Selecting Coolants: Why and How

Editor’s Note: This is the first in a series of columns on coolant-management issues written by William Sluhan, chairman of the board of directors and CEO of Master Chemical Corp., Perrysburg, Ohio. Mr. Sluhan has 29 years of experience in the design, implementation, and evaluation of coolant-management programs.

	Courtesy of Master Chemical Corp.

Since my father, Clyde Sluhan, founded Master Chemical Corp. more than four decades ago, I have witnessed the birth and growth of the cutting-fluid industry. In this series of Articles, I hope to impart some of my observations of what has happened and visions of what’s to come in fluid management. In this, the first installment, I’ll review the decisions and tradeoffs involved in choosing a fluid for a specific application.

To reasonably evaluate the choices available in cutting and grinding fluids, it’s good to remember why you’re using coolants in the first place. Metal-removal techniques generate friction, which in turn generates heat, and excessive heat in metal removal is always a detriment. Consequently, to provide proper size control and desired shape and finish, cutting and grinding fluids must properly fulfill two important cutting functions: lubrication and cooling.

Lubrication is important because external friction, or metal-to-metal contact, generates approximately a third of the heat that results from cutting. Internal friction, or the resistance of the metal atoms to movement when the metal is deformed in the shear zone, is responsible for generating two-thirds of the heat. When the cutting zone is properly lubricated, cutting becomes more efficient, thereby reducing external friction and, to a lesser extent, lowering internal friction.

To reduce internal friction, chlorine, sulfur, and phosphorous atoms in straight oil and in extreme-pressure (EP) coolant additives penetrate the microcracks in metallic surfaces, thereby preventing the rebonding of metal atoms displaced in the cutting process and reducing the power needed to form a chip. Also, by lubricating the chip/tool and tool-flank/cut-surface interfaces, the angle of the shear plane increases and, as a result, the area of the shear plane decreases. As the shear-plane area decreases, the power required to form a chip decreases and so does the heat generated.

The cooling effect provided by cutting and grinding fluids is necessary to remove heat from the tool, chip, and workpiece. This cooling extends tool life primarily by preventing tools from exceeding their critical temperature range while in the cut. Beyond the critical temperature, tools soften and wear rapidly and fail to meet tolerances for surface finish and part size. Cooling effects also help keep the part thermally stable, aiding in the control of part size. With so many parts today being finish-machined (e.g., engine blocks and transmission cases), without subsequent processing, the fluid’s ability to keep the part dimensionally stable during machining is all the more critical. In these cases, there is no offset for heat-treating to fall back on.

The type and degree of lubrication and the level of cooling required for various metal-removal operations vary according to the kind of operation; the rigidity of the part; the type of metal and its hardness and microstructure; the tool material and geometry; and the speed, feed, and DOC selected.

No one type of fluid will provide optimum lubrication and cooling qualities for every metalworking operation. In fact, lubricating and cooling are at opposite ends of the spectrum. Take water, for example. It’s the best coolant of all known substances. It has the highest specific heat (the ability to absorb a large quantity of heat with only a small increase in temperature), it has the highest heat-transfer rate, and it has the highest heat of vaporization. However, it’s a terrible lubricant. Water also rusts iron and steel, and it has a high surface tension (it doesn’t wet and penetrate well).

Conversely, oil is the best known lubricant, but it provides very poor cooling properties and is flammable. Since neither water nor oil is a satisfactory cutting fluid by itself, fluids must be formulated to exhibit as many benefits and as few drawbacks as possible. Water-soluble fluids with appropriate additives often meet or exceed the machining and grinding results of straight oils. Water does the cooling while the additives provide the lubrication and corrosion inhibitors.

Fluid Types

Cutting and grinding fluids can be divided into four categories, each with its own set of properties.

Chemical fluids come in two distinct types, true-solution and surface-active fluids. The presence or absence of wetting agents determines which category a fluid belongs to.

