<p>Longitudinal-torsional (L-T) ultrasonic-assisted milling can improve machining performance by introducing an additional torsional component to the tool vibration. However, current designs of L-T ultrasonic vibration systems mainly focus on the transducer structure, while the tool is often treated as a passive load. In fact, the helical cutting edges of a standard milling tool are geometrically similar to the inclined slots used in L-T transducers, and may therefore affect vibration coupling. In this paper, the L-T vibration output of an ultrasonic vibration unit (UVU) is investigated by considering the helical cutting-edge geometry of the milling tool. The effects of relative helical orientation, transducer slot parameters, tool helix angle, and tool overhang length are analyzed. The electrical characteristics and vibration output of the UVU are further characterized through impedance and acoustic pressure measurements. The results showed that the helical cutting-edge geometry significantly affected the tool torsional amplitude and the torsional-to-longitudinal ratio, with the relative helical orientation being an important factor. The counter-directional configuration produced a larger torsional amplitude than the co-directional configuration, suggesting that the orientation effect was not a simple geometric superposition.</p>