3 keys to reverse engineering

Determining the intended use of scan data, acquiring the data, and then processing it are the three important stages of reverse engineering projects

3D point cloud

Once a 3D point cloud is created, millions of points cleanly represent the part and it’s time to create a model.

Reverse engineering projects are complex tasks requiring pre-planning and proper technology use. However, a three-step process can streamline even the most arduous scanning job.

Determining intent

The first step in a reverse engineering project is determining intent.

During this stage asking a few key questions is paramount in determining the successful path of the data output, and each question is mutually exclusive of the others.

Question 1: How will this image be used, and why should I care how it’s being used?

When approaching a project of any magnitude, the goal is to find the cleanest, clearest path to the desired result. Not knowing the intended use can send a technician down a very long, inefficient path.

Often a customer says they need to scan an entire engine with CAD output. At face value, I can jump to the conclusion that they need every nook and cranny captured digitally by any medium of my choosing, and I spend several weeks meticulously creating detailed models of starter motors, cooling lines, and complex engine block castings.

Ninety-nine per cent of the time, that is not necessary. If I simply ask how the scan will be used, I find that 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 modelling.

Question 2: Will there be any changes?

Why should I care if the customer anticipates changes? Because anticipated changes will drive the method of creation and the type of model that should be used.

Parametric CAD models, for example, contain design intent, a structured combination of prismatic 3D features driven by specific dimensions. An IGES or STEP output is a stripped version of that parametric 3D CAD model typically represented by an outside skin of the CAD surface, allowing you to share 3D data with multiple platforms that do not share a common internal language.

3D CAD model

Not taking shortcuts during the scanning process eliminates time-consuming editing when processing 3D data.

Knowing that changes may be needed is very important so that a part is designed to make those anticipated areas easier to modify without fighting unwieldy surfaces, fillets, or draft in the completed CAD model.

Or, in the example of the engine, there is no need to modify that data, only to check what fits around it. This means a simplified non-uniform rational B-spline (NURBS) can be used to output a lightweight, volumetric skin to bring into the existing CAD file.

Question 3: What are the tolerance requirements?

Tolerances? The final deliverable should be perfect, right? Wrong.

Do you really need a sand casting with tolerances of 0.001 in.? Are you sure you want to see every imperfection in that part? Let’s apply the brakes at this point and back up a bit.

During the process, 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 to the process’s detriment or advantage. The reason: anticipated changes.

If the intent is to create new tooling and develop clean surfaces for machining, then developing it through a traditional CAD work flow makes the most sense. But this calls for an allowance. An allowance of tolerance.

Intended flat surfaces may not be flat anymore on the physical part. Through 3D scanning and interpreting those surfaces back to a clean CAD model, you can correct that discrepancy, therefore influencing the deviation on the actual part back to its new digital CAD counterpart.

However, if you need high precision, you can create high-precision NURBS output to satisfy those exact surfaces and make certain the part is defined properly.

Portable scanning arm

New scanning arms offer high-speed, flexible data capture thanks to wider laser lines and higher hertz rates.

Data acquisition

The second step in the reverse engineering process is data acquisition. Now that you have established guidelines for intended use, you can examine your options based on those choices.

Advancements in technology over the last 20 years have been amazing. Structured light is cleaner; portable arm-based laser scanning is much faster and more accurate; and the time-of-flight and phase shift (long-range) scanners can scan farther distances with substantially higher precision. Metrology-grade 3D CT (X-ray) scanners also are becoming more powerful and financially feasible.

Structured light is clean, and clean data yields a cleaner result.

Structured light is typically a two-camera stereo system. It 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 into 3D data. These sheets of lights are bounced off the part and provide a clean and highly accurate digital representation of the part.

This clarity is regarded as a high standard in comparison to its counterparts. Its only real limitations are translucency/transparency and deep colours opposing the light spectrum of the projected light. Also, both cameras need to see the geometry that is being captured.

Portable CMMs and scanning arms offer high speed and flexibility and are one of the biggest improvements to the industry.

