Quality 101: The Evolving CMM
December 30, 2008
The coordinate measuring machine (CMM) evolved out of a need to know locations of features in manufactured parts. X and Y locations could be coaxed out of smaller parts with a height gage on a surface plate, but with heavy parts this was not practical. Parts would be placed back on a milling machine or jig bore and positions checked with a dial indicator in the spindle. This, of course, took the machine tool away from production. Feature sizes could be measured with hand tools.
Early CMMs, sometimes called stylus machines, were manually operated cantilevered or bridge types used for 2-D checks, usually with a tapered hard probe mounted vertically in a moving slide to center it in bores. A digital readout reported the X and Y locations with some reliability. It was now practical to measure coordinate locations of features in large and heavy parts more efficiently.
Soon the probe mount was equipped with a Z-axis position scale, and 2½-D measurements became practical. By the early 1970s, the touch trigger electronic probe and computer assist for measurement became more widespread, and the 3-D CMM as we know it today had become an accepted means of ensuring the dimensional quality of manufactured parts.
The Modern CMMToday, three elements are found in every CMM: a sensing probe, a computer for data input and analysis, and a precision structure to transport the probe around the workpiece. Manual operation has largely given way to motorized direct computer controlled (DCC) CMMs.
There are many types of CMMs, but the moving bridge design is the most common and generally consists of a bridge member with two legs riding on tracks mounted to a granite plate. A carriage rides across the bridge and supports the vertical Z-axis. In most systems the bridge is considered the Y-axis, and the carriage is the X-axis, because the operator tends to work facing the bridge.
Whether manual or DCC, today’s CMM produces very accurate evaluations of every kind of 3-D geometric dimensioning and tolerancing (GD&T) concern. Consider a bore in a part. The CMM can determine the part coordinate system, with origins, no matter how the part is positioned within the measuring space. It can probe the bore and report its size, position projected into the reference datum frame, cylindricity and the angular attitude of the bore axis in 3-D space.
Given nominal and tolerance data, it can further report conformance to tolerances, whether unilateral, bilateral, true position regardless of feature size (RFS), maximum material condition (MMC) or least material condition (LMC), with or without datum bonuses. It also can report relationships to other features such as distance, parallelism, perpendicularity, angle or intersections, for example. Many of these measurements are nearly impossible to make or repeat using traditional surface plate techniques.
With a program, the DCC CMM will quickly and automatically measure each production part in precisely the same way, at the same locations, with the same probing speeds. For these reasons the CMM is a fast, efficient, accurate and repeatable tool for the management of dimensional part quality for small shops, large-scale production, and high-volume or one-of-a-kind manufacturing.
A Continuing EvolutionThe CMM continues to evolve, as do the sensors and software. For complex parts, CMMs can support fourth and fifth rotary axes, and some are capable of combining different sensors such as contact probes, lasers and cameras. Probe systems have become far more capable, with automatic two-axis indexing heads, auto-probe changing and auto-stylus changing. Continuous contact scanning probes are becoming more common and produce far more data per feature by streaming data constantly while scanning. Calculating a bore, for example, from 1,000 data points is a better solution than from four or eight points.
Software has become friendlier toward computer-aided design (CAD), allowing off-line programming and making measurements of contoured surfaces and profiles that require vector data more practical. In addition, CMMs are regularly used for reverse engineering. This involves measuring an unknown part, then exporting the data to CAD in order to create a CAD model that can be used to produce computer numerical control (CNC) machining programs to replicate the unknown part.
During the years, typical CMM size has been in the range of 30 inches by 40 inches by 25 inches. They are found in inspection or gage rooms and often in clean rooms, operated by specially trained technicians. There is now a trend toward smaller CMMs dedicated to specific production needs in which a small footprint is an asset on a cramped production floor. These are tougher machines that have more inherent protection and guarding, simpler interfaces that give unskilled operators easy access to canned cycles and frequently provide closed-loop feedback to the process.
These systems are beginning to replace hard gages, checking fixtures and handheld instruments on the production floor. OEM providers are expected to bring turnkey systems to customers that include the CMM, holding fixtures, inspection programs and even production interfacing.
Where there is the need to confirm geometric integrity in 3-D and document it, the CMM is a fast, efficient, accurate and repeatable means to that end.