A grass cutting mobile robot includes a body and a blade assembly connected to the body and rotatable about a drive axis. The blade assembly includes blades, a housing to hold the blades, and a spring that connects the blade to the housing. The housing includes a slot in which to mount a blade so that a portion of the blade is movable through the slot towards another blade in response to an impact. The slot slopes upwards in the housing towards the body, thereby enabling the blade to move upwards relative to a ground surface toward the body in response to the impact. The spring is for constraining movement of the blade relative to the housing.