Image Number 4 for United States Patent #5484021.
A method and apparatus for forming a window in well casing. First, using coiled tubing, a two stage whipstock is set in the well casing through the production tubing. Next, a jointed milling assembly is inserted through the production tubing, also using the coiled tubing. The travel of the mill is guided by the two stage guide surface of the whipstock, and the joint in the milling tool allows the direction of the mill to change. Thus, the mill impinges on the well casing at a relatively acute angle to the longitudinal axis of the casing reducing the likelihood that the mill will bite into the guide surface of the whipstock instead of the well casing.