The axle assembly line plants can seem simple, with materials going in one end, and finished vehicles coming out the other. But there’s much more to it, including a very complicated schedule that ensures everything happens correctly and on time.

The moving assembly line, where cars travel continuously between workers who do one job over and over, is efficient. But it requires considerable advanced planning and to-the-minute accuracy, not just inside the factory itself, but also with outside suppliers and even the transports that take away the finished product.

Individual production procedures may vary slightly between companies, but the first step is always figuring out what’s going to be built. “We have regular meetings with our sales and manufacturing groups to put together the forecasts,” says Luai Aown, manager of strategic planning for Honda of Canada Mfg. The company initially sets up long-term forecasts across several years. For each year, the forecast is then broken down into a monthly schedule that determines the exact models and colours of the vehicles being built.

In this March 14, 2014 photo, assembly line workers build a 2015 Chrysler 200 automobile at the Sterling Heights Assembly Plant in Sterling Heights, Mich.
Option configuration can depend on the manufacturer. Some automakers, including Honda, equip their cars with specific features that vary according to the trim level, while others offer items in bundled option packages or as a small number of stand-alone additions. A few smaller-volume automakers, such as Porsche, will let buyers choose from numerous individual options to create a personalized vehicle.

Many – if not most – of these items will be provided by outside suppliers. Each additional layer of customer choice adds complexity, starting with the auto manufacturer, which has to order the right number of parts from its suppliers to match its sales forecast. Something that looks simple enough to the driver, such as a seatbelt, requires such diverse components as the belt itself, the buckle, the mounting hardware, and in some cars, pre-crash safety mechanisms. Each component may be provided by a different company, and the supplier has to order and coordinate the shipments before it can assemble the seatbelt and deliver it to the auto factory.

At the auto plant, the car orders are loaded into a computerized production plan. The car’s specification sheet follows it down the assembly line, through bar codes or paper printouts, which are scanned at each station to be sure the right parts and colours are going on the car. In some production areas, coloured dots on the cars and parts are used as an additional visual aid. To make it easier to coordinate the build, Honda runs sequences of about thirty identical trim levels at a time.

The process typically begins in the body shop, where panels and components are stamped out in presses. Robots weld these together and the car takes shape. Until it’s painted, which is also done by robots, it’s known as a “body-in-white.” After its coat of colour is applied, it’s on to the assemblers to complete the car.

While some parts are put on individually, many components arrive at the line already put together, such as brakes or instrument clusters. Known as sub-assemblies, they’re installed in the vehicle as complete units. Some sub-assemblies are produced on their own smaller assembly lines inside the plant, while others come in from suppliers. The in-house lines and the supplier shipments all have to be coordinated, so there are enough sub-assemblies ready for the number of cars being built that day.

A line of engines awaits cars coming down the line at the Chrysler assembly plant in Brampton, Ontario.
The efficiency of a moving assembly line depends on workers having the necessary parts right beside them. The software that controls the production process keeps track of the number of cars going down the line, which corresponds to the number of parts that have to be available for the assembler as the vehicle reaches each job station. By comparing these numbers as each job is completed, the computer system knows when only a few parts are left in each bin, and it signals that a fresh supply needs to be brought over.

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