52 n ASSEMBLY July/August 2023 www.drivesncontrols.com Replenishment rethink delivers benefits to auto assembly line A large British car manufacturer operates a U-shaped assembly line. The line starts with an empty vehicle shell and ends with a full interior. Between the two are perhaps 50 stations, each fitting parts of the interior, with the car remaining at each station for a set time – the Takt time. When the Takt time expires, the car should move on to the next station. If any station has not completed their task, the whole line stops until that task is completed. Although there may be many acceptable reasons why a task hasn’t been completed, not having the required part at the beginning of the Takt time is not one of them. To maintain a constant supply of parts to each station, a team of people with tow motors move constantly around a path inside of the U. When an operator needs more parts, they step away from their station to flag down one of these people, who then collects more parts and drops them off at the station. They call this the “milk run”. Station operators don’t want to run out of parts and, because the time taken to replenish the stock can vary, they often request parts before they really need them. This leads to crates of parts backing up into the roadway, creating traffic jams which, in turn, increases the time taken to replenish the stations. The car company approached Turck Banner to supply a switch and light system to avoid the operators having to flag down someone on the milk run, because while they were waving to attract someone’s attention, they were not fitting a part, and the Takt time was counting down. Switching on a light above their station would be much quicker and would allow them to continue with their task. While this may sound like a good solution, Turck Banner’s systems team, together with the car-maker, decided to examine the entire parts replenishment procedure, which consists of three sections. The first is a huge, slow-moving warehouse area. Parts are replenished from suppliers, with sufficient stock held to cover the production demand between shipments. The second section is what is known as the “supermarket”, which holds the stock required for one day of production and is replenished daily from the warehouse. The third section is the milk run which delivers the parts to the stations. The aim was to maintain the supply of parts in the most efficient way, ensuring that every station would have the parts it needed at the start of each Takt cycle. The space at each station is limited and some parts – such as seats – are large, so some sort of just-intime system looked promising. The milk run needed a lot of employees, was inefficient in its use of human resources and tow motors, and there was no empirical data on its performance – only anecdotal. The team looked both at the whole replenishment procedure and at the requirements of each individual station, from which they developed a new approach. Because some of the parts, such as door handles, are quite small, it is possible to hold enough stock for a whole day’s production at some stations. Such stations were restocked once a day before the shift started and could be excluded from the milk run. This reduced the traffic on the milk run by 28% and freed up the resources to be used more efficiently in the call-for-parts system. In its original format, the milk run had several weaknesses. For instance, all of the tow motors were constantly in motion, waiting to be flagged down. When they were flagged down, they had to drive to the supermarket before they could collect the required parts. The line operator had to leave A large British car-maker has improved the efficiency of an assembly line by rethinking the way it replenishes parts for the stations on the line. By stepping back and considering the line as a whole, it has cut delays, saved space, increased flexibility and output, and is generating useful data as a by-product.
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