Side-tracking mills

A starting mill for use in sidetracking a borehole, said starting mill comprising a generally tubular body, connecting means at one end of said tubular body for connecting said starting mill to an end of a drill string, and a plurality of cutting blades extending radially from and circumferentially spaced around said tubular body, each said cutting blade having a generally radially extending face which is a forward face in the direction of rotation of said starting mill, each said forward face being defined by an array of cutting elements, each said cutting element being in the form of a respective carbide disc secured to said cutting blade, each said cutting blade being angularly offset from and with respect to the longitudinal axis of said tubular body, said tubular body having another end opposite said one end and said other end being formed as a tapered nose, said cutting blades being positioned on said tubular body behind said tapered nose with front surfaces of said carbide discs lying in planes which are radial with respect to said tubular body and said cutting blades each having a respective radially outer edge inclined at an angle to said longitudinal axis of said tubular body such that the outside diameter of said starting mill across said cutting blades is less towards said one end than towards said tapered nose.