Single-piece flow and work cells
Single-piece flow (also called one-piece flow) moves one unit at a time through the steps of a process. Each step processes one part, one case or one invoice and passes it straight to the next, with no batch and no queue. The work cell is the layout that makes this flow possible: the stations of successive steps stand side by side, most often in a U. Taiichi Ohno built this way of producing at Toyota. James Womack and Daniel Jones made it the third principle of Lean thinking: create flow. The redesign follows six steps, from choosing a family of units to measuring lead time before and after.
Goal
Single-piece flow shortens the lead time of a process, the time between a unit's entry and its exit. It removes the waits between steps, where that time is lost. When work moves in batches, each step processes a whole stack before passing it on: each unit waits while the step processes the rest of its batch.
The US Environmental Protection Agency (EPA) describes this "batch-and-queue" mode in its documentation on cellular manufacturing. The plant produces to forecast rather than to order. Work in process, the units started and not yet finished, waits between departments. Each department makes one product in long production runs to lower its unit cost and change tooling rarely.
The technique decides where the stations stand, how many people staff the cell and which batches to remove. Its deliverable is the cell design: the unit's current route, the new layout, the split of steps between people and the lead time measured before and after.
It applies to the office as much as to the shop floor. An account opening, a supplier invoice or a service request passes through departments organised by function, and each department hands work on in stacks or through in-trays. The redesign shows that this batch is an organisational choice.
Womack and Jones state five principles: specify value from the customer's point of view, identify the value stream, make that stream flow without interruption, let the customer pull it, pursue perfection. Single-piece flow implements the third. Value stream mapping identifies the stream and quantifies its waits before the redesign. Where two steps cannot be chained piece by piece, a kanban pull system links them.
Usage
When to use it
- Long lead time for little work: a case that needs 40 minutes of work and comes out after five days.
- Stacks and in-trays between departments: the visible work in process points to the batch handovers.
- Errors found several steps later: in flow, the next step sees the error on the first unit.
- A family of units with a common route: the same steps in the same order, in enough volume to keep a dedicated team busy.
- Demand that varies: the U-shaped cell adds or removes people without changing its layout.
- A department moving or reorganising: place the stations by the flow rather than by the organisation chart.
When not to use it
- Long changeover between variants: each switch to another variant would add the full changeover time; cut that changeover first with SMED.
- Low volume and routes that all differ: no family justifies a cell; keep the functional layout and cap the queues with a kanban board and its WIP limits.
Description
Batch and flow
The Lean Enterprise Institute (LEI), founded by James Womack, defines continuous flow as producing and moving one item at a time (or a small, consistent batch) through a series of steps as continuously as possible. Each step makes only what the next step requests. The LEI gives one-piece flow and make one, move one as synonyms.
Five cases pass through four steps of 10 minutes, each staffed by one person. In batches, each step processes all five cases before passing the stack on. The first case comes out after 160 minutes and the last after 200. In single-piece flow, the first case comes out after 40 minutes and the last after 80.
The work is the same either way: 200 minutes. In batches, the first case spends 40 of its 160 minutes being processed and 120 in a stack.
Batching also delays the discovery of errors. A repeated error, such as a wrong parameter or an outdated form, affects every case that step 1 processes. Step 2 spots it when it finishes its first case: after 60 minutes in batches, when all five cases already carry it; after 20 minutes in flow, when two cases carry it.
The work cell
The LEI defines a cell as the placement of the processing steps for a product immediately next to each other, so that parts or documents are processed in nearly continuous flow, one at a time or in small batches of constant size through the whole sequence. It notes that many companies use "cell" and "line" interchangeably.
The cell is the opposite of the functional layout, which groups stations by function or by machine: all checks on one floor, all data entry on another. In that layout, each department optimises its own output and the unit moves in stacks from floor to floor. The cell groups stations by product. Among the eight wastes, it removes transport between departments, waiting in queues, work-in-process inventory and the motion of people.
The U shape is the most common. According to the LEI, it reduces operators' walking and lets their number vary with demand. It puts the first and last stations side by side, and the same person can run both, which evens out the pace of work.
Functional
U-shaped cell
In the cell, each person can run several steps: whoever opens a case also closes it. No person's load exceeds the takt time, the pace of demand. The LEI defines it as the available production time divided by customer demand. A 480-minute day for 30 cases gives a takt time of 16 minutes. A person loaded with more than 16 minutes per case builds a queue in front of the station.
