How to Recognize Bad Spindle Symptoms

Expected spindle life depends on many factors. A 1,700 RPM surface grinder could last up to 20 years, while a 120,000 RPM ID grinding spindle could last for three years. There are far too many outside forces and variables to discuss in this article, but keeping an eye on your equipment will certainly allow you to prolong its life and service before catastrophic events occur.

Like any machine, the moving parts will wear more quickly than the stationary parts, but premature spindle failure is usually caused by some external factor. Spindles can fail due to:

• Contamination
• Poor lubrication
• Low-quality lubricants
• Lack of maintenance
• Machine mishandling
• Operator error
• Machine overload
• Spindle imbalance
• Drawbar failure
• Bearing failure
• Machine misuse
• Chiller performance
• Worn/damaged tooling
• Electrical short/spike
• Sensor failure

When you know what to look or listen for, you can save yourself and your shop some major headaches. Even if you’re not positive there’s an issue, it’s worth confirming — you could avoid the following by identifying a potentially bad spindle symptom as soon as possible:

• Unscheduled downtime
• Expensive repairs
• Costly replacements
• Injuries
• Poor product quality
• Dissatisfied customers
• Machine damage
• Longer rebuild times
• More extensive damage

This warning also holds true for delaying routine spindle service. If everything looks and sounds fine when your spindle’s scheduled check-up comes around, it’s best to undergo the routine maintenance procedure rather than risk unexpected downtime later.

What follows are general recommendations for troubleshooting various spindle issues. As always, consult the OEM or a spindle repair facility to verify actual next steps for your application, and certainly before attempting a repair or replacement without proper expertise.

Poor surface finishing
If you’re seeing chatter marks or other finish issues on machined parts, it’s possibly a symptom of a failing spindle. Check to see whether the marks appear on a specific axis — for example, if the X and Y axes look normal, but the Z axis shows chatter marks.

Out-of-spec products 
Similarly, when parts begin falling out of programmed specifications, it can also signal a spindle problem. This can be apparent in the product’s dimensions, roundness, taper, etc.

Vibration
Some amount of vibration is normal in most machining processes, but excessive vibration is usually apparent. You will see, hear, and feel abnormal vibration.

Noise
Just like vibration, noise is a normal part of machining. Today’s high speed spindles run at relatively low volume, so any increased or abnormal spindle noise is noticeable, and likely accompanied by increased vibration.

Temperature
High speed spindle bearings are designed to operate within a particular range depending on many factors. Knowing what is normal for your machine helps you tell when it’s out of range. If you have a spindle chiller, it may alert you to changes in temperature. Excessive temperatures can lead to bearing cage deterioration, lubrication breakdown, tolerancing issues, etc., so this is a serious problem.

Spindle taper wear (Fretting)
Fretting can present as either visible wear or bellmouthing: when the taper is worn so much that the toolholder rattles in the spindle mouth. If the toolholder starts sticking, that can also be a sign of taper wear.

Runout at tooling surfaces
If during your scheduled maintenance checks you note increased runout, this is a telltale sign you have some spindle issues to address soon.

Drawbar force
Similarly, if during your PM you see a trend of dropping drawbar retention force, you need to get this looked at right away. You may be able to have only the drawbar serviced, avoiding a costly full spindle rebuild or, worse yet, the spindle throwing a tool and causing more damage or injury.

High load
Some machines have a display for the spindle motor load. It’s a good idea to keep tabs on this so if it starts to increase you can see there is additional resistance in the spindle even while it’s running a program.

Naturally the first thing we want to do is prove whether or not there is a spindle issue. A machine comprises many moving pieces, and while a symptom that appears at the spindle may make you think that’s the culprit, it may be just that — a symptom of something else.

Every type of machine, machine manufacturer, model, options, year, etc. requires a different diagnostic process, so we won’t get into the weeds on any one particular process here. For the sake of this article, your best strategy is to look at the spindle and prove it is healthy (or not). If you prove you have a good spindle, then you are one step closer to the root cause elsewhere in the machine (for example, a loose ball screw).

Issues such as ball screw backlash, drawbar actuator failure, tool changer misalignment, and spindle drive or motor failure are often attributed to issues with the spindle, but the root cause usually turns out to be something else entirely. The best course of action is to consult a spindle repair expert, who can tell you for certain whether the spindle is actually the culprit.

Regular preventative maintenance is the best way to avoid unexpected spindle failures. Along with a scheduled maintenance program, you can ensure your spindle lasts its full lifetime by taking a few simple steps:

• Measure the drawbar force of your spindle twice per year
• Keep fluid levels normal
• Monitor and maintain the cooling system
• Monitor and maintain the lubricating system
• Keep lubricant free from contaminants
• Only use the manufacturer’s suggested lubrication products
• Make sure your spindle operators know how to monitor the spindle’s vibration and temperature
• Stress the importance of reporting temperature or vibration extremes immediately
• Conducting warm ups and cool downs as the OEM instructed
• If storing for extended periods, follow the OEM’s book as well
• Track tooling runout
• Trend vibration data

Most importantly, it’s wise to have the name of a certified spindle repair facility in your back pocket. Not only can they assist in an emergency, but they can recommend a formal maintenance schedule and offer their expert analysis regarding your spindle’s performance.