Another Interesting Poka Yoke

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Last week Bryan posted about a poka yoke (error proofing) that he found when he bought a kitchen table (here.)  Today I received a piece of office furniture and was frustrated when I looked at the instructions (I read instructions when on company time – I only follow my y chromosome driven instinct to disregard instruction until after I have screwed something up on my time.)  The instructions showed that the back of the chair should be attached to the seat of the chair with a socket head cap screw, a flat washer, and a lock washer.  What was not clear was what went where?  Did the flat washer go between the back assembly and the seat assembly or did the screw go through the lock washer then the flat washer then the seat assembly and finally into the threaded back assembly.  The only thing that was clear was that the back assembly had to be last because it was threaded.

Then I looked at the package that the hardware (screws, washers, and hex wrenches) came in:

The washers and screws where already 'assembled.'

It’s kind of hard to see in my cell phone photo but the manufacturer already ‘assembled’ the washers on to the screws.  The clear plastic of the packaging is of the hard variety not a film that was shrink wrapped, therefore it would not possible (or at least easy) to put the washers in separately.  The shape of the clear plastic might give some sign that something is missing if the washers were omitted, there would be empty bulges in the packaging.  I assumed that the only thing that I had to do was to put the cap screws through the seat assembly and into the back assembly and tighten.  If that isn’t correct then this isn’t a very good poka yoke.

The questions that this poka yoke beckons are:

1. Is this really a poka yoke? According to orthodoxy (per Shigeo Shingo) poka yokes are of 3 types, motion / sequence, counting, or physical contact – physical contact when there is an the abnormal condition.  If the washers were left out or were put on the screw in the wrong order there would be no physical contact that would not allow the part to be ‘passed.’
2. Is it really and example of visuality? Dr. Gwendolyn Galsworth describes the ‘visual workplace continuum’ in her article The Value of Vision (pretty good article, I’d recommend that you read it) for Industrial Engineer. Her continuum is visual order, visual standards, visual metrics, visual controls, and finally visual guarantees (poka yokes.)  Best I can place this would be visual order (the origin of the journey per Dr. Galsworth.)  It tells me where something goes.  Doesn’t really tell me what or in what order.  Abnormality (other than absence of washers) is not intuitive at a glance.
3. Is it an ambiguous visual control? What if the person who put my hardware package together got it wrong?  How would I know?  When the cap screws loosened up?  When the chair falls apart?

What do you think?  Is this a good poka yoke or a good effort at visuality?  It is certainly not a bad effort at something.  It shows more customer focus than simply putting stuff in a bag with inadequate instructions, but how would you improve it?  To me the key has to be that the abnormal condition should be intuitively obvious at a glance – like the shadow on a shadow board tells you when something is missing (like this example from Brady.)  The wrench is missing.

Bruce

Ed. Note – I suppose that we will call the next poka yoke post “Yet Another Interesting Poka Yoke.”  Then I don’t know what we will call them unless one of our readers can suggestion a new naming convention for these types of posts.

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