Dynamic contact length depends on the force between the grinding wheel and the workpiece in the same way that contact area between an automobile tyre and the road depends on the weight of the automobile. Dynamic contact length is given by Rowe, Morgan, and Qi [1993].
where Rr is a roughness factor that depends on the roughness of the grinding wheel. Typically, Rr is of the order of 10. Fn is the normal grinding force, b is the width of contact and is usually equal to the width of the grinding wheel or the length of the workpiece. E* is the effective modulus of elasticity of the grinding wheel and the workpiece materials. The value is given by
 |
 |
 |
The grinding process is affected by the equivalent grinding wheel diameter. The effective diameter in internal grinding is much greater than in external grinding even though internal grinding wheels are normally much smaller than external wheels. Equivalent diameter is defined as the wheel diameter that gives the same contact condition as in flat surface grinding. The equivalent diameter is based on the relative curvature of the two surfaces in contact.
The negative sign applies for internal centerless grinding. Due to the conformity in internal grinding, a softer grinding wheel is required to withstand the increased length of contact and the consequent increase in rubbing contact. The longer geometric contact length increases rubbing wear and tends to cause the wheel to glaze.
19.3.7 Equivalent Chip Thickness
Equivalent chip thickness is a measure of the depth of penetration of the abrasive grains into the workpiece. Equivalent chip thickness is given by
h = a ■-w
eq e v
Equivalent chip thickness is the thickness of the layer of workpiece material removed at wheel speed. Since the speed ratio is typically 100, the thickness of the layer removed at wheel speed is approximately one hundredth of the thickness of the layer removed at workspeed. With 0.05 mm depth of cut, equivalent chip thickness is typically 0.5 pm.
In practice, depth of penetration of the abrasive grains into the workpiece varies over a much greater range due to variable spacing between grains, variable depth of grains below the wheel surface, and surface roughness. A rough worn grinding wheel causes much greater grain depths than the same wheel freshly dressed. However, equivalent chip thickness is useful for comparing grain penetration for different grinding operations using a similar abrasive structure.
19.3.8 Grinding Ratio
 |
 |
 |
Grinding ratio is a measure of the suitability of an abrasive for a particular grinding operation. Grinding ratio is defined as:
Hard work-materials and interrupted cutting operations tend to reduce the grinding ratio. In precision grinding with easy-to-grind materials, a G-ratio of 5,000 or more may be achieved. However, for a difficult-to-grind material operating under adverse conditions, the G-ratio may drop to 1. In adverse conditions, it may be important to increase wheel speed to increase G-ratio.