A drill guide assembly for determining the axis for drilling a bore in a generally dome-shaped bone to receive a component of an orthopaedic joint prosthesis is described. The drill guide assembly includes a drill guide sleeve, a carriage and a platform. The drill guide sleeve is mounted in the carriage towards a first end so that the angular orientation of the drill guide sleeve relative to the carriage can be adjusted about at least one axis. The carriage includes at least one threaded angle-adjustment screw which extends between the carriage and the drill guide sleeve by which the angular orientation of the drill guide sleeve can be adjusted. The platform is fastenable to the bone, and includes at least three feet depending from the platform to engage the surface of the bone with the bone extending towards the platform into the space between the feet.