I’ll probably ruffle some feathers with this comment, but here goes. Kluber greases are very high quality, but NBU15 is meant for high speed spindles, such as found on machining centers. I do not think it is appropriate for knee or bench mills, so you will never find that in my machines. Viscosity is the most important criteria when selecting a grease or oil, at 15, the NBU15 is below what is needed at slower speeds, but appropriate for machines in the 10,000 rpm and higher range. The correct viscosity needs to be calculated based on a variety of factors, including bearing size, operating speeds, and operating temperature. If you live in a hot climate and your shop is not air conditioned, a higher viscosity grease might be necessary compared to me here in CT with my machines in the basement that never gets above 70f. If you spend half your time at 1,000 rpm, and the other half at 5,000 rpm, the viscosity requirements are different at each speed, you would want a grease that is suitable at both speeds. It’s ok to go with a higher viscosity, to a point, but you need to be above the minimum at the lower speeds if you want your bearings to last, and slower speeds require a higher viscosity than at higher speeds and temperatures.
Having said all that, mills in a home shop, or even a job shop, do not see hard service, so a wide variety of greases would be perfectly fine. I would not use less than a 32 viscosity. Bridgeport specifies an SAE10 oil for their spindles, that is equivalent to an ISO32 viscosity. If you want to go with a Kluber of the appropriate viscosity, go ahead, but anything from Mobil, Shell, Fuchs, etc of the right viscosity is going to work too. Kluber does have the advantage that it has fewer impurities than industrial greases, so there are benefits to using it in high precision bearings, just make sure you select the right one.
I have a Burke Millrite with tapered roller bearings. Max speed is 2,500 I believe. I went with Mobil SHC100 with a viscosity of 100 since I had it and have decades of experience with it, and I know the bearings will be properly protected at any speed I run my mill at. I could have easily gone with a grease with an oil viscosity of half that and still be fine, but I don’t know if there are any readily available. For a machine that runs at 5,000 rpm, I would look for something in the 32 range, 46 would be perfectly fine too if that is what you can find. It’s important to not overfill the bearings, and don’t regrease too often. More bearings fail from over greasing rather than under greasing, and the spindle will run hotter with too much grease. I do two or three pumps from my grease gun every 6 months or so, no idea how many hours yet since I haven’t gotten around to installing the hour meter.
Another warning, make sure you know what the base soap is in the grease before switching types. Some of the soaps are not compatible with each other and if you put incompatible soaps together, they can cause a failure of the bearing. If possible, fully wash out the old grease before using the new one.
