TECHNICAL BACKGROUND OF THE INVENTION The present invention relates to an air distribution assembly for a rotary cutting apparatus having a shaft and a mantle, the mantle having at least one cutting member. The invention also relates to a rotary cutting apparatus provided with such an air distribution assembly. Air distribution in a rotary cutting apparatus is previously known and is performed by radial bores formed in the circumferential surface of a solid rotary cutter. Axial bores connect the radial bores with sources of vacuum and/or atmospheric pressure or over-pressure. Drilling of such axial and radial bores is time consuming and expensive, in particular since they have to be made with high accuracy. US-A-4,770,078 discloses in a discussion of the prior art (Figures 1-3) a one piece rotary cutter, which has to be removed from the frame when maintenance is needed. In order to allow the machine to be used during maintenance, a further rotary cutter including its static shaft must always be accessible. In order to overcome that problem, US-A-4,770,078 suggests to divide the rotary cutter into a rotatable shaft and a mantle. The mantle is connected to the rotatable shaft by means of pneumatic pressure. A drawback with this kind of rotary cutter is that it is difficult to index the rotary cutter relative to the anvil. Another drawback is the lack of support of the rotary cutter on the side opposite to the driven side.

SUMMARY OF THE INVENTION An object of the invention is to provide the known rotary cutter with a simplified connection to sources of vacuum and/or atmospheric pressure or over¬ pressure. This has been achieved by the air distribution assembly of the initially defined kind, which comprises a first air distribution part adapted to rotate together with said mantle and a second air distribution part adapted to be connected to the center shaft. Preferably, said first air distribution part is provided with openings adapted to correspond to through holes of said mantle and wherein said second air distribution part is provided with openings adapted to at least intermittently correspond to the openings of said first air distribution part. Hereby is achieved a simplified distribution of air to the exterior of the surface of the rotary cutter. More particularly, said first air distribution part is provided with at least one first opening adapted to correspond to through holes of the mantle intended to influence a region of the mantle relating to the residues of a cut sheet or the sheet to be cut, and at least one second opening adapted to correspond to through holes of the mantle intended to influence a region of the mantle regarding the cut article or the article to be cut. Alternatively, only the region comprising the first or the second opening is influenced. In addition, said second air distribution part is provided with at least one third opening adapted to at least intermittently correspond to said first opening, said third opening being associated with a source of vacuum. Suitably, said second air distribution part is provided with at least one fourth opening adapted to at least intermittently correspond to said second opening, said fourth opening being associated with a source of vacuum. Alternatively, said second air distribution part is provided with at least one third opening adapted to at least intermittently correspond to said first opening and/or to said second opening, said third opening being associated with a source of vacuum; Suitably the radial peripheral surface of the second air distribution part is provided with a groove. Hereby are achieved different possibilities of controlling the air flow from different through-hole on the surface of the rotary cutter. Preferably, said second air distribution part is provided with at least one fifth opening for influencing a region of the mantle regarding the region of the mantle regarding the cut article and/or the residue of the cut sheet, said fifth opening being associated with atmospheric pressure or a source of over-pressure. Suitably, the radial peripheral surface of the second air distribution part is provided with a groove for performing the influence to the region of the mantle regarding the cut article and the residue of the cut sheet. Hereby are achieved different possibilities of controlling the air flow to different through-hole on the surface of the rotary cutter. Advantageously, said first air distribution part is adapted to be arranged radially peripheral to that of said second air distribution part. In particular, said first air distribution part is substantially circular cylindrical and said second air distribution part is substantially circular cylindrical, and wherein said first and second air distribution