Manufacturing Systems: Layouts, Models, and Principles
Core System Challenges
Persistent issues include: variability, congestion, coordination, and flow.
Production System Layouts
Layouts depend on volume, variety, and material flow, determining how work moves through the system between work centers.
Product Line Layout (Flow Lines)
Characteristics: Fast, predictable, and inflexible. Resources are arranged in processing order following a single fixed path. Designed for mass production (e.g., automotive assembly, consumer electronics).
- Pros: Large outputs, high efficiency.
- Cons: Long switchovers, long runs, and limited customization.
Cellular Group Layout
Characteristics: Machines are arranged for similar parts (pseudo-product layouts). Used for automotive components and aerospace part families.
- Pros: Immediate feedback, optimization, customization, and quality.
- Cons: Less flexibility, machine duplication, and higher space requirements.
Process Functional Layouts (Job Shops)
Characteristics: Flexible, slow, and unpredictable. Similar machines or capabilities are grouped together. Jobs are routed based on requirements with many possible paths (e.g., job shops, hospitals).
- Pros: Customization, variety, and flexibility.
- Cons: High inventory, transport needs, and lower utilization.
Fixed Position Layout
Characteristics: The product remains in one place while resources move to the product (e.g., aircraft, ships, construction).
- Pros: Suitable for large builds and contract projects.
- Cons: Requires moving crews and tools; high costs and skilled labor needs.
Performance Metrics
Layouts are selected based on the environment.
- Efficiency: Doing the task right.
- Effectiveness: Doing the right task.
Time Definitions
- Lead Time: Time from placing an order to receiving the item.
- Throughput Time: Time spent in the production system.
- Cycle Time: Time between consecutive batches or starts of the same activity.
System Modeling
- Descriptive Models: Explain behavior (e.g., process flow maps, forecasting, queuing).
- Prescriptive Models: Recommend decisions (e.g., lean models, mathematical programs, linear programming).
- Deterministic Models: Use averages and ignore variability. Pros: Simple. Cons: Less realistic.
- Stochastic Models: Explicitly model variability. Pros: Reflects reality. Cons: Complex.
- Modeling Losses: Perception and measurement, simplification, solution approximation, implementation, and maintenance.
Fundamental Laws of Manufacturing Systems
- Model Utility: All models are wrong, but valid models are useful. Simplification is unavoidable; models are representations, not reality.
- Product Life Cycle: Depends on demand.
- Little’s Law: Applies to stable systems (steady state). Independent of variability or distributions; relates inventory, flow rate, and time.
- Constraint Theory: The weakest link sets the pace. System throughput time is limited by the most constrained process; improving non-constraints does not increase output.
- Flow Conservation: Material balance, capacity, and time.
- System Decay: Systems drift toward disorder. Performance degrades without ongoing effort; stability requires continual attention.
- Reliability Compounding: System reliability is always less than individual reliability. Adding components reduces system reliability; weak components dominate performance.
- Complexity: Exponential growth in complexity.
- Pareto Principle: The vital few vs. the trivial many; a small number of causes account for a large share of effects.
- Setup Efficiency: Drives performance. Large batches lead to longer throughput times, higher WIP, longer forecast horizons, and slower detection of quality problems.