In addition to providing a good machining environment, a cutting fluid should also function safely and effectively during machining operations.
Cutting fluids 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 metalworking fluid will inhibit rust formation to avoid damage to machine parts and the workpiece .
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 fluid must prevent metal, moisture and oxygen from coming together. Chemical metalworking fluids now contain additives which prevent corrosion through formation of invisible, nonporous films. Two types of invisible, nonporous films are produced by metalworking fluids to prevent corrosion from occurring. 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 or Rancidity Control
In the early days of the industrial revolution, lard oil was used as a cutting fluid. 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 metalworking fluids 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 fluid may also increase dramatically if it becomes rancid due to chemical decomposition, possibly causing the fluid to become a hazardous waste. Fluid rancidity shortens fluid life and may lead to increased costs and regulatory burdens associated with fluid disposal. A good cutting fluid resists decomposition during its storage and use. Most cutting fluids are now formulated with bactericides and other additives to control microbial growth, enhance fluid performance and improve fluid stability.
Transparency and Viscosity
In some operations, fluid transparency or clarity may be a desired characteristic for a cutting fluid. Transparent fluids allow operators to see the workpiece more clearly during machining operations. Viscosity is an important property with respect to fluid performance and maintenance. Lower viscosity fluids allow grit and dirt to settle out of suspension. Removal of these contaminants improves the quality of the fluid recirculating through the machining system. This can impact product quality, fluid life and machine shop productivity.
Health and safety Considerations
Workers in machining operations are continually exposed to cutting fluid. A fluid must be relatively non-toxic, non-flammable and non-misting to minimize health and safety risks.
Most metalworking fluids are not highly toxic. Toxicity problems associated with metalworking fluids are usually caused by the fluid becoming rancid, super concentrated or contaminated. The main routes of exposure for metalworking fluid 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 metalworking fluids, it is often very difficult to anticipate whether the fluid will affect individuals constantly exposed to this material. The Material Safety Data Sheet (MSDS) for a metalworking fluid contains important health and safety information and should be reviewed as a first step in fluid selection.
Machining operations typically generate a significant amount of heat which can cause cutting fluids to smoke and/or ignite. A fluid should have a high flash point to avoid problems associated with heat damage, the production of smoke, or fluid ignition.
High speed metalworking operations such as grinding may atomize fluid, 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 fluids provide safer working conditions for the machine operator.