Question: What kind of grease should I put into my Xermac CNC Lube System?
Answer: This is not a simple answer. It’s like asking “should I use a 2-flute or 4-flute cutter on my CNC milling machine. The answer is; it depends. It’s not so much a question of choosing the correct lubricant. It’s more a question of choosing the best lubricant for a given situation. Xermac CNC models spanned several years and included both US built frames and frames acquired from Okomoto – sold as the “I” machines. The ball screw models and linear bearing models differed, but the recirculation technology and sizes are similar enough that the lubricant selection rules can apply to all Xermac CNC machines.
The lubrication system can supply either oil or grease. The decision of what’s best is based on the speed of ball movement. NSK calls it “DM” and it is a function of ball screw RPM, Pitch and Nut diameter. In general, the faster the balls move, the less viscous the lubricant needs to be. For the Xermac CNC, the speed of motion, ball screw pitch and ball screw diameter put the speed class into a low range. The best choice for lubricant is grease.
There are many different types of grease. The next selection criterion will be whether the base oil is synthetic or natural mineral oil. Synthetic grease will hold viscosity better as a function of temperature change. If the EDM is used in a typical temperature controlled U.S. or Canadian plant, traditional mineral oil based grease, such as Shell Alvania 2, is a good choice. If the EDM is in a region of the world such that it will be subject to temperature extremes, synthetic grease, such as Multemp SRL 2 would be a better choice. There is a bit of a trade-off here. The mineral grease is better for high-loads. So if temperature extremes are not a consideration, the mineral grease is the best choice.
There is one special situation in which different, special grease would apply; in situations where a chuck is used to hold the workpiece and long durations are spent with little or no motion in the XY axes, the lack of motion will prevent the balls from recirculating properly and could inhibit lubrication from getting where it needs to be. In these cases, special anti-fretting grease should be used to protect the uncirculated balls from damage. In these cases, NSK NF2 is a good choice. However, in a case where anti-fretting grease is added, the lubrication system should be separated so the anti-fretting grease is delivered only to the X and Y axes. The Z axis ball screw and Z-axis linear bearings should continue to get normal grease.
In addition to selecting the best grease, there are also considerations for frequency of lubrication and the volume of grease to add in each cycle. But that is a discussion for a different Q&A session.