At the beginning of my career, I had 2 choices available, Kerosene or cutting oil. Each had their purpose for different materials, and each had a horrible smell that permeated your clothes and into your pores. Time passed and coolant has mostly replaced the previous options. Coolant is a water-based mixture of who knows what that facilitates making chip piles. I was never a big fan of Kerosene to begin with and gladly used coolant as a replacement; I always felt like a torch waiting to be ignited. Cutting oil on the other hand still has a place in my heart for machining. Unlike coolant, which needs constant attention filtering, refilling, and checking mixture levels, cutting oil stands apart. Cutting oil is just as it sounds but with some huge upsides. Maintenance of cutting oil is all but forgotten since it doesn’t evaporate. A great option for a machine which will be neglected in a corner until it is needed. Have an older machine with leaks your choice cutting fluid onto and into the precision ways and ball screws? Cutting oil will mix with your way lube and lubricate as needed. Of course, every upside has a down.
What makes a fluid a good choice for cutting materials? Lubricity, cooling, and chip control are all you need to know which give use 3 options: cutting oil, coolant, and air. Imagine a knife cutting a piece of wood; the metal blade trying to slice through. That is a simplistic idea of how a lathe and mill function, but in a lathe or mill we use some type of liquid. This liquid has a primary purpose to prevent the metal knife blade from ever touching the wood. The fluid creates a cushion and acts like tiny balls, and compresses against your tool and the wood. It is the fluid which actually cuts.
Cutting oil has great consistent lubricity and is a great choice for long tool life in horrible conditions that doesn’t need constant attention. It does however absorb and retain heat which is not ideal, and an ability to ignite if aerosolized.
Coolant has taken great strides in lubricity to compete, and even out-perform its predecessor. Being water-based means it can quickly evaporate to remove excess heat and keep your tools cool. Cool tools increase tool life. Of course the entire skill of machining is to ensure the heat is transferred to the chip and not the remaining material or cutting tools. The main benefits of coolant are increased speeds and feeds without a flammability rating. Their downsides are the constant maintenance, bacterial growth, and costs to keep the coolant healthy. I will mention to avoid using a paraffin coolant because it can clog up your pumps and lines, a lesson dearly paid for in full.
Lastly, we have air which is seldom a choice for me. As you can imagine, air has no lubricity and relies entirely on the tool’s survivability, but “can” excel at cooling. “Can” meaning if using a method to use super chilled air. The main purpose of using air is chip control. Evacuate the chips quickly from the cutting area. Air is slowly being phased in across America as tooling and machining techniques improve, and “green” practices envelope the country.
I feel that i should clarify something about air, and all machining fluids. One of the goals to machining techniques is specifically to avoid “double-cutting” a chip. When a chip is formed it needs to be ejected from the area immediately. If the chip enters the cutting area and becomes cut again, that is a double-cut; a cut that was not needed. When this happens, surface finish can be damaged, tool life IS degraded, and sometimes, tools will break. All the fluids are helping to evacuate the chips from the area, including air. Sometimes chip evacuation is all you need to cut material.