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Ed Zitney of SKF Machine Tool Services shares universal lessons learned from four success stories of operations that managed to achieve positive outcomes from the machine tool spindle breakdowns they were encountering.

Posted: April 25, 2011


Here are some universal lessons learned from four success stories of operations that managed to achieve positive outcomes from the machine tool spindle breakdowns they were encountering.

True story: An operation recently experienced high failure rates with its machine tool spindles. Productivity constantly lagged. The initial “fix” was simply to replace the spindle bearings every time, which were noticeably compromised from ingress of coolant. But problems with the spindles continued to persist and the spindles would eventually go down once again, every four to six weeks. The organization eventually turned to us for help in turning the situation around with the goal to increase spindle reliability, service life, and subsequent productivity.

From our perspective it became immediately apparent that the ingress of coolant (and resulting damage to the bearings) warranted focused attention. The ingress of coolant through a spindle’s seals, labyrinth, or covers must always be avoided, because the coolant can wash out the grease or oil from the bearings and ultimately attack the spindle’s shaft, motor, and electronics. Our customer was coping with this firsthand and ultimately understood that replacing the bearings and taking no other actions would never resolve the root cause of the spindle failures.

As part of our remanufacturing process, we took a careful look at the spindle’s sealing system and then initiated proactive measures for a long-term solution to the coolant problem. We removed inadequate seals, added an air purge, installed new properly specified seals front and rear, replaced the rear cap with one that includes a seal diameter, and reworked both the shaft and front cap to accept the new seals.

The positive outcome: The MTBF (mean time between failure) for the spindles dramatically increased from a matter of only weeks (before remanufacturing) to 18 months – more than six times longer than before.

Machine tool spindles, in all their designs and sizes, serve as highly advanced technology solutions essential to key operations on the shop floor. They are routinely used to rotate cutting tools, grinding wheels, or parts to be machined in applications ranging from milling, drilling, and boring to grinding, cutting, and sawing. Spindles stand as complex systems integrating dozens of components critical to machine tool performance.

As with any asset, however, spindle breakdowns can occur over time and no component will be immune from potential failure. When a machine tool spindle fails, the expertise of skilled technicians with in-depth knowledge can certainly provide a big assist by performing optimized repairs and rebuilds and getting the spindle back on track.

The following additional cases from our customer files represent true success stories that hold universal lessons for all:

Fixing by Design
Fairly expensive repairs were often required to correct damage to a spindle shaft for one of our customers. Historically, damaged shafts cannot easily be repaired and usually must be replaced entirely.Our detective work determined that the shaft’s integral collet-like end was causing the headaches.

Solution: An all-new design featuring a conversion offered more options, improved accuracy, and less chance for needing a shaft replacement. The shaft was replaced with a collet-type nose that can accommodate several sizes and can often be easily repaired. This conversion did the trick.

Bearing Down
A spindle used by another customer was failing every few weeks. Prior to the spindle’s failure, chatter could be heard and ingress of coolant was noticed. Our attention turned both to the spindle’s bearings and seals. Investigation showed that the standard bearings originally used in the spindle’s design were inappropriate and that at least one seal in front of the spindle was virtually useless.

Solution: Four pairs of properly specified angular contact ball bearings were installed; five new seals were integrated; and most of the spindle’s components were completely reworked. As a result, the spindle performed without incident for more than 16 months, compared with a previous MTBF of only several weeks.

Tooling Along
One of our customers reported that a spindle’s existing tool interface was not fulfilling required accuracy and repeatability demands for the application. In addition, the design was inefficient for tool changes. Compounding the problem, coolant contamination was leading to frequent spindle failure (this was because the spindle was positioned in a nose-up configuration allowing coolant to flood over the top of the spindle).

Solution: The spindle’s shaft design was changed to a manual HSK (hollow shank tooling), which provided a highly accurate, quick, and easy tool change delivering the desired repeatability. In addition, the spindle’s front cap was redesigned and replaced with a rotating slinger that dispenses the coolant away from the spindle and keeps it at bay. These improvements resulted in much higher quality, greater efficiency, and a five-fold increase in MTBF.

As such cases underscore, any number of factors can influence when “a good spindle goes bad.”  Enlisting the expertise of an experienced remanufacturer equipped with working knowledge of all spindle designs, types, and system components can help turn problems into opportunities and promote best-possible outcomes.

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