TECHNICAL BACKGROUND OF THE INVENTION The present invention relates to a rotary cutter for a rotary cutting apparatus, comprising a shaft and a mantle, the mantle having at least one cutting member. The invention also relates to a rotary cutting apparatus comprising a frame part, an anvil and such a rotary cutter. ... uS-A-4 770 078 discloses in a discussion of the prior art (Figs. 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 (stationary) 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 axial side thereof opposite to the driven side.

SUMMARY OF THE INVENTION An object of the invention is to improve the adjustment possibilities of the rotary cutter. It is a further object to allow maintenance to be performed at a reduced cost and to be able to dismantle the rotary cutter in a simplified manner. 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 and removable therefrom. 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 existing 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 relative 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 first and second shaft portions, the mantle having an axial extension and opposite axial ends, wherein said mantle is adapted to be supported by the first shaft portion 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 portion 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.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, preferred embodiments of the invention will be described in greater detail by reference to the accompanying drawings, in which Figure 1 illustrates a rotary cutting apparatus comprising an anvil and a rotary cutter according to a first embodiment of the invention, shown in longitudinal section; Figure 2 is a longitudinal sectional view of a second embodiment of the rotary cutter; and Figure 3 is a longitudinal sectional view of a third embodiment of the rotary cutter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 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 (non¬ rotary) shaft 8, comprising shaft portions or members 8a, 8b, each shaft member being rigidly connected to a respective frame part 10a, 10b by means of screws 12. A drive axle 14 associated with a not-shown power source extends through the shaft member 8a for transmitting a rotational movement to a tool in the form of a mantle 16 via an intermediate transmission member 18a which is rotatably mounted on an outer periphery of the shaft member 8a. The rotary motion is transmitted from the axle 14 to the member 18a via teeth 18c. 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 a stationary frame by means of suitable, not-shown fastening means. The mantle 16 is provided with at least one cutting member 17. During disassembly, the frame parts 10a, 10b are unsecured from the not-shown rest of the frame such that the static shaft members 8a, 8b, plus the transmission members 18a, 18b, can be pulled axially 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 removed one, and the rotary cutting device can be utilised without long stoppage. Of course, it may be enough to remove only one (either) of the static shaft members 8a and 8b, respectively, rather than both. Figure 2 shows a second embodiment of a rotary cutting apparatus 2' and a rotary cutter 4'. The static shaft 8' in this case comprises a single part and is connected to the frame parts 10a1, 10b1 on either side of the mantle 16' by screws 12'. The rotational movement of the drive axle 141 is transmitted to the mantle 16' transmission mechanism which includes gears 20a', 20b1, 20c1, 2Od'. It should be noted that the parts 20b", 20c1, 2Od1 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 figure 2 is unsecured from the rest of the frame (i.e. the left frame part remains secured). The mantle 16' can then be pulled axially away from the tool holder 8'. Figure 3 shows a third embodiment of a rotary cutting apparatus 2" and a rotary cutter 4". 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" which is received in a recess of the mantle. The static shaft 8" is divided into two shaft members or portions 8a", 8b" connected to the respective frame parts 10a", 10b" on either side 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 axially 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).