An articulated milling arm for a milling instrument, which includes a milling arm support movably attached to a frame to allow vertical displacement of the milling arm support; a first vertical axle, rotatably connecting the first milling arm support to a middle portion of the milling arm to allow swiveling of the middle portion of the milling arm horizontally about a vertical axis of the first vertical axle, the first vertical axle being provided with a braking surface; a second vertical axle, rotatably connecting the middle portion of the milling arm to a milling unit support to allow swiveling of the milling unit support horizontally about a vertical axis of the second vertical axle, the second vertical axle also being provided with a braking surface. The milling arm is provided with a movable braking plate, on the middle portion of the milling arm, which movably contacts the braking surface of the first vertical axle and the breaking surface of the second vertical axle. The invention further provides that the common braking plate is pressurized pneumatically or hydraulically via a diaphragm which presses against the braking surfaces of the two vertical axles in order to fix the articulated milling arm horizontally.