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Dan Perreault


Is Your CMM Obsolete?

Not necessarily

Published: Thursday, October 15, 2015 - 15:29

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The CMM, or coordinate measuring machine, has long been the standard for dimensional inspection of complex parts across a variety of industries. But has the CMM outlived its usefulness?  Is this technology now obsolete in the face of new technologies such as 3D scanning?

The CMM was first developed in the late 1950s. With the advent of the touch-trigger probe in the 1970s, the CMM rapidly expanded into U.S. manufacturing companies for use in quality control, leading to a revolution in three-dimensional measurement.

A typical CMM has three mutually perpendicular axes: X, Y, and Z, which allow for movement of a probe in 3D space. The machine records data point readings as indicated by a probe tip. A touch-trigger probe allows for automatic capture of the probe location in space when it makes physical contact with the part being measured.

The coupling of the touch probe with direct computer control (DCC) CMMs means that part inspections can be programmed, allowing for automation of inspection of multiple iterations of the same part design. Add to this the ability to program the CMM based on the computer-aided design (CAD) model of the part, and the entire inspection process can be developed before the first part is ever manufactured.

This concept was revolutionary at the time, and meant that the CMM could quickly and accurately measure a large quantity of manufactured parts. The CMM, however, does have its limitations. To achieve micron-level accuracy, the majority of CMMs are built on a slab of granite and are locked away within a temperature-controlled inspection lab. They are also limited by their size in respect to the types and sizes of parts that can be inspected. Large parts require a large CMM.

Furthermore, CMMs, in general, collect sparse data sets, meaning they only collect a few points in order to calculate geometric features of a part as designated by their programs. Therefore, assumptions have to be made as to whether a measurement is truly representative of the part. For instance, although you can determine the diameter of a circle using only three points, do those three points truly define that feature? The same could be said of measuring a plane. How many points do you need before you are comfortable with the flatness of planar surface measurement?

When you compares these limitations with 3D scanners, you immediately see the advantages that scanners hold over traditional coordinate measuring machines with regard to portability: part size, ease of use, and true representation of a part’s features.

Because the traditional high-accuracy CMM is trapped in the inspection lab, the part must be brought to the CMM. This isn’t true for most scanners, particularly handheld systems, which are extremely portable. This allows the scanner to be brought to the part. Not only is the scanner able to travel to the manufacturing floor to assist in troubleshooting tasks, it can also travel to a supplier location, or a customer location. This flexibility can be extremely useful in solving assembly issues between components fabricated by different suppliers at different locations. A quick scan of each part can be compared to the original design to rapidly illustrate which part or parts in the assembly are causing the problem, the location of the problem, and the magnitude of the problem.

Scanners also have fewer limitations with regard to part size. Scanners can typically collect complete data sets on parts much larger than what can be captured in a single field of view by aligning and stitching successive scanned images together using a variety of different techniques.

One option for aligning multiple scans is the use of optical or photogrammetric targets. Three or more targets must be common between successive scans for the scanner control software to identify which scans match up to each other. Handheld scanners also employ targets, but utilize them somewhat differently. The handheld systems dynamically scan the entire part, as opposed to using successive, discreet images. In this case, the targets are used by the system to establish the position of the scanner in space, relative to the part being scanned.

Another method of aligning scans requires the manual picking of points that are common between data sets, or the manual manipulation of the orientation of any one scan to match it to adjacent scans. Although effective, the manual method can be somewhat time-consuming.

Whereas the CMM needs to be programmed to measure just about anything, most 3D scanners operate in either a “point-and-shoot” mode, similar to a digital camera, or in a dynamic-scanning mode, similar to a handheld video camera. In either case, no pre-programming is required to scan a particular part. The minimal prep time required for scanning as compared to measurement with a CMM means that scanners can be much more flexible as far as their allocation for measurement tasks that arise unexpectedly.

Scanners also capture millions of data points as compared to the CMM, which generally only captures dozens of discreet points and features. This copious amount of data produces either a cloud of points or an STL file that represents the entire part being scanned. Although not technically a CAD model, the resultant data set from a 3D scan does create a near-perfect 3D model or rendering of the part being scanned, allowing for the easy visualization of measurement results. 3D inspection software developed for 3D scanners allows for the creation of a color map that graphically illustrates deviation from nominal when compared to the original CAD. Input an allowable deviation, and out-of-tolerance conditions are immediately displayed so that even novice operators can identify problems. 

Not only can this type of analysis be performed against the original CAD, it can also be performed using a “golden part.” There are instances where the original design of a component doesn’t actually work as intended. To function, that part needs to be modified during the manufacturing process. In such cases, one can end up with parts that work and parts that don’t, and no one knows the difference. A quick scan of each, coupled with a 3D color map, can quickly identify differences. This data can then be fed back to engineering to update the original design to match what works in manufacturing.

Whereas all these attributes do prove that 3D scanners are useful tools for dimensional inspection, they too have some limitations. Because all scanning technologies discussed here are optical, they all suffer from line-of-sight issues, meaning, if you can’t see a feature, you can’t scan it.  A CMM probe, on the other hand can reach into features where line-of-site is impossible. Surface finish or reflectivity can also inhibit scanner performance. Some parts may require dusting or coating with a matte white spray to facilitate scanning. Scanners are also not suitable for every type of part geometry. Highly geometric or prismatic parts can generally be measured more easily by hand, or with a CMM. Scanners are better utilized for those very complex shapes that can’t be measured using traditional methods. Finally, most 3D scanners don’t achieve the same degree of accuracy as the traditional CMM, so they may not be the most appropriate inspection tool for very tight tolerance applications.

3D scanners offer a variety of benefits for many applications of dimensional inspection, but they aren’t necessarily a replacement for the coordinate measuring machine in every instance. Where one tool has strengths, the other has weaknesses. Depending on the variety of work, some shops have both, each utilized to its particular strength. Scanners and CMMs should be considered complementary tools in toolbox of the inspector.


About The Author

Dan Perreault’s picture

Dan Perreault

Dan Perreault has 25 year’s experience as an engineer and 22 years in 3D. He founded NeoMetrix Technologies Inc. in 2003 in the Orlando, Florida, area. NeoMetrix Technologies Inc. specializes in the application of laser-based 3D measurement for digitizing, reverse engineering, rapid product development, and computer-aided inspection. Its engineers are experts at converting raw scanned data into high-quality surface and parametric solid models.