abstract |
A laser beam (15) is directed onto the surface of a flange (10) on the opposite side to the web (11). Radiation detectors (1,2) are located on each side (13,14) of the web and in response to the signals received by the detectors, the beam is moved to the right or left. When the signal (11) is greater than that of (12), the beam is moved to the right. The opposite occurs when (11) is less than (12). |