First of all, speed wins. Faced with ever-increasing global competition, manufacturers are looking to run leaner and faster. They seek to reduce, shorten, or eliminate post-process quality operations that increase the work-in-process burden. Quality assurance is viewed as the top non-value-added activity on the manufacturing floor.
To serve both Lean and Six Sigma disciplines, manufacturers are moving to integrate part inspection and qualification more closely with production operations. They aim to use inspection for real-time process feedback. The goal is to catch and correct process drift before parts are scrapped or rework is necessary. This effort is spurring trends to on-machine probing--using the machine tool to check its own parts--and to moving the CMM into (or nearer) machining operations. Faster inspection is especially vital on large, complex, high-value parts with many critical features. Real-time inspection and process feedback limits problems to only a unit or two, rather than allowing those problems to affect a run of parts.
More inspection can yield higher quality. Along with trying to reduce the cost and process time for quality, manufacturers are doing more inspection--checking more parts more often, checking more features and taking more data points per feature. Needing to produce parts to ever-higher precision, ever-tighter tolerances, they're looking to increased CMM speed for a solution. The faster they can measure parts, the better they can maintain control of their process and optimize production efficiency.
Open metrology can enhance quality. Manufacturers want metrology systems that will allow implementation of best practices across multiple plants and CMMs, along with economical upgrade of legacy machines to new technology advances. They want interoperability--common user interfaces that allow software packages and sensors of their choosing to be used on any make of machine. This flexibility is not currently possible with proprietary machines and control software. Automotive companies, through the Automotive Industry Action Group (AIAG; Southfield, MI), have been pushing for interoperability through the emerging I++ standard. This document will allow them to share programs and standardize practices on all CMM platforms, while making performance upgrades to leverage their investments in legacy equipment.
Metrology companies are responding with new, high-speed measurement technologies. New probing systems entering the market are capable of scanning up to 10,000 data points/sec. Scanning speeds are jumping from conventional rates of 5 - 15 mm/sec to as much as 500 mm/sec--with 1 m/sec a not-so-distant possibility.
Historically, speed has been the physical limiter on accurate measurement as higher acc/dec rates and rapid axis changes induce structural deformations in the CMM. Different approaches are being applied to deal with these machine-dynamic errors. One uses an active head with multiaxis motion capability to manipulate the probe, while the CMM moves at steady speed in a single axis to avoid inertial error. The low-mass head can reposition at speeds magnitudes faster than ever possible by a CMM structure. Another approach uses software to compensate for high-speed machine errors. A part is measured first at slow speed, then again at high speed. Errors at high speed are mapped by the software and compensation is applied, enabling measurement accuracy comparable to the original low-speed inspection. Because machine dynamic errors are repeatable, the compensation can be applied to inspecting all succeeding units of the same part at high speed.
Upgrade packages already available use replacement universal controllers and dynamic compensation software to deliver 5 - 10 X increases in scanning speed without degrading accuracy.
This article was first published in the August 2005 edition of Manufacturing Engineering magazine.