MACHINING OF METALS

EXTREME PRESSURE STRAIGHT CUTTING OILS

Heavy operations in modern machining practice have led to important developments in cutting fluids. Lubrication by a fluid film of oil becomes impossible in these operations because of the high temperatures and pressures involved. Lubricants able to prevent the welding and to reduce the friction under these conditions have consequently been developed. These lubricants consists of mineral oils to which certain chemically active substances have been added. At the high temperatures which are developed where the metal chips press against the tool, these substances attack the metal to form an extremely thin film of a chemical compound which prevents the tool and chip from becoming welded together. Various compounds of sulphur and elementary sulphur itself, are the additives most generally used to produce this result. Certain compounds of chlorine are also used to considerable extent.

Some E.P. cutting oils are black and this is a rather troublesome feature because the work is hidden from the operator. It is possible, however, to prepare transparent types which are much more convenient to use. E.P. cutting oils have much lower cooling power than soluble oil emulsion because their specific heat is only about half the at of water. But, because their lubricating power is much higher, less heat is developed and cooling is consequently less important.

In addition to having high lubricating power, good transparency and stability, these oils must also be sufficiently fluid to be easily pumped and to drain away readily from the metal cuttings so as to avoid waste. These considerations impose an upper limit on the permissible viscosity.

WATER BASE FLUIDS

These are aqueous solutions of chemicals which are diluted further with water before use. Soda solutions are amongst the first cutting fluids used many years ago - and they are still used to a small extent. However, during the past few years, interest has arisen in fluids containing sodium nitrite, which has become prominent because of its rust preventive properties.

At present there are indications of two distinct lines of use for water base fluids. The more common use in grinding operations, where they are used at concentration of 1: 80 or less and where, depending on operating conditions, they can show distinct advantages over clear soluble, oils, they remain clear for longer periods, reduce loading and wear of wheels and provide more rapid removal of swarf from coolant systems. Less commonly, fluids of this type are used, at concentrations of 1:20 to 1:40 instead of soluble oils for such operations as turning, drilling, milling and sawing. Apart from advantages of cleanliness and clarity, there are indications that better cutting performance can be obtained with certain water base fluids; moreover, some success has been achieved with such fluids in the machining of cast iron, an application in which soluble oils have generally shown little advantage.