A down machine tool spindle brings production to a screeching halt. To make matters worse, there is a significant cost for spindle repair that, ironically, may have been avoided or minimized by following a few simple maintenance tips.
Following is a list of maintenance tips:
- Maintain lubricating fluids. Maintenance of lubrication is critical to both a spindle's performance and its longevity. Fluids should be clean and kept at safe operating levels. It is imperative that shops strictly adhere to the machine tool OEM's recommended lubrication schedules.
Many spindles are air/oil types. In addition to keeping up with oil levels, shops must - because these systems also use air - change air intake filters on a regular basis. Conversely, other types of lubrication-only spindle headstocks will require full "oil changes," so to speak.
- Check spindle runout periodically via a ground and calibrated spindle test bar. Many shops fail to conduct this test, even after a minor machine crash. OEMs recommend checking a spindle's runout every six months and/or after any crash. Workpieces that suddenly trend out of tolerance can indicate that a spindle is running untrue.
In line with monitoring spindle runout, shops should also check a spindle's spring force with a draw bar test. Spring force secures toolholders in spindles, and any drop in that force could lead to tool vibration and runout that can not only damage workpieces, but also the spindle itself.
On turning machines, simply check spindle shafts with an indicator to detect excessive runout. In the event of minor crashes, the tooling or turning machine's turrets often sustain the brunt of damage because spindle headstocks are beefy and to a certain extent crash resistant. Nevertheless, runout tests should be conducted after every crash, as well as periodically.
- Keep constant watch on spindle vibration levels. This used to be a physical type check, with operators "feeling" more vibration than usual. Today's machine tools come equipped with vibration sensing technology built into the controls. These systems will trigger an alarm in the event of unsafe spindle vibration. Unfortunately, these alarms are often ignored.
Increased vibration indicates worn spindle bearings. Worn bearings, if ignored, can scorch spindle shafts and even cause spindles to seize. Both of these scenarios are costly major repairs that could have been avoided by simply replacing worn bearings at the first sign of excessive spindle vibration.
- Maintain spindle bearing lubrication. Many spindles are factory lubricated for life and are basically maintenance-free. These are closed units to prevent any contamination. However, other larger spindle headstocks, such as those on big turning machines, bearings should be kept lubricated with oil baths. Shops must periodically check and maintain the oil levels of these baths.
- For high-speed spindles equipped with cooling systems, shops must ensure the system chiller units are always operating properly. A good indicator of potential spindle problems is any rise in chiller system output temperature. If this occurs, a certified technician should thoroughly check the system.
- Re-install any machine covers that have been removed. Many of today's machine tools have spindle areas sealed off with gaskets and/or some types of covers. If these are removed for routine maintenance or any other reason, they should be re-installed because contaminants can infiltrate the spindle, spindle motor or other components that should be sealed and protected.
- When spindle repairs or rebuilds are needed, it is recommended that machine tool OEMs perform the services. This ensures that only factory-certified parts are used and specified procedures followed. Shops that attempt to fix or rebuild spindles themselves often end up doing more harm than good.
When evaluating spindle repair services, shops should make sure that OEMs perform these operations:
- Disassemble, clean, inspect and assess a spindle's overall condition
- Determine root causes of failure and provide reports that detail those causes, as well as repair quotes and diaries of the specific repairs made
- Install any needed replacement parts, along with new bearings, O-rings, springs and seals.
Spindle rebuilds should entail these operations:
- Exchange spindle cores
- Precision grind and lap all critical spindle components for flatness and size, both held to tolerances typically within 2 microns
- Gauge/test spindles at each stage of the rebuild for axial displacement, internal pressure, load settings, tool clamp force and runout
- Balance spindle cartridges to within no more than 0.5 gram for reduced vibration and improved bearing performance
- Conduct a progressive runoff for at least eight hours to gradually work in bearings and monitor temperature.
Authored by Kevin Sekerak, Assistant Manager of Spindle Rebuild, Mazak Corp.
For more information contact:
Tasha Riddell
Mazak Corporation
P.O. Box 970
Florence, KY 41022-0970
859-342-1700
triddell@mazakcorp.com
www.mazakusa.com