To reasonably evaluate the choices available in machine coolants, it's good to remember why you're using coolants in the first place.  Metal‑removal techniques generate friction, which in turn generates heat, and excessive heat in metal removal is always a detriment.  Consequently, to provide proper size control and desired shape and finish,  machine coolants must properly fulfill two important cutting functions: lubrication and cooling.  Refer to our Machine Coolant Index for more articles on Machine Coolant including types of Coolant, health and safety Information, and particle and bacteria removal. 

Lubrication is important because external friction, or metal‑to‑metal contact, generates approximately a third of the heat that results from cutting. Internal friction, or the resistance of the metal atoms to movement when the metal is deformed in the shear zone, is responsible for generating two‑thirds of the heat.  When the cutting zone is properly lubricated, cutting becomes more efficient, thereby reducing external friction and, to a lesser extent, lowering internal friction. 

To reduce internal friction, chlorine, sulfur, and phosphorous atoms in straight oil and in extreme ‑pressure (EP) coolant additives penetrate the micro cracks in metallic surfaces, thereby preventing the rebonding of metal atoms displaced in the cutting process and reducing the power needed to form a chip.  Also, by lubricating the chip/tool and tool‑flank/cut‑surface interfaces, the angle of the shear plane increases and, as a result, the area of the shear plane decreases.  As the shear‑plane area decreases, the power required to form a chip decreases and so does the heat generated. 

The cooling effect provided by machine coolants is necessary to remove heat from the tool, chip, and work piece.  This cooling extends tool life primarily by preventing tools from exceeding their critical temperature range while in the cut.  Beyond the critical temperature, tools soften and wear rapidly and fail to meet tolerances for surface finish and part size.  Cooling effects also help keep the part thermally stable, aiding in the control of part size.  With so many parts today being finish machined, (e.g., engine blocks and transmission cases), without subsequent processing, the machine coolant's ability to keep the part dimensionally stable during machining is all the more critical. 

The primary function of machine coolant is temperature control through cooling and lubrication.  Application of machine coolant also improves the quality of the work piece by continually removing metal fines and cuttings from the tool and cutting zone. 

Temperature Control

Laboratory tests have shown that heat produced during machining has a definite bearing on tool wear.  Reducing cutting-tool temperature is important since a small reduction in temperature will greatly extend cutting tool life. 

As machine coolant is applied during machining operations, it removes heat by carrying it away from the cutting tool / work piece interface.  This cooling effect prevents tools from exceeding their critical temperature range beyond which the tool softens and wears rapidly.  Machine coolants also lubricate the cutting tool/work piece interface, minimizing the amount of heat generated by friction.  A machine coolant's cooling and lubrication properties are critical in decreasing tool wear and extending tool life.  Cooling and lubrication are also important in achieving the desired size, finish and shape of the work piece. 

No one particular machine coolant has cooling and lubrication properties suitable for every metalworking application. Straight oils provide the best lubrication, but poor cooling capacities.  Water, on the other hand, is an effective cooling agent, removing heat 2.5 times more rapidly than oil alone.  Water is a very poor lubricant and causes rusting. Soluble oils or chemicals, which improve lubrication, prevent corrosion and provide other essential qualities must be added in order to transform water into a good machine coolant. 

Removal of Cuttings and Particulates

A secondary function of machine coolant is to remove chips and metal fines from the tool/work piece interface.  To prevent a finished surface from becoming marred, cutting chips generated during machining operations must be continually flushed away from the cutting zone. 

Application of machine coolant also reduces the occurrence of built-up edges.  Built up edges cause increased friction and changes the cutting geometry of the machine tool.  This affects work quality and often results in poor surface finish and inconsistencies in work piece size.  Machine coolants decrease the occurrence of built up edges by providing a chemical interface between the machine tool and work piece. 

Machine coolant Properties

In addition to providing a good machining environment, a machine coolant should also function safely and effectively during machining operations. 

Corrosion Protection

Machine coolants must offer some degree of corrosion protection.  Freshly cut ferrous metals tend to rust rapidly since any protective coatings have been removed by the machining operation.  A good machine coolant will inhibit rust formation to avoid damage to machine parts and the work piece.  It will also impart a protective film on cutting chips to prevent their corrosion and the formation of difficult-to-manage chunks or clinkers. 

To inhibit corrosion, a machine coolant must prevent metal, moisture and oxygen from coming together.  Chemical machine coolants now contain additives, which prevent corrosion through formation of invisible, nonporous films.  Machine coolants to prevent corrosion from occurring produce two types of invisible, nonporous films.  These include polar and passivating films. Polar films consist of organic compounds (such as amines and fatty acids), which form a protective coating on a metal's surface, blocking chemical reactions.  Passivating films are formed by inorganic compounds containing oxygen (such as borates, phosphates and silicates).  These compounds react with the metal surface, producing a coating that inhibits corrosion. 

Stability and Rancidity Control

In the early days of the industrial revolution, lard oil was used as a machine coolant.  After a few days, lard oil would start to spoil and give off an offensive odor.  This rancidity was caused by bacteria and other microscopic organisms that grew and multiplied within the oil.  Modern machine coolants are susceptible to the same problem. 

No matter how good the engineering qualities of a coolant, if it develops an offensive odor it can cause problems for management.  The toxicity of a machine coolant may also increase dramatically if it becomes rancid due to chemical decomposition, possibly causing the machine coolant to become a hazardous waste.  Machine coolant rancidity shortens machine coolant life and may lead to increased costs and regulatory burdens associated with machine coolant disposal. 

A good machine coolant resists decomposition during its storage and use.  Most machine coolants are now formulated with bactericides and other additives to control microbial growth, enhance machine coolant performance and improve machine coolant stability. 

Transparency And Viscosity

In some operations, machine coolant transparency or clarity may be a desired characteristic.  Transparent machine coolants allow operators to see the work piece more clearly during machining operations. 

Viscosity is an important property with respect to machine coolant performance and maintenance.  Lower viscosity machine coolants allow grit and dirt to settle out of suspension.  Removal of these contaminants improves the quality of the machine coolant recirculating through the machining system.  This can impact product quality, machine coolant life and machine shop productivity. 

Health and Safety Considerations

Workers in machining operations are continually exposed to machine coolant.  A machine coolant must be relatively non-toxic, non-flammable and non-misting to minimize health and safety risks. 

Toxicity

Most machine coolants are not highly toxic.  The machine coolant becoming rancid, super concentrated, or contaminated usually causes toxicological problems associated with machine coolants.  The main routes of exposure for machine coolant include inhalation (via vapor, smoke or mist), ingestion and skin absorption.  Dermatitis and respiratory problems are the most frequent health problems of machine shop personnel.  Due to the variety of ingredients contained in machine coolants, it is often very difficult to anticipate whether the machine coolant will affect individuals constantly exposed to this material. The Material Safety Data Sheet (MSDS) for a machine coolant contains important health and safety information and should be reviewed as a first step in machine coolant selection. 

Flammability

Machining operations typically generate a significant amount of heat, which can cause machine coolants to smoke and/or ignite.  A machine coolant should have a high flashpoint to avoid problems associated with heat damage, the production of smoke, or machine coolant ignition. 

Misting

High-speed metalworking operations such as grinding may atomize machine coolant, creating a fine mist, which can be an inhalation hazard for machine tool operators.  Misting also creates a dirty work environment by coating equipment and the surrounding work area.  Non-misting machine coolants provide safer working conditions for the machine operator.