By Jon Miller | Post Date: June 18, 2005 7:43 AM | Comments: 13
Who or what is telling people to look at throughput time (lead-time) through the assembly line when measuring productivity? This number is completely unrelated to assembly line productivity.
I have run into the situation where people charged with doing kaizen on assembly lines (industrial engineers, project managers) are measuring the wrong thing, throughput time. These are trained and educated people. This is happening at name-brand, global manufacturing companies. This week was the third time in two years. Please make this stop.
I don't have a formal IE background so I don't know if this is a standard IE practice or if it comes out of a text book. I do know that the definition of "cycle time" in some textbooks is what we call "manufacturing lead-time" rather than the Lean definition which is something like "hand-time" or "operator cycle time". This can cause confusion at times.
When doing kaizen to design assembly lines for one-piece flow one of the first activities is measuring all of the assembly cycle times (total hands-on time to build one unit, and separately total automatic cycle time). The next step is to calculate takt time for the particular product or product family or value stream. The next step is to balance the workload to takt time and design the layout, work sequence, standard WIP positions, etc (creating Standard Work).
Measuring the time it takes for one unit to make it from the beginning of the assembly line to the end has *no* direct relation to the productivity of the line. While it can be an interesting measure of response time to customer orders, this lead-time number should not be used for designing one-piece flow (or even batch & queue) assembly lines.
After one-piece flow is achieved, only Standard WIP remains in the line so the lead-time calculation is takt time x Standard WIP quantity. If you are doing "fake flow" and pushing, your lead time will be longer and cycle times will be out of balance. However the lead-time of "fake flow" will have no bearing on what Standard Work and one-piece flow should look like.
There is no need to measure this throughput time at all when doing kaizen in assembly since all you are doing is identifying the "wait time" of parts in transit on a belt or sitting on an assembly bench. A quick demonstration has been enough in each case to help people see that throughput time in assembly is irrelevant to productivity.
I have thought about other reasons for this type of measurement but to no avail. I don't plan on researching it further but if someone knows the source of this practice of measuring assembly line throughput time please let me know.Comments are moderated to filter spam and inappropriate content. There may be a delay before your comment is published.