2 card Kanban in pictures - explaining the intricate working and the resultant benefits..

Kanban means visual card. It is used to keep track of work in process inventory on the shopfloor. More than the functions of production and withdrawal which these two kanbans help to serve, it is the other benefits that accrue from the tight coupling between the workstations which brings in many of the benefits.

How the Production (green) and withdtrawal (red) kanbans
work

Movement of the two types of kanbans -
The production kanban (green) in the picture moves only within the workstation and thus is called the Intra-workstation kanban. Withdrawal kanban is the kanban which moves across workstations and is called the Inter-workstation kanban

Resting of the two types of kanbans -
The production kanban has only two places of rest, it is either on the "part-job completed" workpiece or on the production kanban toolpost. Similarly, the withdrawal kanban has only 2 places of rest, either on the part ready to be withdrawn at the outpost of the first (or previous) workstation or on the withdrawal kanban toolpost which is the indicator for the transporter to proceed to the previous workstation and collect the finished workpiece to be brought to the present workstation.

This is how the kanbans move within a workstation and between workstations.

1. When the production green kanban is on the part-job completed workpiece and rests at the outgoing station, it gives signal to the next workstation that it is ready and can be withdrawn to complete the next set of job processes on it at the following workstation, ie. when the production green kanban is on the workpiece at the outgoing station, it gives signal to the following workstation to start the next assembly in that workstation.
2. The second workstation now withdraws the part, removes the production kanban and hangs it on the production kanban post in the first workstation. 
3. After the production kanban was removed at the previous workstation, the workpiece attaches itself with a withdrawal kanban from the withdrawal kanban post. The workpiece has now moved from the first workstation and is traveling to the second workstation with the withdrawal kanban attached to it. 
4. When the part is ready to be worked on at workstation 2, the withdrawal kanban is removed and put on the withdrawal kanban post between the two workstations and a production kanban from the production kanban post in the second workstation is taken out and attached to the new part which has come from the previous workstation (workstation 1).

This process continues between the workstations throughout the production/assembly line. The beauty of having two types of kanbans is that we can have sub-assemblies joining together at an assembly station.

A normal kanban giving the numerous info it carries ..

If and when WIP between two processes or workstations need to be reduced or increased, just have to reduce and increase the kanbans by that amount. 

Is there any purpose in increasing the number of Kanbans other than increasing the inventory between workstations and making the workforce extremely idle and less responsive as now it can get the job done independent of earlier workstation performance or efficiency.

But what will happen if the number of kanbans between workstations is reduced ?

For example, if between WS A and WS B there were 5 WIP inventory stocked, indicated by 5 kanbans. The connection between WS A and WS B is said to be loosely coupled because until all the 5 items are used up, there is no demand for new items. 

By reducing kanbans between workstations, inventory
betweenworkstations can also be reduced , leading to
continuous improvement.
pic courtesy gemba research

If the manager wants to have a tightly coupled system between WS A and WS B, he will remove 2 kanbans from the system. Now there are only 3 WIP between A and B, so the system is said to be tightly coupled than earlier. Any problem that occurs has to be urgently attended to, as otherwise the WIP inventory would get consumed and the workstations would be starved for items leading to stoppage of the production line.

As we reduce the kanbans between the processes, we see that the work-in-process (WIP) inventory reduces.

Even though outwardly removal of kanbans from the system looks like an ordinary inventory reduction problem, the tightly coupled nature between the workstations has different other connotations, which are not visible but inherent in the process. They are as follows -

1. make the system very demanding and can motivate the workers to give their best to the system.  

2. The feeling of accomplishment and extreme satisfaction resulting from such challenging situations is a great motivator for the employees to give their best to the system.

3. Low stock inventory provides faster feedback to the upstream  workstation on functionality and quality from downstream workstations that use that inventory. It thus helps the system to automatically raise the quality levels in the system, without the application of any major QC tools ..

Thus Kanbans, as much as helping keep track of WIP between work stations, also helps to

1. reduce the WIP inventory over time,
2. reducing costs,
3. makes the system very dynamic,
4. responsive,
5. high quality and
6. extremely motivating for the employees.