Booms on construction machinery, flanges on wind tower sections, shells for pressure vessels — plate thickness often starts at sixteen millimetres, and a single groove can take eight, twelve, sometimes fifteen passes. When a shop brings in a welding robot, the questions usually go to whether the path is straight and the bead is even. What decides whether the batch passes ultrasonic inspection the first time is something quieter: interpass temperature, and where each bead of deposited metal actually lands inside the groove.

An interpass temperature that drifts costs you directly. Run it too hot and the weld metal cools slowly, grains coarsen, and impact toughness falls faster than tensile strength; let it sag below the preheat line and the heat sink of thick plate pushes the cooling rate back up, hydrogen has no time to escape, and delayed cracking can surface days after the joint was signed off. In a Truman Robotics welding cell, temperature measurement is written into the cycle as a step of its own: the infrared sensor reads the base metal beside the groove rather than the bead that has just been extinguished, the cell waits while the reading sits above the ceiling and adds heat when it falls below the floor, and that waiting is programmed so nobody has to judge it by feel. Bead layout gets the same arithmetic. Pass count comes from the groove cross-section and the deposit per pass, oscillation width and bead-to-bead overlap are set so each layer fills flat before the next one starts, no unfused valley is left for slag to settle in, and the cap pass runs on tightened parameters to hold undercut and spatter down.

Not every thick joint belongs to a robot. Manual root passes on very heavy butt welds, and fillet welds on seat bosses tucked into cramped structures, still call for a person's judgement. But above twelve millimetres, with a steady batch and a real acceptance standard, the repeated labour of multi-pass welding is precisely what a machine does best: a positioner turns the work into flat position, offline programming generates the whole pass sequence, arc sensing compensates when fit-up drifts, and current, voltage and interpass temperature are filed against the part number so a repair can be traced back to the layer that caused it. If interpass control and inspection yield are what keep costing you night shifts, send Truman Robotics the drawings, the groove design and the acceptance standard, and we will lay out a cell that carries the job from preheat through multi-pass welding to a complete process record.