CUTTING FLUIDS
The correct selection and use of cutting fluids is one of the most important, and often one of the cheapest, factors in enhancing the performance of cutting tools. To obtain optimum rates of metal removal and to maintain optimum tool service life, it is necessary to lubricate and cool the chip-tool interface. Cutting fluids are designed to fulfil one of more of the following functions:
- to cool the tool and workpiece,
- to lubricate the chip-tool interface and reduce tool wear due to friction and abrasion,
- to prevent chip welding (formation of a built-up edge),
- to improve the finish of the machined surface,
- to flush away the chips from the cutting zone,
- to prevent corrosion of the work and the machine.
Experiments have shown that, for the majority of machining operations, the cooling and flushing action of the cutting fluid is most important as the rate of tool wear is extremely sensitive to small temperature changes at the chip-tool interface. For this reason, emulsified cutting fluids are used in most workshop applications since the high water content improves the cooling action and reduces the cost.
However, the lubricating action of the cutting fluid becomes of prime importance in reducing the wear rate of the tool in such operations as tapping, broaching and gear cutting, where expensive form tools are used. In these operations the cutting speeds are relatively low and the chip force on the rake face of the tool is very high, thus straight (undiluted) cutting lubricants are used. These lubricants often contain an extreme-pressure addictive.
Mineral lubricating oils are unsuitable as cutting fluids. Their viscosity is too high and their specific heat capacity is too low to make them effective coolants; their lubricity is inadequate to withstand the high contact pressure between the chip and the tool, and they give off noxious fumes when raised to the cutting temperature. They also represent a fire hazard.
