Friction, Lubrication, and Wear Technology (1997) Part 3 doc

the elastic film of the fluid, thereby allowing it to release the air and drain. Silicones are very effective defoamers. Their major drawback is that if the workpiece is to be coated or painted in a subsequent operation, adhesion may be adversely affected. Nonsilicone defoamers include long-chain alcohols, certain triglycerides, and water-insoluble polyglycols. Antimicrobial agents are materials designed to inhibit the growth of bacteria, fungi, and yeast in metalworking lubricants. All water-based metalworking lubricants are vulnerable to attack by one or more of these agents; even oil- based lubricants containing small amounts of water as a contaminant can be degraded by microbes. Attack of metalworking lubricants by bacteria leads to one or more of the following: buildup of acidic materials, corrosion of machinery and tools, destruction of additives, objectionable odors, and loss of stability in emulsions. Growth of fungi can lead to slimy material coating the machinery and tools, as well as the clogging of pumps and filters. Bacteria, fungi, and yeast are often monitored on a regular basis through commercially available simple culture techniques, and when counts reach a certain level, there is cause for alarm. Microbes can generally be controlled at acceptable levels through use of antimicrobial agents known as biocides and fungicides. Standard practice often calls for the addition of two different biocides to the metalworking lubricant at regular intervals in an alternating fashion, in order to guard against microbes developing an immunity to one of them, resulting in an uncontrolled infestation. Although many types of biocides exist, two of the most common are phenolic materials and formaldehyde-release agents. Phenolic materials, such as 2,4,5-trichlorophenol, destroy bacteria directly. Materials such as 1,3-di(hydroxy-methyl)-5,5- dimethyl-2,4-dioxoimidazole, upon being added to water-based metalworking lubricants, release formaldehyde slowly to keep bacteria in check. Materials such as 2,2-dibromo-3-nitrilopropionamide are useful for controlling bacteria, fungi, and yeast. There are over 50 commercially available antimicrobial agents. In choosing the proper one, parameters such as the required concentration, effect on emulsion stability, and regulations concerning discharge into waste streams must be taken into account. Finally, antimicrobial agents are designed to destroy living organisms. They all display some degree of toxicity toward humans and should be handled with caution. Metalworking Lubricant Types Four commonly used types of metalworking lubricants are illustrated in Fig. 4. Straight metalworking oils, often simply called straight oils, are given this term because they are not mixed with water prior to use. Emulsions are mixtures of either simple or compounded oils with water, stabilized by the use of emulsifiers. Emulsion droplets are similar in size to, or larger than, the wavelength of visible light; hence, emulsions appear milky white. Microemulsions can be similar to emulsions in composition, but, through emulsifier choice, have oil droplet diameters that are much smaller than the wavelength of visible light and therefore appear transparent. Micellar solutions are similar to microemulsions, except that they contain no oil. Fig. 4 Commonly used metalworking lubricants Petroleum oils, as previously defined, are naturally occurring materials that are refined through processes that separate the crude substances into various molecular fractions and remove impurities. In some cases, they are hydrogenated or subjected to reforming catalysts, but are not otherwise chemically altered. The chemical reaction of smaller molecules produces the larger molecules of synthetic fluids, such as poly- -olefins and polyisobutylenes. In metalworking lubricants, the term synthetic is often used to describe either transparent micellar solutions (Fig. 4) or true solutions containing no petroleum oils. Unfortunately, this dual definition for synthetic lubricant has led to confusion. It should be obvious that if the definition of synthetic lubricant is one that contains a preponderance of man-made materials, then the straight oil, emulsion, or microemulsion of Fig. 4 is, strictly speaking, synthetic, if the base oil is a synthetic fluid such as poly- -olefin, rather than a petroleum oil. Straight oils are generally petroleum oil fractions that are normally formulated with either film-strength additives or EP additives or a combination of both. They generally provide excellent friction reduction and workpiece surface finish, good corrosion protection, and a long service life. Straight oils containing certain EP additives will stain nonferrous metals, such as copper, and are commonly referred to as staining oils. The major disadvantage of straight oils is their poor capability for heat removal, compared to water. In addition, straight oils with low flash points, coupled with the high temperatures often encountered in metalworking operations, can create fire hazards. Straight oils are commonly used in metalworking operations where lubrication is a major factor and cooling is a minor factor. Examples of such operations include low to moderate speed metal removal operations where accuracy, tolerance, and workpiece finish are important, and