The Lean Thinker -thoughts and insights from the shop floor
TAKT TIME - CYCLE TIME
這是lean中談到的基礎，但很訝異的是有些lean & sigma工作者並不是很清楚差異，容易混淆。
[Cycle Time] 有許多定義
a. [Cycle time] 可以是在顧客下單到收到物品的總花費時間。這個定義可以用在外部或內部客戶。這個定義實際上是目前大部分在英文的Toyota Production System中被使用。
b. dock-to-dock flow time of the entire process [DTD, 進料到出貨時間] , or some other linear segment of the flow(線性的區間). The value stream mapping in "Learning to See"稱這為[production lead time]生產準備時間，但是有些人也把這叫做同樣的[cycle time]。
這就會變成混淆。在早期的TPS文件中，例如Suzki's New Manufacturing Challange and Hirano's JIT implementation manual, [cycle time]一詞的使用，與現在我們用的[takt time]。所以更是混淆。[cycle time] is also used to represent the actual work cycle which may, or may not, be balanced to the takt time.
[Cycle time] 也被用來表示程序總共的時間the total manual work involved in a process, or part of a process.
當然[cycle time] 也被用來表示一個人的工作時間，但是不包含最終的等待時間。is used to express the work cycle of a single person, not including end-of-cycle wait time.
以上所有的定義都正確。造成混淆的原因是使用者沒有很清楚知道內涵(context)。因此，it is critically important to establish context when you are taking.(要知其所以然）. 精確的形容詞會有幫助，例如[operator cycle time].
Takt time is often over simplified.
The classic calculation for takt time is:
Available minutes for Production/Required Units of Production = Takt Time
這是正確的。但是人總是熱愛知道[available time]包含哪些組成。純粹的定義通成是以total shift time(s) 扣掉breaks, meetings, and other administrative non-working time. 對這沒有人有問題(maybe because that is the way Shingijutsu teaches it, and people tend to accept what Shingijutsu says at face value.)
用例子來看。簡化討論，檢攝a single 8 hours shift on a 5 day work week. There is a 1/2 hour unpaid lunch break in the middle of the day, so the workers are actually in the plant [at work] for 8 1/2 hours.
So we start with 8 hours:
8 hours * 60 minutes = 480 total minutes.
但每天早上有10分鐘start-up時間, 2段10分鐘的休息, 及15分鐘shut-down and clean up at the end of the shift for a total of 45 minutes. This time is not production time, so it is subtracted from [available minutes]:
480 - 45=435
一個非常常見的錯誤是有些人會扣掉30分鐘的lunch break. 請注意，我們一開始的時間並沒有包含這個30分鐘，不要重複扣除。
So when determining takt time, we would use 435 minutes as the baseline. If leveled customer demand was 50 units/ day, then the takt time would be:
435 available minutes/50 required units of production = 8.7 minutes (or 522 seconds)
注意：也可以用一週算 435 minutes * 5 days = 2175 total available minutes
2175 available minutes / 250 required units of production still equals 8.7 minutes
以上的都非常基本，但是我們可以深入以下省思與討論：
因為if you were to run this factory at a 522 second takt time, you will come up short of your production targets. You will have to work overtime to make up the difference, or simply choose not to make it up.
Why??? because there are always problems, and problems disrupt production. 這些中斷會消耗435分鐘的可生產時間，所以end up with less production time than you calculated.
再者，plant manager called an all-hands safety meeting on Thursday. That pulled 30 minutes out of your production time. Almost four units of production lost here.
還有許多可以舉例的....
這裡還有更糟糕的，想一想
When are you going to work on improvements???
If you expect operators to do their daily machine check, when do you expect that to happen?
Do you truly expect your team members to [stop the line] when there is a problem?
以上所有事情，都會將時間生產時間消耗掉...
結果是shop floor的leadership - the ones who have to deal with the consequences of disrupted production - will look at takt time as a nice theory, or a way to express a quota, but on a minute-by-minute level, it is pretty useless for actual pacing production.
All because it was oversimplified.
If you expect people to do something other than produce all day, you have to give them time to do it.
