I don`t think it`s the method im using its a method the company have used for years and its always worked. the job is drawn up in AutoCAD, the same job is then mirrored in AutoCAD and saved as a separate file. Both files are then converted into Excellon drill files. the first file is drilled on the top face of the material and the mirrored file is drilled on the bottom side of the material, the mirroring of the drawing can`t be incorrect in AutoCAD so it must be something on the machine