parts are coaxially arranged in a rotatable interrelationship. Hereby, a suitable shape of the air distributor parts is achieved. Preferably, at least one of said shafts is hollow and is associated with a source of vacuum, said second air distribution part being associated with said hollow shaft. Hereby is achieved a simple and efficient air distribution to the exterior of the rotary cutter. This has been achieved by a rotary cutter and a rotary cutting apparatus of the initially defined kind, wherein the shaft is adapted to be rigidly mounted in a frame part, and wherein the mantle is rotatably arranged relative to the shaft. Hereby, indexing of the mantle relative to the shaft is made easier, since the mantle can be rotated relative to the static shaft. Furthermore, it is only necessary to perform maintenance of the mantle, i.e. the shaft can be used together with another mantle such that the production can be continued while maintenance is performed on the worn mantle. Preferably, the mantle is adapted to be connected to a power source for creating a rotational movement of the mantle. Preferably bearings are provided between the mantle and the shaft. Hereby, a controlled positional and rotational relationship between the static shaft and the mantle is achieved. Suitably, a power transmission means is provided for transmitting the rotational movement to said mantle. Advantageously, the mantle has an axial extension and opposite axial ends, wherein said mantle is adapted to be supported by the shaft and connected to the power source in the region of one of the ends of said mantle, and wherein said mantle is adapted to be supported by the shaft in the region of the opposite end. Alternatively, said shaft is divided into a first and a second shaft member, the mantle having an axial extension and opposite axial ends, wherein said mantle is adapted to be supported by the first shaft member and connected to the power source in the region of one of the ends of said mantle, and wherein said mantle is adapted to be supported by the second shaft member in the region of the opposite end. Advantageously, the frame part of the rotary cutting apparatus further comprises a fastening means for said shaft and a power transmission connection means for said mantle.

DRAWING SUMMARY In the following, the invention will be described in greater detail by reference to the accompanying drawings, in which FIG. 1 illustrates a rotary cutting apparatus comprising an anvil and a cross- section of a rotary cutter according to a first embodiment of the invention; FIG. 2 is a cross-section of a second embodiment of the rotary cutter; FIG. 3 is a cross-section of a third embodiment of the rotary cutter; FIGS. 4A and 4B is an exploded view and an axial cross-section of the first embodiment, provided with an air distribution assembly; FIGS. 5A and 5B are cross-sections in part of the air distribution assembly shown in FIG. 4B provided with first and a second air distribution parts; FIG. 5C is a perspective view of the second air distribution part; FIGS. 6A and 6B are cross-sections in part of alternative air distribution assemblies; and Figs. 7A and 7C illustrate the air distribution parts and the mantle in different angular positions.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 shows a rotary cutting apparatus 2, comprising a rotary cutter 4 and an anvil roll 6. The rotary cutter 4 comprises a divided static (stationary) shaft 8, comprising axially spaced shaft members 8a, 8b, each member being rigidly connected to a respective frame part 10a, 10b by means of screws 12. A drive axle 14 is associated with a not-shown power source for transmitting a rotational movement to a tool in the form of a mantle 16 via an intermediate transmission member 18a. The rotational movement is further transmitted to a rotational support 18b. Cylindrical bearings 19 are provided between the shaft members 8a, 8b and the intermediate transmission members 18a, 18b, respectively, for centering the mantle 16 relative to the shaft members 8a, 8b. The frame parts 10a, 10b are secured to the rest of the frame by means of suitable, not-shown fastening means. The mantle 16 is provided with at least one cutting member 17 (See Fig. 4A) which is endless so as to be able to cut an article from a sheet. During disassembly, the frame parts 10a, 10b are unsecured from the rest of the frame such that the static shaft members 8a, 8b including the transmission members 18a, 18b can be pulled out from the mantle 16. The mantle 16 is taken away and maintenance can thus be performed. Another mantle 16 is mounted in place of the other one, and the rotary cutting device can be utilised without long stoppage. Of course, it may be enough to take away either