16.5.1 Creep Feed Grinding with Vitrified Wheels Containing Alox and Silicon Carbide
In general, as depths of cut increase so do grinding forces, while uncut chip thickness and, therefore, roughness and force/grit decrease. However, when the depth of cut becomes extreme, that is, greater than 1 to 3 mm maximum grinding temperatures can actually fall. Grinding using a combination of slow table speeds and deep depths of cut defines the creep feed (CF) process. Interest started in this field in the 1950s but reached its zenith from a research viewpoint in the 1970s at the University of Bristol, United Kingsom, driven by the aerospace industry and the need to grind highly burn — sensitive nickel and cobalt-based high-temperature alloys.
The creep feed process was developed using soft (E or F), highly porous conventional wheels at relatively low wheel speed of 15 to 30 m/s to keep frictional heat to a minimum, to limit required coolant pressure, and to limit wheel structural strength. The wheels were dressed using formed diamond rolls. Dressing was at first intermittent during the cycle but it was subsequently found that continuously dressing at infeed levels of 0.2 to 2.0 pm/rev of the wheel not only maintained the profile but kept the wheel sharp, thus allowing much higher stock removal rates than had previously been seen. This latter process is the familiar continuous dress creep feed (CDCF).
16.5.2 Coolant Application in CF Grinding
Coolant application is absolutely critical to the process. CDCF is usually carried out using a water- based coolant that fills the highly permeable wheel structure when supplied correctly and under sufficient pressure. The coolant then becomes the primary source of heat removal and maintains the part surface at a temperature at or below 130°C, the boiling point of the coolant under the hydrodynamic pressure conditions in the grind [Howes 1990]. The coolant is excellent at maintaining surface temperatures by the efficient removal of heat until the heat flux exceeds the heat capacity of the fluid. At this point, the fluid boils effectively eliminating all benefits of the coolant and temperatures rapidly climb to those experienced in dry grinding (Figure 16.10). The phenomenon is known as “film boiling” [Howes 1990, 1991].
The heat capacity of the coolant is proportional to the bulk temperature of the incoming fluid. As can be seen from Figure 16.10, reducing the incoming coolant temperature by 40°C to 20°C raises the critical power flux by a factor of 2. Similarly, increasing the coolant pressure to an optimum value where the coolant and wheel velocities are matched maximizes the critical power flux.
It is interesting to note that straight oil, due to its lower heat capacity, is less capable of cooling the workpiece and will cause the part surface to be hotter and much more likely to burn [Ye and Pearce 1984]. However, the higher film boiling temperature of the oil (ca. 300°C) is less likely to cause rapid and catastrophic failure of the coolant. Water is the better coolant, unless finish or wheel wear is the overriding issue because of the large capacity of the grinding wheel pores to hold coolant.
The impact of film boiling can be very marked. As the water turns to steam, there is a rapid rise in temperature of the workpiece causing it to thermally expand. This leads to a sudden increase in depth of cut and additional heat generation followed by massive wheel breakdown. The workpiece then cools to leave a series of deep, usually blackened troughs in the surface (Figure 16.11). The effect can often be detected in process by surges in the wheel head spindle power.