Wider laser lines, higher hertz rates for data capture, and millions of points can be captured in seconds. These pieces allow you to adapt quickly from being set up in a controlled lab environment to packing it up and mounting it on a machine on the shop floor to resolve an issue.

Data from these units is captured via laser, and its ability to adapt to different surface colours and finishes has become highly advanced in recent years. Chrome and deep blacks were always the nemesis, but those days are quickly fading away. With tolerances getting close to matching those of structured light, this technology is taking a good hold on the market and becoming a dominant force and a valuable tool in reverse engineering.

Current limitations are set only by the length of the portable arm, with multiple setups being required for part sizes beyond the arm’s reach.

Long-range scanners can be used for large projects, such as mapping out a building or reverse engineering the outside of an airplane. With the ability to scan geometry that is hundreds of metres away within a reasonable tolerance, long-range scanning is the right application.

These tools send out a laser beam with high precision and record the surface it bounces off back into digital 3D data. That data can be combined with high-res imagery to provide 3D visualization of the objects, areas, or spaces being scanned.

CT (X-ray) technology is becoming more powerful and can now see inside of dense materials, such as steel, and extract internal data. Internal passages that were created with a complex network of sand cores in a foundry process can be seen to correctly validate clearances when re-creating a part. CT also eliminates blind spots, allowing designers to model complex sculptural surfaces with precision and assumptions in filling in missing geometry from conventional scanning methods.

Processing stage

When you have determined the intended use of scanned data and how you will acquire data, you need to know what you will do with it. Today’s processing is a bit of a broad spectrum, but some guidelines are highlighted here to once again lead us down a clear path.

No matter what type of scanner is being used, you need to know your hardware, its capabilities, and its limitations.

At this point in the process, you should already know how the scan is being used, how accurate it needs to be, and what method will be used. Now it’s time to capture data. But it still must be done correctly.

Taking small shortcuts in the scanning process, for example, leads to time-consuming editing when processing 3D data. By not taking an additional scan to capture the bottom of a groove or hole, a lot of assumptions will need to be made when interpreting the scan data into a polygonal mesh. One additional five-second scan saves hours of work. Clean data input streamlines the processing.

Once you have a 3D point cloud, you have millions of points that cleanly represent the part. The common next step is converting the X/Y/Z 3D data points into a polygon model. Simply put, the software connects the dots with a series of triangles to create a representative skin.

Various tools can accomplish this goal. Some hardware suppliers provide this direct output from their scanners, while others rely on third-party software to run the calculations. Many software packages allow you to manipulate the data, including smoothing out imperfections or closing small or even large holes within a reasonable assumed precision.

The ability to quickly edit out clamps or fixtures holding the parts during scanning can now be accomplished easily. This is the step that sets the pace for the next round of modelling.

Next up is conversion, which takes the polygon data, commonly referred to as an STL, and converts it to a usable format with the necessary parameters.

Thankfully, gone are the days of scanning a part, bringing a low-resolution version into a CAD package, cutting cross sections, converting those to complex sketches, and then generating 3D features off those sketches.

Today software packages handle scanning directly into their software, converting large data sets to high-resolution mesh data and generating native parametric CAD features, all in one package, thereby shaving days off the process and achieving a much better result.

Also, the ability to generate NURBS or as-is surface data with extremely high precision has been a dramatic improvement. With complex algorithms able to solve data sets with complex surface geometry with a single button push, the process continues to get faster and faster, as well as more accurate.

The final task is the validation stage. Now that you have two pieces, the 3D scan data and the intended CAD model, it’s time to finish the process. Before passing it to manufacturing, you need to check your work.

Another improvement that continues to excel is the ability to validate data. Validation, used here, is the ability to show deviation of the scanned object back to the CAD model being developed. This deviation typically is represented by a colour map, with each colour representing the 3D distance each point varies from its CAD model counterpart.

Once this evaluation is complete and meets the expectations determined, the CAD model is ready to be delivered.

Greg Groth is division manager for Exact Metrology, 20515 Industry Ave., Brookfield, Wis. 53045, 262-533-0800, www.exactmetrology.com.