Arlington, VA--Dinosaurs. The evolution of gorillas. An Olympic snowboard helmet. A Navy wildcat pulley. A machine part.
What do all of these have in common?
Reverse engineering--the process of engineering backward to build a CAD model, geometrically identical to the original. For all these projects, engineers took electronic measurements of points from a structure's surface, and fed the data into CAD software, which turned the information into smooth surface models.
While the concept of reverse engineering is not new, computer and software advances have made it a powerful tool. The technology is particularly useful for non-planar or curved surfaces. A complicated machine part can be copied with digital accuracy. And done quickly.
The U.S. Navy hired Advanced Marine Enterprises, Inc. (AME), a naval architecture and marine engineering firm, to make recommendations on how to fix a malfunctioning wildcat. Advanced Marine engineers had a mere five-day window to capture the exact physical geometry of the port and starboard side anchor handling systems from an active Navy ship. The system included two 5-ft diameter wildcats, the sprockets that mesh with the anchor chain. To model the wildcats accurately and quickly, the company turned to reverse engineering and HighRes Inc. (LaJolla, CA), a dynamic software company specializing in reverse engineering software.
Braxton Carter, president of HighRes Inc., developed the C++-based software with a Cadkey interface from Baystate Technologies (Marlborough, MA). Although originally slated for "mom and pop" organizations with an entry-level price of $2,995, HighRes now handles several high-profile jobs. In the design halls of Disney, Carter digitized dinosaur fossils for Disney's newest theme park, Tomorrowland. In the jungles of Madagascar, Carter trained a team from the University of Massachusetts (Amherst) to digitize gorilla skulls to determine the animal's evolution based on skeletal models. Now, the U.S. Navy.
The anchor chains on the LSD49 run from chain lockers, up over deck bolsters, around the powered wildcats with about 180 degrees of wrap, and finally, down through another set of bolsters toward the anchors. Each chain link, made from 3-inch diameter bar stock, 18 inches in length and weighing 90 lbs, should sit flat in the pocket of the wildcat whelps (teeth). In this case, the links of the ship's port side chair sometimes ride up the face of the whelps rather than sitting in the pockets, which presents the danger of the anchor falling uncontrollably.
Wildcats are not manufactured with tolerances that allow them to be used with any nominal 3-inch chain. The manufacturer grooms each wildcat to fit a specific chain sample at the factory by chipping, grinding, air-arc cutting, and/or building up weld clad. Several groomings conducted on the port side wildcat over its five-year life did not eliminate the problem.
Enter HighRes. Equipped with a 3DLX 5-axis coordinate measuring machine (CMM) and HighRes STUDIO software, Carter, went to work. "I needed to create a geometric IGES 128 surface data format," says Carter.
Carter only had five days before the ship went out for missile tests. Originally, he took sliced data measurements. "I would have ended up with a rigid model if I continued this way," he says. Instead he used a series of curves and a meshed algorithm technique. Carter drew primary and secondary lines around the surface. "It looked like a tic-tac-toe board," he says. The unique benefit of HighRes is its ability to move around free forms. This method provides x, y, and z coordinates of points on the surface to a tolerance of 0.012 inch.
To accompany the IGES surface model, a miniature wax prototype was machined with FastNURBs software on a Micro CNC mill from Flashcut NC. Machining the miniature part allowed Carter to verify the surface data by holding the actual physical part in his hand. Carter says, "Desktop engineering tools now allow a cost effective integrated set of tools for part verification."
While Carter digitized, Peter Herrman and Jerry Miller of AME, and John Coon of the Navy, took manual 3D measurements using a chalk line plum bob and tape measure. They measured the hawse pipe deck bolster and chain pipe bolster locations relative to the wildcats as well as the profile of the hawse and chain pipe bolster surfaces in the chain contact area.
Miller used these measurements to create 3D models in the IntergraphT environment. When combined with the wildcat models and a parametric model of the chain, AME engineers could assess fit-up and alignment issues.
Virtual simulation. Brian Bergmann, section chief at AME, explained: "The CAD model not only allowed static fit tests to be conducted, but also was exported to a 3D mechanical system simulation code, ADAMST. Since the ride-up problem was originally aggravated at higher wildcat speeds and chain tensions, AME proposed a dynamic simulation to gain a better understanding of the important variables. The plan is to use the model to build confidence that the recommended alterations will work prior to cutting steel."
Michael Bizier, AME simulation engineer, selected the best code and method for modeling the contacts between the complex surfaces of the chain links, bolsters, and wildcat. He says, "We imported the geometry of the components from the CAD model and set everything up to interact." They modeled surface contact normal forces as well as static and kinetic friction. The scale of this contact problem was beyond what AME or the ADAMS developer Mechanical Dynamics Inc., had addressed before, says Bizier. "[So] we purchased a Contact Dynamics Toolkit to supplement the ADAMS code and consulted with Sam MacDonald, MDI's contact dynamics expert." Eventually, Bizier was able to apply tensile forces to represent chain and anchor weight, and then rotate the wildcat on the computer screen.
Bergmann says the company hopes this technique will prove successful. "Perhaps down the road, computer models will replace the current practice of constructing working physical models of anchor, chain, wildcat, chain locker, and bolsters, presumably with significant cost savings."
At the time the author wrote the article, simulation and final report had not been completed.
Turnaround time from proposal to suggested solutions will be approximately four months. Mapping of the wildcats with reverse engineering and HighRes was an essential part of this task. "We considered other methods such as laser scanning and photogrammetry to obtain surface models, but the mechanical arm digitizing method was found to be better suited for this application," says Bergmann.
He adds that the ultimate goal will be to design and test modifications on the computer screen rather than through trial and error with full-size hardware, resulting in tremendous time and cost savings.
What this means to you:Reverse engineering of mechanical parts can be useful when:
- CAD models are unavailable or unusable for parts that must be duplicated or modified
- CAD not used in original design; inadequate documentation on original design
- Original CAD model not sufficient to support modification or manufacturing using modern methods - Original supplier unable or unwilling to provide additional part information
- Shop floor changes to original design