(EN) How to Turn Your Car for Assembly—Part 2

In my last post I started looking in more detail on how to turn a car on an automotive assembly line. This is a deep dive in a specialized topic, but I hope it can inspire you in similar issues with your own assembly line. The big question is how to reduce the overall walking distance for all your operators. But, as we will see in this post, it is also a question for tool and material access. Read on!

A Quick Recap…

Most assembly lines move their cars bumper to bumper. Compared to door-to-door assembly (see below), this makes the walk back along the line slightly longer (the difference between a car width and a car length). However, the distance walking in between the cars is shorter. I talked a lot about this in my last post.

In this post we’ll look in more detail at door-to-door assembly lines, which are sometimes done for segments of the line.

Moving In and Out Between the Vehicles Door to Door

If we turn the car by 90° door to door, the front and rear workers have to walk less than on a bumper-to-bumper line, but the workers for the passenger compartment and the doors have to walk more. The additional walking is not quite half a car length minus half a car width, as they do not work exactly in the middle but on the front or rear door respectively. The distance is from the front of the car to the front door, or from the rear of the car to the rear door. This is probably around two meters and back, as shown below, whereas for bumper-to-bumper assembly it would have been only half a car width (i.e., around one meter and back). Hence the overall walking distance for a person between the cars increases by two meters every time they have to go in between the cars.

Also, do note that there are up to two people at the front and back, but up to four people for the sides (one for each door). Having more workers walk in and out between the cars on the door-to-door line increases the walking distance among all workers. Depending on the car, there may also be more work inside the car than at the front, again increasing the number of people who may have to walk in and out. Hence, overall, the bumper-to-bumper assembly line that is so common seems to have less walking distance than a door-to-door assembly line, unless you have a line segment focusing on the front and rear of the vehicle… which is what Mitsubishi, for example, does, but they then turn the car bumper to bumper for work done on the sides.

Underbody Work

Note that this is a bit easier if the people work underneath the car, as they do not have to walk around the car but can walk underneath. Underbody work is not usually done on a moving platform, and the workers have to walk back and forth on their own. Turning the car sideways would save walking distance sideways, albeit the distance to the material on the side of the lane does not change that much (as this can also be underneath the car). Yet, underbody work is also usually bumper to bumper. The image below shows both situations, bumper to bumper and door to door.

Sideways Rotation

Also for completeness’ sake, when talking about rotating the car, many companies also rotate the car on its side using a c-hanger, as shown here. This is not so much about walking distance, but more about ergonomics, as the workers can now work in front of themselves instead of reaching up over their heads.

And For Something Completely Odd…

Also, just for completeness’ sake, I have never seen the car turned diagonally, and I don’t really see an advantage in it. It also totally goes against my sense of manufacturing aesthetics… but this is not a valid reason. In any case, it could look like this below, but try this at your own risk—or maybe better not try this at all!

How About Tool Access?

However, walking is not the only consideration. You also need space around the vehicle for parts and machines. For example, the dashboard is usually lifted into the vehicle using a robotic arm due to the weight of the vehicle. When the cars are bumper to bumper, the robotic arm has plenty of space on the side of the assembly line to get in. If, for some ungodly reason, the cars would be door to door, then the robotic arm would have to somehow squeeze in between two cars to get into the passenger compartment. This would be much more difficult.

Similarly, many assembly lines have a sort-of seat on an arm for the operator to sit on. Without getting up, the operator simply slides his seat into the car for the work, then slides the seat back out when he is done. This is also more difficult for door-to-door assembly.

How About Space for Material?

Related to the space for tools is the space for parts. As a general rule, there is NEVER enough space around your assembly line (see also my post series Twelve Ways to Create Space around Your Assembly for more). If your cars are moving door to door, then you have much less space per car on the side of the assembly line for parts. Granted, in lean the goal is always to reduce inventory, but you DO need at least some inventory for your assembly line, and a bumper-to-bumper-style assembly line gives you more space here.

Below is an illustration showing the available space for material and tools in a bumper-to-bumper assembly line.

With a door-to-door assembly line, this space for material and tools is significantly reduced. To store the same amount of material, the storage area would need much more depth, which again would increase the potential walking distance to get the material and the tools. This is illustrated below.

In any case, deciding which way to turn your car depends on quite a few factors, and many car makers have their own approach (or may just do it the traditional way of bumper to bumper). My recommendation is that in case of doubt, use bumper-to-bumper assembly, but do consider door-to-door assembly if you have segments with a lot of work on the front and rear of the car. Now, go out, rotate your product in all dimensions to see which one is best, and organize your industry!