referring to the drawings , fig1 shows a cylinder assembly of a rotary web processing machine which is used for forming the crease markings on milk carton blanks . this assembly is removably mounted in the frame of the machine ( not shown ). the assembly has a pair of side plates 1 which rotatably support a pair of cylinders 2 . each of the cylinders 2 is constituted by a cylindrical shaft 3 , a hollow tubular mandrel 4 fixed to the shaft 3 , and four creasing rings 5 detachably fixed to the mandrel . the end portions of the shafts 3 are supported in the side plates 1 by means of bearings 6 . at one side of the assembly the free ends of the shafts 3 are provided with gear wheels 7 . the gear wheels 7 intermesh so that , when one of the gear wheels is driven from the machine by a motor ( not shown ), the two shafts 3 rotate in opposite directions in synchronism . the shafts 3 are mounted in the side plates 1 in such a manner that the creasing rings 5 of the two cylinders 2 define a nip through which a web 88 of board can pass . one of the cylinders 2 is arranged to be vertically adjustable , so that the size of the nip can be varied . this enables webs of different thicknesses to be processed by the machine , and also permits the degree of penetration of the creasing rings 5 into the web 88 to be varied . each of the creasing rings 5 is constituted by two identical parts 5a . each ring part 5a is detachably fixed to the mandrel 4 by means of two screws 8 and 9 . each screw 8 is an axial and radial locating screw and a clamping screw , and each screw 9 is a radial clamping screw . the radial clamping screws 9 pass through aligned apertures 9a and 9b formed respectively in the associated ring part 5a and the mandrel 4 . each axial and radial locating and clamping screw 8 passes through hardened bushes 10 and 11 formed respectively in aligned apertures 8a and 8b formed respectively in the associated ring part 5a and the mandrel 4 . as shown in fig3 the apertures 8a and 8b are stepped , and the screws 8 each have a head 8d , a smooth cylindrical shank 8e and a threaded shank 8f . the threaded shank 8f of each screw 8 engages within a threaded portion 8g of the associated aperture 8b in the mandrel 4 . the cylindrical surface of each ring part 5a is formed with a respective key slot 12 . the two key slots 12 of one part 5a of each ring 5 are adapted to be aligned with the key slots 12 of the other part 5a of that ring . a respective key 13 is provided for engaging within each pair of aligned key slots 12 , and a pair of screws 14 are provided for fixing each of the keys to the associated ring parts 5a . the keys 13 are used to hold the ring parts 5a together prior to assembly of the rings 5 onto the mandrel 4 . as shown best in fig2 the mandrel 4 is provided with a plurality of sets of apertures 8b and 9b , each set having a pair of diametrically opposed apertures 8b for accommodating the screws 8 , and a pair of diametrically opposed apertures 9b for accommodating the screws 9 . the mandrel 4 is provided with a considerably greater number of sets of apertures 8b and 9b than there are rings 5 , so that the rings 5 can be fixed to the mandrel at different locations and spacings . in order to fix a ring 5 on its mandrel 4 , the two parts 5a thereof are loosely held together by engaging the keys 13 in the aligned key slots 12 , and by loosely threading in the fixing screws 14 . the two axial and radial locating and clamping screws 8 are then threaded through the apertures 8a in the rings parts 5a , and into the apertures 8b in the mandrel 4 . the cylindrical shanks 8e of the screws 8 engage within the bushes 10 and 11 to effect accurate alignment of the rings parts 5a both radially and axially with respect to the mandrel 4 . the screw threaded portions 8f and 8g then engage to clamp the rings parts 5a to the mandrel 4 . the radial clamping screws 9 are then screwed into position to clamp the rings parts 5a firmly to the mandrel 4 . the screws 14 are then tightened up to complete the fixing of the ring 5 . if a ring 5 is required to be repositioned ( for example where the rings 5 of a given cylinder 2 have to be repositioned laterally in order to make milk carton blanks of different widths ), it is necessary only to loosen the screws 14 to remove the screws 8 and 9 , to slide the ring along the mandrel 4 to the new position , and then to replace the screws 8 and 9 and retighten the screws 14 . obviously , this procedure is considerably simpler and less time - consuming than the equivalent procedure for replacing the one - piece creasing rings of the known type of web processing machine . in particular , there is no need to remove the cylinders 2 from the assembly to carry out the lateral repositioning of the creasing rings 5 . moreover , the provision of the axial and radial locating and clamping screws 8 ensures that the rings 5 are always accurately positioned both axially and radially . moreover , the repositioning process is both rapid and easy , so frequent ring changes cause no problems . in this connection , the provision of the hardened bushes 10 and 11 substantially reduces wear , and so increases the life of the mandrel . obviously , where the repositioning of the creasing rings 5 requires the addition of one or more rings 5 this can easily be accomplished by fixing the two parts 5a of a new ring onto the mandrel 4 at the required position . similarly , where a ring 5 needs to be removed , this is easily accomplished by completely removing the screws 14 of the ring concerned , and separating the two ring parts 5a of that ring .