Continuous flow 1

Concept:
Continuous flow is the most important concept in lean manufacturing system. Sometime it was called "one piece flow" or "single piece flow" to present the process must flow by batch in one piece or small batch. As we know, lean manufacturing concept came from Toyota and we all know how big the car was. We can imagine that if the car was making in conventional mass production, the workshop would be many ten times bigger than the modern automotive manufacturing factory. Applying one piece flow in automotive manufacturing is the only choose. "single piece flow" looks very similar to "one piece flow", but at my point of view, "single piece" not likes "one piece" to represent absolutely one. It can represent a very small batch size of flow, such as 5 pieces or 100 pieces, the most suitable size in the flow. In making potato chips, can we fire the chip in one piece by one piece? For some industries, the parts is very fine and the process time is very short, such as small plating parts, may be we need to use a small batch instead of one piece concept. Offcause lean manufacturing came from automotive, the concept is good, but we need to analysis and adjust the concept to suitable with your industries rather than just meaningless "copy and paste". Personally, I like to call the concept in simply, "Continuous flow", it is no need to identify how many pieces in the flow, we only need to make the flow in lowest quantity, continuously and non-stop. In this kind of flow, the processes can be operated in more economically and cost-effectively.

Benefit:
From here, I don't want to repeat the benefit for you to apply "continuous flow" because there are so many books to describe the advantages detailly. I stress again here that the most important and powerful function of continuous flow to define value-added and non-value-added process and identify the "7 wastes" in the processes or between the processes in order to shorten the delivery time, improve quality and efficiency. If we want to establish a good continuous flow, we need to eliminate them in the flow. You can also reference to my previous post 7 wastes.

Tact time:
Sometimes, we called it "takt time", in Germany. We often identified tact time as the speed of the process flow and the rhythm of all process. Orchestra needs a conductor to direct when the music rhythm, start and stop. The production leader is working as the conductor to direct rhythm of manufacturing, ensure that the process in the flow working in same speed by tact time. The tact time is calculated by the demand from customer, the other feature of tact time is matching the production output and customer demand. Make sure that we can provide the demand quantity to customer, no more and no less. Ideal situation, the flow is really in one piece, we can explain tact time like this. Unfortunately, not all the process can be put in L or U shape and easily to flow in one piece. I would like to explain the tact time in another way to suitable for the non-one piece flow.
In the situation which all processes can not flow in one piece in every stations. Especially some of the process have a big gap in process time, it is hard to avoid and adjust the process, it is very easy to find the overall process combined with manual assembly and automatic. At this moment, the tact time is using to control the flow rate. Refer to the picture above, the flow rate in process 1 should equal to process 2 and process 3. As we know, the flow speed is high, the flux is low. For example, in process 1 for making parts, the rate is 1 piece / s, 1 piece/s X 20 = 20 piece / 20 s and flow to supermarket 1 for temporary storage. In process 2, the sub-assembly rate to assemble 10 pieces of parts from process 1, 1 piece /10s X 2 = 2 pieces/20s, and flow to supermarket 2 for storage. In final assembly process 3, the process time is 20s to assembly 2 pieces sub-assembled parts from process 2, 1 piece/20s. Every stages of processes have the same flow rate in 20s and formed a good continuous flow. You can find that actually, the process 1 is only one machine and made the 20 parts one by one. The condition in process 2 is the same. At this example, the critical concern is how to setup the quick change-over, the tact time is the flow rate to balance the processes.
In simply say, tact time is same as the heart beat of human, every beat represents a cycle of blooding system.

In the next post, I will discuss some misunderstandings and difficulty for continuous flow.