A fluid infusion pump driver device for pumping fluid from the chamber of a disposable cassette. The pump driver includes a reciprocatable plunger which is driven by a stepping motor. The motor and plunger are coupled through a universal joint which provides a threaded connection translating the incremental angular movements of the motor into incremental linear movements of the plunger, and the plunger is coupled directly to the frame through a flexible leaf spring to prevent wear in the threaded connection from generating undue noise or creating accuracy problems.