Work in process feels like safety: if every machine has a pile in front of it, nothing will run dry. In practice, extra work in process makes every order slower, hides problems and ties up cash, without shipping any more.
Why more work in process means longer lead times
There is a simple rule behind it, known as Little’s law: on average, lead time equals work in process divided by output. If the plant ships the same amount each week, doubling the work on the floor doubles the time each order spends there. Pushing more work in doesn’t raise output, because the bottleneck sets output. It only lengthens the queue.
Signs your plant has too much
- Jobs wait between operations far longer than they take to run.
- Supervisors have to search for orders, or expedite to find them.
- Lead times quoted to customers keep growing while output stays flat.
- Started jobs sit half-finished, waiting for a part, a drawing or an answer.
- Floor space is tight even though the plant isn’t shipping more.
Why it builds up
- Release follows material, not capacity. Work is started when parts arrive, not when the bottleneck can take it.
- Efficiency targets reward keeping every machine busy, so upstream steps run ahead of the bottleneck.
- Incomplete kits are released anyway, then stall on the floor.
- Big batches move work in lumps, so queues swell after every transfer.
How to bring it down and keep it down
- Tie release to the bottleneck. Release an order a buffer’s length before the bottleneck needs it, and not before.
- Release complete kits only. If material, drawings or tooling are missing, the order waits in the office, not on the floor.
- Stop rewarding busy-ness upstream. Steps that aren’t the bottleneck will have idle time; that is protective capacity, not waste.
- Work the buffer reds every day, so the orders that are released actually move.
As the floor clears, problems that were hidden in the piles become visible, and they are much easier to fix.
See the effect for yourself. In the dice game, pushing work in buries the floor while releasing at the bottleneck’s pace ships about the same with far less work in process.
See where your plant stands.
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