Chen, Li and Shady (2010) describe a case study where a Value stream Map (VSM) was used in specifying the focus of a Lean Six Sigma (LSS) project. A VSM shows how both materials and information flow through the system processes.
Company A is a small electrical manufacturing business in Midwestern United States. The company makes industrial switch gears and switchboards. The company had not completed a LSS project, but employees wanted to transform the company using LSS. They called their approach Lean/Sigma, but we will call it a version of LSS. They formed a LSS team. They started with the switchboard unit since it was the major manufacturing section and used the greatest amount of personnel and equipment.
Value Stream Map
The first step was to create a VSM of the current system. The team made two walkthroughs with the manufacturing manage. The first one traced the product flow from the raw material receiving dock to the finished product shipping dock. Then the team walked from the shipping dock upstream to the raw material dock. They collected detailed process information. The VSM appears in the following figure.
The fabrication operation produces 6 parts. The parts are welded together in the welding operation, and then they go to the finishing operation. The last two operations are assembly and wiring. C/T is the average cycle time or the average time a part is in the operation. P/T is average processing time, and C/O is the average changeover time. Chen, Li and Shady (2010) present the data values in the figure.
The VSM shows that the fabrication operation is the bottleneck operation since its cycle time of 140.5 minutes is largest of the five operations. Finishing is the operation with the next largest cycle time, and its cycle time is 128 minutes. The production process begins with fabrication so based on cycle time downstream operations would have to wait for fabrication to complete parts.
The fabrication operation consists of four processes. They are shearing, plasma cutting, de-burring and braking. Two of the 6 work pieces must be processed by the plasma cutter. It has an average machine time of 42 minutes plus a loading time of 3 minutes and an unloading time of 2 minutes. Clearly the plasma cutter was the bottleneck process in the fabrication operation. 94 of its 140.5 minute cycle time is due to the plasma cutter.
In addition to the VSM identifying fabrication as the bottleneck operation, the LSS team noticed 3 days inventory between the fabrication, welding and finishing operations. This inventory was downstream of the fabrication bottleneck operation; thus, the bottleneck was not causing this inventory buildup.
To improve the process and reduce waste, the LSS team created two kaizen events. Each kaizen used the ‘5 Whys’ approach to identify root causes for poor performance.
Kaizen event 1: Reduce cycle time for the plasma cutting machine.
The plasma cutter created defects, and the team designed experiments to identify better operating conditions. The next posting will describe the experiments and their results.
Kaizen event 2: Reduce inventory waste.
They found that inventory between the fabrication, welding and finishing operations occurred due to delays in welding operations. The company only had two welders, and when one of them was absent welding delayed the other processes.
The value stream map is useful in identifying bottlenecks and system constraints. Clearly, large process cycle time is a useful indicator of a bottleneck. WIP is another one. How do we determine which bottleneck is the most significant one affecting system performance? That would be the system constraint.
1. Chen, J. C., Y. Li, et al. (2010). “From Value Stream Mapping Toward a Lean/Sigma Continuous Improvement Process: An Industrial Case Study.” International Journal of Production Research 48(4): 1069-1086.