Metal working is the process of converting bulk met into a component or apart and primarily involves two types of operations: those that produce metal debris and those that produce no debris. The former type is classified as metal removal operations and the latter type is classified as metal forming operations. Cutting and grinding are examples of the first type and drawing, stamping, and bending are examples of the second type. All metalworking operations involve bringing two solids, a tool and a work piece, together to create a new part or a shape. The process involves high friction, high temperatures, and tool wear; and it is the job of the lubricant, or the metalworking fluid, to control them. Metal working fluids accomplish this by providing cooling, lubrication, and protection against corrosion. Therefore, they improve the efficiency of the operation, and hence increase productivity. Metal removing operations are of two types: those where the work piece is moved against the stationary tool and those where the tool is moved against the stationary work piece. In both cases, the tool cuts into the work piece, resulting in chip formation. These fluids, also called metal cutting fluids, are utilized in operations that are used to remove excess metal on the way to manufacturing a new part. Both oil and water based fluids are used for these operations. Oil-based fluids can be of petroleum, synthetic, or biological (vegetable and animal) origin. These fluids are designed to perform four key functions:
- Cooling to prolong tool life.
- Lubrication to minimize friction, and hence improve surface finish.
- Facilitate removal of chips and metal debris.
- Protect freshly exposed surfaces against rust and corrosion.
Heat produced during metal removal is primarily frictional and the most is generated during chip formation. Additional heat results from deformation of the metal and during travel of the chip across the tool surface [10,16]. The primary function of the lubricant in metal removed operations is to reduce friction as well as remove heat quickly. It must also remove melted debris, resulting from cutting and grinding operations, away from the work piece. Otherwise, extensive tool wear will occur. While water is an excellent coolant, it lacks the ability to reduce friction and wear. Therefore, water-based fluids contain friction reducing and wear control additives.
Friction reducing additives primarily generate protective surface films via physical interaction. Wear control additives, on the other hand, generate such films via chemical reaction. The former type includes fatty materials, such as vegetable oils and animal fats, and the latter type includes sulfur, chlorine, and phosphorus derivatives. The presence of these additives also minimizes welding of the generated metal debris onto the tool edge, called the “built-up” edge. This not only reduces blemishing, but it also improves surface finish of the work piece. The material used for cutting tools is selected to facilitate metal (chip) removal. Vibration, metal feed rate, cutting speeds, and lubricant availability (in the cutting zone) also play a roll in this process. These factors must therefore be taken into consideration when selecting tool material. For high speed cutting operations, steel, special cutting alloys, ceramics, and cermets are commonly used. Cermets are composites that contain ceramics and metals bonded together. The material hardness increases in the order listed and so does the generated temperature, from ~ 600°C to 1200°C.
The geometrical shape of the cutting edge also determines chip formation, which in turn affects the deformation and friction zones. That is where the cutting fluid plays a roll by providing lubrication and cooling, hence controlling wear. The selection of a proper lubricant is therefore important because it will affect cutting speed, tool life, surface finish, and precision of the work piece. Tool shape, the depth of cut, and temperature are also important considerations.