We propose ultrasonic rotary cutting, in which ultrasonic vibrations are imparted to a rotating cemented carbide cylindrical tool to cut hardened steel to reduce the cutting resistance and improve the properties of the machined surface, and investigate the machining characteristics. Machining experiments were conducted under dry and wet conditions to verify the effects of the ultrasonic vibrations. The surface produced via ultrasonic rotary cutting was intermittently machined, which is characteristic of ultrasonic cutting. In dry machining, the cutting resistance was reduced by approximately 20%, and the surface roughness of the machined surface was reduced by approximately 30% when the cutting speed was below the critical speed. We also demonstrated that the surface roughness was improved by ultrasonic vibrations when the cutting speed was equal to or above the critical speed. A similar tendency was observed in wet machining with longer cutting lengths. We then applied ultrasonic rotary cutting to machine a straight R groove in hardened steel and showed that the cutting resistance was reduced, and the tool engagement was improved.

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