Grinding Fluids – What's the Solution?
You must consider the type of wheel, its bonding technology, and the application to select the fluid best-suited for the job.
By Stuart C. Salmon
Science and Technology
What sort of fluid should I be using? It's a question often asked by someone who expects a simple answer. Considering all of the technology that goes into designing and building a CNC grinding machine (the basic machine tool technology, however, having been around since the industrial revolution), and the complexity of the abrasive wheel specifications (also a rather mature science—Stone-Age man was making wheels and grinding), the hope is that the fluid science might be relatively simple. Oh no!
There are many types of fluids on the market today: Water-based emulsions; chemical synthetics, semisynthetics, and glycol-based fluids; then there are the straight oils; synthetic, mineral, and vegetable; and gases, and liquid gases. It's even possible to grind dry. See how complicated it has become already? The reader might, after some searching, settle on a fluid, but then there's the question of how to apply it, flood or MQL (Minimum Quantity Lubrication), for instance. Then there are filtration and disposal issues.
The first place to start, perhaps, is whether to use a fluid at all. Why not grind dry? That seems to be the simple choice. Grinding with most coated abrasives is generally a dry operation. Abrasive belts, brushes, and disks are designed to run dry with only the atmospheric air (roughly 20% oxygen and 79% nitrogen with a 1% smidgen of some other, generally inert, gases) to assist in the grinding operation. Dry grinding is not generally used in precision-grinding processes, however. It's used more in surface finishing operations, where dimensional accuracy is not critical, and the surface integrity of the part need not be high.
Dry grinding is performed mostly with hand-held grinding tools, and on manual tool-and-cutter grinders. If liquid fluids were used in these operations, the operator would get very wet indeed. Careful inspection of the grinding wheels used in these operations will show that they are typically resin and polymer-bonded wheels.
Resin bonds are quite different from vitrified bonds. Vitrified-bonded grinding wheels self-sharpen by the action of mechanical force that either fractures the grain or breaks the fragile bond bridges to maintain a more aggressive and sharper face to the grinding wheel. Resin bonds, on the other hand, are quite closed in comparison to vitrified bonds, with far less chip clearance, and are typically made from thermosetting resins. These resins harden, coke, and crumble at high temperatures. Therefore it's important to have the addition of heat in the arc of cut when using a resin bond wheel, as it's the heat that "deteriorates" the bond, and so assists in the self-sharpening effect. There are also some wheels made with thermoplastic resins. In this case, the polymer melts and flows, allowing the grains to move and reorient to present a sharper facet to the arc of cut. Their best application is in high-speed grinding when a conventional abrasive (aluminum oxide or silicon carbide) is used.
Dry grinding is possible with vitrified wheels. To avoid all of the poor-surface-integrity issues, however, the wheel has to be very sharp. To achieve that sharpness, the wheel is typically made with coarse grains and dressed in a manner that creates an aggressive and open structure. The surface finishes achievable with that type of grinding-wheel condition will be rough, and the part will generally be hot to the touch. Therefore it's quite safe to say that, when precision-grinding, a grinding fluid is important and necessary.
The self-sharpening mechanism for resin-bonded wheels, as previously mentioned, is in sharp contrast (no pun intended) to that for a vitrified bonded grinding wheel. Vitrified wheels self-sharpen from an increase in the mechanical force in the contact area of the arc of cut, whereas resin bonds self-sharpen with an increase in heat in the arc of cut. Therefore, a fluid that is a more-effective coolant would be better used in combination with a vitrified wheel, and a fluid that is more of a lubricant would be better used in combination with a resin-bonded grinding wheel.
When a grinding fluid is used in combination with a resin-bonded grinding wheel, it's common to use straight oil. Oil is not a good coolant, but it is an excellent lubricant. The oil will reduce frictional energy, yet not carry away much heat. Resin-bonded wheels can therefore be relatively hard and denser than a vitrified-bonded wheel, and so use finer grains to achieve very fine finishes over a long period of time between dressings. This is all due to heat generation, which helps the self-sharpening effect, in balance with the lubrication of the grains in the arc of cut. The combination of a straight oil and a fine-grain resinbonded grinding wheel can achieve some of the finest finishes required in the industry—mirror finishes, in fact. And this is why one often hears it said that "a straight oil always gives a better surface finish." Of course, that's only half the story. It's not the oil that gives the better finish, but the grinding wheel, the bond system, the grain size, and how the wheel is dressed. Grinding fluid alone, be it oil or water-based, has little-to-no influence on the workpiece surface finish.
