Straight oils are generally considered quite versatile and can be made up of mineral, vegetable, or animal oils. Straight oils are not diluted and are usually used in heavy-duty machining processes. Extreme pressure additives are often used to formulate straight oils to enhance performance.

Emulsifiable or soluble oils are designed to be used as both coolants and lubricants, particularly for cutting and grinding applications. These oils inhibit welding between the cutting tools and the workpiece while also preventing tool wear. Soluble oils contain pure mineral oils (about 40%-70% of the total formulation) as well as emulsifiers.

Semi-synthetic fluids contain anywhere from 5%-30% mineral oil and are diluted with 30%-50% water. Semi-synthetic fluids often contain high concentrations of additives and are designed to cool and lubricate. They are typically used in all metal cutting processes.

Synthetic fluids contain such synthetic base stocks as esters or polyalphaolefins and are used to continually wet the workpiece to prevent mist and smoke, particularly during the grinding process. A significant drawback of synthetic fluids, though, is that they are not as capable of preventing corrosion and can be harsh on the tool over time.

Some of the most common additives used in metalworking fluids are corrosion inhibitors, such as calcium sulfonate, boric acid, fatty acid soaps and amines; extreme pressure additives, like chlorinated paraffins and phosphorous derivatives; anti-mist agents, such as polyisobutylene polymers; emulsifiers, like triethanolamine and salts of fatty acids; dispersants; and odorants. 

Any metalworking fluid that contains any amount of water will incorporate some type of biocide, which is generally considered an additive as well, because water is conducive to the growth of microorganisms. The presence of microorganisms can result in health and safety hazards, separation of emulsions, creation of sediments, formation of biofilms, and metal corrosion, among other things. 

Metalworking Processes and their Lubricants

There are a number of processes used to manufacture metal products. What are some of the most commonly used processes, and what kinds of metalworking fluids do they require? 

Milling. Milling operations include vertical and horizontal milling, boring, honing, hobbing, and drilling processes. Mills contain a rotating tool in which the workpiece being manufactured is secured to a movable table. Depending on the type of cut desired, the tool’s cutter slices from the end or the side. This type of operation can be a final process used to meet tight tolerances (i.e. honing operations) or can remove large portions of metal (i.e. hobbing operations). Milling operations typically employ soluble coolants and lubricants. 

Turning. Classified as a metal cutting operation, turning operations are carried out using screw machines, turning centers and lathes. In these types of machines, the workpiece rotates at low-to-high speeds, and a straight oil or soluble coolant is sprayed on the rotating workpiece and tool to keep both the workpiece and the tool cool. The metalworking fluid in these operations is also tasked with lubricating the cutter, removing chips and swarf, and preventing chip whip and marring of the workpiece surface.

Grinding. Grinding applications include surface grinding, profile grinding, centerless grinding as well as cylindrical grinding. This process is distinct from cutting because it uses an abrasive wheel to remove material instead of simply cutting it away with a tool. In most grinding operations, as with milling, the workpiece is affixed to a movable table with a grinding wheel turning at low-to-high speeds. Most grinding operations use soluble oils at low pressures to cool the surface, flush dust and chips away from the grinding wheel and provide a moderate amount of lubrication.

Heading and stamping. Heading and stamping operations are considered forming operations. In these processes, a piece of metal is repeatedly struck by one or many dies until it is formed into the desired shape. These operations require a great deal of pressure, which generates significant heat and mist. Soluble, neat and synthetic oils can be used for these applications. 

Casting. A cast part is formed by pouring molten metal into a die mold and letting it cool. Aluminum, zinc, and magnesium are the most commonly used metals in die casting. A water-based die lubricant is sprayed onto the die between each casting operation to create a barrier between the die and the cast part. This allows the cast part to be easily removed from the die. This lubricant essentially consists of paraffin wax and water, according to Donaldson Filtration Solutions. 

Drawing. Drawing operations require metal rods to be pulled or drawn through a series of dies that reduce the rod in diameter to form wire-like material. These operations generate heat and commonly use a soluble, wax-based or soap-based lubricant. Straight oils can also be used during this process. 

Health and Safety

It is no secret that metalworking fluids have long been the subject of debate when it comes to occupational health and safety concerns. While the industry has made great strides in ensuring that metalworking fluids are safe for those who work with them, some argue that there are still significant risks associated with them. 

According to Great Britain’s Health and Safety Executive, exposure to metalworking fluids can pose a variety of health risks. These include dermatitis and other irritation to skin as well as lung diseases, like occupational asthma, occupational hypersensitivity pneumonitis and chronic bronchitis. 

In past years, work that required repeated exposure to metalworking fluids carried with it an increased risk of developing several types of cancer, too. However, changes to metalworking fluid formulations seem to have significantly decreased that danger. “Research has shown that [the development of cancer] was due to the use of unrefined oils, which contained carcinogenic substances,” HSE said. “Modern neat oils are highly refined and do not pose this risk of cancer.”

So how can these safety risks be mitigated?

Regardless of metalworking fluid formulation, proper fluid handling is paramount to worker safety. There is the possibility that such carcinogenic substances as polycyclic aromatic compounds (PCAs) and nitrosamines can be formed under certain in–use conditions. For instance, PCAs can be formed when metalworking fluids formulated with mineral oils are used for long periods of time at high temperatures. This scenario can be avoided by changing out the metalworking fluid at the supplier’s suggested interval. 

Metalworking fluid systems that contain any amount of water can become contaminated by harmful bacteria. Because of this, both systems and fluids should be carefully monitored. HSE suggests regular checks of fluid quality, including fluid concentration and pH. This can be done in several ways. For example, microbiological dip slides are a relatively easy way to gauge bacterial contamination. 

Dip slides are plastic carriers coated with sterile culture mediums. They can be dipped into metalworking fluid and then incubated to facilitate microbial growth. Results for this type of test are expressed in colony-forming units per milliliter of fluid. HSE’s recommended threshold for bacterial growth are illustrated in Figure 1. 

Figure 1. Managing Bacterial Contamination in Metalworking Fluids

How else can metalworking operations be made safer? Fred Passman, a certified metalworking fluid specialist and STLE fellow, said in an article published by MSC Industrial Supply Inc. that mist collection is the number one issue when it comes to the safety of metalworking fluids. 

Mist is defined by Donaldson Filtration Solutions, a leading manufacturer of industrial filtration solutions, as “a liquid particle 20 microns or smaller.” 

The United States Occupational Safety and Health Administration’s OSHA Standard 1910 Subpart Z-Toxic and Hazardous Substances for General Industry dictates that the limits for permissible levels of air contaminants should be no greater than 5 milligrams per cubic meter over an 8-hour period. However, some critics think that OSHA’s standard is too lenient, and there are currently petitions to lower the limit to 0.5 milligrams per cubic meter over the same period. 

How can mist be controlled? 

There are a variety of mist collection technologies on the market right now that can aid in meeting safety standards. These systems can be installed on mills, drills and lathes as well as on grinding, casting, and other metal cutting and forming machines to collect mist before it reaches machine operators.  

Sydney Moore is managing editor of Lubes’n’Greases magazine. Contact her at Sydney@LubesnGreases.com