Method of controlling the diameter of a fiber preform including forming a gob on the end and pulling the gob

A method of controlling the diameter of a glass rod drawn from an elongated glass preform, having a long axis and a first end, comprising the steps:

• a) suspending the glass preform in a furnace having a hot zone of adjustable temperature, using suspending means which can move the preform along its long axis,
• wherein, the preform long axis is positioned vertically and a portion of the preform including the first end is in the furnace hot zone;
• b) heating the preform until the first end of the preform softens and a gob of glass, including the first end, begins to move downward to form a glass rod connecting the gob and the preform, the glass rod having a diameter smaller than the preform diameter;
• c) measuring the velocity of the moving gob while the diameter of the drawn glass rod connecting the gob to the preform is at a diameter which differs from a preselected target diameter by no more than 75%;
• d) adjusting, if required, the furnace hot zone temperature to match actual and target gob velocity, the target gob velocity determined from the preform dimensions and a target rod diameter, so that the rod diameter during gob drop is held within +/-75% of the rod target diameter;
• e) gripping the gob using movable mechanical means, the mechanical means having a load cell for measuring tension applied to the rod by the gripping means;
• f) moving the gripping means away from the preform at least at 40% of the last measured gob velocity;
• g) measuring the diameter of the rod;
• h) adjusting the velocity of the gripping means, using the measured rod diameter, and the furnace hot zone temperature, using the measured rod tension, to match the diameter of the rod to a target diameter;
• and,
• i) moving the preform along its long axis into the hot zone at a rate which balances the mass flow into and out of the furnace hot zone.