Oil based metalworking fluids are used in tough operations where lubrication and extreme pressure properties are required. With the correct formulations of additives, cutting oils provide significant improvements in cutting and grinding operations by reducing friction which results in better surface finishes and longer machine tool life. They are particularly ideal for use with grinding wheels and other abrasive tools. Though oil based cutting fluids do not replace water based fluids, where cooling properties are most important, they are instead used in applications where lubrication is more important than cooling.
Grinding and cutting oils are specially designed for optimum performance in many different operations. Some of their specific advantages include:
The two main focuses for metal cutting are boundary lubrication and mixed film lubrication. However, first it’s important to understand how specialty additives work to protect the metal surfaces of the tool and workpiece. Although the metal surface may appear to be smooth to the naked eye, it is not. The surface actually contains asperities. Asperities are high spots on the surface that come into contact during wear or friction as seen in the picture to the right.
As we know, during metal cutting the tool comes into direct contact with the metal workpiece. When two rough metal surfaces come into close contact with each other their asperities will also be in close constant contact.
The local temperature at the point of cut can reach up to ~1,000 oC, which can cause the two surfaces to “weld” together. However, when extreme pressure additives are “activated,” they react with the surface of each metal to create a low shear strength film that prevents welding.
Mixed film lubrication is a combination of two different lubrication regimes: boundary and hydrodynamic. Hydrodynamic is when the lubricant provides a barrier, preventing the two surfaces from coming into close contact. Therefore, their asperities will not touch and there’s no need for extreme pressure additives. In mixed film lubrication, some asperities may still come into contact but not to the extent as boundary, as shown here.
While boundary lubrication happens at the point of cut, mixed film is found around the point of cut. That’s why cutting fluid lubricity additives are very important for protection, providing a cushion between the two surfaces by filling in around the asperities.
To enhance results of the machining process, specialty additives are added to improve cutting performance, corrosion protection, and extend oil and tool-life.
Primary additives include:
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