Production Scheduling

How It Works

1. Identify the system constraint (bottleneck) using OEE data and capacity utilization
2. Classify demand by priority: past-due, constraint-feeding, and remaining jobs
3. Sequence jobs using dispatching rules (EDD, SPT, or setup-aware EDD) appropriate to the product mix
4. Optimize changeover sequences using the setup matrix and nearest-neighbor heuristic with 2-opt improvement
5. Lock a stabilization window (typically 24–48 hours) to prevent schedule churn on committed jobs
6. Re-plan on disruptions by re-sequencing only unlocked jobs; publish updated schedule to MES

Examples

• Constraint breakdown: Line 2 CNC machine goes down for 4 hours. Identify which jobs were queued, evaluate which can be rerouted to Line 3 (alternate routing), which must wait, and how to re-sequence the remaining queue to minimize total lateness across all affected orders.
• Campaign vs. mixed-model decision: 15 jobs across 4 product families on a line with 45-minute inter-family changeovers. Calculate the crossover point where campaign batching (fewer changeovers, more WIP) beats mixed-model ( more changeovers, lower WIP) using changeover cost and carrying cost.
• Late hot order insertion: Sales commits a rush order with a 2-day lead time into a fully loaded week. Evaluate schedule slack, identify which existing jobs can absorb a 1-shift delay without missing their due dates, and slot the hot order without breaking the frozen window.

Core Knowledge

Scheduling Fundamentals

Forward vs. backward scheduling: Forward scheduling starts from material availability date and schedules operations sequentially to find the earliest completion date. Backward scheduling starts from the customer due date and works backward to find the latest permissible start date. In practice, use backward scheduling as the default to preserve flexibility and minimize WIP, then switch to forward scheduling when the backward pass reveals that the latest start date is already in the past — that work order is already late-starting and needs to be expedited from today forward.

Finite vs. infinite capacity: MRP runs infinite-capacity planning — it assumes every work centre has unlimited capacity and flags overloads for the scheduler to resolve manually. Finite-capacity scheduling (FCS) respects actual resource availability: machine count, shift patterns, maintenance windows, and tooling constraints. Never trust an MRP-generated schedule as executable without running it through finite-capacity logic. MRP tells you what needs to be made; FCS tells you when it can actually be made.

Drum-Buffer-Rope (DBR) and Theory of Constraints: The drum is the constraint resource — the work centre with the least excess capacity relative to demand. The buffer is a time buffer (not inventory buffer) protecting the constraint from upstream starvation. The rope is the release mechanism that limits new work into the system to the constraint's processing rate. Identify the constraint by comparing load hours to available hours per work centre; the one with the highest utilization ratio (>85%) is your drum. Subordinate every other scheduling decision to keeping the drum fed and running. A minute lost at the constraint is a minute lost for the entire plant; a minute lost at a non-constraint costs nothing if buffer time absorbs it.

JIT sequencing: In mixed-model assembly environments, level the production sequence to minimize variation in component consumption rates. Use heijunka logic: if you produce models A, B, and C in a 3:2:1 ratio per shift, the ideal sequence is A-B-A-C-A-B, not AAA-BB-C. Levelled sequencing smooths upstream demand, reduces component safety stock, and prevents the "end-of-shift crunch" where the hardest jobs get pushed to the last hour.

Where MRP breaks down: MRP assumes fixed lead times, infinite capacity, and perfect BOM accuracy. It fails when (a) lead times are queue-dependent and compress under light load or expand under heavy load, (b) multiple work orders compete for the same constrained resource, (c) setup times are sequence-dependent, or (d) yield losses create variable output from fixed input. Schedulers must compensate for all four.

Changeover Optimization

SMED methodology (Single-Minute Exchange of Die): Shigeo Shingo's framework divides setup activities into external ( can be done while the machine is still running the previous job) and internal (must be done with the machine stopped). Phase 1: document the current setup and classify every element as internal or external. Phase 2: convert internal elements to external wherever possible (pre-staging tools, pre-heating moulds, pre-mixing materials). Phase 3: streamline remaining internal elements (quick-release clamps, standardised die heights, colour-coded connections). Phase