The process limits define the permissible range of speed conditions for stable grinding. Typical process limits are shown in Figure 19.23. Two examples are illustrated for a 75-kW centerless grinding machine. The gray cast iron is an easy-to-grind work material and specific material removal rates were achieved in excess of 40 mm[10] [11]/s. When grinding EN9 steel, the specific removal rate achieved was 20 mm2/s. These removal rates were achieved using conventional abrasive wheels at a speed of 60 m/s.
The rate of grinding can be increased up to process limits of maximum machine power available, onset of thermal damage to the workpieces, and chatter. Surface roughness depends primarily on the grinding wheel employed and the dressing process. Initial trials to ascertain the size of the operating area is essential as a first step to process optimization.
The results clearly show the benefits of increasing wheel speed. The removal rates at 60 m/s are more than double the rates achieved at 30 m/s.
An interesting feature of the limit charts is the similarity of the shape for two different materials. This supports the conclusion that these diagrams have general validity.
The charts show that high workspeeds increase the probability of chatter. Low workspeeds increase the probability of burn. Low workspeeds concentrate the process energy in the contact zone for a longer period and so increase the susceptibility to thermal damage. Thermal damage, or burn as it is generally termed, can also give rise to a form of chatter that occurs at low workpiece speeds.
High grinding wheel speeds allow higher infeed rates to be employed for the same grinding forces and thus allow higher removal rates.
19.9.3.1 Effect of Infeed Rate
Material removal rate increases with infeed rate. However, as infeed rate is increased, material removed per grit also increases. This is apparent from the equivalent chip thickness expressed in terms of infeed rate and wheel speed.
Increasing chip thickness leads to higher stresses on the grinding grits causing greater wear and fracture. A consequence is “self-dressing” where the grits maintain or increase their sharpness due to the grinding stresses. This yields the benefit of reducing the energy required to remove a volume of material and produces lower specific energy. The disadvantage of the grit fracture process is higher surface roughness and faster wheel wear. The process where specific energy is reduced due to increased feedrate is termed the “size effect.” The size effect can be explained in several different ways but basically the conclusion is that increased chip thickness or increased volume of material removed per grit reduces specific energy.
Increasing infeed rate, with other parameters constant, tends to increase grinding forces, increase roughness, reduce redress life, and reduce specific energy. The process tends to become more efficient until the optimum chip size is exceeded. Excessive infeed rate leads to high wheel wear, a low grinding ratio, and rapid wheel breakdown.