of the static shaft members 8a and 8b, respectively, rather than both. FIG. 2 shows a second embodiment of a rotary cutting apparatus 2' and rotary cutter 4\ The cross-section is such that the cutting member 17' has been omitted, but is of course present (see FIG. 4). The static shaft 8' is in this case a single part and is connected to the frame 10' on either side of the mantle 16' by screws 12'. The rotational movement of the drive axle 14' is transmitted to the mantle 16' via a gear train 20a1, 20b1, 20c', 2Od'. It should be noted that the parts 20b1, 20c1, 2Od' could be produced as two pieces or even one single piece. Centering is performed by means of cylindrical bearings 19'. During disassembly, the frame part 10b1 in the right part of FIG. 2 is unsecured from the rest of the frame (i.e. the left frame part remains secured). The mantle 16' can then be pulled away from the shaft 8'. FIG. 3 shows a third embodiment of a rotary cutting apparatus 2" and a rotary cutter 4". Also in this case, the cross-section is such that the cutting member has been omitted. The drive axle 14" transmits rotational movement directly to the mantle 16" via a coupling member 22". The static shaft 8" is divided into two shaft members 8a", 8b" connected to the frame parts 10a", 10b" on either sides of the mantle 16". The mantle 16" is centered relative to the shaft members 8a", 8b" and the driving axle 14" by means of conical bearings 24". For maintenance purposes, the shaft member 8b" is unsecured from the frame parts 10a", 10b", and then the mantle 16" is released from the shaft member 8a". The mantle 16, 16', 16" may be made of a multiphase material, such as steel, cemented carbide or cermet (hard phase bonded by a metal). FIG. 4A shows a rotary cutting apparatus 2'" and a rotary cutter 4'" similar to the first embodiment (see FIG. 1), so the same reference numerals designating the same elements as in FIG. 1 will be used in FIG. 4A. A major difference between the embodiments of FIGS. 1 and 4A is that in FIG. 4A the rotary cutter 4"' is provided with an air distribution assembly 30 which comprises a first air distribution part 32, a second air distribution part 34, an air connection piece 36 and said shaft member 8b"', now hollow, for interconnecting the second air distribution part 34 and said air connection piece 36. The air connection piece 36 is connected to a section of an air source 35, namely to a source of vacuum pressure 35a (see Fig. 4A). The first and second air distribution parts 32, 34 may be made of a polymer, a metal, a hard metal or ceramics. It is however not necessary that the parts 32 and 34 be made of the same material. As already stated above, cylindrical bearings 19 are provided between the shaft members 8a, 8b"' and the intermediate transmission members 18a, 18b, respectively, for centering the mantle 16 relative to the shaft members 8a, 8b1". The mantle 16 is connected to the first air distribution part 32 by press-fit, fastening means or gluing, whereas the second air distribution part 34 is connected to the connection piece 36 shaft member 8b"', preferably by a fastening means. Thus, during operation the first air distribution piece 32 rotates together with the mantle 16, whereas the second air distribution piece 34 is static. The mantle 16 is provided with first through-holes 40 outside the cutting member 17 and second through-holes 42 inside the cutting member 17. The reason for this will be explained further below. FIG. 4B shows the assembled rotary cutter 4'", the mantle 16 and the first and second distribution parts 32, 34 being coaxially arranged. First and second openings 44 and 46 in the first distribution part are provided for connection to respective through-holes 40, 42 (see FIG. 4A) of the mantle 16. The first and second distribution parts 32, 34 are hollow and substantially circular cylindrical in shape. During operation, the second distribution part 34 defines a coaxial lumen 47 which connects to the interior of the air connection piece 36, which in turn is connected to the source of vacuum pressure 35a. A connector 49a is connected to another section of the air source 35, namely a source of pressure 35b which is at least at atmospheric pressure 35b, i.e., atmospheric pressure or an over-pressure. A bore 49b connects the connector 49a with a substantially radial bore 49c of the second air distribution part. In FIG. 5A, a portion of the first distribution part 32 has been cut away and shows in that relative position of the first and second air distribution parts 32, 34, how the first openings 44 connect to a third opening 48 of the second distribution part 34. The third opening 48 connects in turn to the lumen 47. In FIG. 5B, a further portion of the first distribution