one or more exchangeable longitudinal knives 4 each having a first end 6 and a second end 8； and a support frame 10 for accommodating said one or more exchangeable longitudinal knives； wherein said support frame 10 comprising: a first transverse frame girder 12； and a second transverse frame girder 14； and at least one longitudinal frame girder 16 having two opposite ends 18,20； said at least one longitudinal frame girder at one of said ends 18,20 connecting the first transverse frame girder 12； and said at least one longitudinal frame girder at another of said ends 18,20 connecting the second transverse frame girder 14； wherein the first transverse frame girder 12 as well as the second transverse frame girder 14 comprise one or more openings 22a,22b, wherein said at least one opening 22a in the first transverse frame girder 12 on the one hand, and wherein said at least one opening 22b in the second transverse frame girder 14 on the other hand together in pairs provides for accommodation of one of said exchangeable longitudinal knives 4； and wherein at least one of said one or more exchangeable longitudinal knives 4 comprising a first knife locking means 24 for releasable locking said one or more exchangeable longitudinal knives 4 into a locked position onto said support frame 10； and wherein said support frame comprises second knife locking means 26 for releasable locking said one or more exchangeable longitudinal knives 4 into a locked position onto said frame by engagement of said first knife locking means 24 of said one or more exchangeable longitudinal knives 4 with said second knife locking means 26 of said frame 10.
