Cutting Fluid Selection Criteria
The principal criteria for selection of a cutting fluid for a given machining operation are:
Process performance
Heat transfer performance
Lubrication performance
Chip flushing
Fluid mist generation
Fluid carry-off in chips
Corrosion inhibition
Fluid stability (for emulsions)
Cost Performance
Environmental Performance
Health Hazard Performance
Product Form
Industrial lubricants include low viscosity oils, high viscosity oils, greases, and solid lubricants.
Low viscosity oils offer the least resistance to movement. The reduced shear stress minimizes friction. Load bearing capabilities and fluid film thickness are also reduced. Low viscosity oils may contain additives to prevent lubrication failure during periods of high load or at low speeds.
High viscosity oils exhibit higher shear stresses and thicker fluid films. The thicker fluid film is able to support greater loads and function at lower speeds, although their resistance to flow increases the amount of friction between the mating surfaces.
Greases are semi-solids formed by the dispersion of a thickening agent in a base fluid. The thickening agent serves as a matrix that holds the lubricant in place, while supplying some amount of ingress protection. The oil or base fluid is the active lubricating agent.
Solid, or dry film lubricants, disperse a coating that excludes moisture and reduces friction. Solid lubricants may also contain corrosion inhibitors and are generally used in high temperature applications where liquid lubricants break down.
There are three basic product forms of MWFs: fluids, greases, and solid lubricants or dry film lubricants.
Fluids are water-based liquids, oils, or fluids supplied in liquid form.
Greases, gels, and lubricating pastes are thick, high viscosity products that do not run or flow off surfaces. Greases often consist of oil thickened with a sodium or calcium soap complex or non-soap thickener.
Solid lubricants or dry film lubricants are compounds such as hexagonal flake graphite, boron nitride (BN), molybdenum disulfide, or polytetrafluoroethylene (PTFE) powders.
Composition
Metalworking lubricants, coolants, and fluids vary widely in terms of chemical composition. They may be classified as being either petroleum or mineral oil-based, or synthetic or semi-synthetic.
Petroleum or mineral oils
Petroleum and mineral oil products are functional fluids derived from petroleum. They include a broad range of hydrocarbon-based substances of varying chemical compositions and a wide variety of physical properties. Specific constituents present include aromatic, naphthenic, and paraffinic fluids.
Synthetic or semi-synthetic fluids include fluids with a base of glycol or polyglycol, ester or diester, or silicone-based fluids. They exhibit outstanding thermal and dielectric properties. The characteristics, cost, and heat transfer performance of semi-synthetic fluids fall between those of synthetic and soluble oil fluids.
Other specialized fluids include high water content fluids (HWCF), lithium complexes, aluminum complexes, waxes such as paraffin and stearate, and halogenated hydrocarbons including chlorofluorocarbon (CFC), halogenated fluorocarbon (HFC), halogenated chlorofluorocarbon (HCFC), and perfluorocarbon (PFC).