Back when a 6,000 rpm CNC vertical machining center was considered ridiculously fast, there was little worry about heat buildup in the spindle. Thermal growth of the Z-axis was quite manageable, and the spindle bearings could be expected to last years before servicing became necessary.
That all began to change as spindle speeds hit 8,000 rpm, followed by 10,000 and beyond. Close-tolerance pocketing and hole depths began to be problematic. Spindle heads grew hot to the touch during production runs, accompanied at times by the alarming whine of overheated bearings. Visits from the service technician grew more frequent as bearing replacements became an unpleasant fact of shop life.
The good news is that machine tool builders took notice. Spindle chillers have become a common item on the option list, with some of the more forward-looking builders making them standard on all machining centers above 10,000 rpm.
They make excellent sense. As the name implies, spindle chillers pull heat out of the headstock. There’s no longer a risk of the bearing races expanding to the point that factory-set axial and radial preloads are exceeded, causing premature failure and unexpected downtime. Z-axis growth is minimal even after hours of high-rpm profiling. Routine maintenance becomes just that—routine—and the unwelcome sight of the service tech’s van in the parking lot is now a thing of the past.
If you’re shopping for a new machine, this is one feature you definitely want on your checklist. Look for a chiller that’s plumbed to run fluid through the spindle jacket as well as the mounting block for the spindle motor itself, as the belt and motor generate a substantial amount of heat. The way in which the chiller responds to external heat sources—that is, the shop environment—is also important. Good ones allow the chiller to be set at a certain continuous temperature, or at ambient temperature minus a couple degrees (this is usually the default setting).
Once in place, there’s little to maintain aside from periodic cleaning of the air filter and perhaps an oil change every few thousand hours (be sure to check with the manufacturer on this figure). Our Kent CNC vertical machining centers come equipped with spindle chillers. Our VMC’s starts with 8,000 rpm to 15,000 rpm. Don’t forget, however, that spindle chillers are no different than the air-conditioner in your house or car—there’s a Freon-like substance inside, and this must be checked and refilled when needed if the spindle chiller is to do its job. Stay cool, and happy milling.