Production managementReading time 12 min
Why the promised date is never the real date
The most common exchange on any shop floor is "when will this order be done?", and the most honest answer is usually "no idea". The people are not the problem; the scheduling method is.
01Lead time syndrome
Classic MRP backs into a release date using a fixed lead time. But lead time is an output of the system, not an input, as work in process grows, queue time grows, and actual lead time stretches with it.
- Dates are unreliable → release material early as insurance
- WIP grows → queues lengthen → actual lead time grows
- Someone lengthens the lead time parameter in the system
- Material is released even earlier → more WIP → back to step 2
Little's Law puts numbers on it: WIP = throughput × lead time. Throughput is capped by the bottleneck and effectively fixed, so the only lever on lead time is less WIP.
02The bottleneck decides everything
- An hour lost at the bottleneck is an hour lost by the whole plant, and it never comes back.
- An hour saved anywhere else is not a saving, it just produces more WIP.
- Which means: the bottleneck must not go down, must not make scrap, must not wait for material, and must have the fastest changeovers.
| Position | Strategy |
|---|---|
| Before the bottleneck | Hold a buffer so it never starves |
| The bottleneck itself | Run around the clock, maintain it first, inspect upstream |
| After the bottleneck | No buffer needed, but never let downstream block its output |
03Three ways to schedule
| Method | Core idea | Where it fits |
|---|---|---|
| MRP (infinite capacity) | Backs off the BOM and lead times | Material requirements planning; not for sequencing operations |
| APS (finite capacity) | Accounts for real machine, labour and tooling availability | Discrete manufacturing with many machines and operations |
| DBR (drum-buffer-rope) | The bottleneck sets the beat; release is tied to it | Lines with one clear bottleneck |
| CONWIP | Holds total WIP at a fixed level | Changing product mix, where the bottleneck moves |
| Kanban | Pull replenishment | Stable demand, repetitive production |
DBR: the drum is the bottleneck's beat; the buffer is time protection ahead of it; the rope ties material release to the bottleneck schedule so nothing goes in early. Once release is constrained, WIP falls on its own and lead time shortens with it.
04What the scheduler has to be fed
- Routings, which machine each operation runs on, in what order, with what alternates
- Standard times, run time and setup time kept as separate numbers
- Machine calendars, shifts, maintenance, holidays
- Tooling and fixtures, the second resource everyone forgets
- Operator skill matrix, not everyone can run every machine
05Where to start
- Find the real bottleneck, look at where WIP piles up, not at the utilization report
- Measure setup times and record them separately from run time
- Cap material release, set a WIP ceiling and stop releasing when it is hit
- Put a buffer ahead of the bottleneck and watch how fast it gets consumed
- Use SMED to cut changeover so small batches become viable