Grinding processes generate a great deal of heat. Most of it comes from friction, and it needs to be minimized and extracted from the arc of cut to limit the amount of thermal energy that flows into the workpiece surface. Lubricating the arc of cut so that only a small amount of heat is generated can minimize that heat. Heat can be extracted from the arc of cut by cooling with a fluid that has a high capacity for heat transfer, the most plentiful of such fluids being water. That is the basis of the fluid choice with respect to the cutting functions. Good lubricants will minimize the heat, and good coolants will carry away the heat.
Straight oils are excellent lubricants, and do not allow the generation of as much frictional energy as is produced when using a water-based fluid or when grinding dry. Unfortunately, the heat generated when using a straight oil is not quickly absorbed into the oil, due to the oil's low capacity as a heat-transfer fluid. The workpiece will therefore be warm to quite hot after grinding, which can affect the precision of the process (unless it's compensated for), due to thermal changes that occur in both the workpiece and the machine tool.
Research is being conducted in an area where the heat from the grinding operation can be harnessed for good— grind-hardening. Grind-hardening is a carefully controlled process that uses the frictional energy generated by grinding to heat the workpiece surface. Next, the grinding fluid is used to quench the surface, to create a case-hardened workpiece on the grinding machine.
Straight oils generate a great deal of mist, which is not only a fire hazard but also a known carcinogen. That mist has to be contained and the air filtered, which is relatively easy to do with today's fully enclosed CNC machines. It is essential to have fire detection and suppression on any grinder that uses straight oil. Machine tool builders will generally take such requirements into account, and build-in fire suppression and explosion-proof enclosures. With respect to the mist, it's interesting to note, from carefully controlled tests, that though all oils create mist, there is a wide variation across the spectrum of oils in the amount of mist they generate with respect to the droplet size.
Prior to grinding, a "splashing mist" is generated, comprised mostly of large droplets above 0.00004" (1 µm) in diam. Once grinding takes place and the grinding wheel enters the arc of cut, there is a "grinding mist" comprised of almost smoke-like smaller droplets of submicron size. Laboratory testing shows that for a vegetable oil the proportion of splashing mist is far greater than that for a mineral oil, whereas for the submicron-size droplets generated during the grinding process, the vegetable oil shows substantially smaller amounts of submicron particle mist, when compared to a mineral oil. The specification of the particle size filtered by the air filtration system is therefore critical to having and maintaining a clean, safe, and healthy work environment.
Straight oils work particularly well with superabrasives— cubic boron nitride (cBN) and diamond—and in particular with cBN, providing longer wheel life than most water-based fluids. This should not, however, be the sole justification for choosing a straight oil when grinding with cBN. Since the late 1960s, when cBN came onto the market, much R&D has gone into the chemistry of water-based and glycol-based fluids. In some cases, wheel life is not as low in a water-based fluid as might be expected. Some water-based fluids have been shown to exceed the wheel life expected when using straight oil. Also, the overall cost of manufacture needs to be taken into account. It's clear that grinding-wheel cost may not always be the overriding factor in the overall piece-part cost. There are costs related to the use of straight oils that are associated with mist control, fire suppression, solvent cleaning, and the general cleanliness and upkeep of the workplace.
A grinding wheel is rarely sharp. Only the continuous-dress creep-feed grinding process uses a grinding wheel that is in a continual state of maximum sharpness. Usually, a grinding wheel is sharpest immediately after dressing (if it is a dressable wheel), but thereafter it degrades by loading up with material and/or developing wear flats on the surface of the grains. Wheel loading, particularly in plated grinding wheels that are not dressable, drastically reduces the wheel life by decreasing the chip clearance between the cutting grains. It has been shown that in the presence of a hard lubricant coating on the surface of a plated grinding wheel, wheel loading is virtually eliminated. The wheel life is extended, and the rubbing flats are reduced to only those created from the attritious wear of the abrasive grains. These flats rub and generate frictional heat. It's important, therefore, for a fluid to maintain its lubricating and cooling properties at the elevated temperatures that exist in the arc of cut.
No one has measured the exact temperature at the point of cut of an abrasive grain. There have been many estimates, simulations, and measurements that tell us that the temperature is very high indeed, probably in excess of 1000°F (525°C). This situation brings EP (Extreme Pressure) additives— typically chlorine, phosphorus, and sulphur—into the picture.
Phosphorus is not very common today, whereas chlorine and sulphur are more common. The term sulphur-chlorinated fluid refers to fluids that contain both sulphur and chlorine. These are generally all-purpose fluids. Chlorine reduces the coefficient of friction at low interfacial temperatures, and sulphur reduces the coefficient of friction at high interfacial temperatures. Due to the very high temperatures in the arc of cut, it's a sulphurized fluid that will have the most impact on the grinding process.
