When most people think of lean manufacturing or TPS, they think of a kanban system. While it is a key component to a well established lean manufacturing environment, it is only part of the system.
The basic concept of a kanban is a hand sized card that moves with the product or material. It signals when product is to be built or when material can be moved. A company disciplined in lean manufacturing methods will not build product or move material without the proper kanbans. In order to implement a system using kanbans, you must first understand the types of kanbans, their funtions and most importantly, the rules.
Types of Kanbans
There are two basic classifications of kanbans; generic and brand. Generic kanbans are not tied to a specific product, but will tell you when to move material or build product and the quantity required. Brand kanbans will tell you speciffically what to build or move, when to build it and how many are required. Both types of kanbans may have more information than stated above, such as where the parts are to be delivered, but the main point is to understand the differences between the two types of kanbans.
Within the two classifications there are two main types of kanbans; production kanbans and move kanbans. Production kanbans give the signal to build. Move kanbans (or withdrawal kanbans) give the signal to move product or material.
Examples of production kanbans could be hand sized cards that signal when to build product. A taped off location within a production cell that signals that a part can be built and placed in that location.
Examples of move (withdrawal) kanbans could be a hand sized card that informs when to move material. A full skid may be an indication that the product needs to be moved in favour of an empty skid. A part placed within a taped off location could be the signal that the part is OK to move to the next operation.
The most important function of a kanban is that it instructs when to produce or convey parts. In a lean manufacturing environment, no production or parts movement shoud take place without a kanban. In essence, it is your work order or pick list.
Another key function is visual control; to identify what is in each box. Each box of product (finished goods, raw components or sub-assemblies) should have a kanban attached to it. Seeing a box of parts without a kanban is a clear signal that something is out of place.
The third major function is inventory control. When using a kanban system, it is very easy to control the amount of finished product on hand. The number of kanbans in the system will be determined through levelized production and only that quantity of kanbans will be issued, thus keeping your inventory at the pre-determined level. Regular audits of the system will keep this quantity in check.
Last, but certainly not least, you must define, apply and follow a set of kanban rules (policies, procedures) for your company. I will list some of the major rules that should be included at your company, however you may find it necessary to include some of your own.
1. Never move or produce parts without a kanban.As stated earlier, a box without a kanban will signify a problem.
2. Never produce more parts than you have kanbans for.Kanbans are used to control inventory for many reasons. Producing parts without a kanban will create waste of inventory and will leave boxes unidentified (see rule #1).
3. Never move more parts than you have kanbans for.Storage locations are usually established with a set level of inventory in mind. Moving extra boxes could result in unecessary overflow at the storage location.
4. No one is permitted to send information other than that on a kanban.A brand kanban (which most production kanbans would be) will tell you what to build, when to build it and how much to build. Communicating information other than this will violate one of the other rules.
5. Do not send defective product to the next operation.This is a basic lean manufacturing concept but it must also apply to kanban systems. As the amount of inventory will be low, passing on bad parts could lead to many types of waste with emphasis on waste of defects.
Remember, in order to ensure success when implementing a kanban system, you must train everyone on the proper use and communicate the importance of discipline. Establish your rules and ensure everyone in your company understands them. This way it will be obvious to anyone in the company when something is out of place.