June 2007 Edition

QC/MEASUREMENT

Getting Its Head Out of the Point Cloud

Reverse engineering can be time-consuming, but a company found a way to cut through the process using a laser scanning bureau

Reverse engineering complex shapes can take place several ways. When a consulting design firm was asked to recreate transmission cases from a 1967 Ferrari 330 P4 racer, it knew that it needed help.

 The Austin Group, Quincy, IL, combined the technologies of laser scanning and lost foam casting to duplicate transmission components when drawings and CAD files weren’t available.

 Laser scanning is used to re-engineer complex three-dimensional shapes by creating 3D CAD models from physical components. Lost foam casting is used to cast the complex shapes to a high level of accuracy without the need for foundry tooling. Machined features are often included to further add value to the casting.

 Reconstructing the Past
The Austin Group was contracted to reconstruct a transmission case needed to duplicate the rare Ferrari race car. The part had complex three dimensional shapes that the design firm had to hold to high accuracy levels, yet CAD geometry did not exist for the 40-year-old part. Adding to the challenge, the company was asked to produce small quantities while meeting tight quality and delivery requirements.

 In the past, the Austin Group would have used a digitizer arm to reverse-engineer the component. A technician would have moved the arm around the component and measured the position of individual points. It would have taken perhaps 70 hours to capture a few thousand points. Then a designer would import the point cloud into solid modeling software and stitch the points together into a solid model of the component. Between the capture and stitching, almost three weeks would be invested.

 The problem with this approach is that there wasn’t enough time to capture enough points to accurately define the complex surface to the level needed. This meant the accuracy of the resulting solid model depended on the interpretation process in which the designer attempted to recreate the geometry of the part using the point cloud as the model.

 Time Crunch
  Often points that were needed to determine the precise geometry were missing; so the designer was forced to rely upon intuition and, in some cases, guesswork. The interpretation process typically took about 50 hours for a total of 120 hours to re-engineer the part. The approximate cost to reverse-engineer the part would have been more than $10,000.

 Recently, the Austin Group implemented a reverse engineering method based on laser scanning. Laser scanning systems work by projecting a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser line as it sweeps along, enabling the object to be replicated in fine detail.

 The laser probe computer translates the video image of the line into 3D coordinates, providing real-time 3D coordinate data that give the operator immediate feedback on areas that might have been missed. Laser scanners are able to quickly measure large parts while generating far greater numbers of data points than probes without the need for templates or fixtures. Since the laser has no contact tip that must physically touch the object, the problems of depressing soft objects, measuring small details, and capturing complex free form surfaces are eliminated.

Less Time, Greater Accuracy
  Instead of collecting points one by one, the laser scanner picks up tens of thousands of points every second. This means that reverse-engineering the most complicated parts can often be accomplished in signifi cantly less time with greater accuracy.

Laser scanning can reverse-engineer parts that are so complex that they would be practically impossible one point at a time. The software involved with the scan simplifi es the process of moving from point cloud to CAD model, making it possible to generate a CAD model of the scanned part that faithfully duplicates the original part in a short amount of time.

 Special software can be used to compare original design geometry to the actual physical part, generating an overall, graduated, color error plot that shows where, and by how much, surfaces deviate from the original design. This goes beyond the dimensional checks performed with touch probes on coordinate measuring machines.

 Austin Group managers determined the laser scanning process had potential but were reluctant to make the investment in equipment and personnel that would be required. Instead they worked with Laser Design, Inc., Minneapolis, through its engineering services bureau, GKS Inspection Services Inc., Detroit, which provides laser scanning services along with consultative engineering services for both reverse engineering and inspection applications.

Solid Models

Engineers at the Austin Group sent the parts to Laser Design/GKS. In less than a week, GKS provided solid models of the component that matched the original physical part to a much higher level of accuracy than could have been achieved with touch-probe digitizing, because laser scanning captures millions of points, enough to fully define the surfaces of the parts.

 The cost of reverse engineering to a solid CAD model via laser scanning was about 40 percent less. Also, it was almost twice as fast as using a touch probe arm.

 With the geometry of the part fully defined, the Austin Group was able to proceed with the lost foam casting process. They used the solid model provided by LDI/GKS to produce a foam pattern.

 The lost foam process, simplified, takes the CAD file and uses it to create a foam pattern of the part to be cast. The foam pattern is used to create the form in which molten metal is poured.

Once cooled, the casting is cleaned and treated by other machining processes to get the finish required. Through laser scanning, complex parts can be quickly digitized and duplicated.

 Laser Design/GKS Inspection Services www.rsleads.com/706mn-202

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