A machine for assembling cardboard boxes, with a feeding device for sheets of cardboard comprising a frame to support the sheets of cardboard, where the machine comprises: - an assembling station for cardboard boxes, - a male former located in relation to the assembling station, which descends vertically into an assembling mould, where - the assembling mould is facing the male former and arranged under said male former, - a sucking mechanism for sucking sheets of cardboard from the frame, said sucking mechanism being adaptable to different dimensions of the sheets of cardboard to be used, and - a pushing system able to push the sheets of cardboard towards the assembling station that identifies advancing and stopping positions in relation to the measurements of the sheet inserted by the user in the machine, the machine further comprises: - a first guiding means for guiding the sheets of cardboard along the feeding device, connected to a second guiding means of the sheets of cardboard moving towards the assembling station, so that a single adjusting means is able to completely control said guiding means (10, 13) in order to adapt them to the different dimensions of the sheets of cardboard to be used by regulating the width of said first and second guiding means (10, 13), - where the frame to support the sheets of cardboard is joined to two drag chains independent from each other, - where the drag chains are partially supported on the machine, leaving at least one third of said drag chains hanging and projecting from the machine, - where the drag chains are mounted on at least two spindles that independently regulate the distance between the two drag chains in order to adapt them to the dimensions of the sheet of cardboard, and the sheets of cardboard are able to be stacked vertically on the drag chains; and where the feeding device for sheets of cardboard, the first and second guiding means, the sucking mechanism and the pushing system are interconnected by means of the frame; and the sucking mechanism comprises two parallel guides with suction pads located on the upper part of the parallel guides, where said parallel guides are joined together at the end thereof by means of an axis; and the axis, together with the parallel guides with the suction pads, are actuated by means of a servomotor, where the turning of the servomotor moves a disk and a bushing that make contact with the axis, with said movement of the disk and the bushing resulting in an angular turning of the axis and the parallel guides with the suction pads from a horizontal position to a vertical position, where said suction pads are able to suck the sheets of cardboard from the frame, and are able to place the sheets in a horizontal position; the distance between said parallel guides is able to be regulated in order to adapt to the dimensions of the sheet; and the pushing system comprises a servomotor, which as it turns causes a pushing element to push the sheets of cardboard horizontally along guides. - an assembling station for cardboard boxes, - a male former located in relation to the assembling station, which descends vertically into an assembling mould, where - the assembling mould is facing the male former and arranged under said male former, - a sucking mechanism for sucking sheets of cardboard from the frame, said sucking mechanism being adaptable to different dimensions of the sheets of cardboard to be used, and - a pushing system able to push the sheets of cardboard towards the assembling station that identifies advancing and stopping positions in relation to the measurements of the sheet inserted by the user in the machine, - a first guiding means for guiding the sheets of cardboard along the feeding device, connected to a second guiding means of the sheets of cardboard moving towards the assembling station, so that a single adjusting means is able to completely control said guiding means (10, 13) in order to adapt them to the different dimensions of the sheets of cardboard to be used by regulating the width of said first and second guiding means (10, 13), - where the frame to support the sheets of cardboard is joined to two drag chains independent from each other, - where the drag chains are partially supported on the machine, leaving at least one third of said drag chains hanging and projecting from the machine, - where the drag chains are mounted on at least two spindles that independently regulate the distance between the two drag chains in order to adapt them to the dimensions of the sheet of cardboard, and the sheets of cardboard are able to be stacked vertically on the drag chains; and