Cutting fluids are not fluids that cut materials, although they certainly sound like they are. Instead, cutting fluids are highly specialized fluids that lubricate and cool CNC machines while the machines are in operation. There are several different lubricants, as well as benefits to each.
Grinding oils are typically a mix of water and a special blend of oils. The water "hydro-cracks" the oil so that it blends easily and it thoroughly lubricates grinding components on the machines. Pure water alone is not enough, since water would just vaporize on the grinders because of the heat, friction, and intense speed applied. The combination of these oils with enhanced water keeps the grinders cool, and prevents tiny, scratchy bits of the grinding components from ending up in the materials sent through the grinding applications of the CNC machine.
Mineral Oil-Based Cutting Oils
Mineral-based machining oils are ideal for those times when CNC machines are cutting metal. They help the machines cut, bore, stamp, and perform a wide variety or machining applications on metal. They also prevent overheating, warping of the metal sheets, melting (which can occur with thinner or softer metal sheets), and they help create smoother edges. There are multiple formulas, based on the kinds of applications and machines for which you intend to use these cutting oils.
Vegetable Oil-Based Cutting Oils
These cutting oils are the most eco-friendly. Plant-derived, if they are accidentally spilled on the ground outside, they convert to organic matter over time. These cutting oils have very high flash points, which means that they will not ignite unless exposed to extreme heat that is uncharacteristic of machining. Because of their natural viscosity levels, you use less of these cutting oils than you would with mineral oil-based cutting oils. That makes them doubly eco-friendly because you need less and create less waste. That helps your operating expense sheet, too, because you do not spend as much buying products that you would use up too quickly.
Generally, you could use these cutting fluids on any material, but they work best on really hard-to-cut materials, such as tempered steel or titanium. Viscosity varies, depending on what material you plan to machine, and the machining application you will use. Overall, it is a wise investment of the three cutting fluids, especially if you purchase the oils with no chlorinated additives (in keeping with a "green" approach to machining).