Heart surgery: Turning Performance

Repairing a spindle—the heart of a machine tool—is more economical than buying new, but part manufacturers need to understand the process.

One day you notice chatter marks on parts produced on a machine tool that previously wasn’t giving you a problem. Nothing’s changed—except possibly the condition of the spindle.

Multiple clues indicate a spindle needs to be repaired, or rebuilt, including excessive noise, vibration, loss of part tolerance, rougher surface finish, heat buildup and spectrum analysis readings that indicate the bearings are impaired or destroyed.

Usually, a spindle problem first shows itself in the workpiece, noted Mitchell Kirby, vice president of manufacturing for Riten Industries Inc. “You’ll find you’re not getting the finish you used to get,” he said. The Washington Court House, Ohio, manufacturer of face drivers and live and dead centers also repairs Heald Red Head and other pulley-type ID grinding spindles under 10,000 rpm.

Cause for Concern

The causes of an “unhealthy” spindle are as numerous as the signs something is wrong. According to GMN Paul Müller Industrie GmbH & Co. KG, a German manufacturer of machine tool spindles, a collision, or crash, is the cause for 60 percent of damaged spindles, followed by wear at 11 percent, leakage at 9 percent and lubrication issues at 7 percent, with other reasons accounting for the remaining 13 percent.

Courtesy of Precision Spindle Service

A machine spindle before being repaired (top) and after Precision Spindle Service repaired it.

A machine crash, however, is subjective and difficult to define without witnessing it, according to Doug Kranz, vice president of sales and marketing for Fischer Precise USA Inc., Racine, Wis. The company makes three core spindle brands—Fischer, Precise and Fortuna—and repairs those spindles and ones from other manufacturers. “You can maybe see signs of overload, but the number one contributing factor for spindles coming in for repair is vibration during the cutting operation,” he said. That’s especially the case when machining large, complex components with lots of tight corners, which are often produced on large machines by applying long-reach cutting tools, Kranz added.

Fischer Precise offers the SmartVision monitoring system that allows customers to track vibration and the operating condition of their spindles while running part programs. “Then we can identify some of these high-vibration areas and better match cutting strategies and help them do a better job of keeping the spindle in the part with less vibration,” Kranz said.

Spindle contamination and overload are the second and third most common causes, respectively, according to Kranz. “You can call overload a crash, but a spindle doesn’t necessarily need repairing after simply hitting the bed of a machine,” he said. “Spindles are resilient to some amount of impact.”

According to Kirby, wear and tear are the primary reason a user sends Riten an ID grinding spindle for repair. “The bearing assembly is a rotating element,” he explained. “You run it long enough and hard enough, eventually the bearings begin to wear and need to be changed.”

Unlike other types of machine spindles, an ID grinding spindle is a very specific product and, in some sense, an interchangeable accessory, Kirby emphasized. Such a spindle is distinctly different than a bearing assembly in a tailstock, which is integral to the machine whether it’s a vertical turning center, a lathe, grinder or other type of machine. “I have multiple grinding spindles for some of my machines,” he said. “They’re ID grinders that accept a self-contained grinding spindle.” An OD grinder, on the other hand, may have a live or dead tailstock. “In the case of a live tailstock, it’s still a bearing assembly that rotates, but it’s not interchangeable,” Kirby added.

Being a highly engineered mechanical system, a spindle has numerous components, any of which might be where the damage occurs. GMN stated that the list includes, in order of absolute frequency, a bearing defect, loose rotary feed-through, defective rotary encoder, broken or bent pull rod, faulty clamping sensor and defective temperature sensor. And the list goes on.

At Your Service

Whatever the cause, a problematic spindle will not heal itself. It’s best to use a specialist when repair is needed, according to David Marshick, co-owner (along with William Foy) of Precision Spindle Service Co. Inc., Ferndale, Mich., which repairs spindles with speeds up to 124,000 rpm. The shop was founded in 1946.

While a parts manufacturer might have emergency spindle repair personnel, very few can provide a complete repair, he said. “It doesn’t matter if it’s a horizontal, vertical, grinding or milling machine. Anything in the super-precision ball bearing industry is what we repair.”

