IBAG North America

5 Signs That Your High Speed Spindle Needs Repair

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Shops that use high speed CNC machines, including high speed spindles, realize the many benefits. Above all, when operating at peak efficiency, they provide for a ton of profitable production. However, if the machine is unable to produce due to maintenance issues, the pain is harsh and felt very quickly. A scheduled rebuild is always preferred over a costly catastrophic failure. To avoid unplanned downtime, how do you know when your high speed spindle is ready to fail?

The most critical components in a high speed spindle are the bearings. A high speed spindle relies upon very precise bearings to operate at high RPM. These bearings may be made from steel or ceramic, and are lubricated and cooled. Most spindle failures occur when the bearings fail. So, what are the common causes of bearing failure, and how can you prevent it?

First things first! Make sure your spindle cooling is operating properly. This may be circulating water or oil, and usually includes a chiller. If the spindle bearings are packed with grease, things should be good. If your spindle uses oil mist to lubricate the bearings, make sure the oil type is correct and the pump is properly working to provide the correct oil mist to the bearings. Be certain that your air seal settings are right. And, don’t crash your machine! To avoid that, here are the 5 signs to look for:

1. Increase in Audible Noise
New high speed spindles operate at a relatively low noise level. Although the high speed does create some noise, you quickly get used to what is considered acceptable. However, when a spindle is starting to fail, the noise level increases. This is due to additional vibration caused by bearing wear. It helps to measure and monitor the noise level of your spindle over time, to help predict when things are starting to change for the worse. 

2. Increase in Taper Temperature 
High speed spindle bearings are designed to operate at pretty high temperatures. Some bearings, when running at normal levels, can reach inner race temperatures of over 113°F. That seems pretty hot, but it’s normal. However, when the bearing begins to break down, those temperatures can soar. As the degrees rise, the composite bearing cage begins to deteriorate, eventually breaking and resulting in a complete bearing failure. So, to keep an eye on this, use a non-contact infrared temperature gage to monitor the temperature inside the spindle taper. Run the spindle with a tool holder at maximum speed, stop the spindle, remove the tool holder and shoot inside the taper. This is the best indication of the internal bearing temperature and gives us a pretty good idea of the bearing condition. When the temperature starts to increase, chances are good that the bearings are breaking down. 

3. Spindle Vibration 
As the bearing wears, the balls and race begin to break down and pit. This changes a smooth surface to a bumpy surface, and creates increased levels of vibration. That’s also where that heat and noise come from. Many shops use sophisticated vibration measuring systems to predict bearing failure. These are great tools to use in the shop for all machines, but, especially for high speed spindles. There are even outside contractors that will measure and evaluate vibration signatures to help predict future bearing failure.

4. Accuracy
So, as the bearings begin to wear out, they will exhibit higher vibration levels, increased temperatures and become noisy. What else can happen? One aspect that will also deteriorate is accuracy. High speed spindle bearings are produced to the highest quality and ABEC accuracy class. This is most evident regarding minimal bearing runout. Most high speed spindle bearings have runout in the range of a few microns. As the bearing wears, the runout will gradually increase. It is a good idea to use an indicator to measure the spindle taper runout on a regular basis. When you begin to see the runout value increase, it is a good sign of critical bearing wear.

5. Tooling System
Excessive tool vibration, thanks to a defective or unbalanced tooling, can cause spindle bearing damage. It can also contribute to poor surface finish and reduced cutter life. Another critical component in your tooling system is the draw bar. Over time, the draw bar springs can wear out and reduce the holding force keeping the tool holders in the spindle. As an example, for an HSK63A tool holder, the draw bar retention force should be a minimum of 3,000 pounds. When this drops, you risk losing the tool holder which can be very dangerous. There are holding force gages available that can measure drawbar retention force. The best practice is to obtain one of these gages, check on a regular basis, and rebuild the spindle when the draw bar retention force does not meet minimal values.