True-solution fluids are water solutions of organic alkaline compounds such as triethanolamine and a corrosion inhibitor such as boric acid. They generally are water-clear and nonfoaming, tend to reject tramp oils, and have excellent heat-transfer properties. These fluids are essentially a kind of water that doesn’t rust metal. Unfortunately, like water, they have very poor wetting properties. True-solution fluids are usually restricted to high-heat operations, such as surface grinding and high-speed turning with carbide, where good cooling and low foaming characteristics are paramount.

Surface-active, or synthetic, fluids offer good cooling, wetting, and EP lubricating properties that are best suited for heavy-duty cutting and grinding applications, especially on tough, difficult-to-machine, and high-temperature alloys.

The EP additives are generally chlorine, sulfur, or phosphorous compounds that react with metals at high temperatures and form organic-metallic compounds of low shear strength. These compounds function as lubricants at extreme temperatures and pressures that hydrodynamic and boundary lubricants cannot tolerate.

They usually incorporate corrosion inhibitors similar to those found in true-solution fluids plus anionic and nonionic wetting agents. The wetting agents are soap or detergent-like compounds. They dramatically lower the fluid’s surface tension, making it more lubricious, but at the same time they also are responsible for the surface-active fluid’s tendency to foam and absorb or emulsify tramp oils. These agents tend to form tight, stable, small-bubble foams because of their low surface tension whenever the fluid is aerated. This small-bubble foam is highly persistent.

Like those in true-solution fluids, additives in surface-active fluids are composed of sizable amounts of soluble organic materials. These cause high biochemical oxygen demand (BOD) and chemical oxygen demand (COD) when the fluids are introduced into municipal sewage-treatment systems. Because these organic compounds are water soluble, they do not respond well to treatment normally used for oily waste waters or to ultrafiltration. While they can all be treated by either of these processes, the clear water produced can still have high levels of BOD and COD. The bacteria that break down these materials use up a lot of oxygen, and the resulting oxygen-depleted water can kill fish and aquatic plants. As a result, surcharges may be levied or restrictions applied to the amount being introduced into the sewage system in compliance with federal law.

Emulsion fluids, or soluble oils, do not cool or lubricate as well as chemical fluids. However, because they are composed largely of oil and oil-like materials, they tend to leave a protective and lubricating film on the machined part and the moving parts of the machine tool.

These fluids also are easy to treat for disposal or recycling. That’s because soluble oils are not soluble in water (they merely mix with water), and when the fluid is no longer usable it responds well to both chemical and membrane oily wastewater treatments. The clear water usually has low BOD and COD levels and is readily accepted by most sewage systems. The separated oil, like straight oil, can be re-refined to make new lube feed stock and can be reblended to make “new” soluble oils and cutting oils.

Semichemical fluids, or semisynthetics, essentially are combinations of the chemical surface-active fluids and emulsions. As such, they tend to be broad-application, medium-duty fluids. These concentrates are unable to withstand the freezing temperatures that they may encounter in winter shipping, because they tend to separate into two or more layers that can’t be reblended when the fluid thaws. They also exhibit the same difficulties in disposal that are common among chemical fluids.

The relative strengths and weaknesses of the water-miscible coolants (water mixed with additives) described above are listed in the accompanying table.

Straight (nonemulsifiable) oils provide the best lubrication and poorest cooling properties of fluids commonly used today. Due in part to their general messiness and potential fire and health hazards, their use is restricted mainly to low-speed, heavy-duty cutting and grinding operations where water-miscible fluids cannot provide sufficient lubrication, or to use on older machines that were designed to utilize only straight oils.

PROPERTIES OF WATER MISCIBLE COOLANTS 

*BOD = biochemical oxygen demand
COD = chemical oxygen demand

Selection Criteria

When choosing the best fluid for a specific application, consider both cutting and noncutting functions. Cutting or grinding functions involve lubricity and cooling qualities that provide proper size control and desired shape and finish. Noncutting functions include corrosion inhibition, favorable fluid residues, nonflammability, effective filterability, low toxicity, and recyclability.

