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Poka-yoke worksheet

Poka-yoke, Japanese for mistake-proofing, is a device or method that stops a mistake from being made, or makes it obvious the moment it is made, so it cannot turn into a defect. Shigeo Shingo set out the method in Zero Quality Control (1986): a device either controls (stops the process or will not accept the part) or warns, and it checks by contact, fixed value or motion step. This worksheet takes one job step by step. For each step you write the errors that could happen, what they lead to, where they are found today and what they cost a month, then choose prevention or detection, control or warning, and the method, and describe the device. The Excel version works out the cost per month, the payback of each device and the order to tackle them, and keeps a verification log: a device counts as proven only when someone has made the error on purpose and it was stopped every time, and it flags a re-test that is overdue.

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The poka-yoke worksheetHeader fields, a key to prevention and detection, control and warning and the three methods, and a table with columns for step, possible error, effect, where it is found today, errors per month, type, function, method, device idea, owner and tested.
Steps and possible errorsWhere found today and cost per monthPrevention or detectionControl or warningContact, fixed-value or motion-step methodDevice cost and paybackVerification logRe-test dueWorked example
Filled-in Poka-yoke worksheet: left and right brackets that look the same
The same sheet filled in with the worked example (page 2 of the PDF, illustrative numbers). Open full size (opens the picture in a new tab)

When to use it

When to use a poka-yoke worksheet

  • After a root cause analysis that ends at a human error training has not stopped, such as a part fitted the wrong way round or a step left out.
  • When a defect has reached a customer, to stop the error at the step where it is made. It fits the permanent corrective action of an 8D.
  • When a new product, fixture or station is set up, to go through each step for possible errors before production starts.
  • To decide which of several error-proofing ideas to fund first, by what each error costs and what each device costs.
  • Not in place of finding the cause: if nobody knows why the error happens, start with a 5 Whys or a fishbone diagram.

How to fill it in

  1. 1

    List the steps at the station

    Write each step of the job in order, from the standard work if there is one, and check it against what really happens with the people who do it.
  2. 2

    Write what could go wrong

    For each step, the errors that could happen: a step left out, the wrong part, the wrong way round, the wrong setting, too many or too few. Add the errors that have already happened, from defect records and complaints.
  3. 3

    Note the effect, where it is found and the cost

    What the error leads to, where it is found today (at the step, later, at final inspection, at the customer, or not at all), how many times a month and what each one costs. The Excel multiplies them and ranks the errors by cost.
  4. 4

    Choose prevention or detection, and control or warning

    Prevention makes the error impossible; detection catches it at once, before the part moves on. A control device stops the process or will not take the wrong part; a warning device only signals. Prefer prevention and control. A warning on an error that reaches the customer is flagged.
  5. 5

    Choose the method and the device

    Contact checks shape, size, position or color. Fixed value checks a set number of parts or moves. Motion step checks the steps happen in order. Describe the simplest device that would work, and its cost; the Excel works out the payback.
  6. 6

    Prove it, then keep proving it

    Once the device is fitted, make the error on purpose several times. It passes only if every try is stopped or caught. Log each test, set a re-test interval and put the test in the standard work.

A filled-in example

Illustrative, not a benchmark

An example: a bracket station where the left-hand and right-hand brackets look almost the same (illustrative numbers).

One device per error in the example, and its payback

  1. Prevention · control · contact

    Wrong-hand bracket

    Found at the customer, 2 a month at $1,850 ($3,700). A locating pin only the left-hand bracket's hole fits over, $120.
  2. Detection · control · fixed value

    A screw left out

    3 warranty returns a month at $420 ($1,260). The controller counts four tightenings and the clamp opens only after the fourth, $2,400.
  3. Detection · control · motion step

    Cover fitted before the label

    Found at packing, 9 a month at $35 ($315). The cover bin opens only after the label is scanned, $1,600.

Payback in months = device cost ÷ cost of the error a month

  1. Pin =$120 ÷ $3,700= 0.03
  2. Screw counter =$2,400 ÷ $1,260= 1.9
  3. Cover bin =$1,600 ÷ $315= 5.1
  4. All three =$4,120 ÷ $5,275= 0.8

Wrong-hand bracket, 3,700 dollars a month, prevented by a 120 dollar locating pin: payback 0.03 months. A screw left out, 1,260 dollars a month, detected by a 2,400 dollar screw counter: 1.9 months. Cover fitted before the label, 315 dollars a month, detected by a 1,600 dollar interlocked bin: 5.1 months. All three: 4,120 dollars against 5,275 a month, 0.8 months.

Each device is proven by trying to make the error: the pin and the screw counter passed 5 of 5; the cover bin stopped 4 of 5, failed, and passed 5 of 5 after the latch was adjusted.
  • Wrong-hand bracket fitted: found at the customer, 2 a month at $1,850 each ($3,700 a month). Prevention, control, contact: a locating pin only the left-hand bracket's hole fits over, $120.
  • A screw left out: not found at the station, 3 warranty returns a month at $420 ($1,260). Detection, control, fixed value: the controller counts four tightenings and the clamp opens only after the fourth, $2,400.
  • Cover fitted before the label: found at packing, 9 a month at $35 ($315). Detection, control, motion step: the cover bin opens only after the label is scanned, $1,600.
  • Verification log: the pin and the screw counter passed 5 of 5 tries. The cover bin stopped 4 of 5 at its first test, failed, and passed 5 of 5 after the latch was adjusted.
  • Weekly re-tests: the screw counter passed again 5 days ago; the pin's last test was 8 days ago, so its re-test is a day overdue.

The three errors cost $5,275 a month and the three devices $4,120, a payback of 0.8 months if each device stops every error: 0.03 months for the pin, 1.9 for the screw counter and 5.1 for the cover bin. Two devices are proven and one is flagged for its overdue re-test.

Common mistakes

  • Adding inspection instead

    A check at the end finds defects after they are made. Put the device at the step where the error happens.

  • Settling for a warning when a stop is possible

    A light or a buzzer can be missed or ignored. Where the error reaches the customer, prefer a device that stops the process or will not take the wrong part.

  • Calling a device done once it is fitted

    A device nobody has tried to beat is an assumption. Make the error on purpose, every try, before it goes into the standard work.

  • Never testing it again

    Pins break off, sensors get bypassed and knocked out of place. Set a re-test interval and put the test on a start-of-shift or weekly routine.

  • Blaming the operator

    Mistake-proofing starts from the idea that anyone can make a mistake. Ask how the process lets the error happen, not who made it.

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FAQ

Poka-yoke worksheet: common questions

  1. Japanese for mistake-proofing: a device or method that makes an error impossible, or makes it obvious at once, so it does not become a defect. Shigeo Shingo described the method in Zero Quality Control (Productivity Press, 1986), starting from a switch assembly in 1961 where workers sometimes forgot a spring.

  2. Shingo's three methods. Contact checks a physical feature such as shape, size, position or color. Fixed value checks that a set number of parts or moves has been done. Motion step checks that the steps happen in the right order. Any of them can prevent an error or detect it.

  3. A control device stops the process or will not accept the wrong part; a warning device only signals with a light or a sound and lets the process carry on. Grout's AHRQ guide (2007) ranks forced control, shutdown, warning and sensory alert in that order.

  4. Make the error on purpose: load the wrong part, leave one out, swap the order, several times. It passes only if every try is stopped or caught. Then re-test at a set interval, because devices wear, break and get bypassed.

  5. This worksheet designs the devices for one job and proves each new one works. The error-proofing verification log runs the routine checks of every device in the plant with a master sample, and works out what is at risk when one fails. Use this first, then add each proven device to that log.

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