- April 9, 2014
by Kevin Sekerak
A down machine tool spindle brings production to a screeching halt. And to make matters worse, tack on the hefty cost of a spindle repair that, ironically, could have probably been avoided or minimized by following a few simple maintenance tips.
The easiest tip is to maintain lubricating fluids. They should be clean and kept at safe operating levels. Maintenance of lubrication is critical to both a spindle's performance and its longevity. 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 – service air-intake filters on a regular basis.
Conversely, larger spindle headstocks, such as those on big turning machines, keep bearings lubricated with oil baths. Shops must periodically check and maintain the oil levels of these baths.
Shops should peridocially check spindle runout 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. Also, 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 pull force test. Spring force secures tool holders in spindles, and a sizable 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. But nevertheless, runout tests should be conducted after every crash as well as periodically.
Shops should keep constant watch on spindle vibration levels. While doing so used to be a physical type check, with operators "feeling" more vibration than usual, some of today's machine tools come equipped with vibration sensing technology built in to their 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.
Addressing high speed spindles equipped with cooling systems. For these, 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 type 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, tip 6 recommends machine tool OEMs perform the services. This ensures 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
- Offer exchange spindle units for immediate deliveries
- Spindle rebuilds should entail these operations.
- 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 grams for reduced vibration and improved bearing performance
- Conduct a progressive runoff for at least 8 hours to gradually work in bearings and monitor temperature
Kevin Sekerak is assistant manager of Spindle Rebuild at Mazak Corp.