The theoretical investigation of the cutting force of rotary ultrasonic grinding machining(RUGM) for engineering ceramics in a brittle regime was conducted for the purpose of solving the limitations of the experimental results due to the limited range of process parameters and the number of experiments. Based on the mechanism of RUGM,the formula of the cutting force caused by the bottom and side machining of a single diamond in the brittle regime was deduced by the theorem of linear momentum and kinetic ene...