How Often Should We Change Takt Time?
Dear Gemba Coach,
How often should we change takt time?
Deep question. The answer is: I don't know; it depends! Let's make sure we're on the same page on what takt time is. takt time is commonly defined as daily work time divided by daily customer demand. The idea is to figure out how much time we have to make a part while sticking to customer demand, neither overproducing (and generating the associated waste) nor being late on schedule. Rather than focusing on optimizing the output of every process by looking at parts per minute, takt is a great device to flow by getting every segment of the process running at the same rhythm.
takt time has to be one of the most profound ideas in lean. Not only is it a key driver for kaizen by pushing you to achieve takt with the least resources and the least problems possible (it's impossible to keep the takt if the line has rework or breakdowns), takt time thinking has a huge impact on how you design where you invest your resources. Recently I visited a Post Office operation where mail is sorted for distribution. They had made a ginormous investment in space and machine that only worked at peak for two hours a day (sorry, a night) - and still no better than 3% errors and 15% non on-time-deliver (OTD). takt time thinking would have led to smaller, more flexible investments, which are easier to control. Similarly, takt time thinking can influence engineering, tooling, supply chain, and all aspects of industrial production. Understanding takt time is key to lean thinking.
Nevertheless, takt is a slippery concept. First, what exactly is daily work time? Should we count breaks? Breakdowns? Changeovers? What about banking holidays? And what about daily customer demand? It changes all the time - should we change takt time every day? Isn’t just-in-time about fulfilling exactly customer demand? But what of the impact on production if we do?
What Is the Problem?
The deeper question, as ever in lean is: what is the problem you're trying to solve? takt time is at the heart of the lean industrial model. The idea is to produce with the minimal resources by maintaining a stable volume, while working at high flexibility. The core assumption - generally proved right - is that although customers change their mind about which model they chose, the global volume of demand is fairly stable. First, because volume corresponds to a consumption process, which is essentially a renewal process at the customer, and hence stable. Second, because the law of high numbers applies and thus, relative variation on the overall volume is much smaller than on any of the component parts. takt time thus quantifies the stability of overall volume. Within this, however, we should be able to make any part any time to fulfill specific demand.
The lean industrial model is less wasteful because it manages the inherent contradiction of takt time-based volume stability and spot demand based mix flexibility. In this sense, the first problem we’re trying to solve is to stabilize the overall volume and thus keep takt time as fixed as possible. But what about real customer demand variations? What about financial panics? What about recall crises? What about earthquakes and component rupture? Well, there are a number of ways to flex output without changing takt time:
- Inventory: the assumption is that if customer volume variation is repetitive, we can stick with takt because what customers are not buying today, they’ll purchase tomorrow and vice versa.
- Short work days or overtime: it makes accountants hysterical, but you cannot work one day, and train operators or kaizen, just as you can use a week-end shift.
- Adding or subtracting shifts: this is not something you can do lightly, but it is a possibility.
The core idea is that as demand for new models increase, we should be ample to ramp up the new model while diminishing the old and maintain a stable takt time. Now, this is clearly an ideal that assumes, in particular, that both the old model and the new one can be manufactured on the same line. But it’s one of the insights that make lean so capital free compared to traditional manufacturing which keeps on adding dedicated equipment and facilities for every new project.
takt time Paradox
Yet, there are cases where you deliberately want to change takt time. I was recently visiting a Toyota plant that had to go through the financial crisis, the recall crisis and finally the earthquake crisis. In the first year of the financial crisis, the plant maintained its takt time and reduced production by using short work. But then, a new plant manager arrived from Japan, steeped in TPS practice, who decided the plant was weak in its ability to train employees and conduct kaizen. He then imposed three or four takt time changes within a year. Historically, it took the plant about a month to absorb a takt time change, whereas the standard for a Japanese Toyota plant would be around a week. The problem the new president was trying to solve was to adapt to demand by learning to change.
takt time change is a huge endeavor inasmuch as every operator station has to be rebalanced at takt time. takt time change has also a huge impact on suppliers since parts may be pulled at a slower or faster rhythm. In this case, this meant retraining all the operators of the line and rebalancing all workstations. The first takt time change was quite traumatic, with many quality problems compensated with inspections, with ergonomics issues and so on. But the plant management worked with group leaders, team leaders and members to finally learn to deal with the changes. And so the fourth takt shift occurred without a hitch.
Should we change takt time? This is a core lean question, and a paradox. On the one hand we want to avoid changing takt as much as we can to stabilize the line. For instance, the same plant, when dealing with parts shortages, rather than consume all it had and then stop, preferred to schedule set stoppages to level the impact on its supply base. No takt change would be considered to deal with these conditions. On the other hand, takt is essential to sticking to customer demand, and to adjusting finely the resources on the lines - so takt has to be changed every so often to spur kaizen.
There is no straight answer to your question other than "why?" And then probably, "why?" again. Why do you need to change the takt? What are you trying to achieve? So much of lean is about developing a deeper understanding of capacity, these are critical questions to ask ourselves, so I’d encourage you to keep digging: this is absolutely the right question even though answers are far from obvious. Let me rephrase: what is the problem we’re trying to solve by changing takt time? What benefits do we expect? What issues will we encounter? How will we run the changes with the operators, not against them?