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Lubricant in close-up of a turning operation.

Choosing the Right Coolants and Lubricants for Machining

Dec. 6, 2023
Cutting fluids are essential to the process, and there is a compound for every application, material, and type of operation. The right choice depends on many factors.

Cooling and lubricating the workpiece using cutting fluids is essential to the machining process. There is a formulation for every application, material, and type of operation, from copper plumbing fittings to titanium jet engines and airframes. The right choice for a specific type of cutting fluid depends on many factors, so let’s dive in.

What is a cutting fluid?

Cutting fluid is a type of coolant and lubricant used in machining operations. It cools the workpiece to prevent part warping and coarse surface finish while providing lubricant to reduce friction between the chip and cutting edge.

Machining generates a lot of heat due to friction between the toolhead and the workpiece, making the cooling and lubricating properties of cutting fluid ideal for relieving this stress. Fluid also can flush chips from the cutting edge, which otherwise may build up and interfere with the machining process.

In some instances it's possible to machine without cutting fluid, such as when creating small parts or working with brass or cast iron. However, using cutting fluid increases part quality and prolongs the service life of cutting tools and machinery, making fluids indispensable in almost any machining operation.

Properties of cutting fluids

When choosing a cutting fluid for your machining operation, consider the following properties:

  • Heat absorption capacity
  • Flash point
  • Viscosity
  • Lubrication
  • Non-corrosiveness
  • Odorless
  • Transparency
  • Stability

Ideal cutting fluids have a high heat absorption capacity and flash point, low viscosity, and good lubrication. They should be non-corrosive, to prevent damage to the equipment and parts; odorless; and transparent, to ensure visibility. High stability is also important, meaning it is less likely to oxidize and decompose.

Types of cutting fluids

Cutting fluids can be divided into distinct categories based on their chemical makeup. This makes them suitable for a specific range of processing conditions and workpiece materials.

Neat oils, also called straight or mineral oils, are based on refined petroleum and typically are used for heavy-duty machining at lower speeds. These oils guard against rust and contain extreme pressure or anti-wear additives to optimize performance.

The downside of neat oil is that it is harder to clean, relatively more costly, and cannot be used on copper due to corrosive effects.

Vegetable oils, as opposed to paraffin and naphthenic oils, have a positive environmental effect due to their biodegradability and disposal. However, they leave an unpleasant smell while decomposing. In high-temperature applications, vegetable oils tend to produce smoke and leave a residue on the tool and workpiece.

Synthetic cutting fluids are water-based polymer solutions that do not contain any oils. They offer great corrosion resistance, high-temperature stability, cooling, and lubricity, making them suitable for light machining and high-heat, high-speed operations like grinding and turning.

Semi-synthetic cutting fluids are emulsions of soluble oil and synthetic, water-based fluids. These fluid types are popular as they combine the lubrication benefits of neat oils with the superior cooling quality of synthetics.

Disadvantages of semi-synthetics include the potential to foam (depending on the hardness of the water), cause dermatitis and other skin reactions, and may lead to unexpected contamination with water of differing pH balances.

You can reduce the likelihood of foaming and contamination with active fluid monitoring, such as concentration and contamination checks with a refractometer.

Choosing fluid for the material

The workpiece material selection is a significant factor when choosing cutting fluid. Here are fluid recommendations based on common materials:

Cast iron (and steel) is among the most manageable materials to machine because it is soft and contains graphite and copper, which make it self-lubricating. So, cutting fluid is not usually needed. For high-speed machining, using compressed air is a good method for cooling and chip removal.

Copper and brass are also easy to machine and rarely need the addition of cutting fluid. You can use cleaning liquids to prevent metal particles from getting stuck.

Aluminum is a relatively soft, malleable material, making it perfect for machining processes across a wide range of industries and applications. However, this softness makes aluminum prone to galling, warping, and fusing, and it causes aluminum chips to stick to the tooling. Reduce or avoid these effects by using a cutting fluid with high lubrication properties, like a vegetable oil.

Stainless steel has a high hardness, typically requiring a synthetic for its superior coolant properties.

There are high-performance coolants for specific materials that are yet harder to machine, like magnesium, titanium, and nickel-based superalloys such as Inconel.

Choosing coolant, lubricant for the task

Choosing the right cutting fluid based on the machining operation to be performed calls for the operator to understand the specific requirements of each process. Here's a guide to selecting cutting fluids for various common machining operations:

- The best fluids for turning and milling are emulsifiable oils or semi-synthetic fluids. They provide good lubrication for moderate to heavy cuts and are effective in cooling, which is important for high-speed operations.

-  Straight oils or heavy-duty emulsifiable oils are ideal for drilling and tapping. These cutting fluids have excellent lubrication for reducing friction and preventing tool breakage in these high-stress operations.

-  The high-level cooling capabilities of synthetic and semi-synthetic fluids make them great for grinding operations. This is because grinding produces a lot of heat, making cooling critical to preventing thermal damage to the workpiece. Note that the fluid should be low-viscosity for effective heat dissipation.

-  High-viscosity or heavy-duty emulsifiable oils provide a strong lubricating film, allowing them to withstand the high pressure of broaching and prevent tool wear. The fluid also helps in flushing away the metal chips from the workpiece.

Optimizing with technology

Machining operations are rapidly evolving with technological advancements driving new and improved processes and outcomes.

Recent supply chain statistics reveal that 78% of industry leaders reported actively seeking out new technologies to increase operational efficiency in new product development. This surge in technology adoption is transforming traditional machining practices, enabling higher precision, faster production times, and reduced costs.

Innovations in automation, advanced computer numerical control (CNC) systems, and artificial intelligence (AI) are examples of this progress. For example, automatic fluid control systems streamline machining operations by analyzing variables like pH, EC hardness, oil concentration, and temperature to maintain the optimal mix for the cutting fluid.

These technologies not only improve the efficiency and quality of machining operations but also introduce possibilities in the design and manufacturing of products, making manufacturers more agile and responsive to market demands. As a result, those who embrace these technological advances are finding themselves at a significant competitive advantage.

Doug Walker is a digital marketing specialist at Fictiv, a manufacturing platform for sourcing design and engineering teams, to streamline prototyping and accelerate product development. Contact him at [email protected]

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