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Don’t get tied in knots over spindle maintenance

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A basic 3-axis CNC router used in the woodworking industry is a substantial piece of equipment. The frame and gantry are large pieces of heavy-gauge steel welded together; the motion control systems, rails and the trucks, are made of precision-machined high-quality steel, as are the linear-drive systems.

For most of these moving parts, tolerances are measured in thousands or ten-thousands of an inch depending on the part and its function. Except for one part: the spindle.

According to Bob Barone, national sales manager of Precision Drive Systems, the tolerances inside a variable-frequency-drive electrospindle are measured in microns. A micron is 1/25,400 of an inch. To put a micron in perspective: the tiny bits of sawdust floating around in your shop range from 2-10 microns and a human hair is between 40 and 50 microns wide.

Barone mentioned microns to illustrate the care and precision that the 37 employees at the PDS facility in North Carolina work with when repairing a spindle. Spindles wear out over time, which is what can be expected from a piece of equipment that runs at 8,000 to 50,000 rpm with significant axial and radial loads.

The process to rebuild or repair a spindle demands attention to detail because of the tolerances and the complexity of the spindle and this is why it is so time-consuming and expensive.

What happens?

Attention to detail begins when the spindle is received in the evaluation room, where a technician records information from the spindle’s data plate and takes dozens of photo for reference. Then, before disassembly, runout is measured on the spindle shaft and the stator windings are tested. More measurements are taken after disassembly. The parts then go to the washroom for a thorough cleaning.

The customer is given an estimate to rebuild or repair. If accepted, parts are pulled from inventory or ordered and kept in a bin until needed.

Machining is done next, which can include additive hard chroming to build up areas that have worn over time or have been damaged because of a catastrophic failure. The spindle parts move to the build room for reassembly.

Testing includes a gradual warm-up of the spindle. The process lasts for an hour or more to properly heat and distribute grease in the bearings. Balancing at low and high speeds is also performed. For spindles used with an automatic tool changer, the draw bar tension force is tested and adjusted. Sensors are also tested and reset.

At this point, the rebuilt spindle is ready to go back to work. It can either be shipped back to the owner or, if it’s a spare, stored in the company’s climate-controlled room, a place Barone calls “The Spindle Hotel.” It’s tested again before shipment.

Performance tips

The company also helps customers get better performance from their spindles for longer periods of time. The primary way is to properly warm up the spindle after it has been shut down for even a short period of time. This is usually done during the start cycle of the CNC machine and programmed into the controller. Some less-sophisticated machines rely on the operator to warm up the spindle.

Many spindles fail because they aren’t properly cooled during operation, Barone says. Liquid-cooled spindles require a proper mix of glycol and water for best results. Tap water is a no-no, since it will cause corrosion in the internal cooling passages of the spindle body. Operators should also regularly clear the fins on the spindle body, particularly if it is air-cooled. This should be done with low-pressure air and a brush. Barone was adamant that the worst way to clean the fins is to use a high-pressure air gun, which can force wood dust past seals and contaminate the bearings.

From an operations point of view, Barone indicated that watching the amperage draw when the spindle is cutting provides an indication of potential problems, especially when higher than normal. Usually this indicates that the feed and speed rates need to be examined as the chip load is too high. For a machine that does not have a built-in ammeter, it is a wise investment to add one.

Your operator is the first line of defense against a costly spindle repair. Most can hear the difference when something goes wrong. Don’t wait to make a repair. The spindle should be sent in for evaluation immediately because when the bearings fail, the whole spindle has to be repaired.

Most spindle rebuilders offer on-site services that can significantly extend the life of a spindle, which in turn prevents production interruption. These services can include factory-level preventative maintenance, predictive maintenance and on-site vibration analysis. These types of services are not expensive.

Collets and tooling, too

Barone also says collets and tooling might need servicing and replacing. Collets won’t last forever and a scheduled replacement cycle costs about $25 per collet, compared to $3,000 spindle repairs. Tooling needs to be sharpened and balanced. Vibrations from out-of-balanced tooling, usually identified by poor cut quality or burning, will eventually damage the shaft and bearings.

For spindles with automatic tool changers, the cone and clamp group need to be cleaned and lubricated regularly. A cone that has material buildup will be out of balance and, therefore, vibrate.

A spindle can be rebuilt multiple times. PDS has successfully worked on spindles more than 10 years old that are running 24/7. At some point, though, it’s more cost-effective to replace.

This article originally appeared in the May 2016 issue.

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