metal forming operations such as aluminum foil rolling where strip surface quality is highly important. Emulsions. In commonly used metalworking emulsions, oil globules are finely dispersed in water, and this oil-water combination is employed as a lubricant-coolant. In these types of emulsions, oil is said to be the dispersed phase, and water, the continuous phase. Oil-in-water mixtures are thermodynamically unstable; that is, their state of lowest free energy is total separation. Because of this, oil tends to separate, and emulsifiers are added to stabilize the emulsion. Emulsifiers concentrate at the oil-water interface and inhibit coalescence of oil globules. This is illustrated in Fig. 5. The structural formula of sodium oleate, an anionic emulsifier, is shown in Fig. 5(a). A simplified "straight pin" is depicted in Fig. 5(b). An oil-in-water emulsion stabilized by sodium oleate is shown in Fig. 5(c). The hydrocarbon chain of sodium oleate is compatible with the oil globules and penetrates them. The carboxylate head of sodium oleate is compatible with water and lies at the surface of the oil droplet penetrating into the water phase. Because the carboxylate head carries a negative charge, the surface of each oil droplet is negatively charged, and since like charges repel, the oil droplets tend to stay dispersed. Because the state of lowest free energy of the emulsion is still total separation, the emulsion is said to be kinetically stabilized. Fig. 5 (a) Sodium oleate. (b) "Straight pin" depiction of sodium oleate. (c) Oil-in- water emulsion stabilized by sodium oleate emulsifier Emulsions can vary in stability over a wide range, depending on the nature of the oil phase and the nature and concentration of the emulsifier package. Emulsion oil globule size can vary between about 0.2 m (8 in.) to as high as 10 m (400 in.) or more. Therefore, emulsions appear as off-white to white opaque solutions. The globule sizes within a given emulsion are polydispersed; that is, they vary over some distribution. Stable emulsions have smaller average globule size distributions than unstable ones. A major factor in lubricating with emulsions is the availability of the oil phase to lubricate. Two factors control oil availability: the emulsion stability and the concentration of oil in the emulsion, which is often called "percent oil." In general, the less stable the emulsion and the higher the percent oil, the greater is the availability of oil for lubrication. Unfortunately, the less stable an emulsion is, the higher is the tendency for stability to change, sometimes rapidly, over time. This can lead to undesirable instability in some metalworking operations, such as rolling. Also, as the percent oil in an emulsion increases, cooling capability decreases. Therefore, the stability and percent oil in an emulsion must be carefully balanced to satisfy the lubrication and heat removal needs of a particular metalworking operation. The fact that emulsions are kinetically, rather than thermodynamically, stable leads to other factors in their behavior. One of these is called the emulsion "batch life." New emulsions are generally the most stable and have the least oil available for lubrication. Metalworking operations are often not optimal when a new emulsion batch is introduced. Over time, as debris is generated in the emulsion, providing nucleation sites for oil globule coalescence, and as emulsifiers are depleted, the emulsion becomes less stable and performs at its best. At yet a later time, the emulsion becomes so degraded and unstable as to be rendered useless and is discarded. A new batch is introduced, and the process repeats itself. In general, emulsions that are initially less stable have shorter batch lives. A second factor is the care that must be taken upon introducing foreign substances into the emulsion. For example, introduction of a biocide to combat a microbial infestation or contamination by acids or bases can greatly affect emulsion stability and therefore the consistency of the metalworking operation. Metalworking lubricant emulsions are often complex mixtures of emulsifiers, film-strength additives, oxidation inhibitors, corrosion inhibitors, and coupling agents. Coupling agents are generally lower molecular weight diols and triols that aid in the initial emulsification. Emulsions can also contain various mixtures of EP additives. The formulated oil mixture is called the concentrate and is added to water with agitation to form the emulsion. The quality of the water is extremely important. Distilled or deionized water should be used whenever possible. Metalworking emulsions generally operate at levels between 5 and 10% oil. Metalworking emulsions are maintained in a variety of ways. The percent oil is determined by breaking the emulsion in a graduated bottle with an acid or salt solution, and it is maintained by adding new concentrate during the life of the batch. Nonemulsifiable tramp oils (those that have leaked into the metalworking lubricant) are skimmed off, and the emulsion is subjected to continuous filtration to remove fine debris. In the case of metal removal fluids, chips are often removed by mechanical means. Microbe levels are monitored and controlled by the addition of appropriate antimicrobial agents at prescribed intervals. Emulsions have good lubricating and heat removal qualities