Although there have been several studies on machining of semiconductor materials, most of them are concerned with abrasive finishing (such as chemical-mechanical polishing, mechanical polishing and lapping), and only a few have reported on optimizing rotary grinding conditions (such as the grinding wheel rotational speed, wafer rotational speed, wheel diameter, wafer diameter, and feed rate). In this study, to gain further insight, we define a dimensionless number and use it to evaluate our experimental results. This dimensionless number — called the “grain approach angle” — is the ratio of the grain running length of the machined wafer surface and the depth of cut due to grain during the grain running time. For this study, our evaluation coefficients are taken to be surface roughness, grinding force, and grinding ratio. We found it more suitable to use the grain approach angle rather than a previously defined dimensionless number (which was evaluated as the ratio of the wheel rotational speed to the wafer rotational speed). We also found that, by using the grain approach angle, trends in surface roughness exhibited invariant similarities under varying conditions.