part 32 has been cut away and shows how the second openings 46 connect to a fourth opening 50 of the second distribution part 34. The fourth opening 50 connects in turn to the lumen 47. Furthermore, in the rotational direction after the fourth opening 50, a groove 52 is provided in the second distribution part 34. A longitudinal portion 52a thereof connects to the second openings 46, whereas a circumferential portion 52b continues in the circumferential direction of the second air distribution part 34. As can be seen in FIG. 5C, the circumferential portion 52b of the groove 52 continues with a further longitudinal portion 52c and continues with a substantially radial bore 49c, which in turn is connected to the connector 49a via the bore 49b (see FIG. 4B). The size of the second opening 46 is substantially constant in order to fit the size of the fourth opening 50. However, in order to fit the form of the article to be cut, i.e. the shape of the knife member 17, an axial groove 54 is arranged in the surface of first distribution part 32. In the same manner, the size of the second opening 44 is substantially constant in order to fit the size of the third openings 48, and in order to fit the form of the residue of the sheet, i.e. also in this case the shape of the knife member 17, an axial groove 56 is arranged in the surface of first distribution part 32. In FIG. 6A, an alternative embodiment of a second distribution part 34a is presented, according to which the third and fourth openings 48, 50 have been interconnected by a longitudinal groove 60. In FIG. 6B, the groove 60 is a radial opening, i.e. it projects radially through the part 34a, whereby the openings 48, 50, 60 form a single opening. FIGS. 7A - 7C illustrate how the openings of the air distribution parts 32, 34 correspond to the through-holes of the mantle 16 in different relative positions. Consequently, in FIG. 7A broken lines A and B indicate different circumferential positions of the first and second air distribution parts 32, 34 and the mantle 16 of a pre-determined angular position of the first and second air distribution parts. The through-holes 40 outside the knife member 17 are connected to the third openings 48 via the first openings 44 along the line A. Similarly, the through- holes 42 along line B and inside the knife member 17 are connected to the fourth opening 50 via the second openings 46. Consequently, the through-holes 40 as well as the through-holes 42 will be subjected to a vacuum. In FIG. 7B is shown that the through-holes 42 along the line C are connected to the groove 52, whereas the through-holes 40 along the lines D are connected to third openings 48. Thus, the through-holes 40 will remain subjected to a vacuum, whereas the through-holes 42 will be subjected to atmospheric pressure or an over-pressure. However, at line E, also the through-holes 40 along the line E will also be subjected to atmospheric pressure or an over-pressure. It should be noted that along lines F, the openings 46 are closed, i.e. they do not face an opening or a groove in the second air distribution member 34. In FIG. 7C is shown that along lines G, the openings 44 as well as the openings 46 are closed. Thus, during cutting of a sheet, e.g. a web, a cardboard or a metallic foil, and due to vacuum distributed to predetermined through-holes 40 and 42 (see the lines A and B in FIG. 7A), the whole sheet will stick to the surfaces both outside and inside the knife member 17, while the knife-member cuts against the anvil roll 6 (see FIG. 1). After cutting the article, the mantle 16 and the first air distribution part 32 has rotated away from the contact with the anvil roll 6, and to another position of the second air distribution part 34 (see the lines C in FlG. 7B). The article will come loose from the mantle 16, due to atmospheric pressure or over-pressure distributed to the same predetermined through-holes 42, whereas the rest of the sheet will stick to the mantle 16, due to the vacuum distributed to the same predetermined through-holes 40. A slight further rotation will cause the openings 46 to close (see the lines F in FIG. 7B). Further rotation of the mantle 16 and the first air distribution part 32 relative to the anvil roll 6 and to the second air distribution part 34 will cause also the rest of the sheet to come loose from the mantle 16, since the same predetermined through-holes 40 will then be subjected to atmospheric pressure or an over¬ pressure (see line E in FIG. 7B). A slight further rotation will cause the openings 46 and then the openings 44 to close (see line G in FIG. 7C). With minor modifications of the rotary cutter shown in FIG. 2, it would also be possible to use the air distribution parts 32, 34 in that embodiment.