their leading faces arranged in a plane which is radial with respect to the mill centreline as seen in fig1 the outer edge of each rib 18 is inclined at a small angle u03b1 in the range 1 u00b0 10 u00b0 such that the outside diameter across the ribs decreases in the upward direction this has the effect that in use with the tapered nose portion 10b bearing on the angled face of the whipstock as indicated at 24 the ribs initially contact the casing indicated at 26 at their lower corners thus the cutting action is initially effectively along a circumferential line as cutting proceeds and as the tool wears the area of contact will increase somewhat but it will remain a restricted area approximating to a circumferential line in prior art starting mills in contrast the outer edges of the cutting ribs are parallel to the tool centreline which produces a relatively large contact area leading to high resisting torque the tapered nose portion 10b is provided with tungsten carbide pads 27 preferably as shown in the form of axially aligned strips set in recesses the pads 27 provide wear resistant bearing surfaces for contact with the whipstock the trailing portion 10a of the body is hollow for receiving drilling fluid from the interior of the drill string the drilling fluid is applied to lubricate the blades 18 and flush away cuttings via eight angled bores 28 each exiting in front and near the lower edge of a respective blade 18 a typical starting mill in accordance with this embodiment has a maximum diameter over the blades of 8 504 inch 216 mm and blade length of 6 793 inch 172 5 mm on an overall body length of about 46 5 inch 1 18 m and diameter.