Production and Operations Management

Transcription

1 Production and Operations Management Norman Gaither Greg Frazier Slides Prepared by John Loucks 999 South-Western College Publishing 0 Chapter Shop-Floor Planning and Control in Manufacturing B [ ] E5 [ P9 [---] D [ X8 ----] C6 [- -

2 Overview Scheduling Process-Focused Manufacturing Scheduling Product-Focused Manufacturing Computerized Scheduling Systems Wrap-Up: What World-Class Producers Do Process-Focused Manufacturing Process-focused factories are often called job shops. A job shop s work centers are organized around similar types of equipment or operations. Workers and machines are flexible and can be assigned to and reassigned to many different orders. Job shops are complex to schedule. 3 -

3 Pre-production Planning Design the product in customer order, Plan the operations the product must pass through... this is the routing plan, Work moves between operations on a move ticket, 4 Common Shop Floor Control Activities The production control department controls and monitors order progress through the shop. Assigns priority to order Issues dispatch list Tracks WIP and keeps systems updated Controls input-output between work centers Measures efficiency, utilization, and productivity of shop 5-3

4 Input-Output Control Input-output control identifies problems such as insufficient or excessive capacity or any issues that prevents the order from being completed on time. Gantt charts are useful tools to coordinate jobs through shop; graphical summary of job status and loading of operations 6 Assigning Jobs to Work Centers: How Many Jobs/Day/Work Center Infinite loading - assigns jobs to work centers without regard to capacity.. large queues Finite loading - uses work center capacity to schedule orders... popular scheduling approach... more 7-4

5 Assigning Jobs to Work Centers: Which Job Gets Built First? Forward scheduling - jobs are given earliest available time slot in operation... excessive WIP 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 8 Order Sequencing Problems We want to determine the sequence in which we will process a group of waiting orders at a work center. Many different sequencing rules can be followed in setting the priorities among orders. There are numerous criteria for evaluating the effectiveness of the sequencing rules. 9-5

6 Order Sequencing Rules Sequencing rules include: First-Come First-Served (FCFS) Shortest Processing Time Earliest Due Date Critical Ratio - (time to due date/total remaining production time) Least Changeover Cost 0 Evaluating the Effectiveness of Sequencing Rules Average flow time - average amount of time jobs spend in shop Average number of jobs in system - Average job lateness - average amount of time job s completion date exceeds its promised delivery date Changeover cost - total cost of making machine changeovers for group of jobs -6

7 Experience Says: First-come-first-served has many shortcomings. Shortest processing time does perform well; supervisors like it but have to watch out for long processing time orders 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 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 Time Time to Promised Completion A 6 hours 0 hours B 6 C 9 8 D 4 4 E

8 FCFS Rule A > B > C > D > E Processing Promised Flow Job Time Completion Time Lateness A B 6 8 C D E FCFS Rule Performance Average flow time: 4/5 = 8. hours Average number of jobs in the system: 4/49 =.88 jobs Average job lateness: 90/5 = 8.0 hours 5-8

9 SPT Rule A > E > C > B > D Processing Promised Flow Job Time Completion Time Lateness A B C D E SPT Rule Performance Average flow time: 7/5 = 5.4 hours Average number of jobs in the system: 7/49 =.59 jobs Average job lateness: 76/5 = 5. hours 7-9

10 Critical Ratio Rule E > C > D > B > A Processing Promised Flow Job Time Completion Time Lateness E (.875) C (.889) D (.00) B (.33) A (.67) Critical Ratio Rule Performance Average flow time: 48/5 = 9.6 hours Average number of jobs in the system: 48/49 = 3.0 jobs Average job lateness: 93/5 = 8.6 hours 9-0

11 Comparison of Rule Performance Average Average Average Flow Number of Jobs Job Rule Time in System Lateness FCFS SPT CR SPT rule was superior for all 3 performance criteria. 0 Product-Focused Scheduling Two general types of product-focused production: Batch - large batches of several standardized products produced Continuous - few products produced continuously... minimal changeovers -

12 Scheduling Decisions How large should production lot size be for each product? How many products should have passed each operation if time deliveries are to be on schedule? EOQ for production lots: Batch Scheduling DS p C p d Does not consider production capacity Run-Out Method Only so much capacity available each week so determine size of lots for all orders at the same time Based on most current demand and production rates... not annual estimates 3 -

13 Computerized Scheduling Develops detailed schedules for each work center indicating starting and ending times Develops departmental schedules Generates modified schedules as orders move Many packages available... select one most appropriate for your business 4 Wrap-Up: World-Class Practice In process-focused factories: MRP II refined... promises are met, shop loading is near optimal, costs are low, quality is high In product-focused factories: EOQ for standard parts containers, this sets S, lot sizes are lower, inventories slashed, customer service improved Scheduling is integral part of a computer information system 5-3

14 End of Chapter 6-4