1.5 Assembly Flow Design

Assembly flow design is wherethe manufacturingprocess turnsof fromstructuring theorythe intophysical rhythm.sequence By matching takt time to real, balancedof work and shapingallocating thelabor layoutto formaximize both speedthroughput and ergonomics,ensure aconsistent linequality. canThe runflow steadilymust insteadbe ofintentionally lurchingengineered, fromnot fireallowed to fire.evolve Whether built as a U-cell for flexibility or a conveyor for volume, flow only stabilizes when tests actorganically, as the pacemakerphysical andarrangement bottlenecksof stayworkstations feddictates andcycle visible.time, Thematerial resultmovement is not just higher output, but calmer operators and fewer surprises.

1.5.1 Goal (in one line)

Build a line that moves at customer pace (takt) with no drama: parts arrive when needed, hands travel little,efficiency, and the slowestmanagement stepof bottlenecks. Effective flow design is fed,mandatory free,for achieving predictable delivery schedules and visible.minimizing idle time.

1.5.1 Flow Principles and Layout Design

The physical layout of the assembly line directly impacts communication and material transit time.

A) Sequential vs. Cellular Flow

• Sequential (Linear) Flow: Workstations are arranged in a straight line. This is suitable for very high-volume, low-mix products where the same operation is performed endlessly.
• Cellular (U-Shaped) Flow: Workstations are arranged in a U or C shape, placing the start and end of the line close together.
  • Mandate: Cellular flow is preferred for high-mix, medium-volume Box Build operations. It reduces the walking distance between stages, promotes cross-training, and improves communication among technicians.

B) Integration of Quality Gates

Quality checkpoints must be integrated into the flow, not delegated to an end-of-line inspection.

• In-Process Checks: Audits must be performed immediately after high-risk operations (e.g., after harness routing and before the enclosure is fully closed).
• Pre-Testing: Sub-assemblies (e.g., display units, power modules) must be tested before final installation. Finding a failed component after the entire box is sealed results in high tear-down and rework costs.

1.5.2 Takt Time and Line Balancing

The design of the assembly flow is governed by the required production rate (Takt Time). Achieving a stable flow requires eliminating bottlenecks through effective line balancing.

A) Takt Time Calculation

Takt Time is the required pace of production needed to meet customer demand. It is the absolute target for the output rate of the line.

Takt Time = Available Work Time / Customer Demand Quantity

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• Example:

  If 480 minutes are available per shift and 60 units must be built, the Takt Time is 8 minutes per unit. The line must deliver a finished product every 1.5.28 Startminutes.

B) Line Balancing

The objective of Line Balancing is to distribute the total work content across the assembly stations so that the work time at each station equals (or is just under) the calculated Takt Time.

• Bottleneck Identification: The station with the mathlongest (tiny, honest, and powerful)
  • Available time/shift = shift minutes − breaks/meetings/cleanups.
  • Takt time = Availablework time ÷ Required units.
  • Cycle time (per station) = real average seconds to do the work, including normal micro-stops.
    
     Design so every station’s cycle ≤ takt, withis the bottleneck. justThis undertime taktdictates the actual maximum output rate of the entire line.
  • Work Content Distribution: Work Instructions (WIs) must define work content that can be evenly grouped. If a single task (e.g., complex wire routing) takes 12 minutes, that task must be broken down or reallocated across multiple stations to fit the 8-minute Takt Time.
  • Mandate: The flow design must ensure that the work content assigned to any single station does not exceed the Takt Time.

  1.5.3 Workstation and protected.Material Integration

  The flow design must support the material handling principles established in Section 5.3 (BOM and Kitting).

  A) Material Presentation

  The flow design must ensure that materials arrive at the workstation in the sequence of installation.

  • Kitting Integration: The flow is designed around the Operation Kit (Task Kit) delivery sequence, minimizing the inventory held at the workstation.
  • Poka-Yoke (Error Proofing): The physical layout of component bins and jigs must prevent mis-orientation. Bins for similar-looking fasteners must be separated or designed to release only the correct Part Number for the current WI step.

  Example

  B) Standard Work

  Available:For 7.5maximum hconsistency, =the 27,000assembly s.flow Demand:must 300 units →enforce taktStandard =Work.

