5.5 Assembly Flow Design

Assembly flow design is the process of structuring the physical sequence of work and allocating labor to maximize throughput and ensure consistent quality. The flow must be intentionally engineered, not allowed to evolve organically, as the physical arrangement of workstations dictates cycle time, material movement efficiency, and the management of bottlenecks. Effective flow design is mandatory for achieving predictable delivery schedules and minimizing idle time.

5.5.1 Flow Principles and Layout Design

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

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.

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.

5.5.2 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.

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

• 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 8 minutes.

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 longest work time is the bottleneck. This time dictates 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.

5.5.3 Workstation and Material Integration

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

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.

Standard Work

For maximum consistency, the assembly flow must enforce Standard Work.

• Definition: Standard Work defines the exact sequence of assembly steps, the required time, and the in-process inventory necessary to perform the operation.
• Mandate: Assembly technicians must execute the sequence defined in the WI identically every time. This minimizes variation and stabilizes the flow, improving the C_pk of the assembly process.

Final Checklist