Case Studies: Ballbar System Key in Company's Lean Philosophy

October 1, 2005
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Ballbar testing of machine tool positioning accuracy gives Rockwell Automation Power Systems a fast, simple diagnostic tool for determining machine capability and tracking performance for preventive maintenance. Source: Renishaw Inc.


The Renishaw ballbar enables a 15-minute check of machine accuracy, minimizing downtime and simplifying maintenance scheduling. Source: Renishaw Inc.

Rockwell Automation Power Systems (Greenville, SC) specializes in providing technologies and services for optimizing power transmission performance and productivity. In 2000, the company turned its analytical and creative skills to developing an integrated process for continuous improvement of the manufacturing processes in its own plants. Named Power Lean, Rockwell Automation's program integrates the tools and methodologies of lean enterprise, Six Sigma and total productive maintenance.

Focused on fast, bottom-line results, Power Lean was implemented across multiple power systems plants. In one facility, the program has reduced total inventory more than 20%, cost of quality more than 25% and cycle time by almost 80%, while increasing on-time delivery by 99% and productivity by more than 35%, along with freeing valuable manufacturing space.

A key tool in the Power Lean arsenal is just that-a tool. The company began using Renishaw Inc.'s (Hoffman Estates, IL) QC10 Ballbar kit to test performance of the company's machine tools. "The Renishaw ballbar is an incredibly powerful tool for us," says Larry Sykes, Lean program manager. The QC10 ballbar gives maintenance technicians an internationally recognized standard for assessing the dynamic motion and positioning precision of machine tools and production machinery, he explains. "The ballbar gives us a preventive and diagnostic tool for making sure that our machines are capable of producing to required part specifications."

This provides vital benefits to both Six Sigma quality and lean efficiencies. "Machine capability is critical when you're machining parts to tolerances in the tenths," says Bill Murphy of group maintenance. At the same time, ballbar tests generate trend data to help technicians implement predictive and preventive maintenance to keep machines functioning correctly and part quality within specification.

The ability to do proactive maintenance greatly reduced unexpected downtime to support the move to a high-throughput, lean work scheduling, Sykes says. Ballbar tests are done periodically for performance tracking under the predictive maintenance program, as well as when a machine problem is suspected. "The great thing is ballbar testing is simple and fast, so you're minimizing disruptions and downtime," Sykes says. "It only takes about 15 minutes to check out a machine."

Literally a telescoping bar with machined balls at either end, the ballbar affixes magnetically to socket devices mounted to the machine's spindle and bed. As the machine runs the ballbar through a sequence of programmed routines, a precision transducer tracks machine movements. Renishaw software converts the data into a polar plot of the machine's true movement. The software tracks machine movement to ±0.5 micron, allowing precise assessment of machine geometry, circularity and stick/slip error, servo gain mismatch, vibration, backlash, repeatability and scale mismatch. The ballbar software not only provides diagnosis of specific errors in accordance with ISO 230-4 and ASME B5.54 and B5.57 standards, but also ranks errors according to their overall effect on machine accuracy.

The 15-minute ballbar routine compares with circle/diamond/square tests previously used to check machine capability and accuracy. The features would be machined into test piece, which would then go to the quality department. The quality department would check all the dimensions to make sure the machine was able to produce parts correctly. If there was a problem with a feature geometry, however, the cause might not be obvious, Sykes says.

By comparison, he says, "With the ballbar quick check system, you can evaluate a machine in 15 minutes and be able to tell if it can produce a good part. If there is a problem, the ballbar software gives us the ability to check through all the different possibilities, telling us which factor we need to address on the machine tool. Sometimes we're able to just make adjustments to parameters in the machine controller to offset some ball screw issues in the machine."

Renishaw is constantly coming out with new and more powerful diagnostic capabilities, Sykes says. "We particularly like the trending abilities of the software." A machine history function enables monitoring of an individual machine through its life cycle to help predict and schedule future maintenance, he explains.

Responsible for that maintenance, Murphy appreciates the ability to use the trend data to allocate resources. "We can prioritize the maintenance and make decisions on what needs to be done to extend machine asset reliability," he says.

Another new software feature, the Ballbar Plot Simulator allows "what if" scenarios by simulating machine adjustments, letting maintenance technicians view the effects on ballbar data before making any actual machine adjustments. "Today, if there's a problem with a part, we can run a ballbar test and either eliminate or target the machine tool," Sykes says.

The ballbar has helped Rockwell Automation take corrective maintenance to a higher level with lean timesavings. Sykes cites a computer numerical control lathe with a damaged way that needed to be replaced. "Through working with the OEM and with Renishaw, we were able to capture all the accuracy information we needed and actually do the replacement and re-alignment of the way right here in the plant. Before, we'd have sent the lathe back to the factory for the repair, which could have taken a couple months. Our own maintenance people did the job on the shop floor in three weeks using the Renishaw ballbar," he says.

Ballbar use spread to other plants of Power Systems after plant and lean managers saw it demonstrated at a Champions Forum instituted under the Power Lean program. "The idea is to bring people from all the different plants to share best practices," Sykes says. "After we demonstrated what the ballbar was doing for us at one plant, it was quickly adopted by our other plants."



Renishaw Inc.

(847) 286-9953



• The ballbar system is a preventive and diagnostic tool for ensuring machines are capable of producing required part specifications.

• The ability to do proactive maintenance reduces unexpected downtime to support the move to high-throughput, lean work scheduling.

• The software tracks to ±0.5 micron.

• Technicians can evaluate a machine in 15 minutes and be able to tell if it can produce a good part.

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