Important characteristics of oil-based cutting fluids

发布时间：2022年07月01日 12:30

Oil itself has the property of viscosity, which plays an important role in the formulation of oil-based cutting fluids. Low-viscosity oils are thinner and exhibit better penetration and wettability. By selecting appropriate additives, the oil can reach the cutting zone more quickly. Moreover, because it’s thin, it offers superior cooling and cleaning performance. High-viscosity oils, on the other hand, are denser and composed of larger molecules, providing better lubricity and the ability to separate metal surfaces. However, such high-viscosity oils have poorer fluidity and cooling performance compared to low-viscosity oils.

1. Viscosity

Oil itself has the property of viscosity, which plays an important role in the formulation of oil-based cutting fluids. Low-viscosity oils are thinner and exhibit better penetration and wettability. By selecting appropriate additives, the oil can reach the cutting zone more quickly. Moreover, because they are thin, these oils offer superior cooling and cleaning capabilities. High-viscosity oils, on the other hand, are denser and composed of larger molecules, providing better lubricity and the ability to separate metal surfaces. However, such high-viscosity oils have poorer fluidity and cooling performance compared to low-viscosity oils.

2. Lubrication

In metal cutting, different materials and different cutting speeds generate varying levels of heat and pressure. Oil-based cutting fluids primarily provide lubrication on the tool surface in the sliding zone.

a. Fluid lubrication

(Physically separated) Fluid dynamic lubrication refers to a situation in which the lubricant oil physically separates the tool face from the workpiece surface without undergoing any chemical reactions. Oil-based cutting fluids with higher viscosity or greater thickness exhibit better separation performance due to their larger molecular size. When significant loads and pressures are present within the sliding zone of the tool, the viscosity of mineral oils increases, thereby enhancing their lubricating properties—this phenomenon is known as "elastohydrodynamic lubrication." However, within the sliding zone, if the pressure generated between the tool and the workpiece becomes excessively high, the oil-based cutting fluid can still be squeezed out. Therefore, using pure mineral oil—with its inherent physically separated characteristics—as a lubricant is not a very effective approach.

Oil-based cutting fluid

b. Marginal lubrication

In boundary lubrication, adding polar substances to mineral oil creates a thin film of organic material with chemical bonds on both the workpiece surface and the tool surface. This structured film adheres strongly to various metalworking surfaces, resulting in superior wear resistance compared to the effect achieved solely by oil molecules in conventional oil-based cutting fluids, which rely on isolation between the workpiece and the tool.

Lipid substances have long been used as additives in mineral oils to formulate oily cutting fluids that can produce suitable organic agricultural films for industrial applications. Lipids have a remarkably significant effect on improving cutting performance, which in turn helps extend the service life of tool systems. Commonly used lipid-based oily cutting fluid agents include oleate esters, stearate esters, rapeseed oil, and their derivatives; currently, a large number of synthetic lipids are also being employed.

Natural lipids, fatty acids, and their derivatives can form a monomolecular film on metal surfaces. The carbon–carbon and hydrogen–bonded film acts as an outer skin for the metal, arising from the reaction between the metal and the lipids; this phenomenon is referred to as “soap metals.” Lipid additives create an organic film that prevents direct contact with the metal and provides protection until the temperature rises to the film’s melting point.

c. Extreme-pressure lubrication

In most machining processes, the temperature at the cutting-edge tip of the tool exceeds the temperature range of the edge’s oil-based cutting fluid. Therefore, it is necessary to use additives that can generate higher melting points. These additives are inorganic; chlorine and sulfur are two of the more commonly used elements. When additives such as chlorine and sulfur are employed, they react with the metal surface to form metallic derivatives that exhibit low friction characteristics. The resulting thin film provides lubrication similar to that of dry oil-based cutting fluids, effectively preventing wear and melting on metal surfaces. Chlorine-based films can withstand temperatures up to 600°C, while sulfur-based films can endure up to 1000°C. Both chlorine and sulfur are extreme-pressure additives. Chlorine is typically added in the form of chlorinated mineral oil, whereas sulfur can be incorporated into cutting fluids in various forms; sulfuration is a common method of addition. For more information on oil-based cutting fluids, please visit Qingjie.

The two main functions of oily cutting fluids used in machining

Next episode

An industrial lubricant manufacturer tells you how to choose industrial lubricants.

/ Related News