Manufacturing Operations: Scheduling, MRP, and Quality Control
1. Scheduling: Machine and Worker Assignments
Why it’s the final planning step: Strategy → Capacity → Aggregate Plan → MPS → MRP → then scheduling. Scheduling translates high-level decisions into time-based machine and worker assignments.
Why Job Shops Are Complex
Unlike assembly lines where every product follows the same path, job shops feature unique routing per job. This creates competition for common resources and requires constant re-prioritization. Dr. RS identifies four core difficulties: variety of jobs, distinctive routing, large concurrent orders, and resource competition.
Loading and Priority Rules
- Infinite Loading: Assigns work ignoring capacity (fast but often impossible).
- Finite Loading: Assigns work respecting actual capacity (realistic but computationally hard).
- Priority Rules:
- FCFS: Fairness in low-capacity situations.
- SPT (Shortest Processing Time): Minimizes wait time and WIP; avoid if jobs require later assembly.
- LPT (Longest Processing Time): Useful when subcontracting is expected.
- DDATE (Earliest Deadline): Minimizes tardiness.
- CR (Critical Ratio): (Due Date − Today) ÷ Remaining Time. CR>1 (ahead), CR=1 (on track), CR<1 (behind), CR<0 (overdue).
Why CR beats DDATE: CR factors in remaining work, whereas DDATE only considers the deadline. CR provides a more accurate urgency metric that updates daily.
Shop Floor Control Loop
- Loading: Allocate jobs to machines (e.g., Hungarian Method).
- Sequencing: Apply priority rules within queues.
- Monitoring: Track progress using Gantt charts.
2. MRP, MRP-II, and ERP Systems
MRP handles dependent demand (derived from end products) rather than independent demand (forecasted). Unlike EOQ, which assumes smooth demand, MRP manages the lumpy demand typical of batch production.
The MRP Engine
MRP is a PUSH system driven by the Master Production Schedule (MPS). It requires four inputs: MPS, BOM (Bill of Materials), Inventory Status File, and Time Buckets.
The Three-Step Process:
- Explode: Multiply requirements down the BOM.
- Net: Subtract on-hand inventory and scheduled receipts.
- Offset: Shift orders back by lead time.
Evolution of Systems
- MRP: Material planning only.
- MRP-II: Integrates finance, marketing, and purchasing.
- ERP: Enterprise-wide integration (HR, CRM, Supply Chain) on a single database.
3. Statistical Quality Control (SQC)
SQC distinguishes between natural process variation and assignable causes. SPC (Statistical Process Control) is preferred over acceptance sampling because it monitors the process in real-time.
Control Charts and Limits
Using ±3 sigma limits balances the trade-off between Type I errors (false alarms) and Type II errors (missing real problems).
- Variables: Use X-bar (mean) and R (range) charts.
- Attributes: Use p-chart (proportion defective) or c-chart (count of defects).
4. Project Management: CPM and Crashing
The Critical Path Method (CPM) identifies the longest sequence of tasks. Any delay on the critical path delays the entire project.
The Project Triangle
Crashing (reducing duration) involves three trade-offs:
- Time vs. Cost: Adding resources increases direct costs.
- Scope vs. Time: Rushing may require cutting features.
- Quality vs. Cost: Rushing often leads to errors and rework, which are more expensive than the initial savings.
5. Advanced Strategies
Servitization
A win-win model where manufacturers sell outcomes (e.g., Rolls-Royce ‘Power by the Hour’) rather than products. This aligns incentives for reliability and creates recurring revenue.
Cellular Layout
Groups machines into cells to process families of parts. It offers the efficiency of an assembly line with the flexibility of a job shop.
Six Sigma
A methodology targeting 3.4 DPMO using the DMAIC framework (Define, Measure, Analyze, Improve, Control). It treats quality as a profit center that generates revenue through premium pricing and brand loyalty.