QUICK FACT GUIDES FOR MACHINISTS
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FUNDAMENTALS OF DRILLING & TAPPING
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FUNDAMENTALS OF THE LATHE 
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FUNDAMENTALS OF MILLING
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FUNDAMENTALS OF SAWING
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METALWORKING SHOP TERMS
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STRAIGHT CUTTING OILS

Straight cutting oils are "straight" only in that they are not mixed with water for use in metal cutting. They are available in many formulations, ranging from straight mineral oil to high-compounded blends specifically formulated for severe operations.

       Straight cutting oils, are classified either as "active" or "inactive". These terms relate to the oils chemical activity, or ability to react with the metal surface at elevated spot temperatures to protect it and improve machining.

       An active oil is defined as one that will darken a copper strip immersed in it for three hours at 212 F. An inactive oil, under the same conditions, will not darken the strip.

        Both types of cutting oils are used for machining, but in somewhat different applications. The difference between the two types of oils is that in the active oils a given amount (usually about 2%) of sulfur has been added to the oil to produce an active sulfurized oil, which upon reheating (in the cutting zone) will release some of this added sulfur to react chemically with the metal work surface.

                On the other hand, the entire content of sulfur in an inactive oil is natural sulfur contained in that oil. Because it is tightly bonded within the hydrocarbon structure of the oil, the sulfur is not released and has no value chemically in the functioning of the cutting fluid during machining.

Also included in the inactive oil group are such fluids as fatty oils, fatty oil-mineral oil blends, and sulfurized blends of these oils. These are considered inactive because the sulfur is so firmly attached that little is released to react with the work surface.

       Active cutting oils are generally used in machining steel. These oils may be dark or transparent, straight sulfurized or sulfo-chlorinated, mineral or fatty compounded. The dark oils generally contain more sulfur than the transparent sulfurized oils, and are usually considered better for heavy-duty jobs. Today, however, the newer additives and concentrates make the transparent oils just as suitable for tough machining applications.

Chlorine compounds, if chemically active at tool temperatures, act similarly to sulfur compounds, but usually at lower temperatures, to form a metallic chloride film at the tool-work interface. Both sulfur and chlorine, among other chemicals that are added to cutting oils, help to provide extreme-pressure lubrication and anti-welding properties under the conditions usually found in metal-cutting operations, high unit pressure, low sliding motion, high spot temperature.  

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Last modified: January 08, 2002