A few weeks ago I went on a 2.5 hour tour of the BMW Munich Plant. The tour takes you through the major steps of building 3-series sedans and hatchbacks, including the press shop, body welding, paint shop, engine assembly, final assembly, and testing. While it all looked pretty cool and high-tech, my primary interest was in how BMW organizes this fairly complex process. Below are some of the interesting tidbits I got out of the tour guide (separate tours are given in both German and English).
The press shop uses off-the-shelf presses with the tools (actual parts that deform the flat metal sheet into a body part) made in house by BMW. The same set of presses is used to make different parts. The shop makes a number of parts of one kind then the tools are changed and the same presses make a different part. Quite a bit of the floor space is taken up by the tools, massive pieces of metal about 2 by 2 meters and half a meter thick. It takes 30 minutes to change the tools in a press. It takes about a week to move the shop to a different factory. Parts that come out of the presses are first inspected manually for any visible defects.
The parts made by the press shop are fed by conveyers to the body welding area. The welding itself is done by robots (made by KUKA) with occasional humans moving parts from a conveyer to the robot’s intake tray. Due to space constraints several robots are working simultaneously at any single station. In one station 12 robots are working at the same time which is apparently the industry record. Each robot normally performs several functions, for example, lifting and carrying a panel, applying glue, and welding. Synchronizing these robots’ movements so that they don’t hit each other must be an interesting job.
What’s notable is that the body shapes (sedan vs hatchback) are not aggregated into batches. Instead you see a sedan body followed by a hatchback into the same welding area and the robots pick up different parts and weld them in different places. I asked the guide how the robots know which type of body they are working on. Apparently each body is fitted with a transponder that contains the body configuration. When the body arrives at the station this information is read and the appropriate program is selected.
This sounds quite smart and simple but in reality there are probably quite a few complications down the line. For example, here and later on during the final assembly, different parts need to be delivered to the station depending on the car being built. And since most of the parts are delivered by conveyers, it needs to be scheduled well ahead of time.
After the body is welded it undergoes multi-stage paint work. Here everything is also automated with robots opening doors, painting inside, and closing them back. The bodies are aggregated into batches based on color. You still see a sedan following a hatchback with the robots painting them accordingly. At this stage the bodies do not belong to any particular customer. Instead BMW uses statistics to anticipate how many bodies of a particular shape and color will be ordered.
The engine assembly is mostly manual work and is somewhat disconnected from the rest of the factory in that the engines built at the Munich plant are not put into the cars built there. Instead they are shipped to other plants and engines needed for the 3-series are shipped from other plants to Munich.
In the final assembly a chassis and a body are attached to each other (called marriage). After this point the car belongs to a particular customer. The rest of the line is mostly manual work of installing and connecting various bits and pieces.
After the car is assembled it is tested. A person drives the car to a special booth where the wheels can spin freely on rollers. There is a screen in front of the driver with test instructions and the driver has some sort of a device to confirm completion of various operations. The driver tests basic functionalities like lights and the horn. Then the engine is started and the driver “drives” the car through each gear with the screen showing which gear and speed he should be at. To me the test seemed surprisingly superficial, lasting only a couple of minutes. At the end of the test the car is loaded onto a train wagon for delivery (the tracks come right into the plant).