Selection of the fluid is impacted by the company philosophy: Does the organization prefer an all-purpose fluid that mills, drills, and turns, as well as grinds reasonably well, rather than paying more attention to the detail of each process, and achieving optimal performance and part quality with more than one fluid in the plant? That choice is becoming a little easier in some parts of the US, as chlorine additives are being outlawed. Obviously the fluid manufacturers are seeking substitute chemistry for the lower-temperature processes. It's peculiar for the government that looks after us to rule in the work environment, through OSHA and the EPA, that chlorine is bad, whereas in the home, under dispensation of the FDA, we are washing our clothes with bleach, diving into chlorinated swimming pools, and bubbling in hot tubs filled with chlorinated water. Where you live may, therefore, impact your choice of fluid.
And so to "green" fluids: They may not be so eco-friendly; eco-nomically as well as eco-logically. Green fluids, at the outset, sound like a good idea to the environmentally conscious, but the reality of the situation is not favorable. If the fluid is truly biodegradable, then it will not last very long in the tank. If the green fluid is used to grind a material that contains a designated hazardous substance, then the contaminated green fluid becomes a hazardous waste. It may have been green and eco-friendly out of the barrel, but that's about as far as the green goes.
Filtration is a most-important part of the proper use of a grinding fluid. Not only is there grinding swarf to remove, but particles of abrasive that break free from the grinding wheel during dressing and grinding must also be removed. Vitrified wheels have pockets of porosity that make them a little more forgiving than a resin-bonded or a plated grinding wheel. Plated grinding wheels have no porosity. Proper filtration for them is critical. Particularly where the finest surface finishes are required, it makes sense that the grinding fluid is filtered to an extreme level of cleanliness. The removal of even the smallest sub-micro particles would be beneficial.
Should an emulsion be used, however, the size of the oil droplet that has been emulsified and is suspended in the water will be 0.00012–0.0002" (3–5 µm) in diam for a properly mixed, premium fluid. The oil droplets coalesce with time and with increasing contamination. In the best of fluids, those emulsified oil droplets may grow to perhaps 0.0002–0.00032" (5–8 µm) or more in diam. Therefore a filter paper or screen that filters in the 0.0002 to 0.00032" range will strip the suspended oil/lubricant from the fluid. Full synthetic fluids that are all chemicals and have no oil in suspension can be filtered to submicron levels without any stripping problems. The choice of fluid type may, therefore, be a compromise with respect to lubricity and fluid cleanliness.
The toxicity of the material ground may also impact the fluid choice. Cryogenics are an option when grinding such materials. Liquid nitrogen (LN2) is probably the best one to use. Oxygen is quite flammable and dangerous under the wrong conditions, so with respect to cost and safety, LN2 is a good choice. The only danger, other than frostbite, is asphyxiation. Nitrogen is odorless and colorless, and so goes undetected in the work environment. A buildup of nitrogen in the atmosphere such that the oxygen content goes below about 18% may cause dizziness, unconsciousness, or even death.
This should not be so alarming, however, as to prevent an operation from giving it a try. Liquid nitrogen has been used successfully in machining operations, turning in particular, where a system called Ice-Fly is readily available from Air Products and Chemicals Inc. (Allentown, PA). The liquid nitrogen not only cools, but also "freezes" the part, if it is a small enough mass, making it more rigid and thus minimizing deflection that may occur with delicate parts. Liquid nitrogen is not for the run-of-the-mill type of grinding operation; it's often reserved for toxic workpiece material, like beryllium and depleted uranium, where large tanks of contaminated liquid need to be avoided.
Once the proper fluid has been selected, then comes the challenge of applying the fluid to the grinding wheel and workpiece interface. It's essential to apply the fluid properly, otherwise all of the benefits of the chemistry are lost. A key parameter is to have the velocity of the fluid equal-to or slightly greater than the grinding wheel's peripheral speed. It is important not only to have the velocity correct, but also to have the stream of fluid remain laminar.
