Metalworking lubricants perform several functions. They protect or improve surface finish, provide corrosion protection, reduce frictional forces, cool the tooling, prevent metal pick-up on the tooling, and they flush away the chips created in a machining operation. To perform these functions they contain complex blends of a carrier, lubricity additives, corrosion inhibitors, biocides, and antifoams.
The major components of a lubricant are the carrier, lubricity additives, and in emulsions an emulsifier. Carriers are petroleum oils, water, or solvents. Their function is to provide cooling, act as a vehicle for the other additives, and flush away chips. Water based lubricants are preferred in metal machining because water removes heat caused by friction and metal removal almost twice as fast as oil. Solvents are used in vanishing lubricants that leave no or little residue.
1. Petroleum oil
2. Polar additives
3. Extreme pressure additives
Petroleum oils by themselves are not very good metalworking lubricants. Their lubricating films can take only light loading. They are use on single spindle automatics such as screw machines, lathes where free machining brass and steels are the main metals being machined. They are seldom used in stamping and drawing.
Polar additives are of two kinds: those derived from fatty oils such as lard oil and long carbon chain polymers. They have a positive charge at one end of the molecule and a negative charge at the other end. The negative end is attracted to the positive charged metal, and aligns itself at right angles to the metal surface. The positive ends of the molecules, both on the metal being worked and the tool (cutting or die) repel each other, preventing metal-to-metal contact. This is termed polar lubricity.
Extreme pressure additives are sulfurized or chlorinated fatty oils, fatty oils that have both sulfur and chlorine on the same molecule, and phosphorus compounds. They function by reacting with the metal surfaces of the cutting tool or die to form a sulfide, phosphide, or chloride compound. These compounds have plate like structure, similar to graphite, allowing the work piece and the tool to slide against each other. This reduces friction and in the case of metal cutting reduces the cutting force and in stamping and drawing lowering friction and controlling metal flow into the die. Phosphorus compounds also provide antiwear properties.