13 years ago
On higher-power machines, including most modern CNC machines, the cooling properties of the water often are the dominant thing you need. On a smaller, older manual machine, it's the lubrication. But there are at least three different "lubricating" properties you're looking at, the tribology folks have told me, and motor oil, for example, can lead to skating of the tool.
Lard oil is pretty hard to beat on my old South Bend. It is pretty good in the high-pressure department but the film can be punctured, so it doesn't cause tools to skate. As a lubricant for the tool, where it might bind due to poor clearance, or the miniscule amount of springback that occurs just after the cutting edge passes a particular spot, lard oil is not bad. Not great, but you don't want more slipperiness there, or you may get skating with very light cuts.
I should point out that I have little direct experience with other cutting fluids, having used mostly lard oil, sulphated oils, and water-soluble all my life (plus kerosene, which I don't like, on aluminum), but I spent many hours with the cutting experts formerly at Carnegie-Melon, Purdue, and Master Chemical, and I picked up most of it from them -- including plenty of high-speed film that showed what was happening. (I never knew Gene Merchant but I've read much of his material, too.)
There is enough variation in performance between machines and tools that none of this is hard and fast, but it does give some guidance.
-- Ed Huntress