Grinding fluids are commercially available with different property profiles to meet the requirements of specific machining tasks. DIN 51 385 divides grinding fluids into
• Water-immiscible
• Water-miscible
• Water composite fluids
The more general fields of application of cooling lubricants are cutting and partial forming processes [DIN 51 385].
Water-immiscible cooling lubricants are generally not mixed with water for any application [DIN 51 385].
Water-miscible cooling lubricants are emulsifiable, emulsifying, or water-soluble concentrates, to which water is added before use.
Water-composite cooling lubricants are ready-for-use composites of water-miscible cooling lubricants with water. Within the group of water-miscible cooling lubricants, DIN 51 385 subdivides:
• Oil-in-water emulsions
• Water-in-oil emulsions
• Cooling lubricant solutions
For cutting, mainly oil-in-water emulsions and solutions are used, whereas water-in-oil emulsions are less common [Eckhardt 1983].
There are differences within the group of water-immiscible cooling lubricants according to the fraction and the type of the active substances contained [Bartz 1978, VDI-Richtlinie 3396 1983]. Classification within the group of water-miscible cooling lubricants is carried out according to the
content of active substances or to droplet size in rough disperse and fine disperse emulsions, as well as in fine colloidal, micellar, and molecular disperse solutions [Bartz 1978, VDI-Richtlinie 3396 1983].
The group of water-immiscible cooling lubricants also comprises natural and synthetic hydrocarbons such as mineral oils or poly-alfa-olefins, synthetic and vegetable ester, as well as water — and oil-soluble polymers such as polyglycols or composites of these substances [Konig et al. 1993]. To improve their lubricating properties and their pressure absorption capacity, either chemically active Extreme Pressure (EP), substances, or polar agents binding the lubricating film can be added to the base oils. Furthermore, water-immiscible and water composite cooling lubricants may contain corrosion, foam, and oxidation inhibitors or anti-fog additives [VDI-Richtlinie 3396 1983, Korff 1991, Konig et al. 1993].
Oil-in-water-emulsions are mainly stable disperse composites of water and mineral oil or esters, which contain finely dispersed oil droplets in a water phase [Konig et al. 1993]. The appropriate concentration of cooling lubricant emulsions must be determined for every single case of application depending on the corrosion protection capacity of the emulsions and on the cutting conditions. Conventional emulsion concentrations for grinding are in the range of 2% to 6% [Eckhardt 1983], or in special cases up to 20% [Klocke 1982]. Oil and water can be amalgamated with the help of bipolar surface-active substances. These substances favorably dissolve at the interface of the oil and water phase of an emulsion reducing the surface tension. Emulsifiers thus reduce the natural striving of the disperse phase to minimize the surface area [Mang 1983, Spur 1983, Moller and Boor 1986, Konig et al. 1993, Kassack 1994]. Through the variation of the emulsifier content or of the emulsifier type, different grades of dispersion can be set. Water-miscible solutions are made of completely water-soluble inorganic (e. g., water-soluble salts) or organic (e. g., polyglycols, boron acid amide) active agents for the improvement of corrosion protection and wetting capability, and are free of mineral oil [Bartelt and Studt 1992, Moller & Boor 1986, Konig et al. 1993]. Usually, cooling lubricant solutions are used as emulsions in low concentration [Eckhardt 1983]. Water composite cooling lubricants contain EP-additives, polar active agents, stabilizers, solution agents, preservatives, and corrosion and foam inhibitors in order to improve their functional properties [Bartz 1978, Spur 1983, VDI-Richlinie 3396 1983, Moller and Boor 1986, Konig et al. 1993].