Optimize Spindle Performance by Analyzing Failure

An in-depth failure analysis report documents findings and includes recommendations for corrective actions.

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When high performance equipment such as a CNC spindle underperforms or fails, the owner of that machine can have a million dollars’ worth of equipment that is not being used. It is therefore critical to get the spindle repaired or replaced as quickly as possible. At the same time, repairs provide the opportunity to identify exactly what led to the spindle’s failure so the scenario can be avoided in the future.

There are several components and systems related to spindles that can contribute to or cause spindle failures. Many of these causes can go undetected and instigate recurring failures, resulting in weeks of downtime and costly damage to related equipment, unless the root cause of failure is precisely determined and reported by the spindle repair shop.

“What is really needed after a spindle is damaged, and especially after a catastrophic failure, is a detailed failure analysis report, including the data accumulated through comprehensive inspection and testing,” says David Kirkpatrick, president of Superior Spindle Service in Taylor, Michigan, a company that specializes in repairing and remanufacturing machine tool spindles. “Unfortunately, not every repair shop can provide that.”

He says a comprehensive failure analysis report should clearly show when the real cause of failure is not the spindle itself, but something related to the motor, bearings, gages or even improper spindle mounting.

Generally, failure analysis amounts to the process of collecting and analyzing data to help determine the true cause and/or causes of a failure.

“It is often assumed that failure analysis is an intensive, complete process, as it should be,” he says. “Yet, when applied to a CNC spindle, to be truly comprehensive, the failure analysis process has to extend to all equipment that interacts with a spindle. Our technicians have discovered things such as faulty lubrication lines and control issues that would actuate the drawbar and encoder issues.”

Mr. Kirkpatrick says these problems were all solvable, but it was necessary to work with the machine shop customers to identify each issue and correct each one of them.

The critical phase of an in-depth failure analysis process is the report that documents findings and includes recommendations for corrective actions. Mr. Kirkpatrick recommends an intensive program that encompasses not only attention to the spindle, but also motors, bearings, air purge and other associated components. His company treats failure analysis as a primary service function.

“During the tear down procedure, every part is inspected to identify why and how the spindle failed,” he says. “Once the equipment has been thoroughly cleaned, every part of the spindle’s geometry is examined, referencing OEM standards for dimensions and concentricity. Every step of this process is photographed, documented and included with all findings in a comprehensive failure analysis report, along with specific recommendations on how similar failures can be avoided and spindle life can be extended.”

In some cases, particularly when it appears that customers might be perplexed by failure analysis reports that indicate complex root causes of failures, the spindle repair shop should provide another level of validation via the component manufacturer or supplier.

“In such instances, it’s advisable to reach out to OEM resources—the bearing suppliers, for example—to provide another level of failure analysis,” Mr. Kirkpatrick says. “Suppliers that provide tooling drawbar systems and other third-party experts can also be called upon to complement the failure analysis effort by substantiating report findings as they pertain to their products.”

When customers experience repeated failures, Mr. Kirkpatrick advises that a spindle specialist visit the customer’s production line to get a firsthand view of the CNC center on which the spindle is running. When technicians perform on-site inspection, problems not pertaining specifically to the spindle itself may be discovered. For example, one of Superior Spindle Service’s major automotive customers was experiencing repeated failures, which had caused considerable downtime, as well as more than $120,000 in spindle service costs.

“Each time we inspected one of the customer’s wrecked spindles, we determined the most likely cause was a bearing problem related to a malfunction of the spindle’s air supply,” he explains. “Although we suggested in each failure analysis report that the customer re-check the air supply, each time they checked, the air supply seemed fine. So, our technician inspected the air supply pressure gage and found that the gage needle was stuck, and the air wasn’t turned on. So we installed a new gage, and the customer hasn’t experienced a spindle problem for more than two years.”

Other on-site visits have revealed setup and alignment problems, improper spindle mounting, motor-related problems and controller malfunctions.

Mr. Kirkpatrick is convinced that intensive failure analysis can lead to extended spindle service life. For example, when customers are working with softer metals, decreasing spindle rpm and increasing feed rates can increase spindle life with no impact on cycle times, he says.

In another instance, one of his customers was chronically burning bearings from running a spindle continuously at high speed. So, the spindle technician recommended a different bearing setup, and the spindle service life improved dramatically.

Superior Spindle Service has an inspection center that is dedicated to spindle motor electronics. This includes motor mega-ohm value, thermistor value, encoders, motor proximity sensors, and analog sensors.

“We not only bench check the motor, but we confirm that it communicates accurately with whatever type of machine controller the customer has installed at their CNC machine,” says Mike Davis, who manages the center. “During this process, we also determine motor life expectancy and can predict the failure mode. This enables us to suggest that the customer have the motor rewound or baked, in order to avoid potential failures.”

Mr. Kirkpatrick believes that, when a spindle is underperforming in a specific application, it is possible to improve performance through spindle design modifications such as spindle metallurgy or shape.

Through the use of air gaging equipment, the company is capable of detecting when a customer’s spindle taper is out of OEM specification and can help return the taper to the proper spec within 50 millionths of an inch. However, if a somewhat modified taper will improve performance for the customer’s specific application, the air gage equipment can precisely verify the geometry of that modification as well.

Superior Spindle Services LLC, superiorspindle.com.

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