Scheduling Shop Floor

1. Scheduling Shop Floor ISQA 459/559 Mellie Pullman

2. Learning Objectives • Explain the different kinds of scheduling • Describe different shop loading methods • Describe priority rules • Describe scheduling performance measures

3. Definitions • Routing: • The operations to be performed, their sequence, the work centers visited, & the time standards • Bottleneck: • A resource whose capacity is less than the demand placed on it • Due date: • When the job is supposed to be finished • Slack: • The time that a job can be delayed & still finish by its due date • Queue: • A waiting line

4. High Volume Operations • High volume flow operations generally have fixed routings • Bottlenecks are easily identified • Commonly use line-balancing to design the process around the required tasks • Examples?

5. Low Volume Operations • Low volume job shop operations are designed for flexibility. • Each product or service may have its own routing (scheduling is much more difficult) • Bottlenecks move around depending upon the products being produced at any given time • Examples?

6. Gantt Charts • Graphical tools used to illustrate workloads & help monitor job progress • Load charts: • Illustrates the workload relative to the capacity of a resource • Progress charts: • Illustrates the planned schedule compared to actual performance

7. Load Chart

8. Progress Chart

9. Loading Workstations • Infinite loading: • Ignores capacity constraints, but helps identify bottlenecks in a proposed schedule – enabling proactive management • Finite loading: • Allows only as much work to be assigned as a station should be able to handle – but doesn’t prepare for inevitable slippage

10. Example • Infinite: • Schedule in time period needed • Finite: • Schedule according to capacity limits

11. Forward Vs. Backward Scheduling Start processing when order is received regardless of due date Schedule the job’s last activity so it is finished right before the due date

12. Operations Sequencing • A short-term plan of actual jobs based on available capacity & priorities • Priority rules: • Decision rules to allocate the relative priority of jobs at a work center • Local priority rules: determines priority based only on jobs at that workstation • Global priority rules: also considers the remaining workstations a job must pass through

13. Common Priority Rules • First come, first served (FCFS) • Last come, first served (LCFS) • Earliest job due date (EDD) • Shortest processing time (SPT) • Longest processing time (LPT) • Min Critical ratio: (CR) • (Time until due date)/(processing time remaining)

14. How to use priority rules • Decide which rule to use. • List all the jobs waiting to be processed at the work-center and their job time • Job time includes setup and processing time • Using your priority rule, determine which job has the highest priority and should be worked on 1st , 2nd, 3rd ,etc.

15. Machine Shop with 6 jobs waiting • Use SPT (shortest processing time) • Determine sequence of jobs

16. Performance Measurement • Job flow time: • Time a job is completed minus the time the job was first available for processing • Average jobs in system: • Measures amount of work-in-progress =Total job flow time/Make-span • Make-span: • The entire time it takes to finish a batch of jobs • Job lateness: • Whether the job is completed ahead of, on, or behind schedule • Job tardiness: • How long after the due date a job was completed

17. More Data on the jobs

18. Completion Data Negative lateness means job is finished ahead of due date. 0=on time

19. Group In-Class Activity • Each team should use each of these rules calculate all performance measures: • Critical Ratio • Longest Process Time • EDD

20. Worksheet for your method:____________ Make-span ____________ Job flow-time ____________ Average jobs in system ____________

21. What are the company’s objectives?What priority rule works best? • Fairness? • Minimizing mean job tardiness? • Minimizing mean job flow time, lateness, and average jobs in the system?

22. Sequencing through 2 work centersJohnson’s rule • All jobs are processed through 2 work centers sequentially • List the jobs and processing times • Find the shortest activity processing time among all jobs (not yet schedule). • If it is the first activity, put job needing that activity in the earliest available position in the sequence • If it is the second activity, put the job needing that activity in the last avaiable position in the job sequence. • One you schedule a job, it’s eliminated from further consideration 3. Repeat step 2 until every job is on the schedule

23. Sample Problem

24. Sample Problem (days) Ties: pick either

25. Group In-Class Problem