Finely Tuned

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by Kip Hanson | photos courtesy Renishaw

There’s more to machine tool maintenance than topping off the way-lube 


Few manufacturing people would dispute that routine maintenance is an important part of machine tool operation, which is why machinists are taught early on to check the filters, keep the way-lube reservoir full and wipe the machine down at the end of their shift. These are clearly necessary tasks, but is it really sufficient to keep expensive CNC equipment operating at its full potential? 

Good machine, good parts
Granted, CNC lathes and machining centres aren’t like the family car; you can’t simply drive one into the local garage every few months for a checkup. But there is a better way to monitor their performance than waiting for bearings to grumble or parts to fall out of tolerance. It’s called ballbar inspection, and Philip Smith, vice president of sales and marketing at Renishaw Canada Ltd., will tell you it’s easier, less expensive and more important than you might think. 

The QC20-W wireless ballbar system can be used to diagnose a variety of machine ailments.“A lot of people don’t realize it, but Renishaw is a significant manufacturer—we machine around one million components per month across our different facilities,” he says. “And because we know that you can’t make a good part on a bad machine, we inspect our CNC machine tools every two months with a ballbar, and shoot them with one of our XL 80 laser measurement systems if there appears to be a problem.”

Renishaw’s not alone. Smith says aerospace shops throughout North America follow a similar practice. Doing so helps them avoid surprises such as poor part quality and unexpected downtime, while extending the lives of their machine tools. “Every time you ballbar a machine, you get a snapshot of its performance which can be compared to previous tests. Over time, you can identify trends—maybe there’s a small amount of Y axis backlash one day, and two months later it’s grown worse. By extrapolating those values, it’s pretty easy to see that six months from now the machine will be unable to make good parts.”

Having a ball
Tom Ellis, marketing manager of Renishaw plc’s Laser and Calibration Product Division, agrees. He describes the company’s QC20-W wireless ballbar system as a diagnostic tool that companies can use to easily and quickly check the health of their machine tools. It works by magnetically attaching each end of a dumbbell-shaped sensor—the ballbar—to cup-like mounts affixed to the machine spindle and table (or turret, on a lathe). 

One end of the ballbar is then rotated [interpolated] through a circular motion, allowing the ballbar to detect microscopic deviations from the intended path. Depending on the size of the equipment and the number of axes involved, an average test takes approximately fifteen minutes, during which problems such as axis orthogonality-worn ballscrews, tired bearings, way misalignment and even out-of-level conditions can be identified. 

Ball bar inspection can also detect one of the more common causes of machine inaccuracy—crashes—something machine operators are not always transparent about. “Some customers are very honest about their mistakes, telling us up front that, ‘Yep, I bumped it. My bad,’” explains Clive Plummer, senior applications engineer at machine tool distributor Elliott Matsuura. “No one likes to admit it, but at least you then know where to start. This ultimately saves the customer time and money.”

Routine ballbar inspection is a valuable tool for shops concerned about traceability, part accuracy and preventive maintenance.Aside from the occasional uh-oh, there are environmental reasons to perform ballbar inspections. Plummer notes that concrete floors often settle after machine installation, or the season has changed from summer to winter and the shop is now ten degrees cooler. Also, some shops routinely machine heavy parts at one location on the machine table, leading to wear in that particular area. And just like breaking in the engine on a new car, machine tools tend to wear more quickly when new. 

Cascade of errors
“Machines can fall out of square, the bearings settle in, pivot locations might move a bit, all of which means it’s a good idea to ballbar inspect machines a few months after installation,” says Plummer. “To be clear, the errors we find on new equipment are generally on the order of microns, but for very fussy shops, and on more complex machine tools, it might be enough to notice.”

The complex machine tools he’s talking about are multitasking lathes and especially five axis machining centres, where “axis stacking” tends to magnify microscopic inaccuracies—if the X axis on a trunnion style machine has a “tenth or two” of error, it can lead to problems with mismatch and true position, particularly on taller workpieces, or where the workholding positions the part high above the A and B axis pivot points. 

Elliott Matsuura is one of Renishaw’s customers, and Plummer and his colleagues have access to specific software designed to identify these errors. Renishaw’s Smith says AxiSet Check-Up utilizes a Renishaw spindle probe, a datum sphere and software that is used to calculate and then adjust for pivot errors, eliminating blending issues and any other five axis faux pas that might otherwise occur.

Here, the QC20-W ballbar is being used to calibrate a CNC drill-tap machine.Practicing what you preach
Plummer had a chance to use AxiSet Check-Up recently at D-M Precision Products, an Ontario job shop specializing in high precision medical and scientific components, which recently purchased its second Matsuura MX-330 PC10 five axis machining centre from Elliott Matsuura. 

Shop manager Chad Plath says they installed the first 10-pallet MX-330 in November of 2018, and the company was so happy with it they invested in a second one a few months later. In each case, Elliott performed a routine inspection of the machine using its AxiSet system, followed by some actual test cuts to verify the results. 

Plath admits that he hasn’t seen a need for routine ballbar inspection at D-M Precision. The new machines are just that—new—so a follow-up visit from Elliott hasn’t been necessary. He adds that the shop floor is kept within a few degrees at all times, eliminating any temperature-related variation, and since D-M measures “pretty much everything,” any machine inaccuracies are quickly discovered and dealt with. Perhaps most importantly, Plath says the Matsuuras have proven to be “dead accurate,” and there simply hasn’t been a need yet to worry about machine inspection. 

Despite D-M Precision’s success, Renishaw’s Ellis encourages all shops to take a more analytical view of their machine tools. “As I said, the ballbar is a diagnostic tool you can use to check the health of your equipment. Just like going to the doctor for a routine physical, it gives you a baseline against which to measure any changes, while providing clues as to what might happen in a month or ten months, potentially avoiding extended machine downtime. Let’s face it: there’s room for improvement in any manufacturing company, and inspecting your machine tools on a regular basis is an excellent place to start.” SMT

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