In the bank in the figure, three people run five stations. If demand falls to 24 cases a day, takt time rises to 20 minutes and two people are enough: one runs steps 1 and 3 (18 minutes), the other steps 2, 4 and 5 (20 minutes, exactly the takt time). At 40 cases a day, takt time drops to 12 minutes, the duration of step 3. Four people are then enough: A keeps steps 1 and 5 (11 minutes); steps 2, 3 and 4 take one person each. Beyond that, step 3 exceeds takt time and its station takes a second person.
A team that works partly from home builds the cell in its workflow software: a dedicated team, one queue per family and a work-in-process limit of one case per step, visible on a kanban board.
Running the redesign
The EPA describes three steps: understand the current state, move to a process-based layout in a U or C shape with short changeovers and right-sized equipment, then improve continuously. A team can redesign one family in a kaizen event of a few days, with people from the departments concerned.
- Choose a family. The units that follow the same steps in the same order: private-client account openings, supplier invoices matched to a purchase order. Rare cases stay in the existing organisation. Quantify daily demand and derive the takt time from it.
- Follow one real unit from end to end. Record for each step its work time, the wait before it, its department and its floor. A spaghetti diagram drawn on the floor plan shows the back-and-forth. According to the EPA, the current state covers the product mix, the flow, the cycle times and the mobility of equipment.
- Ignore the current layout. Lay the steps out in their order, as if they could touch. Walls, floors, the organisation chart and the assignment of stations to departments are constraints to discuss afterwards, one by one.
- Group the steps into a cell. Place the stations in step order, in a U, with the entry next to the exit. Split the steps between the people so that no load exceeds the takt time. Set the transfer batch to one case: each station processes one case at a time; between two stations, a slot holds only one.
- Size resources for small batches. Equipment designed for large batches brings the batch back: the production scanner that digitises fifty at a time, the central printer, the IT job that runs only at night. The cell gets right-sized equipment with short changeover times. SMED shortens those changeovers.
- Measure and improve. As the EPA asks, measure the cell against the state recorded in step 2: lead time, work in process, load per person. Write the standardised work for each station and train each person on several stations. For continuous improvement, the EPA cites kaizen, total productive maintenance (TPM) and Six Sigma.
Small batches, when a single unit is out of reach
A batch of one is the target. The LEI definition allows a small, consistent batch. Some steps impose a batch: a machine with a long changeover, an external laboratory that returns its results once a day, a supplier that delivers every morning. The cell then cuts the batch to the smallest size the constraint allows and measures what it costs in lead time. Flow resumes after the step that imposes it.
Pitfalls
Moving the desks together without breaking the batch. The stations sit side by side, but the transfer batch is still the in-tray of twenty cases. The cell then keeps the lead time of the old organisation.
Judging each person on individual output. A person judged that way fills stacks to stay busy. The cell is measured by its finished units and its lead time.
Leaving a station above takt time. Rebalance: split the steps differently, add a second person to the station or shorten the step.
Leaving the batch in the IT systems. Software that hands out cases in bundles imposes a batch that the furniture does not show. A nightly run imposes a batch of one day. The redesign reconfigures the tools as well.
Losing a control when grouping steps. In the office, the four-eyes principle and the segregation of duties remain mandatory once the steps are grouped into the cell: the person who enters a case does not approve it. The split of steps respects both, and the compliance department signs it off before the move.
Training too few people. A cell where a step has only one trained person stops at the first absence. Every step has at least two trained people, and a skills matrix shows it.
AI considerations
The event log of workflow software gives, for each case, the time it entered and left each step. A process mining tool derives the lead time from it and, for each step, the work time, the wait and the work in process in the queue. Batches show up there: when a group of ten or more cases leaves a step together at a fixed time, those cases have moved in a stack. A language model given the step times and the takt time proposes splits of the steps between people and drafts a first standardised work sheet for each station.
The log records only what passes through the software. The paper stack, the walk to the scanner and the call to a colleague do not appear in it: the team still follows a real case on site. The model knows nothing of people's skills or of the segregation-of-duties rules until someone supplies them. In a spreadsheet, the team recalculates each proposed load and checks it against the takt time. The team and compliance settle the split.
In a bank, banking secrecy covers client data: art. 47 of the Banking Act (BankA) punishes the employee who discloses a secret learned in that capacity. Before anything goes to an external model, the team extracts only the timestamps and the step codes. It aggregates times by step rather than by person: art. 26 of Ordinance 3 to the Employment Act (OLT 3) prohibits systems intended to monitor workers' behaviour at their workplace.
