning provides a better way to collect site data than traditional methods such as manual and photogrammetric surveys.
ConclusionsFor decades, 3D input devices have played an important role in 3D CAD because they can deliver the least common denominator of any 3D geometry—coordinate points. Today's 3D CAD application developers are recognizing how important 3D point data is becoming to product and industrial designers and civil engineers. They're starting to provide the necessary tools to work with dense point datasets so that users can more easily convert them to Class A surfaces.
Don LaCourse (firstname.lastname@example.org) has spent the last 25 years working with and writing about CAD/CAM operations. He is a Cadalyst contributing editor, principal partner of eDocHelp (www.edochelp.com) and founder of www.3DCADTips.com a free, independent, and objective information resource for 3D CAD users, administrators, and managers.
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