<p>Chatter in long-overhang boring severely degrades surface quality and shortens tool life. Although a tuned mass damper (TMD) can alleviate this vibration, a conventional fixed-parameter TMD is optimized for one specific frequency and quickly loses effectiveness when the excitation frequency changes. To overcome this limitation, previous studies have proposed vibration-reducing toolholders that adjust a single TMD parameter—damping, stiffness, or mass—but their adaptability remains inadequate when cutting speed, feed rate, or depth of cut varies. This paper presents a novel boring-bar system whose TMD enables independent, high-precision adjustment of both stiffness and damping, thereby achieving superior vibration suppression under variable machining parameters. The absorber stiffness is provided by a cantilever whose effective length—and hence stiffness—is altered by repositioning its support, whereas the damping is regulated in real time through eddy-current dissipation. By analyzing the vibration behavior of the boring process and the governing principles of TMD control, explicit relationships between the TMD parameters and diverse excitation frequencies are established, and the effects of cutting load and system parameters on vibration-reduction performance are evaluated. Experiments are conducted on the boring bar system to verify the accuracy of vibration reduction capability and theory.</p>