Assembly lines make the same product over and over again. One example is a typical ball point pen. A more complicated assembly line might require controls and processes that become quite involved. Imagine the assembly lines that build 35 millimeter film for cameras. With all of the chemical handling, and chemical impregnation onto the film surface, it is easy to imagine that the process is not simple. There are assembly lines with complicated assembly steps in them. Imagine building photocopy machines. Lastly, there are flexible assembly lines. These are typically where automobiles are manufactured. The assembly line might be used to build small sedans, but one is red, the next is blue, the next has the 6-disc CD stereo, the next has the cassette deck, the next has the automatic transmission, the next has the manual transmission, the next has the V6 engine, the next has the I-4 engine, etc.
Manufacturing cells are next in line. They offer more flexibility than the assembly line, but tend to operate at lower speeds. Some home furniture is built in cells. The first item of the day is a full length couch with removable bottom cushions. The next is a shorter version of the same platform, but with fixed cushions. The next is a small loveseat with removable cushions on the bottom and the back. The next is . . . one can imagine the variations. The distinction between cells and flexible assembly lines can easily become blurred.
There is the common thread to all the above styles of manufacturing. The work-center-to-work-center routing is either the same, or is very similar, from product to product. Also, the work content tends to be similar. The work content for each product won�t vary too much from the other products, and the work content at each work-center will be somewhat similar for each product. The more dissimilar the routing or the more dissimilar the work content, then the slower the line or cell will tend to run. The reason for this tendency for decreased speed is due to one or both of the following: 1) one high work-content job arrives in the line or cell, causing other work behind it to stop to await its turn, and/or 2) jobs with dissimilar routings will have a statistical tendency to �pile up� in front of one work-center. This �big pile� of jobs then takes longer to work back down to a smaller backlog.
Job shops, however you define them, are not any of the above types. Job shops will have any one or more of the following characteristics:
1. Work-center-to-work-center routings are generally random, for at least a portion of the routing
2. Work content can vary drastically from one job to the next.
3. Work content at any given work center can vary drastically for any two jobs.
4. A job may be assigned to one individual (or small group of workers) to work. This worker may take the job from start to finish, or may accomplish a significant portion of the work content, without routing the job from work-center to work-center. This is sometimes called �Project manufacturing.�
5. A job shop may be a small link in a large supply chain and may not be allowed to refuse jobs that don�t conveniently fit into its processes or equipment.
6. A job shop may be a contract manufacturer who builds on behalf of one or many OEM customers. This work will tend to be sporadic.
7. A job shop will typically have many finished-product part numbers that are �active� or �recent�
8. A job shop may have one �center� product for which they have to design and build custom interfaces.
9. A job shop will tend to have variable and unstable demand, limited control on supplier lead times and due dates, and very little visibility of work that is shortly forthcoming.