Laminar flow will ensure that the divergence of the fluid stream is minimized. John Webster is a great proponent of laminar flow nozzles, supplying them through his company, Cool-Grind Technologies LLC (Storrs, CT). A further assistance to the flow of fluid is the "Megasonic" nozzle designed by Kiyoshi Suzuki of the Nippon Institute of Technology in Japan. Using an ultra-high-frequency, ultrasonic cone to pulse energy through the fluid as it exits the nozzle, Suzuki has shown that the stream of fluid may be made coherent, as well as assist in de-loading the wheel surface. Further research done in Japan has shown that microbubbles in the fluid benefit both wheel life and surface integrity. On the other hand, consultant Ken Doenges (Warrenville, IL) sees bubbles in the fluid as detrimental, and supplies sensors to recognize fluid aeration, so that it may be detected and eliminated. Perhaps the jury is still out on fluid with bubbles.
It's important to note that employing a large volume of grinding-fluid flow does not mean that the grinding fluid will effectively get into the arc of cut. Open-structure vitrified grinding wheels can "pump" 50–70 gpm (189–265 L/min) through their open pores quite easily. Not only does this take a large quantity of fluid through the arc of cut, but upon exit the flow of fluid, out of the pores, helps to clean and de-load the wheel periphery. It is a different matter for a plated wheel, where there is no porosity; the maximum amount of flow of fluid through the arc of cut is minimal. For example, a 1" (25-mm) wide wheel, made of 60 US-mesh cBN, has a "gap", between the arc of cut and the bottom of the plated matrix, of approximately 0.003" (0.076 mm). Even at high wheel speeds, say 24,000 fpm, (120 m/sec), the maximum flow of fluid through the arc of cut is in the order of only 3.75 gpm (14.2 L/min). It is pointless and costly to pump hundreds of gallons of fluid into a grinding machine in the hope that doing so will push some of it into the arc of cut.
Let us assume that the best fluid has been purchased, and the proper nozzle arrangement has been employed to apply the fluid. The limit of the grinding process now becomes the amount of energy that can be dissipated in the arc of cut. Should the grinding cycle be too aggressive, even under the best fluid conditions, the fluid will boil off and the process will perform no better than dry grinding. The part will burn, there will be burrs, the grinding wheel will load up with material, and the surface integrity of the workpiece will be very poor.
Here is an important concept to understand when purchasing a new grinding machine: Doubling spindle power does not mean that the grinding cycle time can be reduced by half. If, with a 10-hp (7.5-kW) machine the grinding cycle has been optimized and an increase in productivity is required, a 20-hp (15-kW) machine will not be able to double the feed rate. It will put twice the amount of energy into the same area of the arc of cut, which means that the grinding fluid will have to dissipate twice the amount of energy over the same area of cut. A quart cannot be put into a pint pot without a mess. Yet the industry still sees an increase in spindle power as an avenue to increased productivity. That is not always the case.
When choosing a grinding fluid, another area of concern is the fluid's noncutting properties. These include odor, color, skin irritation, fungus, foam, mist, bacteria, residues, cost of disposal, and the fluid's reaction with resin-bonded grinding wheel bonds and the polymers and rubber compounds that make up the seals and windows of the machine tool. The heavy environmental regulations under which industry must operate in the US has many fluid companies concentrating more on the noncutting properties, in order to be compliant, rather than the cutting functions of lower power, longer wheel life, better part quality, and surface integrity, which are the primary requirements for a successful grinding fluid.
The choice of a grinding fluid depends upon both its cutting and noncutting characteristics. There is no substitute, however, for data, and those industries with large central systems will not convert from one fluid to another on a whim. Manufacturing Engineering magazine presented an article in its February 2000 issue entitled "Customize Your Grinding Fluids," which showed how fluid performance can be measured. The grinding fluid is only a part of the total piece-part cost. Only with test data in hand can a decision be made that meets the economic, ecological, and productivity requirements of an application. Without data there is only opinion.
Grinding and abrasive machining is going to be a wet process for a long time to come. Where large-chip machining is moving toward dry and near-dry applications, abrasive processes are remaining wet. From straight oils to water-based and glycol-based fluids, there is a spectrum of fluids available to manufacturers. A fluid suited to each and every application is available to work in an optimum fashion with either the type of abrasive being used or the material being ground. Much R&D is going into improving grinding fluids today. With grinding and abrasive machining being a materials-driven process, the fluids must keep pace with materials technology. The proper fluid will serve the user well if it is properly applied and properly maintained. If, however, a shop purchases the most inexpensive fluid and improperly applies it to the arc of cut, it may as well be just muddy water.
Many of the R&D programs referred to in this article were reported at the Tenth ISAAT at the SME International Grinding Conference held in Dearborn, MI in September 2007. The ISAAT proceedings are a good source of information on the latest in abrasives and grinding research.
This article was first published in the February 2008 edition of Manufacturing Engineering magazine.