Reverse Engineering: A Basic How-To
An inspection specialist imparts principles for successfully going about the reverse engineering process with 3D scanning equipment.
Note from the editors: This article was originally published in the June 2017 issue of Quality Magazine. We provide it here, with edits, for your convenience.
Step 1: Determine Your Parameters
How will a scanned image be used? Do you believe the customer will want to make changes to a part after you’ve already created a CAD model? What are your tolerance requirements? All these questions are paramount in determining the successful path of the data output.
Why Do I Care How It’s Being Used?
After being in the 3D scanning service industry for over 18 years, I find significant value in this simple question. Here’s why. When approaching a project of any magnitude, the ideal goal is to find the cleanest, clearest path with an optimal desired result. Not knowing the intended output can send a service tech down a very long, inefficient path. Here is a scenario I encounter on a regular basis. Customer: “I need to scan an entire engine for a vehicle, with CAD output.” At face value, I can jump to the conclusion that they need every nook and cranny digitally captured by any medium of my choosing and that I can spend several weeks meticulously creating detailed models of starter motors, cooling lines and complex engine block castings. Ninety-nine percent of the time, that is not necessary. Usually, a simple volumetric representation is all that is needed to determine fit and clearances. This can be accomplished quickly and efficiently, using simplified scanning techniques and rapid solid modeling.
Why Do I Care If the Customer Wants to Make Changes?
Knowing what changes may be needed is very important because it informs your design decisions. A user can now design a part to make areas easier to modify without fighting unwieldy surfaces, fillets or drafts in the completed CAD model. In the example involving an engine, there is no plan to modify that data, only to check what fits around it. This can be resolved with a simplified NURBS (non-uniform rational basis spline) to get a lightweight, volumetric skin to bring in to the customer’s existing CAD design.
How Do You Manage Tolerances?
Tolerances can be managed through use of the correct hardware and software in the hands of a skilled tradesman. Modern scanning hardware possesses the ability to capture high surface detail, sometimes to the detriment or advantage of the reverse engineering process, depending on other factors. For example, if I intend to create new tooling and develop clean surfaces for machining, then creating a model through a traditional CAD workflow makes the most sense. However, this calls for an allowance of tolerance. While I may intend to create a flat surface in my model, that surface may not be flat anymore on the actual part. Using 3D scanning, I can revert my CAD model to a previous version and correct the discrepancy. In addition, if I am developing something to grab or hold a part and need high precision, then I can revert my design to take a different approach to the modeling output. In either scenario, we can create high-precision NURBS output to generate those exact surfaces and make certain the part is defined properly.
Step 2: Acquisition
Now that we have the guidelines established for intent, let’s examine our options. As I mentioned earlier, the advancements in technology over the last 20 years have been amazing. Structured light is cleaner and portable arm-based laser scanning is much faster and more accurate. Additionally, time-of-flight and phase shift (long range) scanners can scan greater distances with substantially higher precision. Metrology grade 3D CT (X-ray) scanners are becoming more powerful and financially feasible.
Why Structured Light?
Structured light is clean, and clean data yield a cleaner result. Structured light is typically a two-camera, stereo system. The system uses a digital projector to project a fringe pattern onto the surface of the part. Thus, displacement of the fringe pattern along the part is correlated back to 3D data. These sheets of light bounce off the part to provide a clean and highly accurate digital representation of it. This clarity is of a higher standard in comparison to CMM, long-range and CT scanning. Its only real limitations are translucency/transparency, deep colors opposing the light spectrum of the projected light and the fact that both cameras need to see the geometry being captured.
Why Portable CMM/Scanning Arms?
Most of the improvements in the reverse engineering process have been made in the portable CMM industry, including wider laser lines, higher hertz rates for data capture and millions of points captured in seconds. Now, portable scanning arms offer speed and flexibility. They can also be set up in a controlled lab environment or quickly packed up and mounted on a machine on the shop floor to resolve an issue. Data from these units are captured via laser, and the ability of lasers to adapt to different surface colors and finishes has become highly advanced in recent years. Chrome and deep black were always very problematic, but now lasers can accommodate them. In addition, with tolerances getting close to matching those of structured light, lasers are taking a good hold on the market and are becoming a dominant force and a valuable tool in reverse engineering. The only current limitation is the length of the portable arm, with multiple setups being required for part sizes beyond the arm’s reach.
Why Long-Range Scanning?
Do you need to map out a building? Do you need to reverse engineer the outside of a 747? Long-range scanning provides the ability to scan geometry hundreds of meters away within a reasonable tolerance. Long-range scanners send out laser beams to record, with high precision, the dimensions of the surfaces they bounce off. Then they turn that information back into digital 3D data. That data can be combined with high-resolution imagery to provide 3D visualization of the scanned objects, areas or spaces.
Why CT (X-Ray)?
Now that 3D X-ray machines have become more powerful, they are able to see inside of dense materials such as steel and record internal proportions. CT also eliminates blind spots, which allows designers to model complex sculptural surfaces with precision and to fill in missing geometry using conventional scanning methods.
Step 3: Processing
Now that we know what we’re trying to accomplish with our model and the methods we can use to acquire the necessary data, what are the best practices of the reverse engineering process? I would like to highlight some guidelines.
Take Your Time
At this point, we know how accurate scanners need to be and how to use them properly. However, taking small short cuts in the scanning process leads to time-consuming editing when processing 3D data. For instance, not taking an additional scan to capture the bottom of a groove or hole leads to missing information, which means that a lot of assumptions will need to be made to interpret the scan data for a polygonal mesh. One additional 5-second scan can save hours of work. Clean data input streamlines the processing.
Convert Your Data into a Polygon Model
There are various tools that can accomplish this goal. Most hardware suppliers provide direct output from their scanners, while others rely on third-party software to run the necessary calculations. Many software packages allow you to manipulate data, including smoothing out imperfections and closing small to large holes with reasonably assumed precision. When you create your model, you should also edit out the pesky clamps or fixtures that held the parts during scanning. Doing this sets up the next round of modeling.
Now that we have our 3D scan data and our CAD model, let’s wrap up the process. Before you throw your project over the wall to manufacturing or another division, you need to check your work. Doing this has become significantly easier because our ability to validate data has greatly improved. Validation is the ability to show deviation of the scanned object back to the CAD model being developed. This deviation is represented typically by a color map, with each color representing the 3D distance that points on the model vary from its physical counterpart. Once this evaluation is complete and meets the expectations determined, the CAD model is ready to be delivered.
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