This paper addresses the development of a tool for optimising assembly workcell layout in the context of an industrial robotic CAD/CAM/CAE software product. The criterion to be minimised is the cycle time for completing a given sequence of operations, which is achieved by determining the relative positions of peripheral machines on the cell floor. The algorithm is constructive: the machines are placed one at a time in the robot neighbourhood, by means of a modified simulated annealing (SA) method. This method yields several possible and optimal positions for a machine, and several layouts are thus obtained at the end of execution. The optimisation tool has been implemented in IGRIP, and a case study illustrates its performance.