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We get a lot of questions about high speed spindle life such as, “How long will a new (or repaired) spindle last?” or, “What can I do to extend the life of my high speed spindle?” It’s no wonder why - spindle failure often leads to avoidable costly repairs or complete replacement. Worse yet, it means your machinery stops running, production halts, labor is wasted, and delivery dates are missed. In an environment where customers are expecting precise lead times, there is little room for error. All of these issues can be avoided by following these relatively simple tips to extend high speed spindle life, while maximizing performance and output. Although no spindle will last forever - it makes both practical and economical sense to keep them running as long as possible.

Keep in mind that high speed spindles require the use of high precision, lightly preloaded bearings to achieve the high RPM required. These bearing setups are not identical to a traditional high torque, high stiffness spindle found in most conventional CNC milling machines. They require more care and attention to optimize performance and extend their useful life. Here’s what you can do:

1) USE GOOD TOOL HOLDERS FOR YOUR HIGH SPEED SPINDLE

By good tooling, I mean high precision tooling that is both balanced and accurate. The toolholders should be balanced to a minimum of a G2.5 balance rating when new. Also, the cutter clamping method should be a high quality, accurate collet system. The collet should not have a large collapsing range, which can create tool runout.
 
Completely avoid using end mill holders with a side lock set screw. While these are fine for low speed spindles, they introduce too much imbalance in high speed applications. Imbalance creates vibration that can reduce tool life, and causes terrible surface finish.
 
If you have access to a balancing machine, use balanceable tool holders. If not, the best type for high speeds is a mechanical clamping or heat shrink fit system.

2) USE GOOD CUTTING PRACTICES

Improper cutting conditions can cause very high levels of chatter and vibration. Vibration caused by improper feeds and speeds can lead to substantial mechanical bearing damage in a short time. The bearings used in a high speed spindle are typically configured with a light preload. This allows very high RPM, but reduces the stiffness of the spindle. High stiffness is required for high chip loading, so, remember that most high speed spindles are designed to take light cuts very quickly, not heavy cuts very slowly.

This should be reflected in the fixturing and tool path programming as well. Tool lengths should be kept to a minimum. And, when a long tool is absolutely required to reach a feature on the part, the feeds and speeds should be adjusted to reduce the cutting forces seen by the spindle bearings.

3) FOLLOW RECOMMENDED HIGH SPEED SPINDLE MAINTENANCE PRACTICES

High speed spindles often have complex oil-air bearing lubrication systems, are liquid cooled and have air purge systems that must be properly maintained. Observe all machine tool builder recommended warm-up procedures before using the spindle. This will not only extend your spindle bearing life, it will also optimize spindle accuracy.  
If the bearings are lubricated with oil-air mist, make sure you use the oil that is recommended by the machine or spindle builder. Using another oil, can affect bearing performance, and, worse yet, invalidate the spindle warranty. If the spindle bearings are permanently lubricated with grease, it is extremely important to strictly observe warm-up procedures as the grease must be at operating temperature (liquified) when the spindle begins cutting or reduced bearing life can occur.
 
If the spindle is water or oil cooled, make sure the fluid is maintained at the proper temperature and flow rates. Often users reduce the cooling liquid temperature, thinking this will benefit the spindle. In reality, lower temperatures cause spindle parts to shrink which can adversely affect fits and tolerances. Maintain the chiller and monitor the coolant level and quality as well.
 
Cleanliness is also a very important factor. The tooling systems used on high speed spindles (usually HSK or BT) must be highly balanced. If the spindle taper is contaminated with dirt and chips, balance and accuracy are compromised. The spindle taper and tooling should be kept clean.
 
Every high speed spindle has a cleaning blast of air directed through the taper during a tool unclamp to blow out any chips. Make sure this is working properly.

4) MAINTAIN GOOD QUALITY SHOP AIR

Most quality high speed spindles have a positive-air-over-pressure system to keep contamination out of the spindle. Low air pressure is supplied inside the spindle housing and flows out through the front rotating labyrinth. This prevents dust, coolant and chips from migrating into the spindle and bearings. Make sure this system is functioning properly. Also, ensure that the quality of compressed air supplied to the machine is clean. This may require the addition of filters and traps to catch contamination, extract water and remove any oil from the air supplied to your expensive spindle.

Following these steps will help increase the life and performance of your high speed spindle. Taking the time to work these tips into your day to day processes and maintenance routines will certainly pay off in the long run. Keep your machines running and your customers happy! As the old saying goes, an ounce of prevention is better than a pound of cure.

Author:
Bill Popoli

IBAG North America