and are widely used in most metal removal operations and many metal forming operations. Microemulsions are clear-to-translucent solutions containing water; a hydrophobic liquid, that is, an oil phase; and one or more emulsifiers, which are often referred to as surfactants and co-surfactants. Microemulsions in which water is the continuous phase and oil is the dispersed phase are called oil-in-water microemulsions and are the type generally used as metal-working lubricants. Microemulsions employed as lubricants are commonly called semisynthetic fluids. Several parameters differentiate microemulsions from emulsions. Most importantly, microemulsions are thermodynamically stable; that is, the state of lowest free energy is dispersed rather than separated. Therefore, the stability problems associated with emulsions are nonexistent. Microemulsions remain stable indefinitely, as long as they are maintained in appropriate ranges of pH, oil-to-water ratio, and temperature. These ranges may be very narrow to very broad, depending of the nature of the microemulsion. The diameters of the dispersed oil globules in microemulsions range from about 0.01 to about 0.2 m (0.4 to 8 in.), depending on the nature of the oil and the types and concentrations of emulsifiers. This small oil globule size is the feature that makes them appear clear to translucent. Additionally, the oil globule diameters are much more uniform in microemulsions than they are in emulsions. Microemulsions are generally produced in the metalworking environment by adding a concentrate to water with agitation. The concentrate generally contains oil, the emulsifier package, film-strength additives, corrosion inhibitors, biocides, and in some cases, EP additives. Dilutions range from about a 10:1 ratio of water to concentrate to as high as 60:1. Lower dilutions are used in operations were lubrication is more important, whereas higher dilutions are used where cooling is more important. Concentration is commonly determined with a hand-held refractometer and a calibration chart that relates the instrument reading to concentration. Microemulsions offer good resistance to corrosion and to microbial attack, as well as excellent stability and cooling. They suffer from higher initial cost, difficulty of disposal, and a stronger tendency to foam. Microemulsions formulated with fatty acid soap-type emulsifiers tend to degrade rapidly in hard water, because of the formation of insoluble calcium and magnesium carboxylates. Micellar Solutions. When emulsifier molecules are dissolved in water, they tend to aggregate into larger units called micelles. Micelles are spontaneously formed because of the fact that the lipophilic portion of the emulsifier molecule tends to aggregate in the interior of the micelle, whereas the hydrophilic portion tends to penetrate into the water phase. Figure 6 illustrates, in two dimensions, the relation between a molecule of a typical anionic emulsifier, sodium dodecyl sulfate, and the spherical micelle that it forms in water. Micellar solutions used as metal-working lubricants contain neither petroleum oils nor synthetic hydrocarbons. They contain film-strength additives, EP additives as appropriate, and corrosion inhibitors solubilized within the interior of the micelles. Because micelles have diameters typically between about 0.005 and 0.015 m (0.2 and 0.6 in.), micellar solutions are transparent to the eye and, like microemulsions, are thermodynamically stable. Because virtually all of the components of micellar solutions are obtained by chemical synthesis, they are often referred to as either synthetic lubricants or chemical coolants. Fig. 6 (a) Molecule of sodium dodecyl sulfate. (b) Sodium dodecyl sulfate micelle in water Like emulsions and microemulsions, micellar solutions for metalworking are generally formed by the addition of a concentrate to water with agitation. Dilutions typically range from a water-to-concentrate ratio of 10:1 to about 50:1, depending on the application. As in the case of microemulsions, concentration is determined by refractive index. Micellar solutions that contain no alkali metal soaps or amine fatty acid soaps show good stability in hard water. They are more resistant to microbial attack than either emulsions or microemulsions. They can be formulated to reject tramp oils, which can then be skimmed and collected for disposal or recycling. They have excellent cooling capability, provide excellent corrosion control, and have a long useful life. On the downside, because micellar solutions tend to cost more initially, a total cost-benefit analysis should be performed. Additionally, because they are highly fortified with emulsifiers, foam can be a real problem. Antifoaming agents can be added to control foam. Micellar solutions, in general, have lower lubricating capability than other types of metalworking lubricants. This limits their applications to those metal removal operations with low tool pressures and high tool speeds where cooling is of paramount importance. In such operations, tool life can be extended as much as 250 ° by using micellar solutions, compared to straight oils. One final drawback is waste disposal. It is often very difficult to separate the organic materials from water, because of their nature. The organic materials tend to remain soluble over wide ranges of pH, temperature, and