Cutting Fluid System:
The cooling ways in machining include:
- flood cooling
- chilled fluid cooling
- mist (spray) cooling
- jet cooling
Flood application delivers fluid to the cutting tool/workpiece interface by means of pipe, hose or nozzle system. Cutting fluids may also be atomized and blown onto the tool/workpiece interface via mist application. Cutting. The flood method is the most common method for applying cutting fluids in turning, drilling, and milling process [Jerry P. Byers, “Metalworking Fluids”, Marcel Dekker, Inc.,1994] The Flow rates can be as low as 10 l/min for turning and as 200 l/min for face milling. Chips can be washed away from the cutting region in deep- hole drilling and end milling by using fluid pressures ranging from 700 to 14,000 kPa [P. Byers, 1994].
The primary function of cutting fluid is cooling and lubrication. A fluid’s cooling and lubrication pproperties are critical in decreasing tool wear and extending tool life. Cooling and lubrication are also important in achieving the desired size, finish and shape of the workpiece [Sluhan, 1994]. A secondary function of cutting fluid is to flush away chips and metal fines from the tool/workpiece interface to prevent a finished surface from becoming marred and also to reduce the occurrence of built-up edge (BUE).
The cooling Mechanism
Cooling influences machining in various ways.At the contact between the chip and tool, cooling can reduce the chip temperature and, thus, affect directly the friction force between the chip and tool.However,contact pressures are so high that the cutting fluid has no path by which it can completely penetrate the contact area: cooling is mainly indirect via modified conduction through the chip.Remote from the cutting edge, the coolant always play a major role in maintaining the machined material at ambient temperature, reducing error of size resulting from thermal expansion. The primary reason for cooling is to retard high rates of face and flank wear by curbing the sharp temperature rise which accompanies short ranges of higher speeds for a given tool life can be obtained. Low temperatures were correlated with smaller cutting forces and small chip curl diameters; higher temperatures were associated with high cutting forces, and larger chip curl diameters. The results showed that cutting oils as well as aqueous coolants lowed cutting temperature. The former indirectly by reducing heat generation, the latter by heat removal. The internally_cooled tool produced similar results, but clearly through a thermal mechanism.It was conclude that cooling effects of cutting fluids are of primary importance.
A cutting fluid’s effectiveness depends on factors such as the method used to apply the cutting fluid, temperatures encountered, cutting speed, and type of machining process [S, Kalpakjian, ÒManufacturing Processes for Engineering Materials,Ó 2nd ed., Addision-Welsley, Reading, MA 1991]. The role of a cutting fluid as a coolant or lubricant is very sensitive to the cutting speed. For example, in high-speed cutting operations such as turning and milling where the tool-work interface is small, the cooling characteristic of a coolant is extremely important. Conversely, in low-speed cutting operations such as broaching, threading, and tapping, lubricity is more important since it tends to reduce the formation of a BUE and improves surface finish [Byers, 1994] . The most common theory for the method used by a cutting fluid to penetrate the tool-chip interface is capillary actions [Byers, 1994]. This model was used by Merchant to explain the results from his studies using a transparent sapphire cutting tool. His experiments showed that the cutting fluids entered the interface by seeping in from the sides of the chip. As cutting speeds increase, penetration may be improved since the high temperatures can convert the cutting fluid into smaller gaseous molecules which have good wetting properties, but since these process take time, the capillary action is probably less effective. At high speeds, the cooling function of the cutting fluid is more important. a different theoretical model for the cutting fluid action was proposed by Smith, Naertheim, and Lan [T. Smith, Y. Naertheim, and M.S. Lan, ÒTribol. Int.Ó 21:239-247, 1988.] Their model was based on the capillary flow theory, but the model assumed that fissures in the chip and along the tool-chip interface allowed the cutting fluid to enter into the chip and along the interface.
The lubrication mechanism
Lubricates reduce friction between surfaces which are in relative motion.At any given rate, the force holding the parts together controls the intimacy of contact and thus has a direct influence on frictional force. when normal forces are quite low, the viscosity of an intervening layer of fluid can be the major factor in regulating friction. In metal cutting where normal contact stresses at the chip/tool interface may exceed 50,000 psi.,been estimated as the maximum thickness of fluid films with the low viscosities found at cutting temperatures such as 900 F. Since the average surface asperity height is 10 to 10 in., it seems certain that fluid_film lubrication cannot exist under these conditions. Supposedly ÒsmoothÓ metal surfaces are composed of hills and valleys so that static contact of such surfaces consists of degree of mating of these irregularities when a load (normal force, N) is applied.Greater intimacy of contact is achieved creases until the compressive yield strength of the metal in actual contact is equal to the load. If a tangential force,T, is applied, sliding occurs at a value of T given by the equation:
This is AmontonÕs law, which governs sliding friction-a situation where the real are of contact is : (1) small compared to apparent area, and (2) proportional to the normal force. Under conditions of high temperature and pressure in the contact area, however, a degree of asperity adhesion due to welding (local melting) of asperities occurs, and force is required to break such adhesions.The apparent coefficient of friction under these conditions rises, but the requirements for AmontonÕs law to apply still prevail if a small proportion of the apparent surface is involved.
This is the physical realm where boundary and extreme-pressure lubrication are important.Chemical lubricants are useful under these conditions. The gouging, ploughing, and transfer of metal from one surface to another is opposed by physically absorbed or chemically interposed films,respectively.Such films must be readily sheared at temperatures and pressures lower than those at which the shearing of asperities would normally occur to obtain beneficial effects.Such components in the past have been referred to as conferring Òoiliness,Ó involving modification of the metal surface. Boundary lubricant ingredients, like fatty acids, give films at ambient temperatures, largely by adsorption. Secondary action by these materials, and primary action by extreme-pressure (EP)lubricants, can involve chemical reaction to form surface layers of metal salts.These reactions may, especially with chlorine and sulfur compounds, occur at those temperatures at which their lubrication effects are required: thus, differences in temperature thresholds and subsequent film forming reaction rates are important.The effectiveness of these secondary(chemical) films is understood to be limited by there melting points: fatty soaps up to 100 C.,iron chloride to 600 C., and iron sulfide to 1000 C. Above their respective melting points, in accord with the above interpretation of their function as solid lubricants of low shear strength, only ineffectual fluid_film action could remain, and they would cease to function. This interpretation of EP lubricant action is consistent with the main stream of theoretical viewpoint, as expressed by Bowden and Tabor.Details of the actual operation of EP lubricants varies in opinions ofGrozzek.Rabinowicz and Kohn.It is theorized that much of the favorable effects of WEP lubricants in metal itself, under compressive stresses in metal cutting. It is well established that boundary and EP ingredients in cutting fluids not only reduce cutting forces but can also improve surface finish and provide longer tool life. Most petroleum chemists find it natural to among the familiar precepts of boundary and EP action as a basis for their consideration of cutting oils. The relationship seems a very close one.For example, Brookman and Ham have studied a series of fatty oils, sulfurized compounds, and chlorine derivatives in both sliding_friction tests and simple metal cutting experiments. The results gave a reasonably close correlating between the varying efficiencies in these two kinds of action. In recognition of this fact, the 4_Ball Tester, designed for EP lubricant studies, serves as lavoratory test for screening possible cutting_fluid additives. Colloidally_dispersed solid lubricants such as molybdenum disulfide and graphite are known to be useful as anti_weld agents in EP lubricants, with effects resembling that of iron sulfide (a reaction product of dissolved active sulfur compounds) in the same application.These low shear_strength solids also give superior lubrication effects in metal cutting.