Poka Yoke is a component of lean manufacturing which involves error proofing of the production process.
Like the square peg that doesn’t fit into a round hole, it involves designing processes to eliminate the ability to produce poor quality products.
Each task in a production process is inspected for defects. If the defects emerging from a task are particularly high, the root cause is determined and a method of poka yoke (error proofing) is installed. This can be anything that measures and removes the root cause of defects at source, for example installing a scanner that illuminates a warning light, or posting a checklist on the wall.
Poka Yoke is generally considered a preventative measure, but can be performed as a proactive measure in response to a known problem.
Types of Poka Yoke
In lean manufacturing there are three types of Poka Yoke:
- Contact method
The products coming out of the assembly station are tested for shape, size, orientation, or other physical attributes. For example, a window is measured for the correct length and width prior to installing the frame.
- Fixed Value method (aka Constant Number method)
The process is tested to ensure a certain number of movements have been made. For example, a circuit board manufacturing station senses the number of solders that are made prior to releasing it to the next station.
- Motion-step method (aka Sequence method)
In this method, the prescribed steps are inspected to ensure they have been followed. For example, a checklist in which each box must be physically checked by the operator would be a motion-step poka yoke method.
Poka Yoke Examples
Here are some generic poka yoke examples:
- Product designs that have physical shapes making it impossible to install in any but the correct configuration.
- Parts containers that contain sensors which do not allow the product to move to the next station if the operator has not picked the correct part.
- A parts monitoring system that detects when parts are removed from bins.
- An order input algorithm that flags orders that fall outside the traditional pattern, allowing the order to be inspected to see if it has been correctly inputted.
A manufacturer of steel pipe tests each weld using ultrasonic sensing equipment, and about 15% of the pipe is returned to the welding station for further processing to bring it up to specifications. The plant manager performs a poka yoke by determining why the welds are not to specification. He finds that the welders tend to cut corners by avoiding the lifting of the heavy pipe sections to adequately weld underneath the pipe. The manager develops an improvement to the process by developing a written checklist for the welding process, which includes walking to the lifting equipment, waiting for the pipe to be lifted by 3 feet, and then walking back to perform the weld. All of the welders are trained in this procedure, and the defect rate drops from 15% to 2%..
How to Perform Poka Yoke
There are four steps to performing poka yoke:
- Identify the problem
Many plant managers think their plant is functioning great and there is no reason for concern, even while watching a certain percentage of their products turn out defective or consistently move back to previous stations for re-work. Lean manufacturing specifies continuous improvement (kaizen) which involves constantly searching the production process for waste that does not contribute to the value stream. Even the leanest plants still find and remove waste regularly. The key is to measure for variation in each section of the assembly process. Every time a product must go back and be re-worked at a previous production station there is a problem with variation that could use a poka yoke. In most cases if you have a variation of 5% or more you need to improve the process, but this varies quite a bit from industry to industry and product to product.
- Determine the root cause
Once you have found a variation (or defect) you must search for the root cause of that defect. That is, if too many products coming out of a station are defective, you must ask why. In fact, Taiishi Ohno, the industrial engineer who was the father of the Toyota Production System stated that he would always ask 5 Whys, each time drilling deeper into the root cause. Firstly, why did the defect happen? Secondly, why did that happen?… Why did that happen?… and so on. After five repetitions the root cause is usually near at hand.
- Find the solution
Now that you know what caused the defective products, the possible solutions must be weighed. Like any decision making process, the alternatives must be developed and each one analyzed for their pros and cons, as well as costs. The three types of poka yoke, described above, must be utilized to ensure all of the possible options are on the table. Once all of the options are analyzed, one must be chosen.
- Apply the solution
Often the solution requires training or manufacturing of specialized tools and equipment. Sometimes the production process must be stopped to allow the implementation of the poka yoke. These types of process changes require planning. In the Toyota Production System, the concept of jidoka means that line workers are given the authority to immediately stop the production line if they notice any quality problem that can be fixed. This usually applies to small, minor poka yokes, but all quality issues are targeted.
Most plants think, plan and analyze intensely prior to implementing the smallest changes to the production line. In contrast, lean manufacturing reasons that the elimination of waste from the value stream is more like an emergency – the patient is dying and the waste must be completely eliminated from the production line without delay.
Poka Yoke was originally popularized by Shigeo Shingo of the Toyota Motor Company in the 1950’s and 60’s. After Toyota went from near ruin following WWII to become the world’s largest automaker in the 1990’s, its production system came under intense scrutiny by others wanting to duplicate its success. The Toyota Production System is the forerunner to today’s Lean Manufacturing methodology.