Water-soluble oils are the workhorses of the metalworking industry. They are most commonly used for CNC machining of ferrous metals, but can be used in a wide variety of applications. They leave behind an oily layer on the parts and machines, which acts as a rust preventative. When the coolant is used properly, it is very uncommon to have rust problems. The disadvantage of the water-soluble oils is that they will emulsify tramp oil, so that the tramp oil (hydraulic oil, spindle oil, etc.) now becomes part of the coolant. In effect, this weakens the emulsion stability of the metalworking fluids and can eventually split the emulsion.