  90 s/unit
  • Definition:.

    SplitStandard workWork sodefines eachthe stationexact ≈sequence 80–88of sassembly steps, the required time, and the bottleneckin-process hasinventory anecessary smallto buffer ahead.

    
    

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    1.5.3 Pick a flow pattern (useperform the lightest one that works)

    Pattern	When to use	Pros	Watch-outs
    Single bench	L1–L2 products, low volume	Minimal WIP, flexible	One person = one rate; skill variance
    U-cell (1–6 ops)	High-mix, medium volume	Short walks, shared tools, easy help	Needs cross-training; bottleneck can starve without a small buffer
    Inline (serial)	Medium–high volume, clear sequence	Simple pacing, easy to see flow	Long walks if poorly arranged; changeovers hurt more
    Conveyor / pulse line	High volume, repeatable	Fixed pitch, easy station timing	Less flexible; rework loops needed
    Hybrid (U feeding conveyor)	Mixed families + runner SKUs	U does prep; conveyor finishes	Two rhythms to manage

    Rule: If you change models often, start with a U-cell; if you ship one product all day, build an inline/pulse.

    
    

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    1.5.4 U-cell basics (make it flow in a small footprint)

    • Arrange stations in a ∪ so the product and eyes travel clockwise, materials inside the U, finished goods out the open end.operation.
    • Two-person UMandate:: splitAssembly worktechnicians 60/40;must execute the fastersequence operatordefined floats to helpin the bottleneck.
    • PitchWI boardidentically atevery time. This minimizes variation and stabilizes the exitflow, showsimproving planned vs actual every 30–60 min.
    • Keepthe shared toolsC_pk (label printer, torque driver presets) at the base of the U.assembly process.

    Feeding

    Final the U: module supermarket (22.3) behind operators; carts roll into the U on casters, one cart = one WO/Variant.

    
    

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    1.5.5 Design the line in eight moves (whiteboard to floor)

    1. List work elements with honest times (stopwatch 5–10 units).
    2. Group by skills & tools (torque set, adhesive, programming).
    3. Balance with a Yamazumi (stacked bar) until each station ≈ takt.
    4. Pick layout (bench/U/inline). Sketch reach zones and walking paths.
    5. Place materials: inside the U or right side of inline; heavy parts at waist, fasteners in color cups by torque group.
    6. Add micro-buffers: WIP shelves before/after the bottleneck (1–2 units).
    7. Gate tests: put the pacemaker (functional/safety test) near the end; everything flows to it.
    8. Run a pilot: 10–20 units, time each station, fix the tallest Yamazumi bars, repeat.

    
    

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    1.5.6 Balancing tactics (fast wins)

    • Split the pile: move one or two heavy elements from the slow station to neighbors.
    • Parallelize: build PSU tray/fan wall/display door as L1 modules (22.3) off-line.
    • Change the unit: build in pairs (two units per pitch) if fixtures/tool change time dominates.
    • Kit smarter: move screw hunting into kitting; arrive in torque groups.
    • SMED at changeover (18.3): pre-load labels, images, torque maps by SKU scan; swap fixtures on zero-point pins.

    Mini example (balancing by minutes)

    Four stations, takt 90 s. Times: 120 / 70 / 75 / 60 → Station 1 is the bottleneck.

    Move “fan wall install” (30 s) to Station 3 → new times: 90 / 70 / 105 / 60.

    Then move “label set” (20 s) from 3 to 4 → 90 / 70 / 85 / 80. Done.

    
    

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    1.5.7 Buffers & pacing (Little’s Law without the lecture)

    • Size buffers by variation: start with 1–2 units before/after the bottleneck.
    • Use FIFO lanes with visible max lines; overflow means you just found a problem.
    • Pitch (fixed release every takt) keeps rhythm: a small timer beeps; if red lights persist at one station, rebalance.
      

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    1.5.8 Ergonomics & reach (speed = comfort)

    • Hands work in the elbows-down zone; heavy picks between knee and chest.
    • Two-handed tasks? Put bins split left/right to avoid crossing.
    • Torque drivers on retractors; bits parked in a labeled shadow.
    • Lighting: bright, diffuse; avoid glare on glossy plastics and label windows.
    • Turntables / tilt stands to rotate the chassis instead of the operator.

