A random search and backtracking procedure for transfer line balancing

The optimal logical layout design for a type of machining transfer lines is addressed. Such transfer lines are made of many machine-tools (workstations) located in sequence. On each workstation there are several spindle-heads. A spindle-head does not execute a single operation but a block of machining operations; all operations of a block are executed simultaneously (in parallel). Spindle-heads of the same workstation are activated sequentially in a fixed order. The transfer line design problem considered in the paper consists of finding the best partition in blocks and workstations of the set of all operations to be executed on the line. The objective is to minimize the number of spindle-heads and workstations (i.e. transfer line investment cost). An optimal decision must satisfy a desired productivity rate (cycle time) as well as precedence and compatibility constraints for machining operations. A heuristic algorithm is proposed: it is based on the COMSOAL technique and a backtracking approach. Results from computer testing are reported.