Machinists use a variety of machines to create precision parts, which are typically fabricated from metal. The machines they use to create these parts, such as a milling machine, use metal tool bits. These operations are done at a high rate of speed, and the metal-on-metal fabricating process causes a tremendous amount of friction and heat. For this reason, they can only be operated with coolant or lubricant. Here is a look at the most commonly used milling machine coolants and their advantages and disadvantages.
Using an oil that is composed of 100-percent petroleum does not necessarily mean it doesn't have additional additives — it just means no water is used in addition to the oil. Straight petroleum oil is best used on parts that will be machined at a slower speed. Petroleum provides excellent lubrication, which not only protects the part being machined but the cutting and tooling dies as well, which can be expensive. Because it has no water added, there is no concern about bacterial growth, which can be damaging both to the parts being machined as well as the operator. An additive, such as chlorine, can be added for some metals.
Because this oil-based coolant is 100-percent petroleum, it should be used with caution on parts that are machined at a higher speed. This is because there is the increased potential for things getting hot enough to spark a fire. Another reason 100-percent petroleum is best used on parts that are machined at a slower speed is that it can become airborne, which isn't good for the machinist to inhale.
Soluble Oil Lubricants
Petroleum oil can be used in a lesser concentration and mixed with water. While oil and water don't typically mix, they can be mixed by a special process that creates an emulsion. This emulsified substance is similar in appearance and substance to liquid soap. It is slippery, providing lubrication, but it will also coat the metal and cling to it as it is machined.
Adding water means corrosion and bacteria are a potential issue, so an additive is added to prevent this. Soluble oil lubricants can also make for a slippery and hazardous work area if proper cleaning and safety measures aren't taken.
Most machine shops opt for synthetic coolants. These contain no petroleum, don't cause a fire hazard, and can be used for virtually all machining operations. They can be mixed with water, support additives for specific machining needs, and provide excellent lubrication. The machinist must still take care to not breathe in fine particulate matter that becomes airborne, but in modern machine shops, the ventilation system usually eliminates this concern.