salt concentration. It is often necessary to resort to sophisticated techniques, such as reverse osmosis, for disposal. True solutions differ from micellar solutions in that the molecules of active substances do not form micelles when dissolved in water. Rather, each ion is solvated by water molecules. Because essentially all film-strength additives and EP additives either form micelles or require micelles to dissolve them, true solutions offer lubricating qualities that are little better than those of water. True solutions are used in cases where cooling is the only consideration. In these cases, their advantages are low cost, very high cooling, stability, low foam, and a very long life. Typical true solutions often contain nothing more than a corrosion inhibitor, such as sodium nitrite, in water. Solid-Lubricant Suspensions. Lubricants for specialized uses often contain solids in the form of finely divided powders suspended in a liquid carrier, such as oil or water. The liquid carriers may also contain soluble additives of the classes previously mentioned. One of the most common suspended substances is colloidal graphite, with specific surface areas that often exceed 100 m 2 /g (3 × 10 4 ft 2 /oz). It is used extensively in hot forging and extrusion of both ferrous and non-ferrous metals. Molybdenum disulfide is another commonly used suspended solid. Both graphite and molybdenum disulfide are compounds that possess layered crystal structures with weak forces bonding the layers together so that they are easily sheared. They function by plating onto tools and workpieces such that the weak shear direction is parallel to the surfaces. As the tool and workpiece surfaces are brought together, they form a solid film, preventing tool-work-piece contact and shear along the weak shear plane, thereby reducing friction. Other types of solids that are suspended as fine powders in some metalworking lubricants include mica, polymers such as Teflon, certain metal oxides, and glasses. Mica has a layered structure and functions in a way similar to graphite. Polymers mechanically separate metal surfaces, lower friction, and reduce metal transfer. Hard metal oxides, such as aluminum oxide, have good wear resistance but high friction coefficients. Soft oxides, such as lead II oxide, give relatively low friction coefficients that decrease at higher temperatures, where the mechanism of deformation changes from fracture to plastic flow. Glasses are suspended in lubricants for use in metalworking operations at high temperatures, where they soften on hot die and work-piece surfaces and function as parting agents of low shear strength. Solids can be kept in suspension by using surface active agents, mechanical agitation, or both. Problems can arise if not enough care is taken and the solid is allowed to "settle out." Also, metalworking lubricants containing suspended solids tend to produce buildup on tools and workpieces that are difficult to clean. These problems can be minimized by the appropriate formulation. Metal Removal Lubricants A metal removal operation is shown in Fig. 7. In some cases, such as turning, the workpiece is moved against a stationary tool. In other cases, such as drilling, the tool is moved against a stationary workpiece. Either operation results in essentially the same type of metal removal mechanism. The tool cuts into the workpiece, resulting in the formation of a chip. Workpiece metal is deformed in the metal deformation zone, resulting in about 65% of the heat generated in the operation. The remainder of the heat is generated by friction between the tool and the chip, and the tool and the workpiece. The lubricant penetrates the shear zone and reduces heat by reducing friction and carrying heat away from the tool and workpiece. Heat can also be reduced by increasing the shear angle, thereby reducing the amount of metal deformation that occurs. Fig. 7 Metal removal process One problem that occurs in metal removal, especially with ductile metals, is known as "built-up edge." This problem results when some of the workpiece welds to that part of the tool that is in contact with the chip. Portions of the built-up edge eventually detach, come between the tool and the workpiece, and blemish the workpiece finish. Built-up edge is controlled through film strength and EP additives, which react at the interface to prevent welding, and by choosing an appropriate cutting speed. In addition to reducing friction and removing heat, the metal removal fluid must flush away chips and debris from the metalworking interface. Unremoved chips can retard the progress of the tool and damage the workpiece. Because a significant amount of the heat generated is located in the chip, removal of the chip is a major factor in reducing heat buildup. An important function in prolonging tool life is reducing and removing heat from the operation. Selection of metal removal lubricants depends on the operation and on the type of metal composing the workpiece. In operations where low speeds and relatively deep cuts are used, or where workpiece finish and tolerances are important, straight oils are often used. In these cases, lubrication plays a more important role than cooling. Conversely, in high-speed operations with relatively shallow cuts, cooling is most important, and microemulsions or micellar solutions are often