Marshick added that in-house spindle repair was more prevalent a decade ago, primarily at large manufacturers. However, it is costly to maintain that capability because repair technicians must work in a clean-room environment, ideally with a controlled temperature and atmosphere, and even hair and industrial smoke can damage a spindle assembly. “The repair business is booming because they’ve limited or cut those programs,” he said.

“Cleanliness from start to finish is obviously a top priority,” concurred Fischer Precise’s Kranz. “When talking about tolerances and cleanliness, we are living in the world of microns, and 3µm is big in our world, so the smallest particulate can cause you a problem during a spindle build.”

Kranz estimates that it would cost about $1 million to purchase the equipment needed to properly analyze, test, troubleshoot, balance and support a repair service for high-speed, cartridge-type machine spindles. Although Fischer Precise performs complete spindle repairs at its facility, he noted the company has two field representatives that perform partial repairs at a customer’s site, such as fixing a bad encoder, a failed sensor or a broken HSK tool gripper assembly. In-field repairs can save customers a significant amount of time and money. Although contamination is a concern while executing an on-site repair, a technician can successfully complete the job with proper care and attention to details, he added.

Occasionally, an in-house repair ends up being sent to a dedicated repair facility. An attempted customer repair, for example, shows up when a repair service provider checks the shaft and sees the bearing seats are worn and don’t have the proper fit, Riten’s Kirby explained, noting the shoulders that the bearings shoulder up against must be perpendicular within 0.0001″. “If the bearing is fretting on the raceway, where it’s actually moving, or the shaft itself is flexing,” he said, “then those shoulders get out of square and need to be reworked.”

Sourcing Bearings

Having reliable sources for bearings, which are not reused when a spindle is rebuilt, is also critical for spindle repair companies. Not always easy to locate even in the best of times, bearing availability became particularly difficult during the Great Recession when bearing producers limited inventory to control costs, but has improved along with the economy, Precision Spindle Service’s Marshick noted. However, “it still takes some time and you need a lot of different sources,” he said.

“High-precision bearing supply is of utmost importance,” Kranz said. “If you don’t have it in stock or readily available, you cannot serve customers with the lead times they deserve, and some bearing lead times can stretch to 6 to 8 months.”

He added that Fischer Precise has to predict demand and maintain and invest in its own stock of bearings. “We have about $2 million in bearings sitting in what we call the ‘vault’ in our stockroom just to support our own product line,” Kranz said. “We have such a vast array of [bearings for] our product that we’re able to service third-party product as well—any other spindle.” If a type of bearing isn’t in stock, the company has locations in Europe and Asia that it can pull from.

Courtesy of Riten Industries

A before (top) and after photo of an ID grinding spindle repaired at Riten Industries.

After bearings are changed, the spindle must be balanced to ensure adequate bearing life. “When the bearing assembly is out of balance, it wants to vibrate and you see chatter in the workpiece,” said Riten’s Kirby. “You also put an additional load on the bearing assembly, so the bearings wear rapidly because the unit is out of balance. That becomes more critical as rpms increase.”

In addition to installing new bearings and balancing, a full spindle rebuild at Precision Spindle Service can involve spindle disassembly, remanufacturing of the cone and housing bores, remanufacturing and rechroming the shaft, and work on the hydraulics and electronics, Precision Spindle Service’s Marshick noted. To recondition spindle components via turning, milling and grinding, the company has a “full faceted” machine shop, he said. “We repair whatever the failure mode is. Normal turnaround is 3 weeks.”

Kranz explained that Fischer Precise has a defined process when repairing a spindle and starts by obtaining as much information as possible from the customer should the failure mode or service need not be clearly identified. For troubleshooting purposes, the company requests that any tooling in the spindle when problems occurred be included. “We start from the outside in,” he said. “We have several incoming checks and a 54-point inspection as we disassemble the unit. As we’re taking the spindle apart, we’re checking all the systems, including cooling, bearings, bearing carriers, housing, lubrication condition and supply, shaft, rotor and clamping.”

Typical lead time for a complex spindle repair is 4 to 6 weeks, but the company can execute a repair in 1 to 2 weeks by working extra hours and during the weekend based on customer need, according to Kranz. Replacing a spindle with a new one takes 18 to 20 weeks, he noted. A repaired spindle also costs less, with the rule of thumb being 50 percent or less than the cost of a new one, which equates to significant savings for a high-performance spindle that can cost $75,000 or more, he added.

Maintenance Matters