The apparatus for converting side-to-side to rotational motion for a workpiece includes a spring assembly with a movable base mounting element which is driven in a slightly arcuate reciprocal side-to-side motion and a fixed mounting element spaced apart from the base element and mounted in the appliance so that it does not rotate or move in operation of the apparatus. A spring member, such as two planar leaf springs positioned at an angle to each other, is mounted between the movable base element and the fixed mounting element. A drive shaft member extends from the movable base element through the fixed mounting element and beyond. A workpiece such as a brushhead is mounted on the distal end of the drive shaft. The spring member is configured so that it is substantially less resistant to twisting action than bending action, such that the side-to-side motion of the base element results in rotation of the workpiece. A method for tuning resonant spring assembly systems used in appliances such as power toothbrushes, particularly leaf spring systems, includes the step of measuring the frequency-dependent amplitude of the appliance workpiece, and then removing a portion of the spring member so as to decrease its spring rate sufficiently that the resonant frequency of the spring member closely approximates the drive frequency of the appliance.