Manufacturing systems with forbidden early shipment: Implications for setting manufactured lead times

The authors compare two well-known methods for setting manufacturing lead times (flow allowances) in a general job shop when early shipment of completed jobs is forbidden. One of the methods for setting a job’s allowance is to make it proportional to the total processing time for the job. This method, referred to as TWK, is compared to a second method PPW. With the PPW method, a job’s allowance is obtained by adding to the total job-processing time an allowance for waiting that is proportional to the number of operations that the job requires. The two allowance methods are compared using a computer simulation of an 8-machine job shop. The model is unique in that jobs are not permitted to leave the shop early. This feature of forbidding early shipment (FES) complicates the comparison between allowance methods because it draws the issue of finished-order inventory management into the analysis. Results of computer simulations over a wide range of average due-date difficulty suggest that TWK is the dominant procedure by virtue of providing both lower tardiness and lower inventory.