If you google “Buffer Management” you will get a variety of results that focus mainly on buffers used to pad time for a task in project management or a deliberate amount time built into a project management timeline to facilitate nuances that might occur. That sounds a lot like waste to me, and it in no way captures the essence of this article and how you can employ buffer management to create continuous flow.
As you know from past articles, I am strongly against inventory of all types. My definition of a truly lean organization is one that operates without onsite inventory. That is the ultimate True North in the lean journey. However, the journey to utopia is an evolving process. Discovering what you don’t know, and what you don’t know that you don’t know, involves developing and testing alternatives to each new current state that evolves from the incremental continual improvements that take place along the way. Learning how to better manage inventory while creating flow is one of the areas of discovery. Even though inventory is waste, sometimes a little waste can be beneficial.
Inventory is one of the Seven Deadly Wastes. Procuring and managing inventory takes time. It consumes space and financial resources that cannot be used for other purposes. It requires that someone receive it, transport it, account for it, allocate it for consumption, transport it to the point of use, maintain it again as the transformation steps take place, and finally account for it as a finished item of either a temporary or long-term nature.
Although inventory has a lot of bad side effects, one of the positive attributes is having needed items available so production interruptions are minimized. Properly managing inventory levels so they do not become excessive can be done in a few different ways. One way is through a Kanban system (Google “Kanban” or search the FDMC archives to find out why Kanban is an important lean tool). Buffer Management employs a type of pseudo Kanban approach by ensuring the necessary amount of work-in-progress inventory is available to facilitate flow, while not consuming other valuable resources unnecessarily.
Like all Kanban systems, buffers need to be flexible to be most effective. A rigid buffer system won’t easily accommodate changes in customer preferences or fluctuations in demand. Kanban systems use visual triggers to tell a person when to take action. Buffer systems need to be established in the same way. The intent of Kanban and buffers is to enable and engage people so they can make timely and informed decisions.
One of the hardest parts for many leaders to understand about creating continuous flow is employing the concept in areas where there is a vast difference in the task times from one process to the next. How do you create flow when one process takes three minutes while the next one takes five minutes, and a subsequent process only takes 30 seconds?
A good example of time disparity between process steps is the finishing department. It can take 4 minutes to prep sand a part, but only 1 minute to stain the part. Then it might take 3 minutes to scuff sand it and another half-minute to spray the first lacquer coat. After another 3 minutes of final sanding, the part is ready for a 30-second top coat. The typical response in this scenario is to batch parts and do each step individually.
The downside of batch thinking is that it takes more space to do a large quantity of parts, it produces more transport and motion waste as parts are moved from carts to saw horses and back to carts, and continuing in that manner for every additional step. Finish departments that employ a batch method are usually two to three times larger than they need to be, which exacerbates transportation waste. Buffer management is a better solution.
One perception that finishing people have is that batching allows parts to flash-off and cure before they are handled again, thus enhancing quality. Although it is necessary to allow for that to take place, in reality, batching encourages flash-off and cure times beyond what is necessary because so many parts are being processed at the same time. The larger the batch, the longer the total process time. In a batch process, the first part completed in each process step has to wait for the entire batch to be done before it can move on. Typically, that means parts wait longer than is necessary.
Let’s assume a batch contains 40 doors. Based on the above finish process of 12 total minutes per door, you might believe the total process time to be 480 minutes. Actually, the total process time is 480 minutes plus all of the handling time to load and unload parts to and from the sawhorse platforms, plus the additional movement of the sprayer between rows of parts while untangling the hose that drags along behind him, plus the flash-off time of the last part on the sawhorses. The total process time is at least 60 or more additional minutes.
In a flow scenario where parts are pulled by each subsequent step, a part is processed every 4 minutes. So, the 40 doors could be completed in 160 minutes, plus the time it takes to fill the line until a part is at each station, plus the flash-off and cure time of the final part. The total would be closer to 200 minutes rather than 500 or more minutes.
Achieving flow requires balancing the steps so each is pulling from the prior step. On the surface, it looks like some of the people will be standing around waiting, while others are overwhelmed with work. If the line were staffed with one person at each step, that assumption would be true. However, most finishing departments are comprised of people who are trained to do multiple tasks so the sprayer should know how to do scuff sand and the stain person should be able to prep sand, etc., so they can float between process steps to balance the work content.
Flash-off and cure times are allowed for through the creation of buffers as necessary between process steps. If the longest process time is 4 minutes and the flash-off and cure time is 20 minutes, a buffer of five parts would be required between lacquer and scuff sand. Once the line and buffers are full, a part comes out every 4 minutes.
If my explanation is challenging to visualize, check out videos below.
Lean manufacturing finishing room single piece flow:
Giffin finish department process improvement:
As noted in one of the videos, when buffers are full, the supplying process stops and it is time to investigate how to help the customer break down the barrier causing the interruption in flow.