Straight cutting oils are non-emulsifiable and are used in machining operations in an undiluted form i.e. as supplied by the manufacturer. These are oil-based materials, which generally contain extreme pressure (EP) additives such as chlorine and sulphur added at various levels to provide increased tool life and improved surface finish.
There are several advantages of straight cutting oils viz.: good lubricity, effective anti-seizure qualities, good rust and corrosion protection, and stability. Neat cutting oils as opposed to soluble or synthetic cutting fluids provide high levels of lubrication to help reduce frictional heat as the cutting tool penetrates the metal or rubs against the accumulated fine particles of metal during machining. Neat oils also save energy by reducing cutting forces. Straight oils perform best in heavy duty machining operations and very critical grinding operations where lubricity is very important. These are generally slow speed operations where the cut is extremely heavy. Some examples would include broaching, threading, gear hobbing, gear cutting, tapping, deep hole drilling and gear grinding.
There are few disadvantages also viz.: poor cooling, mist and smoke formation at high cutting speeds, high initial and disposal cost. Straight oils provide the best lubrication and the poorest cooling characteristics among cutting fluids. Straight oils do not work well in high speed cutting operations because they do not dissipate heat effectively
Straight cutting oils containing sulphurised fatty material provide excellent surface finish, extended tool life and low chip welding tendencies. They are suitable for heavy duty machining, milling, tapping, turning and automatic/CNC machining of tough ferrous and stainless materials. They will stain copper and brass
Straight cutting oils containing chlorinated EP additives react more readily than sulphur at lower temperatures to provide a responsive light to medium cutting oil. They are suitable for medium duty machining, milling, tapping, turning and automatic/CNC machining of ferrous and non-ferrous materials and will not stain yellow metals.
Since they are not diluted with water and the carryout rate on parts is high, these oils are costly to use and, therefore, they are used only when other types of cutting fluid are not applicable.