A machine for grinding the outer metering edge contours on the edges of the axially extending grooves of a power steering gear input-shaft (10) in which a substantially cylindrical grinding wheel (40) whose working surface is dressed parallel to the axis of the input-shaft (10) effects the grinding, the distance between the input-shaft (10) and the grinding wheel (40) being cyclically increased and decreased several times during each revolution of the input-shaft (10) in such a manner that each outer metering edge contour so ground has a form which is a mirror image of the form of at least one other outer metering edge contour around the outside periphery of the input-shaft (10), characterized in that the angular velocity of the input-shaft (10) is varied cyclically in a manner co-ordinated with the cyclic increase and decrease of the distance between the input-shaft (10) and the grinding wheel (40) thereby substantially reducing the peak rate of stock removal per unit time compared with the peak rate that would occur if the angular velocity were constant and equal to the mean value of the cyclically varying angular velocity.