Tech Talk
What are "Coolants"?
Coolants absorb heat on contact with the end mil, chips and workpiece and carry it out of the cutting zone. All cutting fluids act as coolants. However, some fluids are naturally more efficient heat absorbers than others. Water, for example, has a "specific heat" which is more than double that of hydrocarbon oils. (Specific heat is a measure of the amount of heat needed to raise the temperature of one gram of a substance by one degree Centigrade.) This means that, in the process of absorbing a given amount of heat, water will increase in temperature only half as much as a like volume of oil. Water also has a higher heat of vaporization, which means that it can absorb more heat before evaporating than an oil. Water also has a higher heat conductivity and can transfer heat from the cutting zone two to three times faster than an oil. Thus, water-based cutting fluids are considerably more efficient as coolants than oil-based cutting fluids. The characteristically high cutting speeds involved in end milling, and the resultant heat, dictate the use of an efficient coolant to reduce cutting zone temperature. Lubrication is considered only for jobs where extreme accuracy or fine finish is critical. Thus, water-based cutting fluids are the type most commonly used for end milling operations. These cutting fluids are made from mineral oils, petroleum oils, marine or vegetable oils, etc., either singly or in combination. While they can be used as-is, they are usually enriched with polar or EP additives or both. Straight mineral oils are usually limited to light-duty cutting on easy-to-machine metals such as aluminum, magnesium, brass and some free-machining steels. Compound cutting oils are typically used for low-speed machining of difficult-to-cut metals where extra lubrication is needed.
