Cutting fluid plays a crucial role in machine tool processing.

In the machine tool industry, cutting fluid has always played a vital role. It is used regardless of the material being machined. The history of human use of cutting fluid can be traced back to ancient times. People knew that adding water could improve efficiency and quality when grinding stone, bronze, and iron tools. In ancient Rome, olive oil was used when turning castings for piston pumps, and in the 16th century, tallow and water were used to polish metal armor. The application of water and oil in metal cutting began in 1775 with the development of a boring machine by John Wilkinson in England for machining the cylinders of Watt's steam engine. After a long period of development, by 1860, various machine tools such as lathes, milling machines, planers, grinders, gear cutters, and thread cutters emerged, marking the beginning of large-scale application of cutting fluid.

Cutting fluid is an industrial liquid used in metal cutting and grinding processes to cool and lubricate cutting tools and workpieces. It is scientifically formulated with various high-performance additives, possessing excellent cooling, lubrication, rust prevention, degreasing and cleaning, corrosion prevention, and easy dilution properties. It overcomes the shortcomings of traditional soap-based emulsions, such as odor in summer, difficulty in dilution in winter, and poor rust prevention. It also has no adverse effects on lathe paint and is suitable for cutting and grinding ferrous metals, making it a suitable grinding product currently available.

Without cutting fluid, machine tools and workpieces are prone to deformation and rust at high temperatures. Therefore, cutting fluid must be used during machining to avoid these problems. Below, we will introduce the advantages and disadvantages of various cutting fluids:

There are many types of cutting fluids. Today, we will discuss some commonly used ones on the market: emulsions, chemically synthesized cutting fluids, and cutting oils with extreme pressure additives. We will focus on these three common types.

Advantages and Disadvantages of Emulsions

Advantages: The biggest advantages of emulsions are strong heat dissipation, effective workpiece cleaning, and high cost-effectiveness, which is beneficial to operator hygiene and safety. Therefore, most metal processing plants choose to use emulsions for processing. Except for particularly difficult-to-machine materials, emulsions can be used for almost all light and medium-load cutting processes and most heavy-load processes, especially for all grinding processes except for complex grinding such as thread grinding and groove grinding.

Disadvantages: The biggest disadvantage of emulsions is that they are prone to bacterial and mold growth, causing the effective components in the emulsion to decompose, resulting in foul odors and spoilage. Therefore, low-toxicity organic bactericides are generally added.

Advantages and Disadvantages of Chemically Synthetic Cutting Fluids

Advantages: Their advantages include high cost-effectiveness, rapid heat dissipation, strong cleaning ability, and excellent workpiece visibility. Workpiece dimensions are very easy to control, and their stability and corrosion resistance are stronger than emulsions.

The disadvantages are: insufficient lubrication, easily causing adhesion and wear of moving parts in the machining center. Furthermore, the viscous residue left by chemical synthesis can affect the movement of machining center parts and cause corrosion on overlapping surfaces.

Advantages and disadvantages of cutting oils with extreme pressure additives:

Advantages: Generally used for cutting speeds not exceeding 60 m/min, effective for machining any material.

Disadvantages: During high-speed cutting, due to the high heat generation, oil-based cutting fluids have poor heat dissipation, leading to excessively high temperatures, smoke, or even fire. Therefore, excessively high temperatures can negatively impact the workpiece, causing deformation and poor machining accuracy.

Differences between various cutting fluids:

Oil-based cutting fluids have better lubrication but poorer cooling. Water-based cutting fluids have relatively poorer lubrication but better cooling compared to oil-based fluids. Slow-speed cutting requires strong lubrication; generally, cutting oil is used when the cutting speed is below 30 m/min.

Cutting oils containing extreme pressure additives are effective for machining any material at speeds not exceeding 60 m/min. During high-speed cutting, the high heat generation and poor heat transfer of oil-based cutting fluids can lead to excessively high temperatures in the cutting zone, causing the cutting oil to produce smoke and even catch fire. Furthermore, the high workpiece temperature can cause thermal deformation, affecting machining accuracy. Therefore, water-based cutting fluids are more commonly used.

Emulsions combine the lubricity and rust prevention of oil with the excellent cooling properties of water, offering both good lubrication and cooling. This makes them highly effective for high-speed, low-pressure metal cutting processes that generate significant heat. Compared to oil-based cutting fluids, emulsions offer advantages such as better heat dissipation, easier cleaning, economic efficiency through dilution with water, and improved operator hygiene and safety, making them more appealing to users. In practice, except for particularly difficult-to-machine materials, emulsions can be used for almost all light and medium-load cutting processes and most heavy-load processes. Emulsions can also be used for all grinding processes except for complex grinding such as thread grinding and groove grinding. The disadvantage of emulsions is that they easily promote the growth of bacteria and mold, causing the effective components in the emulsion to decompose chemically, resulting in foul odor and spoilage. Therefore, low-toxicity organic bactericides should generally be added.

Chemically synthesized cutting fluids have the advantages of being economical, having rapid heat dissipation, strong cleaning properties, and excellent workpiece visibility, making it easy to control machining dimensions. Their stability and resistance to corrosion are stronger than emulsions. Poor lubrication will cause adhesion and wear of moving parts in machine tools. Furthermore, the viscous residue left by chemical synthesis can affect the movement of machine parts and cause corrosion on overlapping surfaces of these parts.

Water-based cutting fluids should generally be used in the following situations: locations where oil-based cutting fluids pose a potential fire hazard; high-speed, high-feed cutting where the cutting zone exceeds high temperatures, produces intense smoke, and poses a fire hazard; situations requiring water-based cutting fluids due to the flow of preceding and following processes; and situations where it is desirable to reduce pollution and dirt around the machine tool caused by oil splashing, mist, and diffusion, thereby maintaining a clean operating environment.

From a price perspective, for some easily machinable materials and workpieces with low surface quality requirements, general water-based cutting fluids are sufficient and can significantly reduce cutting fluid costs.

When tool durability plays a significant role in the economic efficiency of cutting (e.g., expensive tools, difficult tool sharpening, long loading and unloading times); situations where high machine tool precision is required and water contamination is unacceptable (to prevent corrosion); situations where the machine tool's lubrication and cooling systems are prone to cross-contamination; and situations where waste fluid treatment equipment and conditions are unavailable. In these cases, oil-based cutting fluids should be considered.