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The secrets of Poka Yoke

Poka Yoke (“poh-ka yoh-kay”), translated as mistake-proofing, was developed by Toyota manufacturing engineer Shigeo Shingo in the 1960s. (Its original name, ‘fool-proofing’ was changed because some people were offended by its implications.) It’s another preventive technique that I recently promised to explain in more detail.

Poka Yoke is a simple but effective approach to reducing errors and defects in any business or manufacturing process by removing the opportunity to make the mistake in the first place; it eliminates the need for particular concentration or skill or memory to get the process right.

Often several different Poka Yoke techniques are used at the same time on the same process or assembly, each preventing a different potential error so that the process is robust and virtually impossible to get wrong. Having said that, some people also use it for early detection of errors by making them immediately obvious, although I’m not keen on that interpretation, I prefer to keep it purely for preventing the problem occurring in the first place.

Poka Yokes usually involve devices like fixtures, jigs, mechanical interlocks or switches, and warning mechanisms that prevent people from making mistakes even if they want to! They automatically stop machines or mechanisms, prevent components being assembled the wrong way round, guard the users against hazards or warn them if something starts to go wrong.

Yes, they could involve sophisticated computer vision systems, sensors and lots of software but more often than not they use something like a peg fitted to a block of plastic or a mechanical part that is asymmetrical so it only fits in its hole one way round. The most effective Poka Yokes are usually very cheap and very simple.

How to develop Poka Yokes

The great thing about this technique is that anyone can do it; once you have the right mindset it’s something that a bit of common sense, some creative thinking or brainstorming, and a little experimentation can deliver.

The first action is to look at what can go wrong, because – as per Murphy’s Law – anything that can go wrong will go wrong. What sort of mistakes can and should be prevented? Could the wrong number of parts could be used, or the wrong type of parts (e.g. too few screws of the wrong length), could you forget to apply thread-locking compound or miss out the adhesive from an assembly operation? Could you use incorrect machine settings, or make measurement or calibration errors? Could you work to the wrong assembly documentation, or miss a key stage of the assembly, or fit the wrong connectors or cables together or fit them in the wrong orientation?

Do you have examples of where things have already gone wrong? You could have a brainstorm or any other structured creativity session about what errors might possibly occur, however far-fetched. Try using ‘reversals’ – rather than looking at how to assemble it right, look at how you could assemble the item incorrectly if your life depended on doing so.

Then come up with the simplest possible mechanism, or technique, or tool, or jig that would eliminate each error at source. If you can’t possibly stop an error, as a second-best how can you show that it has occurred as quickly and obviously as possible?

Then test the mechanisms or techniques or jigs to see which combination works best, then put them in place and train your staff in their use.

Examples

Good Poka Yokes include things like ‘keyed’ plugs and sockets that prevent the wrong connectors being fitted together or fitted the wrong way round, or asymmetrical hole patterns in matching plates so they can only be screwed together the right way round, or cut-outs in Printed Circuit boards that only allow them to be fitted the correct way into an enclosure. The bevelled edge on a mobile phone SIM card is a good example, as it stops you inserting it the wrong way round, as is a guard over a button that stops it being pressed by accident.

As another example you may decide to pack exactly the right number of nuts and bolts for a given assembly in a container that travels with each assembly; if you have any left over at the end, or if you run out, you can easily see this and look into what has gone wrong – this can save you from leaving fixings off the assembly or from the ‘loose screw problem’ – spare fixings rattling round loose inside the unit because they were dropped in there.

Do a Poka Yoke on your Poka Yoke

Never underestimate the capacity of some folks to get things wrong! People can be ingenious. Although colour coding has its place, don’t over-depend on it as a significant proportion of the population is colour-blind. If you’re designing mechanical interlocks remember that people are stubborn and may force things even when you think it’s obvious they shouldn’t be fitted that way round. Safety interlocks can be defeated – try to defeat yours and see it it’s possible. Do a Failure Mode and Effects Analysis on your design – what would have to go wrong to render the Poka Yoke ineffective?

And when you’ve implemented them, review the effectiveness of your Poka Yokes; keep an eye on them and make sure they are delivering the results you expect after a period of time.

Is Poka Yoke difficult? Frankly, no; as with many Japanese quality techniques it’s mainly applied common sense but the trick is to actually apply it in a structured, planned way and make it stick. Mistakes will be made – people are only human – so find ways of preventing those mistakes leading to defects in your products or services. Educate your colleagues, set up some Poka Yoke sessions, get some quick wins under your belt and show how everyone can help reduce costs and reduce waste through the application of good, sound, common sense Poka Yoke techniques.

Then keep doing it!

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