Quality Scan: Calibrated Machines Produce Good Parts
The only thing better than knowing your machine tool just made a good part is knowing beforehand that it would.That knowledge is worth money—especially to manufacturers aiming for lean practices, faster throughput, and reduced WIP.The most expensive, non-value-added process in manufacturing operations is post-process piece-part inspection. Inspecting good parts—parts that meet all part specifications—is a waste of time, money, and manpower. When the machine tool is correct, the parts coming off the machine should also be correct, barring material imperfections, machine crash, or catastrophic failures of tooling and fixtures.
Being able to document the capability and the accuracy of your machines, proving you have control of the machining process, is the foundation of ISO 9000 and QS 9000. To determine that accuracy, your machines should be inspected to a nationally recognized and accepted standard, such as ISO 230-2 or ASME B5.54. Both standards prescribe how to use telescoping ballbars and laser calibration systems to measure the accuracy of three-axis CNC machines, baseline their capabilities, and perform critical geometric and contouring checks. The prescribed tests cover: linear displacement accuracy; bi-directional repeatability; volumetric performance via diagonal laser displacement measurements; and contouring performance.
The latest laser calibration systems provide linear measurement accuracy better than ±0.5 ppm with a resolution of just 1 nm over linear distances up to 80 m, enabling calibration of both large and small motion systems with precision. To meet the demands of new high-speed machines and devices, slew rates have been cranked as high as 4 m/sec. These systems can deliver this performance even on the shop floor, where an unstable environment can play havoc with lesser equipment. The very best laser interferometer systems can maintain accuracy and performance over an operating range of 0–40°C—think Alaska to Death Valley.
Calibration software enables both static and dynamic data capture, and provides data analysis that meets national and international machine tool testing standards. Besides determining current machine capabilities, linear error compensation software can create new compensation values for a CNC machine's controller to significantly improve positioning accuracy. While many CNC machines are mapped and compensated by OEMs for axis positioning errors, wear, damage, and misuse can cause these errors to change.
One of the latest calibration software developments allows data from the laser to be streamed to a PC at rates as high as 50 kHz and displayed, in real time, on an oscilloscope-style software display. The operator can immediately "see" the effects of vibration and the smoothness of axis motion.
Perform calibration on need, not on time.There is no reason to pull a perfectly good machine out of production for calibration. Routine testing with a telescoping ball bar takes just 15 min to complete, and will clearly show if a machine is still performing to specification.
A compact and portable tool first introduced to industry by our company, the telescopic ball bar can identify 22 different dynamic and geometric errors affecting machine axes. Attached magnetically to the machine spindle at one end and to the tool bed on the other, the ball bar monitors machine movement of the machine to ±0.5 µm as it follows a programmed circular/arc path, and compares the test path to that of a perfect circle to identify errors. Algorithms in the ballbar software produce a comprehensive diagnostic report that enumerates the 22 error values, then ranks them in order of importance to part accuracy.
A software historic review function allows tracking machine motion over time to identify wear/process deterioration, and enables the shop to schedule preventive maintenance. A machine error-simulator function enables maintenance technicians to try out the effect of different adjustments and fixes, without actually touching the machine.
In short-run, high-flex production—where lean manufacturing is trending—inspection can only tell you when there is an error in the part, not if there is a repeatable error traceable to the machine. When part runs are small, there's often no "trend" to monitor with SPC. By defining and monitoring machine capabilities—inspecting before the fact—manufacturers can have high confidence that parts are produced to spec, right the first time.
This article was first published in the February 2008 edition of Manufacturing Engineering magazine.