To be clear, you mean two cylinders, one id of 16 the other 30, ~4 mm and 20 mm long, joined by a cone that tapers between these dimensions - correct? The length of the cone being 75 mm. Does the outside have to taper as well as the inside? for example, could you have a tapered bore but the outside being a cylinder of od larger than 30mm but a shoulder on the end where the bore is smallest down to whatever the smallest od of your cylinder is? Would easier is you didn't have to machine the outside to a taper as well. The tapered bore would be a doddle in a CNC lathe.