Metrology Update: Scanning vs. CMMs
Traditionally, the technology of coordinate measuring machines (CMMs) provided the benchmark for dimensional accuracy of parts and dies. During the last 20 years, however, non-contact scanning devices using laser and X-ray technologies have also proven to be versatile and accurate.
For more than 50 years, the benchmark for accuracy in measuring solid objects – whether machined, molded, die cast, welded or forged – was the coordinate measuring machine (CMM). Typically using a solid granite base table along with a vertical, horizontal, gantry or bridge-mounted arm and touch probe, measurements would be taken and compared in blocks to an engineering file – early on as 2-D drawings and today as CAD files hosted in the cloud. During the last two decades, however, a “new kid in town” arrived on the scene, with power, size, point capability and price value that are a viable option to CMM technology.
When scanning products, you select the technology that delivers the accuracy, speed and resolution you need for the application. You should also consider the size, color and reflectivity of the object being measured. The non-contact technologies typically used today include laser scanners, white-light scanners and CT scanners.
- Laser scanners are usually mounted on an arm or tracked with an optical system. This allows you to capture scan measurements in an area up to 18 feet in diameter and at a rate of nearly 500,000 points per second.
- The optical (white-light) scanners, which are mounted on a tripod, capture more than 1,000,000 points per second. In a single shot, the white-light scanners take measurements in an area ranging from a few square inches to a few square feet.
- CT scanners allow for the easy measurement of the internal structures of nonferrous parts by generating a point cloud from a number of two-dimensional X-ray pictures.
Each of these 3-D scanning technologies offers high resolutions of approximately 0.0005 inch. Combined with accuracies that rival conventional CMMs, laser- and optical-scanning technologies are perfect for tiny objects or large manufactured parts.
Currently, contract measurement services on the market include a variety of short-range (<20 feet) and long-range (20 to 900 feet) technologies that use both contact and non-contact methods. Each of these technologies is portable, so suppliers can bring the scanning services to you.
When your process calls for both high-density scans and high-precision point measurements, the benefit of using a portable coordinate measurement machine (PCMM) is a simple flick of a switch. When your parts demand a longer range than 20 feet, 3-D scanning and PCMM solutions typically grab measurements at 30, 100, 500 and 900 feet.
Benefits of 3-D Scanning
3-D laser scanning plus the latest trend of industrial computed tomography (CT) scanning, which is based on similar technology used for medical MRI, permit an array of external and internal points to be read on a variety of substrates and manufacturing techniques. 3-D scans are made on many types of metal, composite, thermoplastic, elastomeric, wood, fiber and fabric materials. CT scanning permits the internal exploration of a thick aluminum casting, injection-molded plastic or rubber part, prepared foods, packaging, archaeological finds and fine art. The old joke is, with CT scanning we would have known about Venus de Milo’s arm problems long before they occurred. CT scanning can produce results for porosity, dimensional verification, failure analysis, volume and fiber orientation, all while leaving the part undamaged.
In contrast to conventional tactile CMM techniques, laser and CT scanning capture all surface points simultaneously, on even the most complex, convoluted surface areas. As an example, where a typical touch probe might capture 300 points of reference for comparison to a CAD overlay, the laser or CT will capture millions of points. Typical systems today can offer measurement accuracies to 4+L/100µm, referring to the VDI 2630 metrology guideline used to verify the accuracy of a CT metrology system.
With laser line scanners (white light and blue light), CT scanners and long-range laser scanners, this new technology can be used in just about any customer application currently served with even a large-scale CMM. You can make adjustments to your machining processes by reviewing the 3-D color map of your scanned forging.
Benefits of CT Scanning
Besides offering a method to get 3-D views of the inside of a part, another primary benefit of CT scanning is that it is the only way to nondestructively obtain truly dimensional data. If you need internal defect analysis/3-D quantitative porosity analysis, materials structure analysis or assembly control, CT scanning (industrial X-ray) is a viable process.
