forward clamp structure 23 is in this embodiment provided by a clamp member 24 pivoted at its one end at 26 to the leg 2 and its other end 27 to the leg 6 the spacing of said pivot point 26 and its counterpart at the other end of clamp member 24 from the surface of the table 16 is such that the clamp will interfere with and bear against said table when same is rotated in a counterclockwise direction as viewed in fig1 but same can be released from such contact with said table upon rotation in a clockwise direction as viewed in fig1 a resilient device such as the spring 28 is fixed between the clamp 24 and a suitable anchor point such as the anchor point 29 on leg 1 and continuously urges said clamp for rotation in a counterclockwise direction a handle 31 may be provided if desired to facilitate manual pivoting of said clamp in a clockwise direction with the base material above mentioned extending from the roll 17 under the clamp 23 and across the table 16 as above described the hooking operation is carried out adjacent the end of said table normally in the zone z in such case the pin 19 will be inserted into the opening 18 and a selected opening 20 in registry therewith on said roll to prevent rotation of the roll 17 and the clamp 23 being urged by the resilient means 28 against said base will hold said base material down firmly against the table as desired to.