Cutting and grinding fluids typically are measured against five key criteria:

Machinability is the fluid’s ability to generate the desired shape, size, and finish on work materials, while achieving the necessary tool or wheel life. The fluid must aid the manufacturer in producing his parts with the proper dimensions, shape, and finish at required production rates while providing adequate tool or wheel life. Since each part is different from every other part, it is theoretically possible (with plenty of experimentation) to determine an optimum fluid and concentration for each part produced. However, the manufacturer would find that he had selected a different fluid for each part.

The practical considerations of fluid management dictate that the user select one to three fluids that do a good job on all parts. While the user probably won’t get optimum machining results on each part, his ability to do all of his work well with a small number of fluids will generally yield the lowest overall cost. That’s because the less-than-optimum performance will most likely affect only tool life, and the tools or wheels represent only 4% of the total cost to produce a part. Yet the coolant’s performance will be good enough to prevent problems that can bring a machining process to a screeching halt.

Compatibility is the fluid’s applicability to the machining of a wide range of work materials, while minimizing machine tool maintenance costs, resisting the growth of microorganisms, and inhibiting corrosion on both workpieces and the machine tool.

One might think that the ability to cut and grind effectively is the most important criterion in fluid selection. One might be wrong. Most fluids perform adequately from this standpoint, and, as a result, users normally change from one supplier to another because of compatibility problems such as rust, rancidity, and residues and foam. The fluid’s cutting and grinding capabilities pale in significance when valuable, high-quality parts become rusty junk while sitting on the shop floor, or when a half-million-dollar CNC machining center malfunctions because of harmful coolant residues left on the machine’s moving parts.

Acceptability involves operator safety, including the effect of the fluid on the operator’s skin and operator acceptance of the fluid’s odor, feel, and appearance. The operator’s acceptance of the fluid’s appearance, odor, and feel is highly subjective but important, because if the operator doesn’t like it, he’ll use it as little as possible regardless of how well it performs or how kind it is to the machines. And it’s realistic to assume that if a coolant constantly gives off offensive odors, the operator will demand unscheduled, costly coolant changes.

Acceptability is becoming an even more important selection criterion in today’s “chemophobic” atmosphere. The alarmist reports from the media about chemicals have led people to fear chemicals, and any unusual smells make people fear for their lives.

Disposability/recyclability refers to the ease and cost of fluid disposal and the fluid’s ability to be recycled. Recycling systems generally reduce the purchase of new fluid concentrate by 50% and virtually eliminate the disposal of spent fluid. It is important to note that effective, long-term fluid recycling requires the use of high-quality fluids that can be controlled easily and are compatible with a wide variety of work materials and machine operations.

Today’s stringent environmental regulations have raised the cost of legal disposal of spent fluids dramatically since 1980. Today, fluid-disposal costs equal and in some cases even surpass the initial purchase and usage costs of the fluid. For this reason, users are demanding fluids that last longer and can be easily separated into a clear water phase and a clean oil phase for disposal. Since the separated oil phase must still be disposed of, many users are going a step further and are using recyclable fluids that circumvent the costs of disposal altogether.

Financial return involves the fluid’s impact on production costs, including the fluid’s purchase price and its usage and disposal costs. Metalworking fluids are used because they lower the user’s cost of machining parts. Without fluids, tool and wheel life would be drastically reduced (increasing downtime for tool and wheel changes), surface finishes would be severely degraded, the necessary dimensional control of parts would be virtually impossible to attain, and operations such as broaching, reaming, tapping, and gear cutting would virtually cease.

The most economical fluid for any operation is the one that best combines cutting and noncutting functions.

Before evaluating these criteria, remember that to properly evaluate fluid performance, you must test the fluid in your own shop environment. Successful coolant management begins with proper fluid selection based on the kinds of metals being machined, their respective microstructures, the types of machine tools being used, and the specific operations being addressed. The savings in fluid purchase and disposal costs almost always pay for the investment in people and equipment in less than one year.