0 Production and Operations Management Norman Gaither Greg Frazier Slides Prepared by John Loucks  1999 South-Western College Publishing

1 Chapter 12 Shop-Floor Planning and Control in Manufacturing B2 [ ] B2 [ ] E5 [ E5 [ P9 [---] P9 [---] D1 [ D1 [ X8 ----] X8 ----] C6 [- C6 [

2 OverviewOverview l Scheduling Process-Focused Manufacturing l Scheduling Product-Focused Manufacturing l Computerized Scheduling Systems l Wrap-Up: What World-Class Producers Do

3 Process-Focused Manufacturing l Process-focused factories are often called job shops. l A job shop’s work centers are organized around similar types of equipment or operations. l Workers and machines are flexible and can be assigned to and reassigned to many different orders. l Job shops are complex to schedule.

4 Pre-production Planning l Design the product in customer order, l Plan the operations the product must pass through..... this is the routing plan, l Work moves between operations on a move ticket,

5 Common Shop Floor Control Activities l The production control department controls and monitors order progress through the shop. l Assigns priority to order l Issues dispatch list l Tracks WIP and keeps systems updated l Controls input-output between work centers l Measures efficiency, utilization, and productivity of shop

6 Input-Output Control l Input-output control identifies problems such as insufficient or excessive capacity or any issues that prevents the order from being completed on time. l Gantt charts are useful tools to coordinate jobs through shop; graphical summary of job status and loading of operations

7 Assigning Jobs to Work Centers: How Many Jobs/Day/Work Center l Infinite loading - assigns jobs to work centers without regard to capacity.. large queues l Finite loading - uses work center capacity to schedule orders... popular scheduling approach l... more

8 Assigning Jobs to Work Centers: Which Job Gets Built First? l Forward scheduling - jobs are given earliest available time slot in operation... excessive WIP l Backward scheduling - start with promise date and work backward through operations reviewing lead times to determine when a job has to pass through each operation.. less WIP but must have accurate lead times

9 Order Sequencing Problems l We want to determine the sequence in which we will process a group of waiting orders at a work center. l Many different sequencing rules can be followed in setting the priorities among orders. l There are numerous criteria for evaluating the effectiveness of the sequencing rules.

1010 Order Sequencing Rules l Sequencing rules include: l First-Come First-Served (FCFS) l Shortest Processing Time l Earliest Due Date l Critical Ratio - (time to due date/total remaining production time) l Least Changeover Cost

1 Evaluating the Effectiveness of Sequencing Rules l Average flow time - average amount of time jobs spend in shop l Average number of jobs in system - l Average job lateness - average amount of time job’s completion date exceeds its promised delivery date l Changeover cost - total cost of making machine changeovers for group of jobs

1212 Experience Says: l First-come-first-served has many shortcomings. l Shortest processing time does perform well; supervisors like it but have to watch out for long processing time orders l Critical ratio works well on average job lateness criterion.. may focus too much on jobs that cannot be completed on time forcing more jobs to miss promise dates

1313 Example: Sequencing Rules Use the FCFS, SPT, and Critical Ratio rules to sequence the five jobs below. Evaluate the rules on the bases of average flow time, average number of jobs in the system, and average job lateness. Job Processing TimeTime to Promised Completion Job Processing TimeTime to Promised Completion A 6 hours10 hours A 6 hours10 hours B 1216 C 9 8 D 1414 E 8 7

1414 Example: Sequencing Rules l FCFS RuleA > B > C > D > E Processing Promised Flow Processing Promised Flow JobTime Completion Time Lateness JobTime Completion Time Lateness A A B B C C D D E E

1515 Example: Sequencing Rules l FCFS Rule Performance l Average flow time: 141/5 = 28.2 hours 141/5 = 28.2 hours l Average number of jobs in the system: 141/49 = 2.88 jobs 141/49 = 2.88 jobs l Average job lateness: 90/5 = 18.0 hours

1616 Example: Sequencing Rules l SPT RuleA > E > C > B > D Processing Promised Flow Processing Promised Flow JobTime Completion Time Lateness JobTime Completion Time Lateness A A B B C C D D E E

1717 Example: Sequencing Rules l SPT Rule Performance l Average flow time: 127/5 = 25.4 hours 127/5 = 25.4 hours l Average number of jobs in the system: 127/49 = 2.59 jobs 127/49 = 2.59 jobs l Average job lateness: 76/5 = 15.2 hours

1818 Example: Sequencing Rules l Critical Ratio RuleE > C > D > B > A Processing Promised Flow Processing Promised Flow JobTime Completion Time Lateness JobTime Completion Time Lateness E (.875) E (.875) C (.889) C (.889) D (1.00) D (1.00) B (1.33) B (1.33) A (1.67) A (1.67)

1919 Example: Sequencing Rules l Critical Ratio Rule Performance l Average flow time: 148/5 = 29.6 hours 148/5 = 29.6 hours l Average number of jobs in the system: 148/49 = 3.02 jobs 148/49 = 3.02 jobs l Average job lateness: 93/5 = 18.6 hours

2020 Example: Sequencing Rules l Comparison of Rule Performance Average AverageAverage Flow Number of Jobs Job Flow Number of Jobs Job Rule Time in System Lateness Rule Time in System Lateness FCFS FCFS SPT SPT CR CR SPT rule was superior for all 3 performance criteria. SPT rule was superior for all 3 performance criteria.

2121 Product-Focused Scheduling l Two general types of product-focused production: l Batch - large batches of several standardized products produced l Continuous - few products produced continuously.... minimal changeovers

2 Scheduling Decisions l How large should production lot size be for each product? l How many products should have passed each operation if time deliveries are to be on schedule?

2323 Batch Scheduling l EOQ for production lots: l Does not consider production capacity l Run-Out Method l Only so much capacity available each week so determine size of lots for all orders at the same time l Based on most current demand and production rates... not annual estimates

2424 Computerized Scheduling l Develops detailed schedules for each work center indicating starting and ending times l Develops departmental schedules l Generates modified schedules as orders move l Many packages available.... select one most appropriate for your business

2525 Wrap-Up: World-Class Practice l In process-focused factories: l MRP II refined.... promises are met, shop loading is near optimal, costs are low, quality is high l In product-focused factories: l EOQ for standard parts containers, this sets S, lot sizes are lower, inventories slashed, customer service improved l Scheduling is integral part of a computer information system

2626 End of Chapter 12