sCh16Scheduling

Chapter 16: Scheduling

• Author: A. Eller

• Location: Arizona

Overview of Scheduling

• Definition: Timing the use of human activities, equipment, and facilities.

• Benefits of Effective Scheduling:

  • Cost savings

  • Increased productivity

  • Enhanced customer satisfaction and competitiveness

Scheduling Systems

1. High Volume Systems

• Flow System: A repetitive process where all jobs follow the same sequence.

• Flow System Scheduling Goals:

  • Ensure a smooth rate of flow for goods or customers through the system.

  • Achieve high utilization of labor and equipment, which involves line balancing.

2. Low Volume Systems

• Job Shop Scheduling: Scheduling for systems with low volume but many variations in requirements.

  • Focus on make-to-order products.

  • Different processing requirements (materials, time, sequence) create a complex scheduling environment.

  • Firm schedules cannot be established until actual orders are received.

Scheduling Approaches

1. Forward Scheduling

• Involves scheduling ahead from a specific point in time.

• Example Question: "How long will it take to complete this job?"

2. Backward Scheduling

• Scheduling backwards from a due date.

• Example Question: "When is the latest this job can start and still be completed on time?"

Loading Strategies

1. Loading Definition

• Assignment of jobs to processing centers.

2. Types of Loading

• Infinite Loading: Jobs are assigned without regard for center capacity. May lead to overloading (requiring overtime, work shifts, or contracting out).

• Finite Loading: Jobs are assigned considering center capacity and job processing times.

Gantt Charts

• Definition: A visual aid for loading and scheduling, displaying actual or intended resource use in a time framework.

• Types:

  • Gantt Load Chart: Shows loading and idle times for machines/departments.

  • Gantt Schedule Chart: Displays jobs in progress and their scheduling status.

Sequencing

• Definition: Determines the order of jobs processed within a work center.

• Priority Rules: Heuristics for selecting job processing order.

1. Priority Rules

• FCFS (First Come, First Served): Jobs processed in arrival order.

• SPT (Shortest Processing Time): Jobs with the shortest processing time processed first.

• EDD (Earliest Due Date): Jobs with the earliest due dates processed first.

• CR (Critical Ratio): CR = Time Remaining until due date / Remaining processing time. Lowest CR is processed first.

• S/O (Slack per Operation): Average slack time is calculated. Emergency or rush jobs prioritized.

Sequencing Performance Metrics

• Job Flow Time: Time from job arrival to completion, including waiting time.

• Makespan: Total time to complete a group of jobs.

• Job Lateness: Measures how much a job exceeds due date.

• Average Number of Jobs: Calculated as total flow time / makespan.

Application of Sequencing Methods

Examples

• FCFS Example: Computed metrics such as total flow time, lateness, makespan, and average number of jobs.

• SPT Example: Similar metrics calculated, demonstrating efficiency.

• EDD Example: Analyzed to optimize lateness.

• CR Example: Jobs sequenced using Critical Ratio with resulting metrics detailed.

Johnson's Rule in Two Work Center Sequencing

• Definition: Technique to minimize makespan for jobs processed at two work centers with the same sequence.

• Method:

1. List jobs and times.

2. Select job with the shortest processing time for scheduling.

3. Schedule jobs until all are processed.

Assignment Model for Loading

• Goal: Optimal assignment of tasks to resources.

• Hungarian Method: Step-by-step method to find the lowest cost solution, including row/column reductions.

Scheduling Challenges

• Challenges include variability in setup times, processing times, interruptions, and job set changes.

• No exact method for optimal schedules, making it a challenge for managers.

Strategies to Minimize Scheduling Challenges

• Set realistic due dates.

• Focus on bottleneck operations to increase capacity.

• Schedule according to bottlenecks and consider splitting large jobs into smaller batches.