As stated in previous posts, it is very difficult to achieve a continuous flow when demand increases and decreases and having a limited predictability. A strategy to combat the aforementioned problem is by implementing pull/kanban systems.
An effective way to overcome ups and downs in demand is not to make manufacturing or process workforce work faster (“uncontrolled speed makes waste”) but through scaling the process to meet demand or adding or taking away process or manufacturing workforce and shifting process steps up or down the whole process.
This is what the fourth principle of The Toyota Way states:
“Level out the workload (heijunka)”.
As said by the Toyota Motor Corporation President, “when trying to implement the Toyota Production System, the first thing you have to do is balancing production. This should require bringing forward or postponing some deliveries and might have to ask to your customers to wait some more time. Once the level of production remains constant, in one month you will be able to apply pull systems and balance your assembly line. On the contrary, if the production levels change on a daily basis it won’t be effective to apply those systems and you will not be able to establish standardised procedures on those circumstances.“
As an analogy to the fable of the tortoise and the hare, it is better to work slow and steady rather that fast in order to win the race.
When demand fluctuates, maintaining a balanced effort from our workforce is very important. It is crucial not to lower down the efforts when demand goes down and, the opposite, not to increase the effort when demand goes up. So, as it is not trivial, how can we manage this situation in the real life?
For instance, if we have demand doubled or tripled one week compared to the previous one, we do not need to double or triple efforts but to, if we have machinery resources, create a second flexible production line to cope with the additional demand. In case we do not have resources we could extend the schedule by working extra time or weekend occasionally.
However, the reality is not often as exaggerated and, in most cases, demand fluctuates lower values such as 10 % / 20 %.
Using simple example, let us say a process which need 6 operators is balanced and each operator works in 2 process steps to complete the assembly or production of a part. There are ten sequential process steps needed to complete the process.
Let us imagine that the demand is increased a 20%. If process steps are standardized and correctly balanced to cope with the fluctuating demand, one could add an extra operator to the process and move process steps up-stream or down-stream (depending on where the additional operator is allocated) and give each of the six operators one less process steps to work on. Thus, you still have the whole sequential process steps to complete, but the effort and workload given to each workforce remains the same or, in some cases, lower and they can effectively complete each process step in the same amount of time as before.
In reality, not all process steps are always balanced and demand fluctuations are not easily absorbed. There are some strategies that can be applied to balance the workloads such us:
Leveling by volume
It is applied when you use the same production process for a family of products and when demand varies +/- 20 % approximately. Toyota says that when production systems vary in the required output, they consequently suffer from mura and muri with the capacity being forced. Their approach is to balance the long-term demand and hold an inventory proportional to the fluctuation of demand.
The advantage of holding this inventory is that it can smooth production throughout the plant, reduce process inventories and simplify operations which, at the same time, reduces costs.
Leveling by product
In most cases, the value streams, produce a mix of products and have to make a choice of production mix and sequence. It is when the discussions on MOQ (Minimum Order Quantity) take place and have been dominated by changeover times and the inventory which is required. Toyota’s approach is to reduce the time of changeovers so that small batches are cost-efficient and the lost production time and quality costs are not significant.
To simplify the levelling of products with different demand levels a visual scheduling board known as a heijunka box is often used.
The Heijunka box allows an easy and visual control of a production schedule.
A “heijunka” box has horizontal rows for each product. It has vertical columns for the time intervals of production (could be hours, days, weeks, depending on the production schedule and mix). Production control kanban are placed in the holes provided by the box in proportion to the number of items to be built of a given product type during a time interval.
This ensures that production capacity is balanced.