Nov 8 2014
Using Poka-Yoke Techniques for Early Defect Detection | Accelerate Management | Jennifer R.
“Shigeo Shingo developed processes, called “devices,” which made errors much less likely. In one of the examples used by author Harry Robinson, Shingo created a process where workers were required to take two small springs and put them into a dish before assembling a switch (which used the two springs). While this seems like a waste of time, it stopped the workers from forgetting to put the springs into the switch to start with, which saved an enormous amount of time by preventing technicians being sent to customer locations for repair.”
Source: www.compaid.com
What could possibly go wrong? Placing two springs in a dish prior to assembly not only adds a handling step, but it neither physically prevents a mistake, nor immediately detects it once made. A new operator, or one who fills in for another who has the flu, is likely to skip this step, particularly if necessary to sustain the pace.
This example not like any Poka-Yoke I am used to, like the slots in my printer that are shaped so that an ink cartridge of the wrong color won’t go in, or the food processor that is started by pressing on the lid. These devices actually make mistakes impossible without adding any work, so that there is no incentive to bypass them.
And it’s not difficult to imagine methods that might have worked with the switches. For example, the springs, presumably prop the buttons up, and a whisker hanging over the assembly line might be triggered only if the switch is tall enough…
This article made me wonder whether Shingo, the inventor of the Poka-Yoke concept, had actually come up with this dish idea. It is indeed on p. 44 of his book, “Zero Quality Control: Source Inspection and the Poka-Yoke System,” and he does call it a Poka-Yoke, even though he didn’t coin the term until two years later.
It is the only example I remember seeing in the Poka-Yoke literature that does not meet the requirements of being 100% effective and not adding labor.
Devices and methods that make errors less likely are useful too, but not mistake-proof. It is usability enginering. If you make operations easy to understand with intuitive, self-explanatory user interfaces, mistakes may be so rare that you don’t need mistake-proofing. It’s fine, but it’s not mistake-proofing.
See on Scoop.it – lean manufacturing
November 10, 2014 @ 1:37 pm
I had an opportunity in 1989 to discuss Poka-Yoke with Shingo himself. Below are extracts from my notes on his comments, and my own experiences over the last 25 years. Initially he chided me for talking about it as an individual technique, and said it should be seen as the tool for implementing his system of ‘source inspection’ and guaranteeing zero defects.—
Shingo explained that traditional ‘long cycle’ inspection systems wait until an error in action produces a defective item, the defective item is then found by inspecting the output. His concept of source inspection uses the ‘short cycle’ inspection system. In this system the action itself is checked 100% using mechanical means. If an error occurs, immediate action is taken to correct it before a defect is produced. With this methodology we can guarantee zero defects to the final customer.—
The basic system is simple;
The Poka-Yoke methods/devices should be designed to detect deviation from the standard actions and outputs required to satisfy the customer’s requirements.
This can be done in three ways;
a) Physical contact.
b) Fixed values.
c) Motion steps. —
In some cases at the original design stage the part can be made a Poka-Yoke device by ensuring it can only be assembled/used in the correct way.—
They should also check for deviation in the 3M’s of actions and items;
Missing. Action or item not there.
Misplaced Action or item there, but in wrong position.
Malformed. Action or item are there but wrong, size, shape, colour, temp etc.—
When designing Poka-Yoke devices they must check for specific deviations in the 3M’s using; a, b and c. This can be done with a ‘what can be’ 3 M’s analysis.
The Poka-Yoke device should then;
1) Control the operation. Stop the process when an error or defect occurs.
2) Warn the operator. Signal to the operator that an error or defect has occurred.
They should be applied at the following check points;
1) The source action. (source check) This is the ideal as it gives zero defects.
2) Output of the action. (self check) . This is our second choice as the output will be defective if the PY device is activated, but it will not be passed to the internal customer.
3) Before the next process. (successive check). At this stage the item will be defective if the PY device is activated, but it cannot go to the final customer.
With this system in place it is now possible to consistently achieve;
‘Zero Defects in our activities and production processes’. —
This was Shingo’s original goal in 1965.
If applied to safety it is possible to achieve ‘Zero Accidents’. I do not understand why this methodology is not more widely used in this area. —
The most impressive example of Shingo’s system I have experienced was on an assembly line for inlet manifolds in Japan. We were allowed to work on the line and challenged to produce a defective assembly. It was impossible to produce one, and we had some very talented people trying. —
Once our front line people understand this system they become some of the best designers of Poka-Yoke devices.—
Poka-yoke should be seen as the device for implementing Shingo’s zero defects system. The goal is to identify deviation from the desired conditions or actions in any situation. —
A good example is the selector stick on an automatic car gearbox. If the stick is not in the park position the poke-yoke switch is not activated and the engine will not start. Zero defects in all situations. Shingo pointed out to me that this would be impossible to achieve with statistical techniques.
Having had the privilege to study with the Shingo, I do enjoy sharing his thinking.
November 10, 2014 @ 2:25 pm
In your conversation, it doesn’t seem that Shingo brought up the point that Poka-Yoke should not add labor, a point that was brought to my attention when I first learned about Poka-Yoke, about the same time you had that conversation.
To this date, it is often missing in the literature. The key reason to insist on it, however, is sustainability. Any mistake-proofing device that adds labor is guaranteed to be bypassed in an emergency, which makes it as effective as a medication you don’t take.
November 11, 2014 @ 1:16 pm
The idea that there must be no additional labour involved is implicit in the basic concept. The fundamental idea of source inspection and Poka-Yoke is to move physical and mental work from the operator to the machine and PY devices. This should never increase labour costs. The additional cost that is accepted is the process-stop costs involved when a PY device is activated. This is accepted as an investment in the improvement process and a small price to pay for guaranteeing Zero Defects to customer.
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November 30, 2015 @ 8:06 am
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