Abstract: In a fodder harvesting machine, the device disclosed comprises a rotating chopping drum (1) and a fixed counter-knife (25) cooperating with the knives (2) of the drum (1), as well as a retractable wheel (21) adapted to be moved in rotation in order periodically to sharpen the knives (2). According to the invention, at least two positioners (5, 6, 7, 8) located near the free ends of the chopping drum (1) are mounted to pivot about the geometrical axis thereof and connected to an actuation device (a) bringing, by pivoting, the end contact piece (8) of each positioner (7) either opposite the wheel (21), or opposite a corresponding stop (32) of the counter-knife (25) which is fastened to a fixed support (3, 35) via a presser device (34) and a fixing device (33).