In any process, you won’t get the desired results if there are any bottlenecks. In a value stream, such a bottleneck is known as a process block. So you need to identify process blocks as soon as sources of waste have been identified. A process is blocked if cycle time exceeds takt time.
Cycle time is the time it takes to complete a cycle, or task, excluding any waiting or queuing time. And takt time is the time a task must be completed in for production to continue at a rate that matches customer demand, given the work time that’s available.
Determining the time it takes to complete a task is fairly easy but you need to calculate takt time. The formula for this is net operating time per period divided by the customer requirements for that period.
But what’s the net operating time? It’s the number of working hours per shift, minus time taken for breaks and meetings.
Consider an example. At an electronics manufacturer, the team calculates takt time by dividing the available time per shift — 7 hours, which is 420 minutes — by the required output per shift — 35 products. So 420 divided by 35 equals a takt time of 12 minutes.
As the cycle time of the painting process is 14 minutes, there’s a block, because this exceeds the takt time.
A bar chart is useful for finding process blocks. This allows you to chart out the completion time of each process and check these against the takt time. For example, if a bar chart shows the cycle time of three processes as 20 minutes, 15 minutes, and 18 minutes, and the takt time is 15 minutes, the first and last processes are bottlenecks, because their cycle times exceed takt time.
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