The situation

Between the tube bender and the flange welder there are pallets. Many pallets. Some are in the aisle, one is behind a pillar, and one has been there since the last audit. Nobody knows how long a pipe waits between bending and welding. When welding finds a defect, the bender has already made hundreds more of the same.

Someone decided long ago that the welder must never wait. The result is a mini-warehouse nobody planned.

What WIP and FIFO are

WIP is work in process: parts that have been started and are not yet finished or shipped. It is money and space, tied up on the floor.

FIFO means first in, first out. A FIFO lane is a short, fixed-length lane between two machines. Parts enter at one end and leave at the other, in the order they came. The lane has a maximum. When it is full, the upstream machine stops.

Two simple ideas make a lane work:

  • The lane is small on purpose. It covers the upstream machine’s usual interruptions, not every possible problem.
  • The stop rule is absolute. A full lane means the machine upstream stops making parts.

Little’s law, in one paragraph

Little’s law links three numbers: the average work in process, the average rate parts leave (throughput), and the average time a part spends in the system (lead time). In words: lead time = WIP ÷ throughput. If 36 pipes sit between two machines and 80 pipes leave per hour, a pipe waits on average 36 ÷ 80 = 0.45 hours, or 27 minutes. Check it by hand: 80 pipes an hour is one every 45 seconds, and 36 × 45 s = 1,620 s = 27 minutes. This is a planning estimate, not a measurement. It assumes a steady flow and uses averages. It tells you roughly what to expect, and it does not replace putting a time mark on a part and watching it travel.

A short history

John D. C. Little proved the law in 1961, in a paper called “A Proof for the Queuing Formula: L = λW” in the journal Operations Research. FIFO lanes are widely used in lean practice. Mike Rother and John Shook describe them for practitioners in Learning to See (Lean Enterprise Institute, 1999), and Christoph Roser covers buffer sizing in his history of manufacturing (2016). The sources also connect FIFO lanes to the Toyota Production System, but the exact origin of the lane idea is unclear to us, so we do not give a date.

Where it applies, and where it does not

A FIFO lane is a good fit when:

  • two steps cannot be connected as one-piece flow, for example between stamping and welding, or machining and paint;
  • you must keep the build order of parts, for example in high-mix work;
  • the floor is crowded with pallets, lead time is long and defects are found late.

It is a poor fit when:

  • Stations are tightly balanced and parts pass directly from hand to hand.
  • Many customers pull standard parts. That is a kanban supermarket, a different tool.
  • The area is a distribution warehouse with changing picking patterns.

How to do it

Our example is the fictional Vistula Precision Exhausts. Patryk is an industrial engineer. Beata leads the welding shift. The link is Station 10 (CNC tube bender) to Station 20 (robotic flange welder).

  1. Find takt time. Customer demand needs 640 pipes in an 8-hour shift. Takt = 480 min ÷ 640 = 0.75 min = 45 seconds per pipe.
  2. Add up the upstream interruptions. Bender changeover is 15 minutes (900 s). Average repair time after a breakdown is 7.5 minutes (450 s). Total = 900 + 450 = 1,350 seconds.
  3. Minimum lane content. How many pipes does the welder consume while the bender is stopped? 1,350 ÷ 45 = 30 pipes.
  4. Add a cushion. Common practice is 15 to 30% for variation. Patryk uses 20%: 30 × 1.2 = 36 pipes.
  5. Convert to physical size. A tote holds 6 pipes, so 36 ÷ 6 = 6 totes. A tote is 600 mm long, plus 50 mm for handling space, so 0.65 m each. A gravity roller lane for 6 totes is 6 × 0.65 = 3.9 metres.
  6. Post the rule. Paint a line at the end of the lane. Put a sign: “Max 6 totes. When full, the CNC bender stops.”

Now Little’s law gives the lane’s maximum lead time: 36 pipes × 45 s = 1,620 s = 27 minutes. In the old layout Patryk estimated about 4.5 hours between the two machines. That is the effect of the lane, and it is also a promise you should check on the floor afterwards.

One point for the team to discuss: 30 pipes of protection is a response to a 15-minute changeover. If the changeover drops to 5 minutes, the lane shrinks. A smaller changeover is often better than a bigger buffer.

Three ways it goes wrong

  1. The ghost buffer. A supervisor builds a three-day cushion so the welder never starves. The cushion hides breakdowns, stretches lead time from minutes to days and ties up cash. If the buffer is more than one shift of production, you are building a warehouse, not a lane.
  2. FIFO becomes LIFO. Operators stack pallets two deep or take the easy parts from the top. Old parts sit at the bottom, traceability is lost and defects stay hidden for weeks. Use a lane with a clear entry and exit.
  3. Ignoring the stop line. The lane is full, and the upstream operator carries on, stacking totes in the gangway. A rule without a visible limit and a manager who backs it will not hold.

Our recommendations

  • Treat the Little’s-law answer as an estimate. Verify with a timed part.
  • Size from real changeover and repair data, not from fear.
  • Keep the limit visible: a painted line and a sign.
  • Review the size when changeover time or breakdowns change.
  • When defects appear downstream, use the lane to trace which batch is affected. That is one of its benefits.

Try it

The WIP and FIFO tool will do both calculations and print a stop-limit sign for the lane. It runs in your browser. It is coming soon: WIP and FIFO tool.

From the book

WIP and FIFO is Chapter 16. See the book page.