Job shops handle a variety of jobs, where each job is different. In batch production though there is a continuous demand for products, the rate of production exceeds that of demand, and hence there are batches. In batch production, jobs are predictable. Job shop is a different proposition, where jobs and demand both are unpredictable. Job shop handles the unique jobs each time a unique set of operations and processing time.
Job shop machines are general purposes machines organized department wise. The sequencing of each job is unique, depending upon the technological requirement. Job shop is a complex waiting line system — a job exits from a machine to wait on a new machine because of other jobs. Each machine has a waiting line of jobs. The converse is also true. Machines may wait for the job but no job is forthcoming (Idle time). Planning here is a process of prioritizing the jobs at each machine to seek the desired objectives.
Problems of Job production:
The basic question here is that of scheduling. When there are â€˜nâ€™ jobs awaiting processing on â€˜mâ€™ machines, each job having a pre-decided sequence of operation and processing timings, then what should be the order of loading the jobs on machines so as to optimize the expected performance standards?
Expected performance standards could be:
1. Mean flow time.
2. Total processing time.
3. Idle time of machines.
4. Mean earliness and lateness of jobs (Job completed before due date is its earliness. Lateness — actual completion time — due date).
5. Mean tardiness of jobs (Job completed after its due date).
6. Number of tardy jobs.
7. Mean waiting time.
8. Mean number of jobs in the system.
The factors affecting the solution are:
1. Total number of jobs for scheduling.
2. Total number of machines.
3. Manufacturing facilities — flow shop or job shop.
4. Nature of job arrivals — static or dynamic.
5. Evaluation criteria for scheduling.
If n (number of jobs) goes up and m (number of machines) also goes up, the problem of scheduling to that extent becomes more complex. Here there are no optimal solutions then.
The facilities in the job shop are limited, and the jobs crowd up demanding individual processing sequences. It gives rise to some peculiar problems: flow pattern problem, WIP inventories, counting of jobs, long completion time and certain unpredictable problems.
The work in this area mainly relates to static job or flow shop. However, in a large shop, the possible sequences are many. It requires further research work. More work has been in the area of dispatching rules using simulation in a dynamic environment. Scheduling has implications for both the costs and effectiveness.