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Understanding industrial cutting fluids

The fluids used in metalworking are designed to lubricate and cool. Fluids are vital to optimise a wide range of processes, assisting cutting speed, cut quality and tooling service life.

Multi-purpose, metalworking fluids must serve multiple functions. Along with acting as a lubricant and coolant for a machine’s cutting tool and workpiece, they must also clear the cutting zone of any metal shavings and swarf. In a conventional operation, the resistance produced by shearing atoms of a part will generate around two-thirds of heat production. Friction from the cutting tool will then supply the remainder. Sufficient lubrication crucially changes the dedicated shear angle, so it produces a far thinner metal chip. This efficiently reduces heat from the friction occurring even further.

A question of cutting fluid

Water is well known to be an effective coolant. This is because it has the highest heat capacity of any known liquid. However, water is not without its drawbacks as a lubricant. When water is exposed to air and metal that contains iron or one of its alloys, like steel, iron oxide – or rust – can develop.

Water is also an incredibly low-viscosity lubricant, which means it runs directly off of parts it is applied to, making it unable to create a protective film to battle friction and wear. Unlike water, oil lubricates well, as it can adhere to surfaces; however, it cools poorly as heat is retained locally, making it potentially flammable.

Some of the first cutting fluids used were from animal by-products. Lard oil was believed to be an excellent cutting fluid, as was horse oil, wool fat and sperm oil from the whaling industry, before it was banned to protect endangered species.

Synthetic cutting fluid

In 1947, the Cincinnati Milling Machine Company in the USA launched a synthetic fluid, marking the first time a product was formulated to combine the water’s cooling capabilities and oil’s lubricity. This new product promoted higher operating speeds and enhanced tool life.

Modern metalworking fluid formulations

To perform specific roles and suit different applications, today’s metalworking fluids include a whole host of additives, like emulsifiers, antioxidants, anti-microbial pesticides, stabilisers and defoamers to help improve lubricity and prevent rust and other contaminants. They are formulated by considering the metal removal process required, metal type involved and the application advantages from a specific blend and type of fluid.

Water and oil don’t naturally mix. An emulsifier is added to fluids, acting as a border in between the two liquids, so that both their useful properties can serve an application simultaneously. There are a wide range of water-miscible fluid types, from synthetic and semi-synthetic fluids to soluble oil. These products are typically sold as concentrates that are oil-based and include dedicated emulsifiers that can stabilise the concentrate when it’s diluted by water. Products known as lean mixtures that include less oil and more water will provide less lubrication, but have greater cooling properties, while rich concentrations will offer the opposite effect.

Oil is always added last to these formulations, before it is mixed in thoroughly to make the cutting fluid.

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