Examples
A cell design sheet for private-client account openings in a bank. Demand is 30 cases a day; with a 480-minute day, the takt time is 16 minutes. For each step, the table sets the functional organisation beside the cell.
| No. | Step | Work (min) | Functional: department, floor | Functional: wait before the step (working days) | Cell: station, person |
|---|---|---|---|---|---|
| 1 | Intake, completeness check | 6 | Mail room, ground floor | 0.5 | P1, A |
| 2 | Identity check, AMLA check (Anti-Money Laundering Act) | 11 | Compliance, 3rd floor | 1.5 | P2, B |
| 3 | Entry into the core banking system | 12 | Data entry, 1st floor | 1 | P3, C |
| 4 | Validation of the entry (four eyes) | 4 | Client advisor, 3rd floor | 1 | P4, B |
| 5 | Mailing of the contract and cards | 5 | Dispatch, ground floor | 0.5 | P5, A |
| Total | 38 | 4 floor changes | 4.5 | One room, 3 people |
The loads stay under the takt time of 16 minutes: A runs steps 1 and 5 (11 minutes), B steps 2 and 4 (15 minutes), C step 3 (12 minutes). The four-eyes check stays in place: B validates C's entry.
B, at 15 minutes out of 16, sets the pace of the cell: B's load is the first to be split between two people if demand rises. A keeps 5 minutes of margin.
By Little's law, average work in process equals throughput multiplied by lead time. At 30 cases a day and 4.5 working days of waiting, 135 cases sit idle in the in-trays of the functional organisation. In the cell, the day's mail waits in front of P1. Inside, five stations and four transfer slots hold nine cases at most. At B's pace, one case every 15 minutes, the first case comes out after 38 minutes and the thirtieth after 38 + 29 × 15 = 473 minutes, before the end of the day.
Visualisations
The batch-and-flow diagram sets the five cases from the Description on a timeline: batches above, single-piece flow below. The layout diagram shows the bank from the example as two floor plans: functional on the left, with an in-tray at each handover; a U-shaped cell on the right. The design sheet is a table: one row per step, with the functional organisation and the cell in columns. The operator balance chart shows each person's margin below the takt line.
Cost
| Phase | Level | Justification |
|---|---|---|
| Preparation | Medium | Choose the family and follow real units. The department heads whose stations change reporting line must agree. |
| Execution | High | Move the stations, adapt the equipment and configure the software, train each person on several steps. This work costs more than the analysis. |
| Documentation | Low | A plan of the cell, a design sheet, a standardised work sheet per station and the measurements before and after. |
Tools
A floor plan and a pencil are enough to trace the current route. Workplace layout analysis records the distances and movements on it. A spreadsheet holds the step times, the takt time and each person's load. An operator balance chart (or yamazumi) stacks each person's steps against a line at the takt time and shows which stations to rebalance. To test a layout before moving the furniture, the team marks the stations on the floor with tape and runs a few cases through them. Flow simulation software earns its cost when the cell is expensive to build, for example when it means moving machines. In the office, the configuration of the workflow software sets the transfer batch as much as the layout does.
Sources
- Taiichi Ohno, Toyota Production System: Beyond Large-Scale Production, Productivity Press, 1988 (original Japanese edition, 1978): the Toyota production system, where single-piece flow was born.
- James P. Womack and Daniel T. Jones, Lean Thinking: Banish Waste and Create Wealth in Your Corporation, Simon & Schuster, 1996: the five principles of Lean thinking, with flow as the third.
- Lean Enterprise Institute, Continuous Flow, Lean Lexicon: the definition of continuous flow, the small consistent batch and the synonyms one-piece flow and make one, move one.
- Lean Enterprise Institute, Cell, Lean Lexicon: the definition of the cell, the reasons for the U shape and the interchangeable use of "cell" and "line".
- Lean Enterprise Institute, Takt Time, Lean Lexicon: the definition of takt time, available production time divided by demand.
- Lean Enterprise Institute, Lean Thinking and Practice, Lean Lexicon: the five principles of Womack and Jones.
- United States Environmental Protection Agency, Lean Thinking and Methods: Cellular Manufacturing: the batch-and-queue mode; the three steps of the move to cells, from the current state to continuous improvement.
- Ordinance 3 to the Employment Act (OLT 3), SR 822.113, art. 26: the prohibition of systems intended to monitor workers' behaviour at their workplace.
- Federal Act on Banks and Savings Banks (Banking Act, BankA), SR 952.0, art. 47: banking secrecy.