A mowing or grinding device comprising a rotatable shaft which carries a plurality of mowing or grinding tools which form flails, each tool being mounted directly or indirectly on the rotatable shaft by means of at least one hinged connection with preferred positioning, which allows keeping the tool in the operating position to be favored and comprises a mounting aperture and an axial support which passes through the mounting aperture； the device being characterized in that the positioning portion of the axial support has at least one cylindrical boss which has a circular surface and the positioning portion of the mounting aperture has at least one concave housing which has a circular surface which is capable of receiving the circular surface of the cylindrical boss when the tool is in the operating position, the circular surface of the concave housing having a constant curve which is complementary to the circular surface of the cylindrical boss over a periphery which is greater than a quarter circle.
