A Simple Diagnosis: General Industry Coverage

It’s not the most glamorous piece of equipment by any means, but the ball bar is a diagnostic tool used, in some form, to benchmark the performance of virtually every machine tool sold, according to Harold Schoch, vice president of technology and sales for Konecranes, Erlanger, Ky.

“The telescoping ball bar is probably the best thing to happen to a machine tool maintenance guy in all of history,” he said. “It’s not the answer to every problem, but it’s a really nice roadmap to taking some corrective action in a timely manner. It’s the best tool you have for establishing trending in machine tool geometries and performance, and it lets you focus all your time and problem-solving efforts on the biggest contributors to machining errors or inefficiencies.”

A Renishaw QC20-W wireless ballbar performs a test on a vertical machining center. Image courtesy Renishaw.

Operationally, a ball bar is little more than “an indicator on a stick,” said Michael Wilm, business manager of calibration products for manufacturer Renishaw Inc., Hoffman Estates, Ill. Renishaw manufactures telescoping ball bars, which means that as a machine moves the “stick” in a circle around a central point in the machine—typically a machining axis—a transducer on the end moves in and out in a linear motion, acting as a plunger-type dial indicator. The ball bar is then able to monitor the radial deviation. The data can be used to determine machine errors —everything from geometric inconsistencies to servo performance to axial delays.

A telescoping ball bar can also be a valuable part of a shop’s regular routine, Wilm noted. “You put it on a machine and within a matter of minutes you can get a picture of the machine’s performance and how it relates to the last time you measured it,” he said. “In one circle clockwise and one circle counterclockwise, we can determine the linear positioning differential between two axes. It’s a quick, efficient tool for finding errors, and once you identify the errors, you can get the proper tool to address it.”

But while a telescoping ball bar may be quick and easy to operate, the level of accuracy pales in comparison to the even-more-rudimentary rigid ball bar, noted Eugene Gleason, president of Los Angeles-based industrial ball manufacturer Bal-tec.

“We use a telescoping ball bar from Renishaw to certify our machines every 6 months,” he said, “because the [0.015″ to 0.050″] tolerance is fine for those purposes. But, especially on large equipment, there’s just no comparison when it comes to telescoping vs. rigid.”

A rigid ball bar is based on trigonometric measurements, which can be accurate down to ±0.00005″, according to Gleason. The ball bars are made of Invar, known for expanding and contracting less than a 10th as much as steel, and the most common length (2.3m or 7.54′) is part of a number of ANSI standards for measurement.

One manufacturer records the results of ball-bar tests on history charts. Doing this allows the company to predict when maintenance tasks will be required, rather than having to ‘put out fires’ when machine performance becomes so bad that scrap is produced. Image courtesy Renishaw.

“You have a perfect ball that gives you a perfect dimension in 3-dimensional space, a second ball, and a certain distance between it,” Gleason explained. “You can then use trigonometry to measure machines of almost unlimited size, and the uncertainty is almost zero.”