At the Eynsham factory of Oxford Magnet Technology, 750 liquid helium-filled vessels are produced each year for housing the superconducting magnet that forms the heart of magnetic resonance imaging systems for body and brain scanners. It takes an experienced operator 14 hours to weld each stainless steel vessel by hand, but following the installation this summer (2001) of a turn-key robotic welding cell configured by Bauromat, the cycle has been reduced to just two hours. Said Mark Tullett, production engineer in charge of the project at OMT, âThe system has been coded by TíV in Germany as meeting its Pressure Vessel Regulations. It is the first time that any vessel that has been robotically welded has met a relevant European Union standard. âIt has been a painstaking process whereby the first 50 vessels off the robot cell underwent exhaustive tests including cross sectional examination of the full penetration circular welds. From now on, 1 in 100 vessels will be taken for quality assurance testing.â?
One of the problems associated with welding the product by hand is that the code stipulates full penetration of the 6 mm thick 304LN stainless steel, an OMT specification that combines austenitic (non-magnetic) properties with impact resistance at 4 degrees Kelvin. As there is a relatively wide gap â typically 6 mm â to fill with metal, six or seven TIG passes are needed to complete the butt weld. These take a long time and introduce variation in the weld seams. In contrast, the robot equipped with the laser sensor is able to MIG weld the joints quickly and consistently in just one pass.