employed. Many producers of commercial metal removal fluids provide a fluid selection table with their product literature, such as that shown in Table 1. The table is a matrix with metal removal operations listed in order of decreasing severity on one axis, and various workpiece materials listed on the other axis. The matrix is then filled in with the lubricants recommended for a particular operation and workpiece material. In those cases where the lubricant is dispersed or solubilized in water prior to use, the table often gives the recommended oil:water dilution ratio. In general, straight oils would be more commonly recommended for operations at the top of the table, microemulsions and micellar solutions more commonly recommended for operations toward the bottom, and emulsions recommended over a wide range of operations. However, the recommendations are general, and instances can be found where almost every type of lubricant has been employed satisfactorily in almost every type of operation. Table 1 Organization of typical metal removal fluid selection chart In addition to this type of table, commercial suppliers furnish information about each lubricant in specification sheets. These sheets customarily contain: recommended uses; physical properties, such as viscosity and specific gravity; staining tendencies on nonferrous metals; and chemical data, such as percent sulfur, chlorine, and fat. Information on materials that are not contained is also provided, such as "contains no nitrites, phosphorus, chromates, or heavy-metal salts." For those materials that are diluted with water prior to use, waste disposal information, such as chemical oxygen demand, biological oxygen demand, oil and grease, and alkalinity, is often given for a particular ratio of water to oil. In addition, commercial suppliers provide material safety data sheets for each lubricant in their product line. These describe potential hazards associated with the use of the lubricant and safe handling procedures, such as required protective clothing like safety glasses and gloves. Lubricant Application. In most metal removal operations, the lubricant floods both the tool and the workpiece. The lubricant is supplied through high-volume low-pressure spray nozzles to maximize cooling and minimize splashing and foam. The lubricant is customarily directed into the contact zone between the tool and the workpiece. The lubricant is also directed at other positions on the workpiece, where appropriate, to enhance cooling. In Fig. 7, for example, lubricant would be directed into the contact zone to reduce friction, prevent metal transfer, and facilitate cooling. Lubricant would also be sprayed over the back of the chip to further enhance cooling. Lubricant Maintenance. Lubricants can be maintained through a variety of practices. In water-based coolants, tests for percent oil, pH, tramp oil contamination, suspended solids, microbes, and corrosion are made on a regular basis, and corrective action is taken as required. For example, an increase in pH beyond a specified range might signal contamination by a very basic substance. On the other hand, a decrease in pH might be due to oxidation, microbial infestation, or contamination by an acidic substance. Corrective action could involve adding an oxidation inhibitor, adding a biocide, or finding and eliminating sources of contamination. During their use, metal removal fluids become contaminated with metal chips and fine debris, such as metal particles and insoluble metal oxides. If not removed, these contaminants can accelerate degradation of the coolant by promoting oxidation and, in the case of emulsions, providing nucleation sites to destabilize the coolant. These contaminants can also damage the workpiece when circulated back to the contact zone. Metal chips are generally allowed to settle and are then removed. The use of centrifuges can accelerate this process. Fine debris is usually removed by filtration, often in two stages. The first stage involves filtration by paper or cloth to remove coarser debris, followed by filtration through filter aids, such as diatomaceous earth or fine volcanic ash, to remove fine debris. The leakage of hydraulic oils and other lubricating oils (tramp oils) into a metal removal fluid is very detrimental. In the case of straight oils, tramp oil contamination can change the viscosity and dilute the additive package. In the case of emulsions, tramp oils are often emulsified and increase the oil globule diameters and destabilize the emulsion. Some emulsions and many microemulsions and micellar solutions reject tramp oils. In these cases, the tramp oils are removed by skimming. Reclamation and Disposal. In metal removal operations, lubricant adheres to the chips and is available for reclamation. The chips are moved to a central location with care to segregate chips that come from operations employing different lubricants. The oil can be collected simply by allowing the lubricant to drain from the chips and collecting it. The use of a centrifuge accelerates the process and results in more complete removal of oil. The oil is then cleaned by filtration, as required, and returned to operation. Most metal removal fluids in use will reach a point in time at which they can no longer be maintained and must be disposed of. No metal removal fluids should be released into the environment