Inspection and testing companies are increasingly using scanning devices such as lasers, white lights, computed-tomography machines, and articulated-arm touch-probes to capture 3D shapes at lower costs. Another technology called cross-sectional scanning can provide a useful alternative for first-article inspection of small, complex, injection-molded parts.

3D models from 2D layers

CSS captures 2D profile images layer-by-layer and assembles them into a 3D digital representation that defines the physical object. It works like this:

A part is placed in a set-up frame and covered with a special curing material that holds the part together for future operations. Once the material sets, the part gets mounted on the platform of a cross-sectional scanning machine.

An industrial flycutter (a rotary tool that uses one or more single-point tools for plane surfacing) then mills away thin layers while the scanning system captures each newly exposed 2D profile. The milling and imaging repeat until the part has been completely consumed. System software post-processes the collection of 2D images to generate a high-density point cloud that digitizes every corner, edge, slope, angle, and void.

The point cloud is ready for export as an STL, OBJ, ASCII, or IGES file. Unlike other scanning methods, CSS eliminates the need for scan alignment, hole filling, and surface smoothing. Inspection can begin almost immediately after the scan completes.

The method uses proprietary, patented software called Spec.Check and scanning programs such as Geomagic, RapidForm, and PolyWorks. Spec.Check handles feature-by-feature inspection and go-no go reporting while the scanning program generates color maps and reports point deviations. These two programs provide a quick, yet thorough, analysis of part quality.

Users can save a part inspection as a template in Spec.Check, and apply the template to other instances of the same part. In contrast to traditional CMM reporting, Spec.Check produces tabular data along with a 3D image of the point cloud and a graph of inspection data. This visual format provides a quick reference - green for pass and red for fail. Also, the point cloud allows interrogating any feature of the part at any time, unlike conventional inspection processes that require development of a thorough inspection plan before the work is done.

Comparing technologies

Compared to other data-acquisition technologies, CSS combines many benefits of contact and non-contact systems while delivering complete part definitions. For example, with templates and batch processing, CSS is extremely fast when performing inspection on multiple parts. CSS is the only technology besides the more expensive and more complex CT scanners to generate 3D data from internal geometry and other inaccessible features.

Users preparing a CMM inspection must define all features of interest and potential trouble spots prior to measuring because it is not a trivial matter to re-inspect a part. Building a plan takes time and careful consideration. And the not-always-correct assumption is that users can foresee or predict elements that might affect part quality and performance. Furthermore, parts must be fixtured. The machines must also be programmed to capture each point of interest, which takes a lot of employee time.

In contrast, CSS allows general inspection plans because the dense point cloud it generates can provide unlimited quality interrogations without having to rescan parts. This speeds inspections and reduces time-demands on inspection staff. In fact, some users report a 10:1 labor savings.

CSS improves on lasers and white-light scanners in that it does not use triangulation to calculate spatial data. Thus, the technology can handle a wide range of materials, including those that are opaque and transparent. There are few limitations on part color and CSS can inspect parts with glossy or matte surface finishes.

Cross-sectional scanning (CSS) is being used by some medical-device manufacturers to inspect small injection-molded components with complex internal and external geometry. CSS's method and data-set have been extensively tested and verified. But adoption has been slow because the technology is a radical departure from time-tested tools common in quality labs. Change is rarely easy. Still, CSS provides flexibility in quality procedures, reduced inspection costs, and faster approval cycles.

An accurate first-article inspection is of the utmost importance. CSS is precise, yielding an average error of just 6 microns and an overall error range of 18 microns. The technology works with any machinable part, including aluminum alloys, plastics, steel, cast iron, stainless steel, and copper. In addition to first-article inspection, the method works well for reverse engineering, failure analysis, and tooling qualification.

CSS captures detail in complex parts and defines internal geometry. It requires no fixturing, simultaneously processes multiple parts, and is capable of working unattended. CSS uses white light to produce a complete digital definition of a physical object by generating an ultra-dense, 3D point cloud. As in laser and white-light systems, a CCD digital camera captures point data.

Although CCS is a powerful way to generate 3D data, it is not always the best technology for every application. Scanning time is a function of part size, not complexity, so an ideal application is injection-molded parts that span tiny to medium sizes, from small connectors to objects the size of a softball. And because CSS consumes the part, the technology does not suit one-of-a-kinds. This might rule-out many prototyping applications.

To determine appropriate applications, consider the number of data points. When only a few points are of interest, contact systems such as CMMs are a best option. But for users needing hundreds, or even thousands of data points, or when the same measurements are needed over multiple parts, CSS is likely the most efficient process.

Keep in mind the technology can capture data from parts not suitable for contact or optical systems. For example, soft, elastomeric parts are difficult for contact systems since they often deform under the pressure of the probe. And clear parts are not suited for optical systems because the light passes through the part without returning to the scanner sensor. However, both materials work well with CSS.