The Quality Cycle in a Precision 3-D Scanning Process

With the proliferation of 3-D scanning technologies, many options are available for obtaining an electronic 3-D file of a scanned object. The quality of results varies widely. Of those that have experienced poor results, some complain about the accuracy of the data. Others have voiced concern over the usability of a completed file, having found that it does not work for their particular application.

In a recent poll, 57 percent of respondents that had used scanning services said they received “successful” results less than 80 percent of the time. A full 21 percent said that success was achieved less than half the time. While this information may be shocking to some, it is no surprise to others. It has become all too common for first-time users of 3-D scanning services to find themselves shopping for a better solution, or worse yet, discouraged from using scanning services again.

What is shocking is that such low levels of quality exist in a process that is quite critical to the product development and manufacturing cycle. Scanning technologies are used at the very beginning of product development for capturing reference geometry. Then again in the product validation phase to compare first production samples to the new CAD model. And finally as a problem solving tool to improve tooling performance or product reliability. Quality results of less than 80 percent would not be tolerated in any other part of the product development/manufacturing cycle. Yet this level of quality is commonplace with scanning services.

Without exploring all of the potential reasons why, one thing is very clear. Whether it’s laser scanning, white-light scanning, computed tomography, or another 3-D scanning process, even fundamental disciplines of quality are little-used with a technology that has come of age.

As a customer of 3-D scanning services, one should become well-informed about the quality of the service that will be provided by inquiring about the service with regard to the four steps in the Shewhart cycle (plan, do, check, act). Asking questions and supplying the critical information associated with the plan-do-check-act improvement cycle will dramatically increase the success rate of a 3-D scanning output. Most scanning operations are first-time events, so use of the Shewhart cycle becomes a prevention tool before the first scan is performed.

Plan

Planning is perhaps the most critical stage in the reverse engineering process. It’s also the point at which many errors are made.

The first step in the plan is to define what you expect to do with the output from the scan. What will be the next step? That is, what will you do with the file in the very next sequence of your process?

Once the next step is defined, how do you see the file being used in the future? Will it be edited at some point? The answers to these questions will help define what type of file needs to be developed from the scan. You may be using the file to compare the part to your existing CAD as a first-article inspection method. That expectation requires one output of a file. Your goal may be to perform a finite element analysis on the object. That will require another type of file. Perhaps you need to offset a feature from the surface of the part; that will require yet another type of file to produce a successful result. Define the next step and future use of the file very clearly before any scanning work begins.

Second, work with an engineering team that not only has complex modeling expertise, but understands the product development cycle and precision manufacturing in general. This depth of knowledge is indispensable in producing a robust file that can be put to use quickly.

Third, clarify what surface preparation may need to take place to obtain an accurate point cloud. Most precision parts require some sort of surface preparation before laser scanning or white-light scanning can begin. Scanning technology has improved dramatically in recent years such that less preparation is necessary. Even so, to achieve the best positional accuracy from the scan, the surface should be flat (nonreflective), opaque (nontransparent), and white.

The ideal surface to scan is an eggshell. Unfortunately, few eggs require laser scanning. Therefore, some preparation is usually required to replicate a nonreflective, nontransparent, white surface. If the part can be painted, one of the best prep materials is a flat white spray paint. This adds 0.0002 in. to 0.0004 in. of thickness to the surface when properly applied. Paint is best used on metal parts such as castings that can be cleaned with a paint remover, or on parts that are disposable.

A more common prep material is developing powder used for die-penetrant inspections. It adds 0.0005 in. to 0.0010 in. of thickness to the surface and can be washed off with soap and water.

Perhaps the project is too precise to add a layer of paint or powder to the surface. Some parts can be made less reflective by glass bead-blasting. A negligible amount of material is removed from the object while the surface layer is "disturbed" slightly to reduce reflectivity.

If none of these options are possible, a combination of scanning (for reference) and precision dimensional inspection can be employed.

Proper surface preparation is a critical step in the scanning process.

Fourth, confirm the level of accuracy being provided. Know what error is inherent in the preparation and scanning process and how much error may be introduced by the surfacing/modeling steps. The sum of these two will be a good estimate of the overall error to expect in the finished file.

Do

The second step, “do,” is the implementation of each of the decisions and adjustments that were made during the plan. What’s most critical here is that a robust quality system is in place and followed in the development of the scanned point cloud and resultant CAD file.

The system must include a bullet-proof method of translating the communications that took place during the planning stage to the technician or engineer performing the work. How does the information exchanged during the quote process develop into a clear set of work instructions? Perhaps this is an obvious step; yet in a process such as scanning where every project has a unique set of requirements, this point of information exchange holds one of the highest potentials of error. A process of continuous improvement is indispensable in this area. As a user of scanning services, ask how the project details get translated into work instructions.

Another important component in this second step of the Shewhart cycle is that there is thorough training to validate skill sets for the individuals performing the scanning and modeling processes. The training should also include actual industry experience. Not only experience in scanning and modeling components from different industries, but hands-on manufacturing and designing work experience in a variety of industries. This will bring a depth of understanding to the completion of the file you will need for your project.

Check

Checking is all about analyzing the data on the solution. Data analysis begins with verifying that the point cloud came from a quality scan. This includes evaluating stray points and checking to ensure there was no unplanned movement during the scanning cycle. In the image in figure 1, a clean scan is compared to an invalid scan where scan layers do not match with one another.

Figure 1

During the CAD file construction, analyzing the data repeatedly is an integral part of the reverse engineering process.

Multiple color comparisons such as the one shown in figure 1 will ensure that all features are present, constructed to the correct size, and that they are in proper position. A complicated component may require more than one hundred color-compare checks before the file is ready for final inspection.

The final inspection should be accomplished with a second set of eyes. Preferably by someone else that has manufacturing or tooling experience associated with the product under review. This final check must be a full review of the plan that began when the project was initially quoted.

Be well informed about how your provider performs the check step throughout the build of your model.

Now the project is ready for presentation to the end user or customer. This live demonstration of the finished file at completion is a final check with the intent of helping the customer understand what decisions were incorporated into the design and process that was used to build the model tree. Sometimes corrections or adjustments come out of this review that need to be acted upon.

Act 

A good reverse engineering process integrates a loop of checking and acting until the final electronic file meets the intended use. The last “act” step is the adoption of the final solution, putting closure on the project, and including any additional adjustments the customer may need. Perhaps more important, this is an opportunity to identify systemic changes that can be implemented to standardize the solution. That way all future projects benefit from those changes.

As a user, be sure you will be afforded the opportunity to review a live presentation of your reverse-engineered result. This way you’ll have an opportunity to identify those items that need to be acted on before final delivery. Discuss the incremental improvement process your provider uses to ensure the highest quality scan results.

The continuous improvement quality cycle as an integral part of a robust reverse engineering process.

About The Author

David DeVowe

David DeVowe is the general manager of The QC Group in Burnsville, Minnesota. He has more than 24 years of manufacturing, engineering, quality, and business leadership experience in precision manufacturing environments. With a bachelor's degree in mechanical engineering from Michigan Technological University, his experience includes aerospace, automotive, die casting, telecommunications, and precision metal stamping. He is actively involved in laser scanning, reverse engineering, and dimensional inspection services at The QC Group.