Abstract: A drawn lightguide fiber is coated with a layer of a plastic material by moving the fiber through a container of the coating material and a die. The die is made of a relatively flexible material and is mounted so that a lower end extends through an opening of a diaphragm which is adapted to apply substantially coplanar forces to the lower end of the die to reduce the size of an exit orifice. The coating on the fiber is measured and if necessary, as a result of that measurement, the diaphragm is adjusted either manually or automatically to change the size of the die orifice and provide a coated fiber having a predetermined diameter.