1. Mistake proofing concepts and case studies
John Holmes

2. Mistake proofing
“The basic principle of mistake proofing is that a defective component will not be manufactured – and if by some chance it is, it will be noticed by the operator before the next operation is carried out” [McKellen 2004].

3. Mistake proofing levels
Source inspections + Poka-yoke
Judgment inspections
Informative inspections
SQCS SuCS SeCS
>0 absolute defect rate
Zero QC (0 absolute defect rate)
Increasing timeliness of feedback
Inspections closer to source

4. Poke yoke classification: Regulatory
Level of sophistication
Regulatory function
Definition
Example
Level 1
Warning
Device uses audio or visual queue to alert
Buzzer activated by optical sensor when wrong part bin selected
Level 2 (best)
Control
Device shuts down machines or lock clamps to halt operations [Shingo 99]
Barcode scan of wrong product shuts down line [TLC]

5. Poke yoke classification: Setting
Setting function
Definition
Example
Contact
“sensing device detect abnormalities in product shape or dimension by whether or not contact is made between the products and the sensing devices”
Vending machine rejecting foreign coins
Fixed-value
“abnormalities are detected by checking for the specified number of motions in cases where operations must be repeated a predetermined number of times” [Shingo]
Filling an egg carton [McKellen]
Motion step
“abnormalities are detected by checking for errors in standard motions in cases where operations must be carried out with predetermined motions” [Shingo]
Airplane takeoff checklist [McKellen]

6. Case study: Toyota Camry Headrest Adding a poke yoke
Regulatory function
Setting function
Simple and inexpensive?
Error condition
Control (Car will not start until limit switch detects headrest installed)
Contact (shape must fit)
Yes
-Headrest does not prevent removal and allows backwards installation.
-Can cause whiplash injuries [NHSTA].
Use half- circle to prevent insertion error

7. Case study: Kel-Tech Engineering Source inspection + poke yoke
Poke yoke jig to detect weld nut at source workstation
Visual queue
Air gun turned off (cannot proceed with assembly)

8. Case study: Kel-Tech Engineering Source inspection + poke yoke
Poka-yoke type?
Inspection type
Error condition
Improvement
Contact (jig senses all 4 weld nuts are present) and control (air gun disengaged to prevent further operations)
Source
Missing weld nut
None