Batch-and-Queue

Batch-and-queue is the default flow pattern in most manufacturing shops, which is why most manufacturing shops have long lead times and high WIP. The pattern is so common that most teams never name it as a deliberate approach; it is just how things work. Lean treats batch-and-queue as a specific system with predictable failure modes, and naming it as a system is the first step toward replacing it with something better.

"Batch-and-queue makes each machine look productive. It makes the whole stream slow."

How batch-and-queue works

The mechanism is straightforward. Each operation processes a large lot of identical parts in an uninterrupted run, amortizing setup time across the lot. The lot then moves to the next operation, where it sits in a queue until the next operation finishes whatever it is currently working on. The next operation processes the lot, passes it forward, and so on through the value stream.

The logic that produces batch-and-queue is local efficiency thinking. Each operation has a setup cost, usually changeover time. Running a long batch amortizes that setup across many parts, which makes each part look cheaper. Each machine looks fully utilized, which feels efficient. The schedule is built to keep every station as busy as possible. From the perspective of any single station, this looks optimal.

The cost is global. Lots wait between operations, and the waiting is most of the lead time. A part that has eight hours of actual machining might take three weeks to flow through a batch-and-queue shop because the lot sits in queue at each operation. Total work in process across the shop is huge because every operation has lots waiting in front of it. Defects propagate because by the time the next operation sees the defective batch, the upstream operator has moved on and cannot help diagnose what went wrong.

The lever to escape batch-and-queue is shrinking the batch size, which requires shrinking changeover. Quick changeover is the precondition. Once changeover is short, small batches become viable, and small batches start to behave like continuous flow. The transition is not all-or-nothing; a shop can shrink batches incrementally and see incremental flow improvement at each step.

Where batch-and-queue fits on the shop floor

Picture a small machine shop running CNC parts through five operations. Without lean, the shop runs batches of 100 parts at each station. Setup at each station is 45 minutes, so the long batches amortize setup to under a minute per part. Each station runs full-time. From the outside, the shop looks productive. From the inside, parts take four to six weeks to move through the shop because each lot of 100 sits in queue at every station for days.

The shop starts a quick changeover project on the bottleneck operation. After two months, setup at that station drops from 45 minutes to 10. The team can now run batches of 25 instead of 100 at that station without losing much capacity. Smaller batches mean shorter queue times downstream, because the next operation does not have to wait for 100 parts before it can start. The benefit propagates. Within a quarter, the shop has rebalanced batch sizes across all stations and lead time has dropped from five weeks to two and a half. WIP is roughly half. Output per shift has stayed the same; the shop just stopped wasting time waiting.

This is what coming out of batch-and-queue looks like in practice. The transition is gradual. The shop does not need to install continuous flow on day one. It just needs to shrink batches as changeovers shrink, and the flow improvement follows automatically.

Common mistakes with batch-and-queue

• Not naming it. Most shops do not recognize batch-and-queue as a deliberate system. Naming it is the prerequisite for evaluating alternatives.
• Trying to fix it with bigger batches. Bigger lots mean more amortized changeover but worse flow. The fix is smaller batches with faster setups.
• Treating batch size as fixed. Batch sizes should shrink as changeover times shrink. A long batch that made sense five years ago may be lazy now.
• Confusing it with capacity. Batch-and-queue is a flow problem, not a capacity problem. Adding machines without changing batch size just gives you more places for lots to sit.
• Skipping the changeover work. Small batches without quick changeover means too much time lost to setup. The two have to move together.

Batch-and-queue and related Lean tools

Batch-and-queue is the opposite of continuous flow, and it is the underlying flow pattern of most push systems. The lever to escape it is shrinking batch size, which depends on quick changeover. Batch-and-queue is also one of the structural sources of muda, the lean waste category, because the queue time between operations is pure waiting that adds no customer value and shows up as long lead time.