Ask the Manufacturing Execution Systems (MES) Expert: Scheduling Assistance
 Asked on Mar 30 2007 1:11:04:000PM | Q | We are in the process of defining a lean manufacturing system for an MTO industry. The key to this industry is that all the orders are unique items and are based on the customer specifications. How do we set up Kanbans for this industry? I have heard that other processes, e.g., CONWIP, can handle such a situation. Has the industry accepted the CONWIP philosophy? Sunil Roy, Coppell, TX |
| A | CONWIP (Constant Work-in-Process) is a well-accepted alternative to Kanban, differing from Kanban in subtle but important ways. In a Kanban production line, pull signals are sent by each work cell to the preceding station, maintaining a buffer at each cell. In a CONWIP system, replenishment orders are sent by the last cell in the line to the first cell as parts from the finished goods inventory. There is only a single set of cards in a CONWIP line — representing finished goods rather than individual Kanban cards circulating between each work cell. Both approaches are pull-based, responding to actual customer demand. But the resting state of a CONWIP production line has empty buffers at each station, with the exception of the finished goods buffer. Sending a single pull signal from the finished goods buffer — rather than individual signals from each cell — has an impact on the way the line responds to failure. In Kanban systems, when a cell experiences failure, the entire line stops. In a CONWIP system, signals continue to be sent from the finished goods inventory as parts are pulled and the cells upstream of the failed equipment continue to produce work-in-process, filling buffers above the failed machine. This allows faster recovery once the failed machine comes back online, since buffer stock is immediately available to continue production. CONWIP is generally considered to be more effective in systems with high process variability — the make-to-order scenario which you have described. It is also possible to construct hybrid systems that utilize both CONWIP and Kanban. Production is initialized as units are pulled from finished goods inventory, while intermediate buffers on the line are maintained by secondary Kanban cells. This is particularly useful when a line has bottleneck points. |
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