Job Shop Scheduling
Definition
Job shop scheduling is the task of sequencing many different jobs across shared work centers in a job shop, where each job follows its own routing, so that due dates, queue times, and machine utilization stay in balance.
In Practice
A job shop makes low-volume, high-variety products — 50 open orders might follow 50 different paths through the same milling, turning, and finishing centers. The scheduling problem is deciding which job each work center should run next, typically using dispatching rules such as earliest due date, shortest processing time, or critical ratio.
Because queues form wherever routings converge, job shop scheduling is as much about controlling input as sequencing: releasing work faster than the shop's output rate only lengthens queues and lead times. Finite-capacity scheduling tools and simple priority rules both aim at the same goal — predictable completion against promised dates.
Frequently Asked Questions
What dispatching rules are used in job shop scheduling?
Common rules include first-come-first-served, shortest processing time (maximizes jobs completed), earliest due date, and critical ratio (time remaining divided by work remaining). No single rule wins everywhere: shortest-processing-time boosts throughput but can starve long jobs, while due-date rules protect delivery performance.
Why are job shop lead times so long relative to run time?
Because every work center serves many competing jobs, most of a job's life is spent queuing — often 90% or more of total lead time. Cutting lead time therefore depends less on faster machines and more on shorter queues: controlled order release, sensible priorities, and smaller transfer batches.
Related Terms
Forward scheduling is a planning method that starts a job at the earliest available date — usually today — and adds each operation's lead time to project the earliest possible completion date.
Priority PlanningPriority planning is the function of determining what to make and when — setting and maintaining valid due dates and quantities for orders — as distinct from capacity planning, which determines whether the resources exist to do it.
Critical RatioCritical ratio is a dispatching priority index calculated as time remaining until due date divided by work time remaining; a ratio below 1.0 means the job is behind schedule, and lower ratios get worked first.
BottleneckA bottleneck is the resource or process step with the least capacity in a system, which limits the output of the whole chain. Total throughput can never exceed the bottleneck's rate.