The two major enemies of the spindle are: (1) heat and (2) contaminants (namely, chips and coolant invading the bearing system). Find out what design features are included (or available as options) that protect the spindle. Historically,

the most common cause of spindle failure has been bearing failure due to contamination from coolant ingress, condensation, contamination or chip damage. You want the spindle temperature to stay cool and you want to make sure contaminants stay out.
In most cases, contaminants enter the spindle because the spindle seal failed. Find out what design measures the machine tool manufacturer has taken to keep the seal tight. An air purge system uses a labyrinth seal and purges the seal with positive air pressure to keep contaminants out.

  A dual air purge system, a system with two ports (usually upper and lower) is one design feature that works well to keep contaminants out of the way.
Temperature is the other factor that leads to spindle problems. Because heat causes steel to expand, manufacturers should explain what measures they have taken to protect the spindle from head growth—which leads to mostly Y and Z axis changes.

  Heat exchangers or chillers (most common) are used to keep the spindle cool and control spindle growth as well as head growth. This type of system adds life to the spindle and reduces head growth, and is typically used when you’re running long cycles or high duty cycles. The selection of the chiller is dependent upon the application. For extended high-speed applications, you may want to investigate a thermal stabilization system. This system uses a thermostat with an oil chiller to automatically cool the spindle as needed.

  Another contributing factor to spindle performance is the tooling used. Using unbalanced tools, worn tools and/or tools that are too long can affect the longevity of your spindle.

  Tooling Considerations
Like the spindle, temperature can have a negative impact on tooling. Find out if the spindle comes with a coolant ring or uses flexible coolant nozzles. With a coolant ring, you want to find out how many nozzles there are and whether they are adjustable. Obviously, the more nozzles the better and having the ability to adjust the direction of the nozzles is an advantage to cover a large range of tool lengths without frequent adjustments.

  Coolant Through the Spindle (CTS) is generally recommended when machining at 12,000 rpm or more, and you have custom tools or expensive tools that you want to ensure are protected. CTS also is recommended at lower rpms for certain applications and duty cycles. Prices vary for this feature depending upon the pressure of the CTS and how the spindle was designed.

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