Grinding device (1) for an automatic coffee machine, comprising: a support body (2); a lower grinder (3) and an upper grinder (4) arranged coaxially with respect to an axis (X) and delimiting a gap where the grinding is performed; a motor (21) for driving at least one of said grinders (3,4) in rotation about said axis (X); an adjustment ring nut (6) for adjusting the width of said gap having means for transforming a rotation thereof about said axis (X) into a translation of at least one of said grinders in the direction of said axis (X); and a gear for driving said ring nut (6) in rotation; where said gear comprises a toothed wheel arc (7) engaging a pinion (8), and a mechanism for fixing said toothed wheel arc (7) in an adjustable position on said ring nut (6).