Defining and generating a working drawing of a conical spur gear to replace an original piece requires geometrical and dimensional constraints, especially when the cutting module is determined by reverse engineering. We propose an approach using Computer aided design (CAD) to establish the drawing definitions and to verify the assembly of the conical spur gear. We have developed the model drawing according to the standard International Standard Organisation (ISO), in which we introduce a new non-existent coast into the current dimensioning and check the assembly using the Tredgold method, which transforms a conical spur gear into a model cylindrical spur gear, to which the equations for parallel cylindrical gearing can be applied. This method determines a good estimate if the couple of gear verifies the specified geometrical conditions of assemblies. We present the developed algorithms, as well as the results of applications of various programs.