Production Schedule Slip on a Mixed-Model Line
A cell builds three variants (A, B, C) on two identical CNC machines. Weekly demand: A = 120, B = 80, C = 40 units. Each machine runs one 8-hour shift, 5 days/week, with 45 min/day of planned downtime (breaks, tool changes). Standard cycle times per unit: A = 18 min, B = 24 min, C = 30 min. Changeover between variants takes 40 min and is currently done whenever a variant runs out. This week the cell is behind: it has 2 days left and has completed 90 A, 50 B, and 20 C. A customer expedite for 20 units of C has arrived for end of week. Setup time is charged against machine capacity. Both machines are currently set up for A.
1. Calculate remaining capacity and whether the week's demand plus the expedite can be met.
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Available time per machine this week (2 days): 8 hr − 0.75 hr downtime = 7.25 hr/day × 2 = 14.5 hr = 870 min per machine; two machines = 1,740 min.
Remaining demand: A = 120 − 90 = 30; B = 80 − 50 = 30; C = 40 − 20 = 20, plus expedite 20 = 40 C.
Run time: A 30 × 18 = 540; B 30 × 24 = 720; C 40 × 30 = 1,200 → 2,460 min.
Changeovers: both machines currently on A. To run B and C, at minimum one changeover to B and one to C per machine that runs them. If both machines run all three variants, that is 2 changeovers each × 40 min × 2 machines = 160 min; a smarter split (one machine A+B, one machine C) needs fewer. Even with a minimal 2-changeover plan (~80 min), total load ≈ 2,540 min vs. 1,740 available.
Conclusion: demand cannot be met on straight time - shortfall ≈ 800 min (~13.3 machine-hours). The expedite alone (20 C = 600 min) is not absorbable without overtime or schedule changes.
2. Propose a schedule and capacity action to satisfy the expedite and maximize on-time delivery.
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- Prioritize the expedite: dedicate one machine to C (40 units × 30 = 1,200 min) - but that exceeds one machine's 870 min, so split C across both machines or add overtime. Run C first on one machine after a single 40 min changeover, then the other machine continues A.
- Sequence to minimize changeovers: group by variant (A then B on one machine; C on the other) rather than alternating. Two changeovers total (~80 min) instead of four.
- Add overtime: shortfall ~800 min ≈ 13.3 hr. Two machines × ~6.7 hr overtime, or one machine ~13 hr over two evenings. Confirm labor availability and overtime cost.
- Defer or split non-critical demand: if overtime is capped, ship partial A/B and communicate revised dates; the expedite C is the priority.
- Re-check the plan: after sequencing, confirm each machine's load ≤ 870 min + assigned overtime, and that C's 40 units finish before end of week.
3. What change would reduce this recurring risk?
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Move from run-to-exhaustion changeovers to a level/fixed-interval schedule (e.g., daily mixed-model sequence sized to demand), which cuts changeover frequency and buffers expedites. Also: pre-stage tooling and fixtures for fast changeover (SMED) to cut the 40 min; hold a small finished-goods buffer of C given its expedite history; and track schedule adherence weekly so slippage is caught before the last two days. Verify standard times against actuals - if C's 30 min is optimistic, capacity planning is overstated.