Tech Talk
What is the “Role of the Cutting Fluid”?
The cutting fluid performs four main duties: assists in the dissipation of the grinding energy, chemically assists the material removal process, lubricates the cutting action, and washes away the grinding debris from the wheel/workpiece interface. There are three main types of cutting fluid: 1) Gases, most commonly air; 2) Water-based fluids, where the great majority of the fluid (over 80 percent) is water; 3) Straight oils, where the fluid is 100 percent oil.
Water-based fluids are the best conductors of heat, whereas straight oils are the worst. Conversely, water-based fluids do not lubricate the abrasive process very well, whereas straight oils lubricate exceptionally well. Here, therefore, are the reasons why straight oils generally produce a better surface finish than water-based fluids. A dull wheel, with large wear flats, will rub and burnish the surface of the workpiece. The large flats will be efficiently lubricated by the oil, allowing a high degree of surface finish to be generated by the smoothing action of the flat grains. Oil has a boiling point higher than that of water by approximately 200 C, depending on the type of oil, so it can support higher surface temperatures before thermal breakdown occurs. Water-based fluids lubricate poorly and have a lower boiling point, so they suffer thermal breakdown under lower levels of heat, which is generated in the arc of cut. This can be illustrated by the phenomenon called “film boiling”. It can be seen by flicking water onto a very hot stovetop. The beads of water will not turn to steam, but will be supported on a film of steam gas, so that none of the heat from the stove top will be conducted or convected into the water. The beads of water will simply run off the stovetop, having taken little heat from the surface. In the grinding arc of cut, the heat is generated more at the top than at the bottom of the arc of cut. Therefore, particularly in long arc of cut processes like ID grinding and creep-feed grinding, the cutting fluid warms up as it travels around the arc of cut. It is most important, therefore, to keep the temperature of the cutting fluid constant as it enters the arc of cut. There are a number of heat sources which can increase the temperature of the cutting fluid. There is a tremendous amount of energy released from the pumping action of the system pumps, as well as from the churning of the fluid by the grinding wheel rotation. A significant amount of heat also comes directly from the energy of the grinding operation. A constant cutting fluid temperature is an important control over the process. If the cutting fluid is allowed to increase in temperature, it will rise closer to its boiling point, leaving little room for the extraction of the heat before the fluid turns to steam, in the form of film boiling, across the arc of cut.
One of the assumptions made, in the application of cutting fluid and the extraction of the grinding energy from the arc of cut, is the cutting fluid is indeed applied properly. There is a myth in the industry that there is an impenetrable air barrier around a rotating grinding wheel, which can be broken only by the combination of extremely high-pressure jets and deflector shields, etc. It is simple to dismiss the myth. It is true that there is a turbulent layer of air around a rotating grinding wheel. The surface of the wheel is rough and the surface speed can be in excess of 115 km/h (70 mph). If one takes a table tennis ball and drops in onto a rotating wheel, catching it before it hits another object, there will be scratches on the ball, proving that it must have touched the surface of the wheel. If a very light, spherical object can "burst" through the air barrier, then there is no need for ridiculously high-pressure jets of fluid or vast flow rates. In most grinding applications the cutting fluid is "dribbled" in the general direction of the arc of cut, and, although there are some surface tension effects which can assist in carrying the cutting fluid into the arc of cut, the great majority of the fluid merely cools the surface of the workpiece after it has been machined. This is a very bad situation, as cooling the surface directly after it has been machined means that the surface is quenched, causing surface tensile stresses, and initiating cracks and generally poor surface integrity. The key to proper application of the cutting fluid is to apply the fluid at wheel speed, then all of the open pores can be amply filled with fluid and transported around the arc of cut. If the fluid is applied in large volumes at anything less than wheel speed, then the wheel will not inherently suck the fluid into its pores. At best, some surface tension effects will drag a very small amount of the fluid applied into the arc of cut, but the rest of the fluid will be blown away by the "spindryer" effect of the rotating wheel. Therefore, it is important to manufacture cutting fluid nozzles so that the speed of the fluid is equal to or slightly faster than the speed of the grinding wheel.The reason for a slightly faster speed is that the nozzle will not be positioned close-up against the wheel. Over only a short distance the velocity of the fluid can drop off quite rapidly. A special case arises with very high-speed, plated wheels, where there is no porosity. A manifold and shoe is the best method for applying the cutting fluid to the wheel periphery. A large volume of fluid fills the manifold, is dragged up to wheel speed in the show, and then is taken into the arc of cut. Nonporous wheels also suffer from hydrodynamic lift or aquaplaning. The oil is compressed in the gap between the grain bond and workpiece surface. As the swarf fills the gap the pressure increses and lifts the wheel away from the surface being machined, creating a dimensional error as well as a change in chip geometry, which will affect the surface finish. Perforations can be made in metal-bonded and plated wheels in the form of slots or radial holes to dissipate the pressure and in some instances actually assist in the application of the fluid. The pattern of holes and slots on the periphery of a grinding wheel is critical. Peculiarly, when a series of slots are made in a grinding wheel, they are put in symmetrically and equally spaced, thus forcing a vibration from interruption in the periphery into the system, just like a milling cutter as it cuts into the material with each tooth. Random spacing of slots and holes is preferred in order to prevent a vibration or siren effect. The slots, and particularly the pattern of the holes, will most definitely affect the longevity of the form profile and, of course, decrease the amount of abrasive on the wheel periphery. When problems arise, one of the last things to do is break up the wheel periphery with slots and holes.
Maintaining the flow of fluid through to the end of the arc of cut is also a very important part of fixture design. As the grinding wheel exits the workpiece, the cutting fluid will splash off the back face of the part and cause starvation of the fluid in the arc of cut. The support dam, built into the fixture, helps to hold a volume of fluid in place through to the end of the cut. It is well to notice that as the grinding wheel exits the cut, there is less bulk of workpiece material which can act as a heat sink. The cutting fluid dam is an essential part of good grinding fixture design. In creep-feed grinding, in particular, where the arc length of cut is long, it is quite evident when a dam is necessary since a ramp will occur at the end of the cut. Heat from the grinding is conducted into the workpiece material as the wheel exits the cut. The heat causes the material to expand into the grinding wheel and the wheel machines the expanded material, which, once it has cooled to ambient temperature, shows up as a ramp off the end of the workpiece. It is important not to confuse such a ramp with deflection in the system. A ramp at one end (the exit end) of the workpiece signifies a thermal problem, whereas a ramp at both ends of the cut indicates a mechanical deflection.
