The Operations-Time Chart: A graphical tool to evaluate the performance of production systems – From batch-and-queue to lean manufacturing

In this paper, we describe the spreadsheet modeling of manufacturing systems by means of the Operations-Time Chart (hereafter, OT-Chart), a graphical tool for an automatic time-phased representation and measurement of the operation of production systems, developed by the authors. In order to improve the design of a production system, it is necessary to know the key performance metrics of the system (productivity, lead-time, inventories, downtimes and wait times) and identify the effects of design parameters on system performance. Calculating some of these magnitudes can be very complicated, especially for production systems involving multiple and confluent processes, with different cycle times and lot sizes. The OT-Chart permits a visual tracking of the aforementioned parameters throughout each process, and like a simulation tool, the program calculates and displays the effects of changing input parameters. A special version of the Chart has been designed for lean manufacturing environments, where visual tools are much appreciated. The OT-Chart provides tracking of different types of waste and supports inventory supermarkets and pull scheduling. The paper includes a case study: a plant is redesigned from a conventional batch-and-queue production system into a lean manufacturing system with the help of the OT-Chart (it is used to test the performance of each layout) allowing managers to evaluate and refine their designs.

► The Operations-Time Chart models production systems with different transfer batch sizes.
► It calculates and displays the evolution of work over time and work-in-process.
► It can handle performance time including allowances for transportation, stoppages.
► It supports lean manufacturing (takt time, pull flow, WIP supermarkets).
► Users can implement their own chart.