Lean Manufacturing Explained: Principles, Example and How to Start

Lean manufacturing means cutting out everything the customer would not pay for and letting the rest flow at the customer's pace. That sounds like theory, but you can show it in numbers on any line. In this lesson you will see the principles applied to the textbook steering bracket example: 11.3 days of lead time before, 4 days after.

What is lean manufacturing?

Lean manufacturing is the production system Toyota developed from the 1950s under Taiichi Ohno. The term "lean" comes from the MIT study of the auto industry, made famous by the book "The Machine That Changed the World" by Womack, Jones and Roos (1990). Lean management means the same principles applied to the whole company, including the office, engineering and leadership.

The core is not "work faster" but "wait less". In most plants, a part is worked on for only a few minutes while it sits in inventory for days. Lean targets that waiting: the inventories, batch sizes, schedules and changeover times that create it.

The five principles of Womack and Jones

In "Lean Thinking" (1996), Womack and Jones sum up lean in five principles:

• Specify value from the customer's point of view: what would they pay for, and what not?
• Identify the value stream: all steps from raw material to customer, with material and information flow.
• Create flow: parts move from step to step without interruption.
• Establish pull: produce only when the customer or the next process takes something.
• Pursue perfection: improvement is not a project but a routine.

Principle flow: less sitting around

The example line makes steering brackets for a vehicle plant: stamping, two welding stations, assembly, shipping. Inventories sit between the processes; the steel coils in front of stamping alone cover five working days. The value stream timeline adds all waiting times and cycle times into the production lead time: 11.3 days. During that time the part is actually worked on for only 2.6 minutes.

Flow means shortening that waiting time: link processes with similar cycle times into a cell, remove the inventories between them, and reduce batch sizes. The ratio of processing time to lead time shows how far a line is from that goal.

Principle takt: at the customer's pace

Takt time sets how often a finished part must leave the line. It is the yardstick for every process: whoever is slower becomes the bottleneck, whoever is much faster produces to stock. On the example line, the customer needs 920 parts per day with 920 minutes of net available time, so one part every 60 seconds.

Assembly takes 62 seconds per part, or 63.3 seconds effectively with 2% scrap. It is above takt time and is the bottleneck. Without takt time you would not see that, because every station is "busy".

Principle pull: consumption drives production

In the current state, several stations get their own schedules from production control and produce whether or not the next process needs the parts right now. That is push, and inventories grow behind it. In the future state, only one process gets the daily schedule: the cell combining welding and assembly, the pacemaker process. Stamping upstream refills a supermarket as soon as the cell withdraws parts.

This way, inventory is controlled by actual consumption, not by a forecast. How supermarket, kanban and FIFO lane work together is explained in the lesson on push vs. pull.

Principle perfection: improvement as a routine

The future state is not the end. The next steps are on the map as kaizen bursts: cut the changeover time at stamping to 10 minutes and bring the cell's uptime to 95%. Every action needs an owner, a due date and a metric; otherwise the burst stays a symbol on paper.

Before and after: the result in numbers

• Production lead time: from 11.3 to 4 days.
• Inventory in the value stream (excluding purchased parts): from 10,380 to 3,680 parts, about two thirds less material in the system.
• Bottleneck: the cell meets takt with an effective 59.5 seconds against a 60-second takt time.
• Control: one daily schedule at the pacemaker instead of weekly schedules for individual stations.

How to start in your own plant

• Pick one product family, not the whole plant.
• Count inventories on the shop floor instead of copying them from the ERP system.
• Put production lead time next to processing time. This one comparison convinces almost any management team.
• Better three actions with an owner and due date than thirty without.

Common misconceptions

Lean is not a headcount reduction program. If you tie improvements to layoffs, you stop getting ideas from the line. Lean also does not mean "zero inventory": supermarkets and FIFO lanes are deliberately limited inventories in the right places. And lean cannot be copied as a toolbox. Kanban without stable processes or 5S without daily leadership will fizzle out.

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