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For most manufacturers, the productivity of their computer numerically controlled (CNC) machines can be measured in fractions of seconds, including not only the cutting or grinding cycle times but also the intervals between tool changes. The performance of the spindle – the workhorse of the CNC machine – has a dramatic impact on that productivity.
When operating problems, such as excessive vibration or chatter, hinder machining efficiency, productivity may be crippled until the spindle is replaced. If no spare is available, repairing or remanufacturing the spindle could take weeks, effectively putting a machine out of business in the meantime.
Achieving higher performance and extended life from CNC machines will require a combination of practices aimed at preventing premature spindle failure and insisting that the remanufacturer provide comprehensive CNC spindle repair services after a failure occurs as well as modifications and operational recommendations that can ensure longer spindle wear life.
The following guidelines can help machine operators achieve those goals:
Normally when a spindle is pulled, it is because of part quality issues and/or intensified noise level.
“Noise often results from damaged bearings creating vibration that may eventually lead to chatter, a more severe vibration that feeds upon itself,” explains David Kirkpatrick, president of Superior Spindle Service, a company that specializes in rebuilding, grinding and machining of machine tool spindles. “Chatter often creates waves in machined surfaces to the point where the parts are out of tolerance and must be scrapped.”
Another immediate and serious consequence of chatter relates to spindle efficiency. Many manufacturers deal with chatter by setting their machining parameters low. Therefore, instead of tool strength and spindle horsepower defining the metal removal rate, chatter becomes the limiting factor that keeps the process from reaching its potential.
“Cutting tool chatter and spindle vibration are indicators that a spindle requires attention or a catastrophic failure can occur,” adds Kirkpatrick, a CNC machine spindle expert with more than 30 years of experience.
In some cases, when vibration does not cause a part quality problem, operators have been known to run the machine until it catastrophically fails, which can be a very expensive decision.
The solution for operators of CNC machines is predictive maintenance. This requires regular monitoring of operating parameters and maintaining detailed records. For example, vibration analysis should be monitored and recorded at appropriate intervals, perhaps quarterly or even monthly.
“The frequency of vibration analysis depends on the application,” Kirkpatrick says. “The temperature of the spindle should also be monitored. It is important that an expert determine that the spindle should be switched out before parts go out of tolerance or motor damage occurs. Plus, if the spindle can be changed out over a weekend, the shop can avoid critical downtime.”
When CNC spindle repair or replacement is needed, a remanufacturing specialist can often provide more comprehensive services, sometimes also including repair of related motors, encoders, proximity sensors and drawbars. Some remanufacturers even offer one-stop repair services. Because there is a direct relationship between spindle performance and the proper function of other components, it is critical that the repair facility be able to evaluate and service all spindle components.
The evaluation process begins with disassembly of a spindle that is out of specification or has failed. This assessment could include the motor, retention force and all electronics (encoders, proximity sensors, bearing sensors, etc.).
“Service technicians meticulously should disassemble and check each spindle for adherence with the original manufacturer’s specifications,” Kirkpatrick explains. “This is followed by a geometric inspection using electronic measuring equipment that is repeatable within a micron. It is advantageous that a digital photo record be kept to ensure that all repair quotes and failure analysis reports are as accurate as possible. Next, grinding, machining and balancing should be performed by experienced technicians using advanced manufacturing equipment.”
It is also essential that spindles are reassembled in a clean work environment. For example, a class 10,000 clean room can prevent airborne contaminants and humidity from affecting spindle performance and help ensure that each spindle is returned to precise specifications.
Kirkpatrick says there is much room for Kaizen-type improvement throughout the remanufacturing process. In one case, his firm was working with a major automotive manufacturer who was experiencing excessive downtime (more than 31 hours) each time it had to change a spindle. With a few specific modifications, spindle life was able to be extended. As a result, the manufacturer’s average spindle downtime was reduced to less than 10 hours, an accomplishment for which the remanufacturer received a Kaizen award.
“In the manufacturing environment, if you can reduce the cycle time even a fraction of a second, it can make a big difference,” Kirkpatrick says. “So with respect to a 20-plus-hour downtime savings, that is monumental.”
For every CNC spindle repair, customers should expect to receive a detailed failure report that includes text and photos.
“This process provides the customer with documentation of the rebuilder’s findings concerning the causes of failures,” says Kirkpatrick. “This is vital because it can help customers take corrective actions to prevent future failures.”
Recommendations may also be provided to help increase spindle service life. For example, application engineers might suggest ceramic (silicon-nitride) ball bearings be used to replace conventional steel ball bearings. Ceramic ball bearings have 30 percent more mass than steel ball bearings and allow spindles to operate at higher speeds.
After spindles are remanufactured, it is vital they be thoroughly tested and verified. If related assemblies are in question, such as electronics or motors, those should be tested and verified as well.
“Advanced testing includes vibration analysis and running the spindle closed-loop on a Siemens or a Fanuc drive,” Kirkpatrick explains. “Our technicians also align encoders using special software. We also test motor thermistors, bearing thermistors and proximity sensors.”
In addition, each rebuilt spindle needs to be thoroughly “run in” at the operating speeds as they were designed. Vibration analysis and temperatures should be monitored throughout the run-in procedure, which may take up to 18 hours to complete. Before leaving the testing facility, detailed reports should be generated for verification by quality assurance technicians.
To help ensure minimal downtime and maximum productivity of your CNC machines, use the procedures mentioned above to increase your mean time between spindle failures.