Spin doctors: Inspection Efficiency

Machine tool spindles are high-tech products. Technicians maintaining them typically require special diagnostic tools and intense technical training. Those two issues add up to a serious question for shop managers: Should your shop fully maintain its own spindles?

Or, should you largely farm that function out to machine tool builders, spindle builders and spindle maintenance and repair companies? Most experts say don’t try doing complete maintenance in-house. The level of in-house maintenance that’s appropriate depends on several factors.

Colonial Tool Technician Duane Eddie maintaining a spindle for a CNC machine tool. Image courtesy of Colonial Tool.

“Companies should not do their own spindle maintenance,” said Tony Landl, chief engineer for Wm. Sopko & Sons Co., Cleveland. “Machine shops that are qualified to repair spindles are few and far between. You should have a relationship with experts. Sometimes shop maintenance people can do more damage to a spindle by taking it apart.”

Are there any kinds of maintenance, then, that can be performed effectively in-house? “Just about anything that does not involve taking a spindle apart, including checking taper runout, balancing tooling and cleaning tapers,” said John Easley, vice president and general manager, business operations, for Fischer Precise USA Inc., Racine, Wis.

Some shops can “do monitoring, but not complete maintenance. Listen for noise from bearings,” Landl said. “The biggest clue that you may need some outside expertise is noise and chatter. It’s really good-old common sense.”

Hardware and Software Tools

In general, spindle manufacturers suggest at least a partnership with outside experts on spindle maintenance. Increasing precision and sophistication in spindle technology means “not many shops can do it, and they typically wait until the last minute,” Landl said.

Other observers agreed and noted the expensive training and tools required for maintaining spindles. “The average shop may not be able to maintain modern spindles, which are lubricated for life,” said Walter Zic, vice president of Dynomax Inc., Mundelein, Ill. “The expense of tools to do this and the higher skills required mean shops need outside expertise and support.”

Various vibration analysis tools are used in spindle maintenance, said Paul Thrasher, president, Colonial Tool Group Inc., Taylor, Mich. There are several manufacturers and “each one has its own distinct features that are sometimes specifically requested by our customers or chosen by our technicians,” he said. “The data can usually be considered similar from each device. However, there are differences that make one or another more functional due to the operating speeds of the spindles or the frequency of the band being studied.” This type of equipment, including software, can cost around $15,000.

Another maintenance tool is the multichannel analyzer, which uses noncontact probes placed normal, or perpendicular, to each other to study the dynamic orbit of the spindle centerline at high speeds. These devices can cost $40,000. The analyzers plot the spindle orbit, which duplicates the cutting path of a single-point tool. “That allows you to predict with relative certainty the in-process performance of the spindle,” Thrasher said.

Spindle assembly running accuracy is a function of multiple component geometry tolerances, bearing accuracy and dynamic effects of the rotating components, which influence the centerline orbit. “Sophisticated assembly techniques can be used to cancel tolerance stack-up errors that can be picked up with this type of specialized equipment,” Thrasher said.

Devices such as hand-held decibel meters for measuring sound levels, portable balancer/vibration analyzers and indicators with magnetic bases can also be used to check spindle runout and end play, according to Sopko’s Landl. Of the three items, the cheapest is the indicator, followed by the decibel monitor. The most costly is the balancer/analyzer, which can cost $10,000. Other monitoring equipment includes temperature sensors and inspection equipment to verify the sizes of shafts and housings.

If a shop has multiple spindles, it may want to consider purchasing and using some of these monitoring and maintenance devices. However, small- to mid-sized shop typically do not have enough spindles to justify the cost and labor required to operate this equipment.

New maintenance tools that can be used by shops are also becoming available. Zic pointed to his company’s soon-to-be-announced diagnostic tool, Smart Spindle Technology, as an example of how sophisticated testing and monitoring has become. “A 5-axis machine or a large gantry machine can use this and download running data onto a laptop computer.” The program is basically an automated maintenance log for cleaning, checking fluid levels and performing vibration analysis on spindles.

Another new spindle analysis and maintenance program is Fischer Precise’s SmartVision, a software package that “provides multiple parameters in real time with the capability of remote access. It can determine spindle service life,” Easley noted.

Many, if not most, shops do some spindle maintenance, even if it’s only cleaning and checking fluids, and then rely on support when they run into problems they can’t handle.

“When a spindle is first built or rebuilt, it is important to document the spindle’s critical [tool interface] geometric and dimensional characteristics, axial and radial compliance and vibration signatures,” Thrasher said. “These are the spindle characteristics most likely to degrade over time and ultimately affect the tool performance and the part being machined. These inspections can normally be performed periodically on the machine and compared to the original for the degree of degradation. This data would be used to set rebuild intervals.”

Machine Complexity

Spindle maintenance is dependent upon the complexity of machine design. “Normally, a belt-driven, grease-lubricated spindle requires little maintenance except for the ancillary utilities,” Thrasher said. The air purge system needs to be maintained for pressure and cleanliness, for example. “More complicated spindles, such as high-speed, motorized CNC spindles with automatic tool drawbars, normally have several ancillary equipment requirements that make maintenance more complicated and more frequent,” he said.

Ancillary equipment also includes chillers for liquid cooling of the motors, oil lubrication systems for the bearings, hydraulic systems to actuate the drawbars and air-pressure tool seat checks. “All these systems have independent pressure-delivery requirements, flow requirements and are more likely to cause the spindle system to fail prematurely than the actual spindle bearings reaching their life expectancy,” Thrasher said.

Further advice on what shop managers should look for in inspections and spindle maintenance came from Dick Garski, senior sales manager, NSK America Corp., Schaumburg, Ill. “A machinist should always wipe oil and chips off the spindle nose with a rag, rather than blowing them off with an air hose. Blowing chips off the nose can allow contamination to get inside the spindle and damage the bearings,” he warned.

If the spindle system uses an oil lubrication system, a machinist should check daily to make sure the oil bowl has a sufficient supply of oil. He should also make sure clean, dry air is being supplied to the lubrication system. If there is water in the air line, an air dryer should be installed, Garski said.