3-D Scanning and Touch-Probing: Ideal Partners for QC

Reverse engineering and 3-D scanning are often considered synonyms. But 3-D scanning is also an efficient way to compare as-built physical objects to theoretical digital 3-D models (i.e., CAD models), and thus an essential asset to every quality control (QC) department’s tool box. Today’s 3-D scanners offer excellent accuracy, are easy to use, and are fully integrated by all major 3-D inspection software. However, three basic questions remain: When should a part be scanned? What features should be scanned? What features should be hard-probed? Here are a few criteria that can help you make the call.

First, you need to determine the overall shape of the object or feature you need to control. Is it a simple prismatic feature like a cylindrical hole? Or is it a complex, free-form surface like an automobile front hood? Because of the evolution in manufacturing methods, most objects that are now designed and manufactured present complex surfaces and styling lines. Because it can generate thousands of points in a second, 3-D scanning makes it possible—and easy—to measure these complex parts. On the other hand, if all you need to measure are simple geometrical entities (e.g., points, spheres, or plans), a hard-probing measurement methods would be your choice for a simpler and quicker process.

Next, you need to ask yourself if you want to a global view of the part deformation, or would you prefer to know the dimensional deviation in a specific spot? Three-dimensional scanning (still in the context of complex object measurement) will allow you to quickly get a general idea of a part’s conformity and to identify problematic areas, if any. Three-dimensional scanning and 3-D inspection software also feature tools that make it easier for quality control professionals to better understand the dimensional deformation of a part. Plus, the high-density data generated by a 3-D scanner can be really valuable when putting together a new production: the molds, tooling, or the first parts produced don’t have a defined inspection program, and the scan data will allow designers to investigate production issues or simply validate the process before the start-up.

Figure 1: Example of 3-D scanner

On the other hand, if you want to measure a specific point on an object or follow through on an inspection program that contains mainly geometric dimensioning and tolerancing (GD&T) symbols to describe, for example, the dimensional deviation of a part surface as a given distance with relation to an established datum, you will most likely resort to a coordinate measuring machine (CMM) that can be programmed to measure each item of the inspection program. The CMM can also provide real-time measurement information. This can prove quite useful when trying to adjust an assembly jig, for instance. When manually adjusting a stop, it is  convenient to see the actual dimension while you are trying to set the stop at its proper location.

Hard-probing technology such as those on portable CMMs, but especially on CNC CMMs, can be more accurate and offer increased repeatability in a controlled environment. However, the accuracy of 3-D scanners has improved greatly in recent years. The accuracy level and repeatability of the measurement device must be analyzed wisely before choosing one technology over another. For instance, you must determine what the real design requirements and the capabilities of the manufacturing processes involved are. Also, you have to consider the variety of parts (e.g., shape, complexity, geometrical vs. free-form, or tolerances), and you will have to inspect and try to find the best compromise. It is sometimes difficult to find a one-stop solution when faced with a large selection of parts, so you could consider a mixed solution (3-D scanning and hard probing) in these cases.

Figure 2: Example of a hard probe

Here is another technical aspect must be considered when trying to determine what measurement technology should be used. Is the part deformable (like a soft material)? In this case, a noncontact measurement method like 3-D scanning must be used. Is the part transparent or highly reflective? Then you will most likely need to use a hard-probing device to acquire the dimensions of the part because 3-D digitizers use optics to generate their measurement, and transparent and reflective material will usually prevent a 3-D system from functioning properly.

Part size is no longer an issue when it comes to choosing between scanning and probing. It is more a question of choosing the right 3-D scanner or CMM for your application. In the case of a fixed CMM, the working volume is fixed and the price increases drastically with size; consequently, it’s probably not the best solution for larger parts. However, there are other hard-probing technologies that are useful on large parts—laser or optical trackers, for instance.

The ability to automate the inspection sequence is no longer a criterion. At the present time, 3-D inspection and metrology software can easily integrate probing and 3-D scanning operations in their inspection programs. As far as the actual measurement operation, 3-D scanners and touch probes can be interfaced with the conventional CMM, or they can be operated manually.

All and all, probing and 3-D scanning should be considered complementary measurement technologies. Often both technologies can be used at the same time—for example, to measure a large part with a scanner and take very precise point measurements with a probe. One set of data can be used to verify the other or to align the other set. In recent years, most manufacturers and developers of such measurement devices and software have actually spent a great deal of energy making sure that the combination of 3-D scanning and touch probes can be as seamless and as efficient as possible.

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