Both the needs of the job and performance properties must be considered in selecting a cutting fluid. The type of operation combined with the most critical operation in a machining environment may dictate the cutting fluid selection. Threading, reaming, tapping and broaching are some of the most difficult machining operations, and require a cutting fluid that reduces friction. Turning, drilling boring, and similar less difficult operations can be machined with products having low to moderate friction-reducing properties. Some grinding applications can be machined with fluids that provide little friction reduction. Form grinding requires a heavy-duty fluid. Machinability ratings, severity of operation, stock removal rates, feeds, speeds and finish requirements must be considered.
Petroleum Based water soluble general purpose coolants are suitable for most machining applications on ferrous and nonferrous metals with a machinability rating above 70%. Heavy-duty water soluble products fortified with extreme pressure additives to promote better tool life and surface finish on metals with machinability ratings below 70% are also available. These products provide hard water stability, reserve alkalinity, bio-resistance and short term corrosion protection. The emulsified solutions create a milky, opaque appearance of oil in water droplets.
Semi-Synthetics have lower oil content (5 – 35%) and an emulsifier package which results in a smaller droplet formation and a mixture that is translucent or transparent (occasionally dyed). These products are suitable for most machining applications. Extreme pressure additives are required to improve heavy duty machining performance. These solutions provide better cooling (more water) and usually have better settling and cleaning properties than emulsified oils which assists in extending bath life.
Synthetics are true solutions and are completely clear (occasionally dyed); they consist of organic and inorganic salts dissolved in water. Many are used as grinding fluids since they offer rust protection and excellent heat removal. Heavy-duty products are good general purpose coolants that provide good lubrication and fast heat removal. They keep the wheels open and free grinding, enable the operator to see the work and produce considerably less mist than other cutting fluids. Synthetic solutions are the less prone to microbial infestation and have superior settling and cleaning properties that help extend fluid life.