6.4 Pull Systems, Kanban, JIT & Setup/Cycle-Time Reduction
Key Takeaways
The CQIA Body of Knowledge names set-up and cycle-time reduction, pull systems (kanban), just-in-time, and value stream mapping as specific lean tools used to reduce waste.
A push system produces to forecast and creates inventory; a pull system produces only on a downstream signal, so work-in-process is capped by the number of kanban signals in circulation.
A kanban is the authorization signal itself — a card, container, or electronic trigger — and no replenishment may occur without one.
SMED (single-minute exchange of die) reduces changeover time by converting internal setup steps, which require the machine stopped, into external steps performed while it still runs.
Shorter setups reduce economic batch size, which shortens lead time, cuts inventory, and exposes defects sooner — the quality benefit that matters most on the exam.
6.4 Pull Systems, Kanban, JIT & Setup/Cycle-Time Reduction
BoK entry III.A.2 does not simply say "understand lean." It enumerates the tools by name: set-up and cycle-time reduction, pull systems (kanban), continuous improvement (kaizen), just-in-time (JIT), 5S, value stream mapping, and error-proofing (poka-yoke). Sections 6.2 and 8.2 cover kaizen, 5S, poka-yoke, and the waste taxonomy. This section covers the remainder — the flow and changeover tools — which is where candidates who studied only the eight wastes get caught.
1. Push vs. Pull Production
This is the conceptual foundation for everything else in the section.
| Push system | Pull system | |
|---|---|---|
| Trigger to produce | A forecast or schedule issued in advance | An actual consumption signal from the next downstream step |
| Governing question | "What does the plan say we should build?" | "What did the customer just take?" |
| Inventory behaviour | Accumulates wherever the forecast exceeds real demand | Capped by design |
| Response to a forecast error | Overproduction or shortage, discovered late | Self-correcting; nothing is made without a signal |
| Defect exposure | Slow — defects sit in queues before discovery | Fast — small batches move quickly to the next step |
The connection to quality is direct and is the reason lean sits in a quality Body of Knowledge at all. Overproduction is the most severe of the eight wastes because it manufactures the other seven — it creates inventory, motion, transportation, and, critically, it delays defect detection. A push system that builds 2,000 units to a forecast may not discover a process shift until 2,000 defective units exist. A pull system with small batches surfaces the same shift after a handful.
2. Kanban: The Authorization Signal
Kanban (Japanese for signboard or card) is the physical or electronic signal that authorizes production or movement. The essential rule, and the one exam items test: nothing is produced or moved without a kanban. The card is not paperwork tracking the work — the card is the permission.
Common Kanban Forms
- Production kanban — authorizes the upstream process to make a specified quantity.
- Withdrawal (move) kanban — authorizes transporting a container from one location to the next.
- Two-bin system — the simplest implementation: when the first bin empties, it becomes the reorder signal while the second bin supplies demand.
- Electronic kanban (e-kanban) — a scan or system trigger replacing the physical card, common across supplier networks.
- Kanban square — a marked floor space; an empty square is the signal to produce.
Why the Signal Count Controls Inventory
Total work-in-process in a kanban loop is bounded by the number of kanbans in circulation multiplied by the container quantity. This makes WIP a deliberate management decision rather than an accident of scheduling. Removing kanbans from the loop tightens inventory and forces problems to the surface — the classic "lowering the water to reveal the rocks" image. Adding kanbans buffers a known problem while it is being solved.
A basic sizing relationship:
Note what drives the numerator: lead time. Shorten lead time and the required inventory falls proportionally. That is the bridge to setup reduction below.
3. Just-in-Time (JIT)
Just-in-time is the operating philosophy the pull system implements: produce and deliver the right item, in the right quantity, at the right time, and nothing more. Inventory is treated not as an asset but as a symptom concealing problems — unreliable equipment, long changeovers, unpredictable suppliers, and defects.
JIT's Non-Negotiable Prerequisites
JIT is frequently misapplied as an inventory-reduction program, and it fails predictably when its prerequisites are absent. It requires:
- Quality at the source. With no buffer stock, a single defect stops the next operation. This is why JIT and poka-yoke, jidoka, and stop-the-line authority always travel together.
