A bending machine for metal sheets comprises punching means (2) including an upper crosspiece (3) adapted to support a plurality of upper bending tools (40, 41, 42) aligned and reciprocally positionable along the longitudinal bending direction (A) to form definite compositions of upper bending tools; die means (4) including a lower crosspiece (5) adapted to support a plurality of lower bending tools (30-35) aligned and mutually positionable along the bending direction (A) to form respective compositions of lower bending tools; at least one back gauge unit (6) movable inside an operating area (W) of said bending machine (1) and arranged to abut and position a workpiece with respect to the die means (4); the back gauge unit (6) comprises a hooking element (7) arranged to engage at least one lower bending tool (30-35) to move the latter along the bending direction (A) on the lower crosspiece (5); the plurality of lower bending tools (30-35) comprises at least a set of side lower bending tools (30) that are positionable in a respective non-operative configuration (R1) at an end portion (5a, 5c) of the lower crosspiece (5) outside the operating area (W) and unreachable by the back gauge unit (6); the side lower bending tools (30), being reversibly connectible to one another in order to form a group (G) of side lower bending tools (30) selectable in number; at least one of the side lower bending tools (30) of the group (G) is positioned in the non-operative configuration (R) inside the operating area (W) so it can be engaged by the hooking element (7) of the back gauge unit (6) and movable by the latter along the bending direction (A) together with the group (G).