Quality Applications

Renishaw QC10 Ballbar System Benefits Software pinpoints specific machine error characteristics Tracks machine movement to identify dynamic, multi-axis errors Samples up to 250 values per second www.renishaw.com

Ballbar System Catches Errors Sooner

A Boeing tooling shop that manufactures assembly jigs and fixtures for complex aerospace components was looking for a way to cut down on costly rework. With the Renishaw QC10 ballbar system, the shop has found the ideal solution: verifying its computer-numeric-controlled machines prior to the process instead of relying on coordinate measuring machines (CMMs) for after-the-fact checks against the machines.

 "A CMM can tell us only when there is an error on the part, but not if there is a repeatable error traceable to the machine tool," says a Boeing official. With the ballbar system, however, the machines can be trusted to produce parts--no matter how complex or critical--that will meet specifications before removal from the machine.

 The shop's first steps were to baseline its machines through ballbar and laser testing, measure the accuracies of each and flag those in need of maintenance. Next, each healthy machine was fingerprinted over time with periodic tests. By establishing precise positioning and contouring capabilities, inspections allow the shop to assign each machine a set of toolpaths that make up a simulated part. Tests can determine whether a given machine could cut a part to specification before assigning a part to that machine. By monitoring how each machine's positioning and contouring capabilities are trending, the shop avoids assigning a job with fine tolerances to a machine that can't operate within those tolerances.

 In the past, the tooling shop used the circle-diamond-square test, which was helpful in finding errors in the X-Y plane but often missed errors in the X-Z and Y-Z planes. Subsequent ballbar diagnoses on a five-axis machine that passed the circle-diamond-square test revealed following errors as large as 0.065 in., a result of misalignment between rotary and linear axes. The ballbar can assess machine performance in 15 minutes with a dynamic circular test, in which the machine performs two consecutive circular arcs, one clockwise and the other counterclockwise. The ballbar measures deviations in the circle radius, and the Renishaw software system uses advanced mathematical techniques to identify the specific error sources. The ballbar user manual details the likely causes for each type of machine error and offers advice for correcting them. The system's ability to pinpoint the machine's specific faults minimizes the downtime necessary for maintenance.

 The Renishaw ballbar consists of a carbon fiber bar and two precision-ground balls, one at each end. One ball attaches magnetically to a centering cup mounted in the machine spindle, while the second rides inside a magnetic center-mount secured to the machine table. The ballbar follows a machine's movements, tracking them to ±1.1 ppm accuracy, to not only measure accuracy but also plot dynamic, multi-axis errors that are only apparent when the machine is in motion. As the machine table is moved through a two-axis, circular or semicircular tool path in any of the X-Y-Z planes, a linear transducer contained within the ballbar measures deviations from a true circle.

 The ballbar and laser tools complement each other and use the same notebook PC for machine evaluation. The ML10 laser system uses interferometry to measure diagonals and analyze for flatness, straightness, squareness, linearity, angularity and parallelism to detect backlash, scaling errors and general component wear. While the ballbar measures the machine tool's ability to move accurately through curves and contours, the laser system takes a series of four diagonal measurements to track how well the machine can hold a straight line. Ballbar data is sent directly to a PC using a standard RS-232 cable. Renishaw software analyzes the data in accordance with ISO 230-2 or ANSI B5.54 machine performance standards.

 Since implementing the ballbar system, the Boeing tooling shop has eliminated part rework on machines it has fingerprinted. "With fingerprinting, we know immediately when a machine's capabilities decline, meaning we can correct the problem and return the machine to its original performance level," says a company official.

 The system has a resolution of 0.1 mm, with an operating range of 32°-122°F (0°-50°C). The ballbar is accurate to ±0.5 mm (±20 min.) at 20°C (68°F), allowing calculation of circularity error, servo gain mismatch, vibration, stick-slip errors, backlash, repeatability and scale mismatch, and machine geometry. Extension bars are available in 50 mm, 150 mm and 300 mm sizes.