Let's get back to the fundamental purpose of takt time and then see what makes sense.
The purpose of Takt Time
Here is some heresy真理. Running to takt time is wholly unnecessary. Many factories operate just fine without even knowing what it is.
What those factories lose, however, is a fine-grained sense of how things going minute by minute. Truthfully, if they have another way to immediately see disruptions, act to clear them, followed by solving the underlying problem then they are as [lean] as anyone. So here is the second heresy: You don't NEED takt tie to [be lean]
What you need is some way to determine the minimum resource necessary to get the job done (JIT), and a way to continuously compare what is actually happening vs. what should be happening, and then a process to immediately act on any difference (jidoka). This is what makes [lean] happen.
Takt time is just a tool for doing this. It is, however, a very effective tool. It is so effective, in fact, that it is largely considered a necessary fundamental. Honestly, in day to day conversation, that is how I look at it. I made the above statements to get you to think outside the mantras for a minute.
What is takt time, really?
Takt time is an expression of your customer demand normalized and leveled over the time you choose to produce. It is not, and never has been, a pure customer demand signal. Customers do not order the same quantity every day. They do not stop ordering during your breaks, or when your shift is over. What takt time does, however, is make customer demand appear level across your working day.
This has several benefits.
First, is it makes capacity calculations really easy through a complex flow. You can easily determine what each and every process must be capable of. You can determine the necessary speeds of machines and other capital equipment. You determine minimum batch sizes when there are changeovers involved. You can look at any process and quickly determine the optimum number of people required to make it work, plus see opportunities where a little bit of kaizen will make a big difference in productivity.
More importantly, though, takt time gives your team members a way to know exactly what 「success」 looks like for each and every unit of production. (assuming you give them a way to compare every work cycle against the takt time – you do that, don't you?)
This gives your team members the ability to let you know immediately if something is threatening required output. Put another way, it gives your entire team the ability to see quickly spot problems and respond to them before little issues accumulate into working on Saturday.
The key point here is that to get the benefit, you have to have a takt time that actually paces production. It has to be real, tangible, and practically applied on the shop floor. Otherwise it is just an abstract, theoretical number.
This means holding back 「available time」 for various planned (and unplanned) events where production would be stopped.
Further, in a complex flow, there may be local takt times – for example, a process that feeds more than one main line is going to be running to the aggregated demand, and so its takt will be faster than either of them. Likewise, a feeder line that builds up a part or option that is not used on every unit is going to be running slower.
And finally if disruptions do cause shortfalls to the required output, you have to make it up sometime. If you are constrained from running overtime (and many operations are for various reasons), then your only alternative is to build a slight over speed into your takt time calculation. The nuances of this are the topic of a much longer essay, but the basics are this:
- If everything goes well, you will finish early. Stop and use the time for organized improvement of either process or developing people. Continuing to produce is overproduction, and just means you run out of work sooner if you have a good day tomorrow.
- If there are issues, the use the buffer time for its intended purpose.
- If there are more issues than buffer time, there is an operational decision to make. Have a policy in place for this. The simplest is 「hope for a better day tomorrow」 and use tomorrow's buffer time to close the gap. If this isn't enough, then a management decision about overtime or some other remedy is required. 管理/經營決策
What about just allowing production to fall short? Well.. if this is OK, then you were running faster than customer demand already. So pull that 「extra」 out of your schedule, stop overproducing (which injects its own disruptions into things), and deal with what just actually have to accomplish. Stop inflating the numbers because they hide the problems, the problems accumulate, and you end up having to inflate even more.
Gee, all of this seems complicated.
Yeah, it can be. But that complexity is usually the result of having an ad-hoc culture that makes up the reactions as you go along rather than a comprehensive thought-out systems-level approach. The key is to work through the 「what if…」 for what you are doing and thinking about doing, how the pieces actually interconnect and interact, and have a plan.
That plan is the first part of Plan-Do-Check-Act.
Then, as the real world intrudes, you can test your thinking against reality and get better and better rather than just being glad you survived another day.
And that, is the whole point of knowing your cycle times and takt times.
source: http://theleanthinker.com/2010/04/28/takt-time-cycle-time/
