Typical Job Shops

Before we get into scheduling a job shop in detail we need to define what a job shop is.  A job shop is a type of manufacturing process structure where batches of a variety of custom products are made. In the job shop process flow, most of the products produced require a unique set-up and sequencing of processing steps. Examples of a job shop include a sheet metal, machine tool shop, a factory machining center, paint shops, a commercial printing shop, and other manufacturers that make custom products. This type of job shop can be made to order and engineer to order shop.

LAYOUT In the job shop there are few to many machines and those pieces of equipment can vary from being very similar to very unique.  Many times these machines are grouped in such a way that a department has work centers, and work centers have machines.   Similar equipment or functions are grouped together, such as all drill presses in one area and grinding machines in another in a process layout. A typical factory layout would be designed in such a way to minimize material handling, cost and work in process. Unlike production shops which produce standard products with specialized machines, job shops use general purpose equipment. Digital numerically controlled equipment, or CNC equipment,  is often used to give job shops the flexibility to change set-ups on the various machines very quickly. Job shops compete on quality, speed of product delivery, customization, and new product introduction, but are unlikely to compete on price as few scale economies exist.

ROUTING As jobs come in the door at a job shop each job is sequenced to the work centers and machines that are required to produce the part.  When an order arrives in the job shop, the part being worked on travels throughout the various areas according to a sequence of operations. Only a small subset of the machines are used for each job going through the shop because each job is unique. In order for scheduling to work properly this routing must accurately depict the set of machines and operations this job will go through. Scheduling defines these machines as resources.

EMPLOYEES Employees are key assets in a job shop and range from employees doing repetitive work to employees that are highly skilled craftsmen who operate multiple machines.  Job shop scheduling must take into account the finite resources of employees.

SCHEDULING A job is characterized by its route, its processing requirements, and its priority. In a job shop the mix of products is a key issue in deciding how and when to schedule jobs.  Scheduling can come in many forms but the basic scheduling is finite forward, finite backwards and infinite backwards.  There are variations on these scheduling mechanisms which each have their positive and negative results.