The transient thermal property of fiw of the workpiece material is given by
Pw =Vk. p. C
where к = thermal conductivity, p = density, and c = heat capacity.
2.3.4 Workpiece Partition Ratio Rw
Workpiece partition ratio Rw is the proportion of the grinding energy that is conducted into the workpiece. The work partition ratio is a complex function of the wheel grain conductivity and sharpness and of the workpiece thermal property. Ignoring, for the present, coolant convection and convection by the grinding chips, Rw approximates to Rws. Hahn [1962] modeled heat transfer between a sliding grain and a workpiece. It can be shown that
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-1
kg is the thermal conductivity of the abrasive grain and r0 is 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.
2.3.5 Effect of Grinding Variables on Temperature
The temperature equation for conventional grinding can, therefore, be very approximately reduced for a given wheel/work/machine configuration to
T xJa ■ v ■ C ■ r
max ye s
from which it follows that increasing the wheel speed, increasing the depth of cut, or increasing the number of active cutting edges (e. g., by dull dressing) will increase the surface temperatures. Further discussion of temperatures generated when grinding at very high wheel speeds is made in a later chapter.