A comparison of JIT and TOC buffering philosophies on system performance with unplanned machine downtime

The impact of buffering under just in time (JIT) and theory of constraints (TOC) is studied to determine whether a difference in performance exists in systems faced with unplanned machine downtime. Comparisons are based on results obtained from simulation of a five-station cell utilised in computer substrate manufacturing. Analysis of the simulation output suggests that the TOC technique, drum-buffer-rope (DBR), achieves higher levels of performance as measured by total output and lead time while reducing inventory requirements relative to the tested JIT technique, Kanban. Improved system performance stems from the strategic placement of buffers in DBR, which maximises protection of the constraint from variation rather than attempting to protect each individual station. Further, analysis suggests that DBR systems are more robust than JIT systems in that they are able to maintain higher levels of system performance across a range of inventory levels.