Patent AT-E399925-T1: [Translated] SINGLE PASS MILLING SYSTEM

A sidetracking system has a pair of serially connected mills (8,9), each having a plurality of circumferentially arranged blades having a tapered cutting portion (82,93) thereon for cutting a window in a casing and then sidetracking in a formation. A whipstock has at least three axially spaced ramps, each ramp being interspaced by a substantially axially extending portion. Each of the ramps has the same angle of inclination to a longitudinal axis and the distance between the ramps is the same as the distance between the tapered portions on the mills (8,9) so that when, in operation, load is shared between both mills. The mills have a button (83,93) of hardened material located on the tapered cutting portions so that the button abrades the whipstock ramps (31,32,33).