10 Fun Facts about Cutting Fluids
Category: Manufacturing Technology • Jan 7, 2021
Metalworking fluids cool, lubricate, and soothe — wait, that last one is shaving cream. But seriously, this lifeblood of metal removal is essential for optimizing cut quality, cutting speed, and tooling life. To non-machinists, the cutting operation probably looks like skim milk and flying chips, but there’s a lot of science and history behind it. Here are ten facts to help you know more metalworking fluid better.
- Multi-purpose. Metalworking fluids serve many functions, including cooling and lubricating the workpiece and cutting tool. They also remove the metal chips or swarf from the cutting zone. In a typical operation, resistance from shearing the atoms of the part being cut generates about two-thirds of the heat. The friction of the cutting tool causes the remainder. Adequate lubrication essentially changes the shear angle to produce a thinner chip, which further reduces heat from friction.
- Cutting fluid conundrum. Water works well as a coolant. It has the highest heat capacity (the ability to absorb energy) of any liquid. No wonder humans sweat to cool themselves. However, water has two drawbacks. First, the O in H2O produces iron oxide (rust). Second, water runs right off the part because of its low viscosity, so it won’t leave a film to fight friction. Oil lubricates well because it sticks around. Unfortunately, oil cools poorly because the heat stays localized, potentially making it flammable.
- Back in the day, cutting fluid could turn rancid. Consider this: A 1921 “technologic” paper on cutting fluids from the National Bureau of Standards stated that "lard oil is the cutting oil par excellence.” Sources of lard oil include neat’s-foot oil (“neat” was an Old English term for cattle), wool fat (lanolin) horse oil (highly popular in Asian skincare products), and sperm oil (until the 1972 Endangered Species Act banned whaling.
- The breakthrough. Fast forward to IMTS 1947, when the Cincinnati Milling Machine Co. launched a breakthrough synthetic fluid that was the first to combine the cooling capacity of water with the lubricity of oils, promoting higher speeds and better tool life. At the time, IMTS required all machines’ paint to be the same shade of “machine tool gray.” Cincinnati capitalized on this fact by coloring its synthetic fluid a bright pink and offering it to all the exhibitors — talk about early viral marketing!
- Cutting fluid cocktail. Metalworking fluids require a host of additives, such as emulsifiers, anti-microbial pesticides, anti-oxidants, defoamers, and stabilizers that enhance lubricity and inhibit rust. Each metal removal process, metal type, and application benefits from a specific fluid type and blend. Choosing the right one is like ordering a fancy coffee drink. If you’re a black coffee drinker who never ordered a half-caf non-fat skinny triple grande caramel macchiato from Starbucks and felt like you need an interpreter, you get the point.
- Oil and water do mix. Think about mayonnaise and salad dressing. The chef adds emulsifiers, such as egg yolks or mustard, then whips them to create fine stable dispersion. An emulsifier is technically a surface-active agent, or surfactant, that acts as a border between two immiscible liquids, such as oil and water.
- Just add water. There are multiple types of water-miscible fluids in addition to straight oil, including: soluble oil, semi-synthetic, and synthetic fluids. These formulations, sold as oil-based concentrates, include emulsifiers to stabilize the concentrate when diluted with water. Lean mixtures (more water-less oil) provide better cooling but less lubrication, and vice-versa for rich concentrations. Some companies are manufacturing cutting-edge fluids like microemulsions and neo-synthetics that defy current product categorization.
How much water to add? That depends on whether a shop is OK with “good enough” or wants more control over the costs associated with metalworking fluids by evaluating specific applications.
- What type of water to add — tap, distilled, deionized, or reverse osmosis? Anyone who ever stained a shirt because their steam iron spit rusty water (hello again, iron oxide) knows water choice makes a difference. Choose distilled water for steam irons but use chemically pure water produced by deionization or reverse osmosis for cutting fluids. Similar to your iron, minerals in the water eventually cause problems.
- Oil in last. As a rule, add concentrates to the water, then oil, and mix thoroughly. One way to remember the proper sequence of addition is to remember OIL, meaning “oil in last.” There’s nothing funny about this.
- Hold the foam. Fluids can foam when agitated. McDonald’s adds an antifoaming agent (polydimethylsiloxane, a type of silicone) to its fryers to mitigate foaming that can cause hazardous oil splashes. Gas relief products also contain anti-foam material. In metalworking, unstable foam (big bubbles that break quickly) isn’t a problem. Stable foam (small bubbles) that blanket the fluid can recirculate suspended swarf and interfere with filters. To fight the foam, metalworking fluid manufacturers add defoamers, dispersants, stabilizers, and a pleasing lilac scent (OK, maybe not lilac, but they do add scent).
Knowledge of metalworking fluid functions goes back to Leonardo da Vinci’s set-ups to test friction, the wheel’s discovery, and the first greased axle. Today, when shops need to squeeze every minute of uptime out of a 24-hour day, the selection of metalworking fluids deserves as much attention as the choice of machine tool, tooling, speeds, and feeds.
Learn more about providers of high-quality cutting fluids exhibiting at IMTS.