Abstract: A chopper has a straight knife clamped between a clamping member and a counter knife. The knife, clamping member, and counter knife are collectively positioned in a machined area formed in a holder that is in turn fitted within a larger recess formed in a chopping disk. A first fastener secures the knife holder to the chopping disk, a second fastener secures the clamping member to the knife holder, and a third fastener secures the counter knife to the knife holder. A projection formed in the counter knife engages a groove formed in the knife to properly align the knife and to inhibit entry of fibers into the space between the counter knife and the knife. The holder further includes a series of steps that position the counter knife and the clamping member, respectively, with respect to the holder.