The effect of workspeed is defined by Peclet number L. Values greater than 5 represent reasonably high workspeed, although much higher values can be achieved and allow cool grinding. Peclet number L is given by
v • l p-c L = w c 4 • k
where k = thermal conductivity, p = density, and c = specific heat capacity of the work material.
A large depth of cut and a small wheel diameter lead to a large contact angle (Figure 17.7).
The transient thermal property, Д*, of the workpiece material is given by
Pw =Vk p-c
Workpiece heat partition ratio, Rw, is the proportion of the grinding energy that is conducted into the workpiece. Rw is a function of the wheel grain conductivity and sharpness and of the transient thermal property. Ignoring for the present, coolant convection and convection by the grinding chips, Rw approximates to Rws. Hahn (1962) showed that
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-1
where kg is the thermal conductivity of the abrasive grain. Grain sharpness is related to r0 the contact radius of the grain. Rws is relatively insensitive to variations of r0. Typically, Rws for conventional grinding varies between 0.7 and 0.9 for vitrified wheels and between 0.4 and 0.6 for CBN wheels. After allowing for heat convected by the coolant and by the chips, Rw can be greatly reduced below R.
ws
The temperature equation for conventional shallow grinding, can, therefore, be very approximately reduced, for a given wheel/work/machine configuration to
ignoring heat taken by coolant and chips.
In this case, it follows that increasing wheel speed, increasing depth of cut, or increasing the number of active cutting edges (by, e. g., dull dressing) increases the surface temperatures. However, taking account of heat convected by the coolant and by the chips as follows shows that much lower temperatures can be achieved than would be expected.