Scanning vs. CMM

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). 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, first as 2D drawings and today as computer aided design (CAD) files hosted in the cloud.

During the last two decades, however, a new kid in town has arrived on the scene, with power, size, point capability, and price that are rapidly leaving CMMs in the dust.

3D laser scanning and industrial computed tomography (CT) – based on technology developed for medical scanning – permit reading external and internal points on various substrates in many manufacturing techniques. 3D scans are made on many types of metals, composite, thermoplastic, elastomeric, wood, fiber, and fabric materials, while CT scanning permits internal exploration of a thick aluminum casting, injection molded plastic, or rubber part – even prepared foods, packaging, archaeological finds, and fine art. With CT scanning, we would have known about Venus de Milo’s arm problems long before they occurred. The non-destructive system can produce results for porosity, dimensions, failure analysis, volume, and fiber orientation.

In contrast to conventional tactile CMM techniques, laser and CT scanning capture all surface points simultaneously on even the most complex, convoluted surface areas. 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 offer measurement accuracies to 4+L/100µm referring to the VDI 2630 metrology guideline.

An engineer at his 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 quick blue light 3D scan on the object and quickly determined the part was out of spec on several faces, as certain critical points had been missed in the CMM protocol. Our millions of scan data points were fit to the deviation map and the problem was tracked to the production source and rectified.

This example does not indicate that CMM technology has outlived its usefulness. We continue to use it in our operations for certain measurements in many applications.

There are two driving factors for a company when evaluating the service vs. purchase. A 3D scanner is typically half the cost of a CMM for measuring comparable part sizes. The other cost factor is more complex – namely the talent of the personnel doing the work. Experienced technicians can scan and process large data sets on a wide variety of substrates, geometries, and part sizes effectively. Coupled with the part volumes being processed and other factors, it’s often more practical and profitable for a company to use a scanning service as needed.

An additional point in evaluating CMM vs. scanning is the time involved. To derive the 300 touch points referenced above might take four hours, while the 2.5 million points derived from a 3D scan would take 30 minutes. Since Polyworks software can be used on a CMM or 3D laser scanner, time and cost savings quickly accumulate. Plus, in this manner, companies develop a universal metrology scenario by augmenting existing CMM technologies with 3D and CT scanning capabilities, making their quality assurance (QA) department more powerful from the outset. One practical note: We can train QA, research and development (R&D), and production department personnel usually in three days or less to expand their capability from CMM to include 3D and CT scanning.

In terms of the data created, the 3D scanning produces color maps, inspections, first article inspections, or other outputs.

CT scanning today offers the ability to evaluate a wide variety of dual thickness and dual density materials.

Exact Metrology Inc. www.exactmetrology.com

About the author: Matthew Martin can be reached at matthewm@exactmetrology.com or 513.831.6620.