KeywordsTool Wear Contact Zone Lubricant Film Material Removal Process Coolant Flow Rate
Grinding fluids belong to metalworking fluids. These are engineering media which are used to allow for higher productivity in material removal processes, i.e., cutting and abrasive processes.
Theory and Application
Metalworking fluids (Brinksmeier et al. 2015) play a significant role in machining operations (in particular grinding) and have a substantial impact on tool life, shop productivity, and workpiece quality. In machining processes such as turning, milling, grinding, and many other material removal processes, metalworking fluids perform several essential functions. One of the main functions of coolants is to lubricate. This is achieved by reduction of the friction which results from energy conversion in the contact zones between tool and workpiece as well as between tool and chip. Heat dissipation, i.e., cooling of the workpiece and washing chips away from the contact zone, is a further important function of the coolant (Brinksmeier et al. 1999, 2004; Inasaki et al. 1993; Howes 1990). The combined lubrication and cooling effect reduces tool wear, enhances surface quality and dimensional accuracy of the workpiece, and allows for higher material removal rates. Cooling and lubrication requirements differ in every application and mainly depend on process conditions. Coolants should, ideally, be composed to suit each specific case. Every coolant consists of a base fluid, to which other products such as anti-wear, anticorrosion, or emulsifying agents (additives) are added. According to DIN 51385, coolants are divided into oil-based and water-based types (Brinksmeier et al. 1999).
Straight oils without additives
Straight oils with chemically active additives
Straight oils with surface-active additives
Straight oils with chemically active additives and extreme pressure additives (additives that form stable adsorption layers)
For high cooling efficiency and washing-away capabilities, water-based emulsions or solutions are employed. Their main disadvantage is susceptibility to leakage oils and microbial effects making high-maintenance costs unavoidable. Furthermore, the water and oil phases must be separated before disposal. Water-based solutions consist of inorganic and/or organic substances and water and very seldom contain mineral oils.
Water-based emulsion concentrates contain 20–70 % base oil (mostly mineral oil). For metal grinding operations, oil-in-water emulsions are common; the amount of oil determines the lubrication ability of the emulsion. Common oil concentrations in emulsions for grinding operations are between 2 % and 15 % of the concentrate in 85–98 % water. Water-based coolants contain up to 20 different chemical components in which each of the components can themselves be multicomponent mixtures. Defined or (based on microbial effects) undesired changes of the chemical composition of the metalworking fluid considerably influence the thermomechanical load of the process.
Enhancers of physical coolant characteristics
Enhancers of chemical coolant characteristics
Enhancers of chemical and physical coolant characteristics
The main characteristic of grinding in comparison to other machining processes is the relatively large contact area between the grinding wheel and the workpiece and the high friction between the abrasive grits and the workpiece surface. This leads to difficulties in supplying coolant to the grinding arc, thus resulting in a high risk of thermal damage to the workpiece surface layer as well as loading and wear of the grinding wheel. Thermomechanical processes in the contact zone are defined by tribological relationships between the grain cutting edge, the grinding wheel bonding, the workpiece, and the chip as it forms, so that cooling lubrication plays a decisive role during grinding with respect to heat generation and dissipation. In addition, the coolant type, composition and filtration, and coolant supply (nozzle position, nozzle geometry, supplied flow rate, and jet characteristics) can influence process productivity, workpiece quality, and tool wear considerably. Coolants should, ideally, be composed to suit each specific case (Brinksmeier et al. 1999; Huesmann-Cordes et al. 2014).
In grinding, the chip is formed as material is deformed by the grit or grain cutting edge and displaced sideways or forward according to the orientation of the cutting edge. When the material shear stress is exceeded, the chip flows over the face of the grain. The coolant in the contact zone is building up a lubricant film. The evaporation behavior and rheology of this lubricant film help to lower frictional forces and cool both the workpiece and tooling surfaces.
Another important influence of coolants on lubrication is the lowering of friction along the chip flow line, i.e., between the chip, the grain cutting edge, and the grinding wheel bond. This reduces bond abrasion and grinding wheel wear (Vits 1985). Effects of coolant lubrication and cooling, respectively, are influenced by the type of coolant (straight oil, emulsion, solution) and its composition (emulsion concentration, additives) (Brinksmeier et al. 2009).
Cooling Supply in Grinding
The heat flux during grinding can create form deviations of the workpiece and subsurface damage. The grinding fluid in the contact zone between workpiece and tool counteracts these undesirable effects by building a lubricant film which reduces friction forces between the acting partners and by cooling the contact zone. To achieve the greatest cooling effect, a variety of coolant nozzles is available for different grinding processes. In grinding, flooding nozzles like jet nozzles and shoe nozzles (a nozzle which covers the grinding wheel partly) have proven to be favorable. Using jet nozzles the required cooling is often tried to be realized by an oversupply of coolant in front of the grinding arc instead of an optimum wetting of the grinding wheel. The positioning of the nozzles has another considerable effect on the cooling ability of the fluid in the contact zone (Brinksmeier et al. 2000; Heinzel et al. 2015). With increasing coolant flow rate, an improvement of the cooling effect is achievable in principle but reaches saturation when exceeding a critical coolant flow rate (Klocke et al. 2000). The benefit of using shoe nozzles is the significant reduction of required coolant amount (Brinksmeier et al. 1999; Heinzel 1999).
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- DIN 51385:2103–12. Schmierstoffe – Bearbeitungsmedien für die Umformung und Zerspanung von Werkstoffen – Begriffe [Lubricants – Processing fluids for forming and machining of materials – Terms]. Beuth, Berlin (in German)Google Scholar
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