    
    

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    1.5.9 Material presentation (zero hunting)

    • One cart = one unit (or wave); variant color band on handle.
    • Fasteners by torque group (color cups) right at the station.
    • Consumables kits: TIM syringe with bead size card; threadlocker dots by color.
    • Chokepoints (programming, label print) centralized with queue visibility.

    
    

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    1.5.10 Tests, rework loops, and the pacemaker

    • Put the functional/safety test near the end; it becomes the pacemaker.
    • Rework loop off-line: failed units exit to NG-QUAR/REWORK without blocking flow.
    • If test time > takt, add two testers in parallel or run burn-in as a side loop.
      

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    1.5.11 Changeover planning (keep flow during mix)

    • Heijunka (leveling): fix a daily product wheel so shared setups stick around.
    • Changeovers are internal only for the bottleneck; convert all else to external prep (carts, labels, images).
    • Measure CO time = last good → first good; target ≤ 5–10 min for mature cells.
      

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    1.5.12 Control & visibility (what the cell board shows)

    • Takt vs actual count, updated each pitch.
    • Starved/blocked minutes per station (pinpoint the constraint).
    • Top 3 stop reasons with one countermeasure in progress.
    • Quality at source: misses by station (labels, torque, routing) with a tiny Pareto.
      

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    1.5.13 Metrics that prove the layout works

    • Throughput = units/shift at or above plan.
    • OEE (18.4/16.3) with focus on Performance leg (cycle time creep).
    • Changeover time and first-good after change count.
    • Walk distance per unit (aim < 10–15 m in a U-cell).
    • WIP before bottleneck (should hover near target, not explode).
    • Defects at source trending down (labels, torque, routing).

    
    

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    1.5.14 Common traps → smallest reliable fixChecklist

    TrapMandate	SymptomCriteria	FirstVerification moveAction
    Takt Time	LayoutCalculated bybased gut,on notcustomer taktdemand and available production time.	NiceWork pictures,content poorallocation output	Dovalidated against the taktTakt math,Time build the Yamazumi, then drawtarget.
    Bottleneck starvedElimination	Longest single-station work time does not exceed the Takt Time.	IdleWork keyis station	Addre-distributed 1–2or unitmethods bufferimproved beforeto it;level rebalancethe elementsline.
    Layout Choice	Cellular Flow (U-Shape)Long walksimplemented for partsmixed-volume production.	TiredAudit operators,confirms slowlayout pace	Bringminimizes materialsmaterial insidetransit theand U;operator shadow tools; split binsmovement.
    Quality Gates	SharedHigh-risk toolsassembly insteps theare wrongimmediately placefollowed by an integrated audit or test.	QueueingSub-assemblies for torque driver	Duplicate ortested movebefore toolinstallation tointo the bottleneck; preset bitschassis.
    Standard Work	TestWork iscontent and sequence are precisely defined and adhered to by the unseen constraintoperator.	UnitsProcess pileengineer at test	Parallel testers; make testvalidates the pacemaker;assembly reducesequence dwell
    Changeoverdefined thrash	Output dips each SKU swap	Pre-stage carts; zero-point fixtures; MES push of recipes
    Rework blocksin the lane	LineWork stops on failures	Create a side rework loop with NG-QUAR stateInstruction.

    
    

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    1.5.15 Pocket checklists

    Design (whiteboard)

    • Demand → takt calculated
    • Work elements timed; Yamazumi balanced near takt
    • Flow pattern chosen (bench/U/inline); sketch reach & walks
    • Bottleneck identified; buffers sized (1–2 units)

    Pilot (first 20 units)

    • Station cycles measured; tallest bar reduced
    • Materials reachable; torque/label tools placed well
    • Test station sets the pace; rework off-line

    Daily run

    • Pitch board green; starved/blocked < 15% at bottleneck
    • Changeovers ≤ target; first-good in ≤ 2 units
    • One small kaizen moved from board to standard each shift
      

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    Conclusion: Designing lines with takt-driven math, balanced workloads, and clear pacing transforms assembly from reactive to predictable. With smart kitting, ergonomic layouts, and disciplined buffers, production achieves flow that is both efficient and sustainable.