5.4 Lean Tools & Waste Elimination

5.4 Lean Tools & Waste Elimination

Originating from the Toyota Production System (TPS) developed by Taiichi Ohno and Shigeo Shingo, Lean philosophy focuses on maximizing customer value while minimizing waste (Muda). Value is defined strictly from the customer's perspective as any activity for which the customer is willing to pay.

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Categorization of Work Activities

1. Value-Added (VA) Work: Activities that physically transform the product or service toward customer specifications.
2. Non-Value-Added (NVA) Work (Waste / Muda): Activities that consume time, space, or resources without adding value. Must be eliminated.
3. Required Non-Value-Added (RNVA) Work: Activities currently required by regulation, safety, or legal compliance (e.g., filing tax reports, mandatory EPA testing). Must be minimized.

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The 8 Wastes (TIM WOODS)

Lean categorizes process wastes into eight distinct types, remembered by the acronym TIM WOODS:

  T - Transportation : Unnecessary movement of parts, tooling, or materials
  I - Inventory      : Excess raw stock, work-in-process (WIP), or finished goods
  M - Motion         : Unnecessary physical movement of operators (bending, reaching)
  W - Waiting        : Idle time waiting for machines, signatures, or upstream parts
  O - Overprocessing : Performing extra work or tighter tolerances than requested
  O - Overproduction : Producing more or faster than customer demand (Cardinal Waste)
  D - Defects        : Rework, scrap, field failures, and corrective inspection
  S - Skills         : Underutilizing employee capabilities, creativity, & knowledge

Key Lean Principle: Overproduction is considered the most critical waste because it generates and conceals all other seven wastes (causing excess inventory, requiring extra storage transportation, and hiding defect root causes).

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5S Workplace Organization Methodology

5S is a structured visual management framework that creates a clean, efficient, and standardized work environment.

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Value Stream Mapping (VSM)

A Value Stream Map visualizes the end-to-end flow of material and information required to bring a product from raw material to customer delivery.

  [Supplier] -----> [Stamping] -----> [Assembly] -----> [Customer]
                       |                 |
                  (Cycle: 40s)      (Cycle: 90s)
                  (Inv: 2 Days)     (Inv: 3 Days)

  Timeline:
  Lead Time:   |-- 2 Days --|        |-- 3 Days --|  Total TLT = 5.0 Days
  Value-Add:                 |-- 40s --|            |-- 90s --| Total VAT = 130 Secs

Key VSM Metric Formulas

1. Takt Time: The pace of production required to match customer demand precisely: $\text{Takt Time} = \frac{\text{Net Available Operating Time per Shift}}{\text{Customer Demand per Shift}}$
2. Total Lead Time (TLT): Total elapsed time for a unit to move through the entire value stream (includes queue/inventory wait times).
3. Value-Add Time (VAT): Sum of actual processing times where value is added.
4. Process Cycle Efficiency (PCE): $\text{PCE} = \frac{\text{Value-Add Time (VAT)}}{\text{Total Lead Time (TLT)}} \times 100\%$

Worked Exam Scenario

Given:

• Available Operating Time = 8 hours (480 minutes) minus two 15-minute breaks = 450 minutes = 27,000 seconds.
• Daily Customer Demand = 900 units.
• Process Value-Add Time ($VAT$) = 180 seconds per unit.
• Total Lead Time ($TLT$) = 5 days (40 working hours = 144,000 seconds).

$\text{Takt Time} = \frac{27,000 \text{ seconds}}{900 \text{ units}} = 30 \text{ seconds/unit}$ $\text{PCE} = \frac{180 \text{ seconds}}{144,000 \text{ seconds}} \times 100\% = 0.125\%$

Interpretation: Production must complete one unit every 30 seconds to meet demand. The PCE of 0.125% highlights massive non-value-added inventory waiting time.

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Kanban & Pull Production Systems

Traditional manufacturing utilizes Push Systems (batch production scheduled via MRP forecasts), leading to overproduction. Lean utilizes Pull Systems, where downstream operations signal upstream processes to produce only what has been consumed.

• Kanban: Japanese for "visual card." A physical or electronic signal authorizing production or material movement.
• Kanban Calculation Formula: $K = \frac{D \times L \times (1 + S)}{C}$ Where:
  • $K$ = Number of Kanban card containers required
  • $D$ = Demand rate per unit time
  • $L$ = Replenishment lead time
  • $S$ = Safety stock percentage factor (decimal)
  • $C$ = Standard container capacity

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Poka-Yoke (Mistake-Proofing)

Concept developed by Shigeo Shingo to achieve Zero Quality Control (ZQC) by preventing human errors from turning into defective products.

Poka-Yoke Levels & Methods

• Control Poka-Yoke (Prevention): Automatically shuts down a machine or physically prevents assembly if an error occurs (e.g., asymmetric electrical connectors that fit only one way).
• Warning Poka-Yoke (Detection): Triggers an alarm, light, or buzzer (Andon) to signal an operator error, but does not physically stop the machine.
• Technique Types:
  1. Contact Method: Uses microswitches or optical sensors to detect physical shape or size.
  2. Fixed-Value Method: Ensures a precise number of operations are completed (e.g., counting 4 screws taken from a bin).
  3. Motion-Step Method: Verifies that sequential process steps are executed in correct order.

SMED (Single-Minute Exchange of Die) and OEE

BoK Domain V.D specifically lists SMED and Overall Equipment Effectiveness (OEE) among lean tools.

SMED — Fast Changeover

Shigeo Shingo’s SMED method converts internal setup (must stop the machine) into external setup (can prepare while running), then streamlines remaining internal steps.

1. Observe current changeover and separate internal vs external work.
2. Convert internal to external (pre-stage tools, preset fixtures, standard carts).
3. Streamline remaining internal steps (functional clamps, one-turn fasteners, parallel tasks).
4. Standardize and time the new method.

Target metaphor: complete changeovers in “single-digit minutes,” though the engineering principle applies to any lengthy setup.

OEE — Overall Equipment Effectiveness

$OEE = Availability \times Performance \times Quality$

Worked example: Availability 0.90, Performance 0.95, Quality 0.98 → $OEE = 0.90 \times 0.95 \times 0.98 = 0.8379$ (83.8%). World-class benchmarks are often cited near 85%, but the exam cares about correct factor definitions and multiplication of the three ratios.