A dishwasher includes a pump assembly having a chopper blade and apertured plate arrangement which function to chop soil particles entrained in a flow of washing fluid. The chopper blade floats or axially slides relative to a drive member from a first position spaced from the apertured plate to a second position closer to the apertured plate in order to macerate the soil particles. The chopper blade is actually mounted to a hub member including a shaft portion that extends into and abuts an inner portion of an impeller to maintain a desired spacing between the chopper blade and the apertured plate, while transferring drive from the drive member to both the chopper blade and impeller.