Taiichi Ohno created a categorization of the seven main forms of waste commonly found in mass production:
- Overproduction: producing more than what the next customer or process actually needs. This is the worst form of waste because it fuels the other six.
- Waiting: operators who have nothing to do when machines are running, equipment is malfunctioning, needed parts are not coming in and so on.
- Transfer: unnecessary movement of parts and products, such as through a warehouse from one machining step to the next machining step when the second step could also take place immediately after the first step.
- Machining: performing unnecessary or incorrect machining steps, often due to poor design of machinery or products.
- Stock: having more than the minimum stocks necessary for a precisely controlled pull system.
- Movement: operators making movements that are burdensome or unnecessary, such as searching for parts, tools, documents and so on.
- Correction: inspection, repairs and outages.