without prior treatment. Petroleum oils, natural fats, and greases, as well as synthetic organic materials, are contained in most metal removal fluids. These materials, when released into streams and rivers, float on the water and thereby slow the adsorption of oxygen from the air into the water. In addition, they consume oxygen in the water through direct oxidation and by promoting the growth of bacteria that consume oxygen as they metabolize. Because aquatic organisms require oxygen for survival, they will die if any substantial amount of that oxygen is depleted by the processes noted. There are strict state and federal laws that regulate disposal of metal removal fluids into streams and rivers, with heavy fines for violators. Spent straight oils are often added to heavier fuel oils and burned to generate heat or power. Emulsions and some microemulsions, as a first step, are treated with acids or salts to separate the organic phase, which is skimmed off. In a second step, the water is then sent to aerated tanks containing aerobic bacteria, where remaining organic materials are consumed. Most plants using large amounts of metal removal fluids have facilities to carry out at least the first step. The water from first-step treatment is often clean enough to be sent to municipal waste treatment systems, where the second step occurs. Small amounts of microemulsions and micellar solutions that resist breaking in the first step can generally be sent directly into the second step as long as their organic content does not overwhelm the system. A third step is sometimes used when the effluent from the second step does not meet water quality standards for discharge. This third step may include such processes as reverse osmosis, chemical oxidation, or oxidation by ozone. Waste from more concentrated microemulsions and micellar solutions can be effectively treated by these methods. A complete metal removal fluid program, including fluid selection, maintenance, handling, reclamation, and waste disposal, is a vital part of a metal removal operation. Such a program can increase profitability by reducing lubricant costs, allowing increased feeds and speeds, reducing tool wear, and improving workpiece finish, while being environmentally responsible. Metal Forming Lubricants In metal forming operations, the desired shape of the workpiece is obtained through plastic deformation. Most metal forming operations employ liquid lubricants consisting of petroleum oil or synthetic oil fortified with additives. These lubricants form films that partially or completely separate the tool from the workpiece, thereby reducing friction and minimizing metal transfer. In many metal forming operations, cooling is also desirable. Therefore, the use of emulsions, and in certain cases, microemulsions and micellar solutions, is common. Certain metal forming operations also use solid suspensions. Lubricant films in metal forming operations are either "wedge films" or "squeeze films." Wedge films result when two nonparallel surfaces converge under relative motion in the presence of a lubricant. Both surfaces may be in motion, as in the case of rolling, or just one surface may be in motion, as in the case of wire drawing. Squeeze films result when two parallel surfaces approach each other with a liquid lubricant between them. An example of a metal forming operation that develops a squeeze film is an open die forging method called "upsetting." Wedge Films. Most metal forming operations that employ liquid lubricants develop wedge films. When surfaces converge under relative motion, the lubricant is swept along the moving surface. As the surfaces approach each other, a gap in the form of a wedge develops. Consequently, the entry of the gap is greater than the exit. As lubricant molecules approach the gap entry, they are slowed because the volume they are permitted to occupy is decreasing. Because the number of lubricant molecules that enter and exit the gap must be the same, the molecules exiting the gap are moving faster than the molecules entering the gap, and indeed are moving faster than those molecules prior to approaching the gap entry. The forces that cause the entry lubricant molecules to decrease in velocity and the exit lubricant molecules to increase in velocity also act to push the surfaces apart. In metal forming operations, the minimum separation between the tool and workpiece surfaces is referred to as the lubricant film thickness, h. For a given operation, h increases as the velocity of the operation and the viscosity of the lubricant increase, and decreases as the force driving the converging surfaces together increases. In rolling, for example, h [...]... 25 255 2,717 30 (a) 107(a) 210 - 53 (- 63) 35 5 -66 (-87) -15 (5) 1.894 1.851 1.92 139 . understand and exit tensions, and then coiled. The coil is typically annealed, allowed to cool, and is then cold rolled on single-stand or multistand mills, under tension, to a gage of about 0 .3. between the tool and the chip, and the tool and the workpiece. The lubricant penetrates the shear zone and reduces heat by reducing friction and carrying heat away from the tool and workpiece about 0.2 and 10 m (8 and 400 in.). A histogram can be constructed that gives the frequency of oil globules with diameters between 0.2 and 0.5 m (8 and 20 in.), or between 0.5 and 1.0 m (20 and 40