We consider a scheduling problem arising in a shipyard subassembly welding process. There are four welding robots of gantry type, which perform the welding process for the subassemblies. Because the robots perform the welding operations at the same time, there is a possibility of collision between adjacent robots depending on the welding schedule. In this paper, we propose a heuristic method to find a welding schedule which does minimize the welding completion time while avoiding collision among the robots. The method consists of two phases: assignment and scheduling. In the assignment phase, we assign each welding line to a proper robot. In the scheduling phase, we determine the welding schedules for the robots so that collision is avoided. Computational experiences with the data which reflect the real situation are reported.