Coolant such a fun topic, you can get a real good rise over in the CNC forum. Will a water soluble oil or synthetic mixed with water rust your equipment. Maybe, depends, and possibly, though not very likely. There are a lot of factors that can go into it. Most coolants contain some kind of rust inhibitors, some are better than others.
A few simple rules of mine, NEVER NEVER NEVER EVER buy coolant from a place that sells floor wax. It will be crap, and you will have no technical support and believe me there is a lot of technical aspects to coolant, its actually fascinating.
My second rule is NO contaminants, if you have a coolant that requires aditives to not smell don't use it, or a coolant that has "biocides" in it. I've said it before and I'll say it again, treat your sump like a fish tank (with the exception of fish food), contaminants, such as bleach, soap, sulfer based oils, food, spit, trash, gum, tobacco etc.. can make your coolant do funny things.
Is it better to pump coolant or cutting oil while machining?
This is also a fascinating topic. More and more people, including me are going to a lot dry machining, not for enviromental reasons, but for performance reasons. When your really hauling ass, you want the heat to go out with the chip, this can get your tool pretty darn hot and carbide, ceramic and all the new polycrystaline stuff don't take thermal shock too well. Not as big of a concern on a lathe when the tool is in the cut for a long time, but on a mill, its critical since the cutting edge is in and out of the cut constantly. Its a really neat concept, you end up with really hot chips, sometimes red or white, sometimes just really hot, the tool is warm and the part is still cool to the touch or at least touchable.
So why do you use coolant to begin with, it keeps things cool and lubricated. How does it keep stuff lubricated, chlorinated parafins, they act the same as sulfur in a sulfur based oil, but come into play at a lower temperature. Under heat and pressure the chlorinated parafins create aluminum chloride, or titanium chloride or whatever your cutting and this is almost the same as a TiN coating just thinner, and consistantly forming and disappearing.
So when would you want to use coolant, when running softer metals, or when simply not hauling ass. With HSS you'll almost always want to use coolant to get the most out of it, since your probably not going to get to the point of keeping the heat out of the part or tool, just too slow. When the tool stays in the cut, you'll usually but not always benefit from coolant.
Now if your just having fun on open machines, and not trying to squeeze every dollar out of your spindle, run it dry and run it a little slower, use an air blast or a spray mister, keeps the mess down.
Hope that helps a little.
