Mistake proofing concepts and case studies John Holmes

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].

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

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]

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]

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

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)

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