SC QUALITY GUILDError-proofing (poka-yoke) worksheet
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List each error and the device that guards against it. The worksheet ranks the devices from designed out to caught downstream, and checks that each one is verified and actually works.
ON THE EXAM · CQE V.F, CQPA II.C.1, V.B, CSSBB VII.B.1, CSQP IV.A.3, CMDA V.B.3 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
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What error-proofing is
People make mistakes, however careful and well trained they are. Error-proofing, or poka-yoke, accepts that and designs the process so a mistake either cannot happen or cannot turn into a defect that reaches the customer. It is a preventive action: it works on the cause, not on finding the bad parts afterwards.
The hierarchy of approaches
- Eliminate. Design the error out. The part is symmetrical, so it cannot go in backwards; the label is printed from the scanned model, so there is no wrong label to pick.
- Prevent. The error cannot be made: a fixture that accepts only the right part, a keyed connector.
- Detect the error before it becomes a defect: a sensor that sees the missing part before the press cycles.
- Detect the defect at the station, before it moves on: an in-station test that will not release a failed unit.
- Detect the defect downstream: inspection later in the process or at the customer. This is the weakest; the defect may be made many times before it is caught.
Each device also has a function. A control stops the process until the problem is corrected. A warning (a light, a buzzer) relies on someone noticing and acting, every time. A control is stronger.
The strength score here combines the two, from 10 (designed out, stops the process) to 1 (caught downstream, warning only). It is a ranking aid, not a standard scale.
The three methods
- Contact: the device senses shape, size, position or color. A locating pin, a limit switch, a vision check.
- Fixed-value: a set number of parts or moves must happen. A bin of four screws for a part that takes four; a counter on the torque tool.
- Motion-step: the steps must happen in the right order. A pick-to-light sequence; an interlock that will not start until the guard is closed.
Is it working?
A device that is never tested can fail or be bypassed without anyone knowing. Each one needs a verification method, often a known-bad part tried at the start of the shift, and a record that it was done. Escapes before and after the device show whether it is effective. If a prevention device still lets errors through, look for a bypass or a second failure mode.
Where it fits
The errors usually come from a PFMEA: high-severity failure modes with weak detection are the first candidates. A device that works is then recorded as the control method in the control plan, and the FMEA's occurrence or detection rating is updated.
On the exam
The CQE covers error-proofing and poka-yoke as preventive action, including analyzing their effectiveness (V.F). The CQPA lists poka-yoke among the lean tools (II.C.1) and error/mistake-proofing in preventive action (V.B). The CSSBB includes poka-yoke under waste elimination (VII.B.1), and the CSQP (IV.A.3) and CMDA (V.B.3) name error-proofing among the lean tools. Expect to tell prevention from detection, identify contact, fixed-value and motion-step devices, and choose the strongest approach for a given error.