TIPS TO EXTEND SPINDLE LIFE
Here are some of the proactive tricks of the trade that can go a long way toward contributing to the health of a spindle.
Machine tool spindles have evolved over the years into highly precise, reliable, and productive systems to rotate cutting tools, grinding wheels, or parts to be machined in applications ranging from milling, drilling, and boring to grinding, cutting, routing and sawing. But as with any critical asset, spindle performance and anticipated service life can be jeopardized in the long run from a basic lack of care and preventive actions.
What are the keys to extending spindle life? Here are some proactive steps that can make a big difference:
Use balanced toolholders. When a toolholder assembly spins during an operation, imbalance in the spinning components will create centrifugal force – increasing exponentially with the speed. As the force increases, excessive and potentially troublesome vibration will follow and result in poor machining and difficulty in holding close tolerances.
The causes of a toolholder’s imbalance may be inherent in its design, or the toolholder may not have been properly balanced from the beginning, or the toolholder may have been damaged or modified by the end-user. The best approach for any spindle application is to calculate a “balance tolerance” for the toolholder by factoring in the intended operating speed and then properly specifying the maximum allowable imbalance within the speed parameter. Balancing within the designated “tolerance” range will help avoid problems, promote performance and accuracy, and minimize wear on spindle bearings that can shorten spindle life.
Check vibration. Excessive or abnormal machine vibration often will indicate a spindle problem. Significant variations in frequencies always should be cause for concern. If such is the case, the spindle should undergo a checkup. This should consist of a trained technician taking a series of vibration readings. If the spindle cannot be balanced due to the circumstances, then it may need to be rebuilt. When vibration frequencies ultimately are brought into line, they should be logged to develop a baseline for future comparative reference.
Protect the spindle from coolant. One of the most common causes of spindle failure is contamination from intense tool coolant spray or flood coolant. The ingress of coolant through the spindle seals, labyrinth, or covers must be avoided. The coolant can wash out the grease or oil from the bearings and attack the spindle’s shaft, motor, and electronics. The lesson is to keep the coolant directed at the tool and away from the spindle. This will give you the best chance for long spindle life. Extra steps of controlling non-direct splash back to the spindle should be addressed.
Monitor temperature. An unexpected increase in spindle operating temperature is often a surefire telltale warning sign of a looming problem. A good rule of thumb: Know the normal operating temperature of the spindle in the particular application and then record baseline temperatures for the spindle’s front and rear bearings. This will enable operators to monitor and compare any changes or deviations.
Watch the surface finish. The parts being machined will tell you much about your spindle and you should pay close attention to surface finish, chatter, inconsistent size, and/or shorter than normal tool life. Any one of these problems can be a sign of trouble. Any noticeable buildup of chips or debris should be removed regularly to mitigate surface finish marring.
Listen. Unusual and/or noticeable noise during spindle warm-up and during operation should sound an alarm as an advance warning of possible trouble. Listening to perceived changes in noise levels and then performing the necessary detective work to pinpoint the cause (or causes) can help keep spindles up-and-running as intended.
Keep clean and proper oil flow. Lubricants should always be high quality, properly specified, carefully stored, and clean and free from moisture or other contaminants. The exact amount of lubricant should be applied and lubricant containers and all transmission lines should be inspected to rule out pre-existing and harmful contaminants. The correct delivery rate and pressure should always be maintained. And, if and when lubrication lines will be removed to perform machine service, operators should bleed the lines before reconnecting.
Supply clean and dry compressed air. Having clean, dry compressed air is critical if you are using air for air/oil lubrication, air purging, or actuators, to name a few. The best practice is to have a sophisticated drying system at the compressor with a number of inline filters. Compressed air creates moisture that can quickly destroy spindle bearings, components, and seals. Eliminating the moisture and contaminants from your supply lines will represent a huge step in prolonging your spindle life.
Watch feeds and speeds. Feeds and speeds are a significant part of any machining operation and finding the perfect combination is always a challenge. In striving to reduce cycle times, machines tend to be pushed to their limits – or greater. That translates into faster feed rates at higher speeds. By watching your spindle’s rpm and feed rates – not pushing the spindle to the point where excessive load is placed on the bearings – you ultimately should be able to expect a longer MTBF (mean time between failure).
Handle with care. Spindles should always be properly handled and protected from forces that could cause damage and subsequent failure. Any radial or axial impact to a spindle shaft can cause permanent “denting” (or brinelling), which will rapidly deteriorate a spindle’s precision bearings.
Properly executed, these tips can go a long way toward contributing to the health of a spindle. Shops additionally can benefit from the introduction of other preventive maintenance initiatives, including regularly scheduled on-site “checkups” conducted by experienced and qualified technicians trained to spot and remedy problems before a good spindle goes bad.
About the Author: Edward Zitney, Jr., oversees SKF Machine Tool Services, a business unit of SKF USA Inc., www.skfusa.com. He has 29 years of experience in the repair, reconditioning, and upgrading of machine tool spindles and is responsible for SKF’s state-of-the-art Spindle Service Center in Mentor, OH, and the recently opened SKF Solution Factory in Houston, TX. For questions or comments on this column, contact Ed at 800-589-5563 or Edward.Zitney@skf.com.