It all starts with Takt Time
As time goes by fact becomes legend and legend becomes myth. Takt time is one of the core concepts of lean, which origins are now misted in myth – uncertain and unknowable, but thought-provoking anyhow. Legend has it that Ohno hit upon Takt time thinking when trying to improve productivity. Toyota was assembling trucks for the US army, and Ohno realized they’d spent three weeks in the month getting all parts in and then producing like crazy for the last week they started again. He figured out that rather than be an end-of-month company, if they were a end-of-day company they could triple productivity. Takt time was originally a device to make all the parts supply processes move at the day rhythm. The important aspect of this just so story is that Takt here is linked to productivity.
Another legend has it that Toyota (and other automakers) did really well in the fifties because the Japanese engineers trained by German engineers for the production of war plane during WWII migrated to the two second-most interesting industries: building ships and building cars. “Takt” therefore supposedly is a local interpretation of cycle time in Kiichiro’s Toyoda just-in-time frame: the pace at which the market takes a product, calculated as production time divided by average customer demand. Now, customer demand is not for just one product, but many. As Toyota did not have the capital to invest in dedicated lines, it started building different options, then different models on the same lines, which means following the same Takt whilst mixing models to also follow each model Takt. Back in the eighties when my father visited the Toyota plants he figured out that they scheduled the line to have a mix of high work content vehicles and short work content vehicles in order to stick as close as possible to the overall Takt. “And the kanbans” kept saying the Toyota engineers. It took him a while to figure out they were also leveling the pull of parts on suppliers, as per the previous intuition: each part assembled had to follow a regular Takt. Which gets us to the second key idea of Takt: productivity and flexibility.