A forging or any other workpiece can be rotated 360 degrees in the X-ray beam’s path as multiple readings from various angles are taken. Once the CT gray-scale images are converted into voxel-based 3-D point clouds, it is possible to generate a CAD-to-part comparison for the customer. This leads to a considerable reduction in time to first article, which can occur in a fraction of the time needed to do a CMM point scan.
We use sophisticated 3-D scanner software programs to generate first-article inspection reports, statistical process reports and 3-D models for computer simulation and analysis programs. Finally, one can verify that your forging does not have any structural defects, such as cracks, before or after the machining process while keeping the part intact.
Buy the Service or Buy the Equipment?
One important point to note is that the same software that manipulates the data from the CMM can be used with 3-D scanners and CT equipment alike. There are substantial cost savings to be realized in the purchase of a 3-D scanning device or the new generation of tabletop CT scanners, depending on the parts being examined. The transition from CMM to 3-D or CT is more economically feasible today. There are two driving cost factors for a company to consider when evaluating the service versus purchase issue.
The easier factor is raw cost, where a 3-D scanner is typically half the cost of a CMM for measuring comparable part sizes. The other, more-complicated factor is the level of expertise of the personnel doing the work. Experienced technicians can effectively scan and process large data sets on a wide variety of substrates, geometries and part sizes. Coupled with the part volumes being processed and other factors, it is often more practical and profitable for a company to use a scanning service on an as-needed basis.
An additional point to note in evaluating CMM versus scanning is the time involved. To derive those 300 previously referenced touch points might take four hours, while the 2.5 million points derived from a 3-D scan would take 30 minutes. Since Polyworks software can be used on a CMM or 3-D laser scanner, the time and money savings quickly accumulate. Plus, in this manner, companies can develop something of a “universal metrology” scenario by augmenting the existing CMM technologies with 3-D and even CT scanning capabilities, making their quality-control (QC) department more powerful from the outset.
One practical note: We can train QC, R&D and production-department personnel usually in three days or less to expand their capability from CMM to include 3-D and CT scanning.
In terms of the data created, 3-D scanning produces color maps, inspections, first-article inspections or other outputs. What you can do with the data after completing the scanning is endless.
On the topic of substrates, it should be noted that CT scanning offers the ability to evaluate a wide variety of dual-thickness and dual-density materials, with disparate examples ranging from an automotive firewall that might have differing thermoplastics to rubber and TPE materials co-molded or even a candy bar with chocolate and nuts!
A customer recently came to us with the following scenario. An engineer at their company was boasting of having produced the “perfect part” based on CMM deviation points. Our contact had used our scanning services in the past. We performed a blue-light 3-D scan on the object and quickly determined the part was out of specification on several faces because certain critical points had been missed in the CMM protocol. Millions of scan data points were fit to the deviation map, and the problem was tracked back to the production source and rectified. Our services are now being used regularly by this company.
This analysis is not indicative that CMM technology has outlived its usefulness. We continue to use it in our operations for certain measurements in many applications. Our mission is to point out the difference between these 3-D scanning technologies to best fit customers’ needs.
The 3-D measurement data from scanners and PCMMs offer a comprehensive definition of a physical object that is used for measurement, inspection, comparison and reporting. When a part is defined by millions of points, you can see subtle deviations, slight variations and fine details that give the confidence necessary that a part, die or mold meets specifications. To deliver the best of both worlds, we combine 3-D scanning with more conventional contact, touch-probe measurement tools to deliver precise dimensions on geometric shapes.
Author Matthew Martin is Division Manager for Exact Metrology, Inc., Cincinnati, Ohio. He can be reached at 513-831-6620 or at firstname.lastname@example.org. For additional information visit www.exactmetrology.com.
About Exact Metrology
Exact Metrology, with facilities in Cincinnati and Milwaukee and affiliated offices throughout the Midwest, is a metrology services provider. It offers customers 3-D laser and CT scanning, reverse engineering, quality inspection, product development, 3D printing and 2-D drawings. The company also provides turnkey metrology solutions, including equipment sales and lease/rental arrangements. Exact Metrology is an ISO 9001:2015, AS9100-certified, FFL and ITAR-registered company.