Poka-Yoke and Kanban

Kanban

Taichi Ohno of Toyota Motor Company was the originator of the kanban method. This idea occurred to Ohno on a visit to the United States when he visited a supermarket. In the supermarket, a product is “pulled” from the shelf and the missing item is replenished. Liker (1997) suggests that the story of Ohno visiting an American supermarket to develop Kanban is fiction. Kanban is the Japanese word for “sign” and is a method of material control in the factory. It is intended to provide product to the customer with the shortest possible lead times. Inventory and lead times are reduced through Heijunka (leveling of production).

For example, if plant production goal for day 1 is 8 units of A and 16 units of B. And on day 2 it is 20 units of A and 10 units of B, the usual method is to produce all of A, followed by all of B. This may be the most efficient use of time for the plant machinery, but, since production will never go according to plan, the customer may change their mind on day 2 and order less of A. This causes a pile-up of inventory and possibly increases cycle time.

To reduce the WIP and cycle time, the goal is to be able to produce each part, every day in some order such as 2 As, 1 B, 2As, 1 B, etc. The factory must be capable of producing such an arrangement. It requires control of the machinery and production schedule, plus coordination of the employees. If a kanban system is used, with cards indicating the need to re-supply, the method of feeding an assembly line could be achieved using the following process:

• Parts are used on the assembly line and a withdrawal kanban is placed in a designated area.

• A worker takes the withdrawal Kanban to the previous operation to get additional parts. The WlP kanban is removed from the parts pallet and put in a specified spot. The original withdrawal kanban goes back to the assembly line.

• The WIP kanban card is a work instruction to the WIP operator to produce more parts. This may require a kanban card to pull material from an even earlier operation.

• The next operation will see that it has a kanban card and will have permission to produce more parts.

• This sequence can continue further upstream.

Poka-Yoke (Mistake Proofing)

Shigeo Shingo (1986) is widely associated with a Japanese concept called poka-yoke (pronounced poker-yolk-eh) which means to mistake-proof the process. Mr. Shingo recognized that human error does not necessarily create resulting defects. The success of poka-yoke is to provide some intervention device or procedure to catch the mistake before it is translated into a non-conforming product.

Shingo (1986) lists the following characteristics of poka-yoke devices:

• They permit 100% inspection

• They avoid sampling for monitoring and control

• They are inexpensive

• Poka-yoke devices can be combined with other inspection systems to obtain near zero defect conditions.

Errors can occur in many ways:

• Skipping an operation

• Positioning parts in the wrong direction

• Using wrong parts or materials

• Failing to properly tighten a bolt

There are numerous adaptive approaches:

Gadgets or devices can stop machines from working if a part or operation sequence has been missed by an operator. A specialized tray or dish can be used prior to assembly to ensure that all parts are present. In this case, the dish acts as a visual checklist. Other service-oriented checklists can be used to assist an attendant in case of interruption. Numerous mechanical screening devices can be utilized. Applications can be based on length, width, height, and weight. Cash registers at many fast food outlets have descriptions or schematics of the product purchased.

This system, in addition to the use of bar codes at super markets has eliminated data entry errors and saved time. Obviously, mistake proofing is a preventive technique. Mistake proofing can also be accomplished through control methods by preventing human errors or by using a warning mechanism to indicate an error.

Some of the control methods to prevent human errors include:

• Designing a part so it cannot be used by mistake

• Using tools and fixtures that will not load a mispositioned part

• Having a work procedure controlled by an electric relay

• A signaling mechanism can alert a worker of possible sources of error

Several applications include:

• Having the parts color-coded

• Having tool and fixture templates in place to only accept correct parts

• Having mechanisms to detect the insertion of a wrong part

• A buzzer or light signal that an error has occurred, requiring immediate action

• Root cause analysis and corrective action are required before work resumes

Other than eliminating the opportunity for errors, mistake proofing is relatively inexpensive to install and engage the operator in a contributing way. Work teams can often contribute by brainstorming potential ways to thwart error-prone activities. A disadvantage is that, in many cases, that technical or engineering assistance is required during technique development.

Design improvements to mistake-proof products and processes include:

• Elimination of error-prone components

• Amplification of human senses

• Redundancy in design (backup systems)

• Simplification by using fewer components

• Consideration of functional and physical environmental factors

• Providing fail safe cut-off mechanisms

• Enhancing product productivity and maintainability

• Selecting components and circuits that are proven

Everyday Examples of Poka-Yoke:

• Gas cap attached to a car

• Gas pumps with automatic shut-off nozzles

• 110V electrical plugs and polarized sockets

• Microwave automatically stops when door is opened

• Seatbelt buzzer to warn drivers and passengers

• Elevator electric eye to prevent door from closing on people

• Lawn mower safety shut-off when bar is released

• Car keys ground symmetrical to allow two-way insertion

• Product drawings on cash registers at fast food restaurants

• Barcodes for product identification during distribution

Poka-yoke techniques are especially effective when:

• Vigilance is required in manual operations

• Mis-positioning of components can occur

• Attributes not measurements are important

• SPC is difficult to apply

• Turnover and training costs are high

• Special cause failures occur frequently