A vacuum drum assembly for a printing machine has a drum with an array of passageways distributed along its length and around its periphery. The passageways permit air to flow from outside the drum to inside the drum in response to reduced air pressure inside the drum. The assembly also has an array of valve members, each valve member being movable between a closed position in which that valve member restricts at least one of the passageways and an open position in which the restriction of that passageway or those passageways is reduced. The assembly is arranged such that, when a partial area of the drum is wrapped with a sheet of material, at least some of the valves for the passageways adjacent the edges of that area are open, and the valves for the passageways which are not covered by the sheet and are not adjacent the edges of that area are closed.