- Reliable equipment. Unplanned downtime with no inventory buffer halts the line, which is why total productive maintenance accompanies JIT.
- Short, reliable changeovers. Small batches are impossible if a changeover takes four hours.
- Dependable suppliers. Frequent small deliveries require supplier quality and delivery reliability — the direct link to Section IV of the Body of Knowledge.
- Level demand (heijunka). Wildly variable demand cannot be met without buffers.
The exam trap: cutting inventory before fixing quality, uptime, and changeover time does not create JIT — it creates stoppages. Inventory reduction is the result of JIT, not the method.
4. Setup and Cycle-Time Reduction (SMED)
SMED — single-minute exchange of die — is Shigeo Shingo's method for reducing changeover time, with the target of completing a changeover in single-digit minutes (under ten). The entire method rests on one distinction:
| Setup type | Definition | Objective |
|---|---|---|
| Internal setup | Steps that can only be done while the machine is stopped | Minimize — this is the time the machine is not producing |
| External setup | Steps that can be done while the machine is still running | Move as much work here as possible |
The SMED Sequence
- Observe and document the current changeover, timing every step. Video is standard practice.
- Separate internal from external. Simply identifying which steps do not require a stopped machine — fetching tools, staging the next die, pre-heating, gathering paperwork — typically cuts changeover time substantially before anything is engineered.
- Convert internal to external. Pre-stage and pre-position materials; pre-assemble fixtures; use duplicate tooling so the next setup is built while the current job runs; pre-heat dies.
- Streamline the remaining internal steps. Replace threaded fasteners with quick-release clamps and one-turn devices; use standardized shims and stop blocks to eliminate trial adjustment; run parallel operations with two people; eliminate test runs by setting to a known position rather than adjusting to a measurement.
Why Setup Reduction Is a Quality Topic
Long changeovers force large batches, because the setup cost is amortized over the run. Large batches produce four quality consequences:
- Delayed defect detection. A process shift discovered after 5,000 units means 5,000 units at risk.
- Long lead time, which weakens the feedback loop between a process change and its observed effect.
- High inventory, concealing the problems JIT is designed to expose.
- Scrap on the trial-and-error setup itself, since adjusting to a measurement wastes parts that adjusting to a fixed position does not.
Halving changeover time roughly halves the economically justified batch size. That is why SMED is not a maintenance topic — it is a quality lever.
Cycle-Time Reduction Beyond Changeover
Cycle-time reduction also targets total elapsed process time, and applies as readily to an insurance claim or a hospital discharge as to a stamping press. The standard levers: remove non-value-added steps identified in the value stream map, eliminate waiting between steps, do steps in parallel instead of in sequence, and remove approval loops that add delay without adding control.
A plant reduces raw material and work-in-process inventory by 60% to implement just-in-time. Within three weeks the line stops repeatedly because a supplier's incoming defect rate of 3% now halts production immediately instead of being absorbed by stock. What went wrong?
Inventory reduction was implemented before the quality prerequisite that just-in-time depends on was satisfied
Just-in-time cannot be applied to an operation that uses external suppliers
The plant should have reduced inventory by 90% rather than 60% to force faster improvement
Kanban cards should have been used instead of just-in-time
During a SMED analysis of a 90-minute press changeover, a team finds that 35 minutes are spent retrieving tooling from the crib, locating the setup sheet, and staging the next die. Which SMED action applies to these 35 minutes?
Eliminate them entirely, because staging activities add no value in any form
Accept them as irreducible, since material handling cannot be performed in advance
Reclassify them as cycle time rather than setup time
Convert them from internal setup to external setup so they occur while the press is still running
In a kanban-controlled loop, what determines the maximum work-in-process inventory between two processes?
The forecast issued by the production planning department each month
The physical floor space available between the two operations
The number of kanban signals in circulation multiplied by the container quantity
The reorder point calculated from the previous year's average consumption
Why is reducing changeover time treated as a quality improvement rather than only an efficiency improvement?
Shorter changeovers are required for ISO 9001:2015 certification
Faster changeovers reduce the number of operators needed on the line
Changeover time is one of the eight wastes in the DOWNTIME taxonomy
Shorter changeovers permit smaller batches, so a process shift is detected after far fewer units are produced
Sections you finish are checked off in the contents.