Production Planning and Scheduling
Core concepts
Production planning decides what and how much to make; scheduling decides when and in what order. Candidates should understand demand forecasting, capacity planning, master production scheduling, material requirements planning (MRP), and shop-floor scheduling rules. Key vocabulary includes lead time, work-in-process, bottleneck, changeover, and safety stock.
What to know cold
- MRP logic: requirements explode from the master schedule through the bill of materials, offset by lead times.
- Capacity: rough-cut versus detailed; the bottleneck constrains throughput.
- Scheduling rules: earliest due date, shortest processing time, first-come-first-served; each trades due-date performance against efficiency.
- Lean concepts: takt time, pull systems, setup reduction.
- Lot sizing: larger lots reduce changeover cost but raise inventory and lead time.
Traps in questions
A frequent trap is optimizing a non-bottleneck resource - local efficiency at a non-bottleneck does not raise throughput and can increase WIP. Another is confusing lead time with processing time; queue time often dominates. Questions may also test whether adding capacity everywhere helps - it does not if the bottleneck remains.
Rule of thumb
Throughput is set by the bottleneck. Apply improvement and scheduling focus there first; protect it with buffers and reduce its changeover time. A useful check: system output cannot exceed bottleneck output regardless of upstream capacity.
Worked example
A line runs 100 units/hour at the bottleneck but a downstream station runs 120/hour. Raising the downstream station to 150/hour changes nothing; raising the bottleneck to 110/hour raises system output to 110/hour, provided upstream supply keeps pace.