Grinding ratio is the quick method of evaluating the grinding wheel performance. Grinding operation removes metal very delicately, leaving a finished surface of superior dimensional accuracy and good surface texture. If it is carried out properly, there will be very less metallurgical damage due to the rise of pressure and temperature of the material being ground.
Comparing with any other machine operation the damage depth of the finally ground surface is very small; grinding can remove a surface layer, previously damaged by any machining operation without introducing any serious damage to the new surface.
The term grinding ratio is defined as the ratio between the volume of metal removed from the workpiece to the wear of grinding wheel. If a graph is drawn between the wear of grinding wheel to the volume of metal removed then the tangent to the curve will represent the grinding ratio.
In Fig. 20.12 a graph is drawn between the wear of the wheel taken on X-axis and the volume of metal removed taken on Y-axis, both in cm3.
In the beginning of curve it is observed that the volume of metal removed is much more in comparison to wear of the wheel, the reason being that in the beginning the abrasive particles are firmly wound on the wheel and the surfaces are rough so that the removal of metal takes place very rapidly.
After some time, the abrasive particles begin to loose contact due to the rise of pressure and temperature and the surfaces also attain moderate finish and hence the removal of metal is also moderate. In this region the graph is having a very less slope and the curve is normally a straight line.
During the latter stage the wear of wheel is at much faster rate than the removal of metal. This is because more and more abrasive particles start falling down due to wear and tear and the surfaces also become smoother and smoother. The smoother is the surface, the more difficult it becomes to remove any layer from it and the curve takes a sloppy form.
In the curve a and b are two points. Drop perpendicular from b on X-axis and a line parallel to the X-axis from a thus obtaining point c.
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The length ac (δx) represents the volume of metal removed and the length be (δy) represents the volume of wear of tool and so the grinding ratio will be as follows:
Volume of metal removed / Volume of wear of tool = δx / δy
The grinding ratio normally varies from 75 to 125. The greater the grinding ratio the better wheel is.