Scheduling

Scheduling is the time-phased aspect of Production Planning and Control. Once routing defines what operations are needed and where, scheduling determines when these operations should start and finish. It involves creating a timetable for production activities.

Definition:

• Scheduling is "prescribing of when and where each operation necessary to manufacture the product is to be performed."
• It establishes the timing of resource usage (machines, labor) and the flow of work through the facility.

Purpose of Scheduling:

• Meet Due Dates: Coordinate activities to ensure final products are completed by their required delivery dates.
• Optimize Resource Use: Minimize idle time for machines and labor by planning a coordinated flow of work.
• Minimize Work-In-Progress (WIP): Efficient scheduling can reduce the time jobs spend waiting between operations.
• Reduce Lead Times: Shorten the overall time it takes to produce an item from start to finish.
• Provide Basis for Control: The schedule serves as the benchmark against which actual progress is monitored.

Inputs to Scheduling:

• Routing Information: Sequence of operations, assigned machines, standard setup and run times.
• Job Information: Quantities required, due dates, priorities.
• Resource Availability: Capacity of machines and work centers, labor availability (shifts, skills).
• MPS/MRP Outputs: Planned order release dates and quantities.
• Current Shop Status: Information on jobs already in progress, machine breakdowns.

Types/Levels of Scheduling:

• Master Scheduling (MPS): Scheduling end items over weeks/months.
• Detailed Operations Scheduling: Determining specific start/finish times for each job on each machine (often complex and dynamic).
• Forward Scheduling: Starts jobs as early as possible. Calculates the earliest possible completion date. Used when speed is key or for "as soon as possible" orders. Can build WIP.
• Backward Scheduling: Starts jobs as late as possible to meet the due date. Calculates required start dates by working backward. Minimizes WIP. Common in JIT environments.

Common Scheduling Techniques/Tools:

• Gantt Charts: Visual timeline representation of jobs scheduled on resources.
• Priority Rules (Sequencing): Rules used to decide which job to run next when multiple jobs are waiting at a work center (e.g., FCFS - First Come First Served, SPT - Shortest Processing Time, EDD - Earliest Due Date, LPT - Longest Processing Time, CR - Critical Ratio).
• Johnson's Rule: An algorithm to find the optimal sequence for minimizing makespan (total completion time) for n jobs that must be processed on two machines in the same order. (Can sometimes be extended to three machines under specific conditions).
  • Basic Idea (2 machines): Find shortest time overall. If on Machine 1, schedule job early. If on Machine 2, schedule job late. Repeat.
• Finite Capacity Scheduling (FCS): Computerized systems that schedule jobs considering the actual limited capacity of resources, preventing overloads shown in simpler MRP/CRP systems.

Indian Example: A custom fabrication shop in India, like one making specialized industrial equipment, heavily relies on scheduling. They receive unique customer orders with specific due dates. Routing defines the sequence (e.g., cutting, welding, machining, assembly, painting). Scheduling then determines when each job will be on each machine, considering job priorities (like EDD), estimated processing times (SPT potential), and machine availability. Backward scheduling might be used from the customer due date to determine required start dates. Johnson's Rule could potentially be used if a set of jobs primarily uses two key bottleneck machines sequentially.

Scheduling translates the production plan into a concrete operating timetable, essential for coordinating activities and meeting delivery commitments efficiently.