Copyright 2009 by The McGraw-Hill Companies, Inc. All Rights Reserved.

Transcription

1 Chapter 16 McGraw-Hill/Irwin Copyright 2009 by The McGraw-Hill Companies, Inc. All Rights Reserved. : Establishing the timing of the use of equipment, facilities and human activities in an organization Effective scheduling can yield Cost savings Increases in productivity 16 Part i-2

2 Context is constrained by multiple system design decisions System capacity Product and/or service design Equipment selection Worker selection and training Aggregate planning and master scheduling 16 Part i-3 Hierarchies 16 Part i-4

3 High Volume Systems Flow System High-volume system with standardized equipment and activities Flow system scheduling for flow systems The goal is to achieve a smooth rate of flow of goods or customers through the system in order to get high utilization of labor and equipment 16 Part i-5 High-Volume: Difficulties Few flow systems are entirely dedicated to a single product or service Each product change requires Slightly different inputs of parts Slightly different materials Slightly different processing requirements that must be scheduled into the line Need to avoid excessive inventory buildup Disruptions may result in less-than-desired output 16 Part i-6

4 High-Volume Success Factors The following factors often dictate the success of high-volume systems: Process and product design Preventive maintenance Rapid repair when breakdowns occur Optimal product mixes Minimization of quality problems Reliability and timing of supplies 16 Part i-7 Intermediate-Volume Systems Output fall between the standardized-type output of high-volume systems and the make-toorder output of job shops Output rates are insufficient to warrant continuous production Rather, it is more economical to produce intermittently Work centers periodically shift from one product to another 16 Part i-8

5 Intermediate-Volume Systems Three basic issues: Run size of jobs The timing of jobs The sequence in which jobs will be produced * Q p DS 2 H p p u 16 Part i-9 Intermediate-Volume Systems Important considerations Setup cost Usage is not always as smooth as assumed in the economic lot size model Alternative scheduling approach Base production on a master schedule developed from customer orders and forecasted demand 16 Part i-10

6 Low-Volume Systems Job shop scheduling for low-volume systems with many variations in requirements Make-to-order products Processing requirements Material requirements Processing time Processing sequence and steps A complex scheduling environment It is impossible to establish firm schedules until actual job orders are received 16 Part i-11 Low-Volume Systems: Loading Loading the assignment o jobs to processing centers Gantt chart Used as a visual aid for loading and scheduling purposes Purpose of the Gantt chart is to organize and visually display the actual or intended use of resources in a time framework Managers may use the charts for trial-and-error schedule development to get an idea of what different arrangements would involve 16 Part i-12

7 Gantt Charts Load chart A Gantt chart that shows the loading and idle times for a group of machines or list of departments 16 Part i-13 Gantt Charts Schedule chart A Gantt chart that shows the orders or jobs in progress and whether the are on schedule 16 Part i-14

8 Loading Approaches Infinite loading Jobs are assigned to workstations without regard to the capacity of the work center. Finite loading Jobs are assigned to work centers taking into account the work center capacity and job processing times. Infinite loading Capacity over over Finite loading Capacity Part i-15 Approaches Forward scheduling ahead from some point in time. Used when the question is: How long will it take to complete this job? Backward scheduling backwards from some due date Used when the question is: When is the latest this job can be started and still be completed on time? 16 Part i-16

9 Managing Work Flows Input/Output (I/O) control Managing work flow and queues at work centers Without I/O control: If demand exceeds processing capacity, a work center overload is created If work arrives more slowly than a work center can handle, work center underutilization results The goal is to strike a balance between input and output rates in order to minimize queues and maximize utilization 16 Part i-17 Sequencing Sequencing Determine the order in which jobs at a work center will be processed Priority rules Simple heuristics used to select the order in which jobs will be processed The rules generally assume that job setup cost and time are independent of processing sequence Job time Time needed for setup and processing of a job 16 Part i-18

10 Priority Rules FCFS - first come, first served SPT - shortest processing time EDD - earliest due date CR - critical ratio S/O - slack per operation Rush - emergency 16 Part i-19 Priority Rules: Assumptions The set of jobs is known; no new orders arrive after processing begins and no jobs are canceled Setup time is independent of processing time Setup time is deterministic Processing times are deterministic There will be no interruptions in processing such as machine breakdowns or accidents 16 Part i-20

11 Sequence: Performance Metrics Common performance metrics: Job flow time This is the length of time a job is at a particular workstation or work center It includes not only processing time but also any time waiting to be processed Job lateness This is the length of time the job completion time is expected to exceed the job due date Makespan The total time needed to complete a group of jobs from the beginning of the first job to the completion of the last job Average number of jobs Jobs that are in a shop are considered to be WIP inventory 16 Part i-21 Two Work Center Sequencing Johnson s Rule Technique for minimizing completion time for a group of jobs to be processed on two machines or at two work centers. Minimizes total idle time Several conditions must be satisfied 16 Part i-22

12 Johnson s Rule Conditions Job time must be known and constant for each job at the work center Job times must be independent of sequence Jobs must follow same two-step sequence Job priorities cannot be used All units must be completed at the first work center before moving to second work center 16 Part i-23 Difficulties Variability in Setup times Processing times Interruptions Changes in the set of jobs No method for identifying optimal schedule is not an exact science It is an ongoing task for a manager 16 Part i-24

13 Service Operation Problems Service scheduling often presents challenges not found in manufacturing These are primarily related to: The inability to store or inventory services The random nature of service requests Service scheduling may involve scheduling: 1.Customers 2.Workforce 3. Equipment 16 Part i-25 Service Operations customers: Demand Management Appointment systems Controls customer arrivals for service Reservation systems Estimates demand for service the workforce: Capacity Management Cyclical Employees are assigned to work shifts or time slots, and have days off, on a repeating basis 16 Part i-26

14 Cyclical In many environments, the scheduling requirements are fairly similar e.g., hospitals, police/fire departments, restaurants, supermarkets Rotating schedules Set a scheduling horizon Identify the work pattern Develop a basic employee schedule Assign employees to the schedule 16 Part i-27