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© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Module 11 Operations Scheduling Chapter 16 (pp ) Work Center and definitions Objectives of scheduling Job sequence Priority Rules and Techniques Scheduling n jobs on ONE machine Scheduling n jobs on TWO machines Shop Floor Control Tools for shop floor control Principles
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 3 Work Center Work Center is an area in a business in which productive resources are organized and work is completed. Work Center may be a single machine, a group of machines, or an area where a particular type of work is done Job shop (by function) Flow (by product)
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 4 Capacity and Scheduling Capacity Consideration Infinite loading (no consideration to capacity) Finite loading Type of Scheduling Backward scheduling (MRP) (latest starting date) Forward scheduling (earliest completion date) Resources to Schedule Machine –limited Labor -Limited Refer to Exhibit 16.1
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 5 Typical Scheduling and Control Functions Allocating orders, equipment, and personnel to work centers (short run capacity planning) Determining the sequence of orders to be done (job priority) Initiating work of the scheduled orders (dispatching) Shop-floor control (Review Status and Expedite)
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 6 Objectives of Work-Center Scheduling Meet due dates Minimize lead time Minimize setup time or cost Minimize work-in-process inventory Maximize machine and/or labor utilization (w/o affecting flow?)
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 7 Priority Rules for Job Sequencing Job sequencing is to determine the order of which jobs to be done on a work center 1. First-come, first-served (FCFS) 2. Shortest operating time (SOT) 3. Earliest due date first 4. Earliest start date first (due date minus lead time) 5. Least Slack Time Remaining (STR) first
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 8 Priority Rules for Job Sequencing 6. Least slack time remaining (per operation as opposed to per job-STR/OP) first 7. Smallest critical ratio first (due date-current date)/(number of days remaining) 8. Smallest queue ratio first (slack time remaining in schedule)/(planned remaining queue time) 9. Last come, first served 10. Random order
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 9 Performance Measures of Priorities Meeting due dates of customers or downstream operations. Minimizing the flow time (the time a job spends in the process). Minimizing work-in-process inventory. Minimizing idle time of machines or workers.
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 10 Job Sequencing Example First-Come First-Served FCFS Schedule Late? On-Time? Orders submitted at beginning of week n-jobs on one machine Total completion time= Average= Average lateness=1515/4=3.75 ?
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 11 Job Sequencing Example Shortest Operating Time Orders submitted at beginning of week Shortest Operating Time Schedule Late? On-Time? n-jobs on one machine Total completion time= Average= Average lateness=1515/4=3.75?
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill In-Class Exercise Develop LCFS and earliest Due Date schedules? Orders submitted at beginning of week n-jobs on one machine
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 12 Job Sequencing Example Last-Come First-Served Orders submitted at beginning of week n-jobs on one machine Last-Come First-Served Schedule Jobs Late? 2 Average lateness = (1+10)/4 = 2.75 days
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 13 Job Sequencing Example Earliest Due Date First Orders submitted at beginning of week Earliest Due Date First Jobs Late? Jobs On-Time? n-jobs on one machine Average Lateness = (2+5)/4 = 1.75 days
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Job Sequencing n Jobs on 2 Machines (Johnson’s Rule) C Jobs M/c 1 (Min) A B D M/c 2 (Min) Suppose you have the following four jobs with time requirements in two stages of production. What is the job sequence using Johnson’s Rule? 1.Select the shortest time 2.If the shortest time is on first machine schedule it first; if it is not, schedule it last 3.Repeat 1 & 2 until completion of sequence on the first machine 4. Develop machine 2 schedule based on machine 1 schedule. (End time from machine1 will be start time for machine 2)
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Example of Job Sequencing Johnson’s Rule First, select the job with the smallest time in either stage. That is Job D with the smallest time in the first stage. Place that job as early as possible in the unfilled job sequence below. Drop D out, select the next smallest time ( Job A), and place it 4th in the job sequence. Drop A out, select the next smallest time. There is a tie in two stages for two different jobs. In this case, place the job with the smallest time in the first stage as early as possible in the unfilled job sequence. Then place the job with the smallest time in the second stage as late as possible in the unfilled sequence. Job Sequence Job AssignedD A B C Time in Hours Jobs Stage 1Stage 2 A B C D
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill 17 Principles of Work Center Scheduling Planned input should never exceed planned output (Exhibit 15.8) Focuses attention on bottleneck work centers InputOutput Work Center Input/Output Control
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Principles of Work Center Scheduling 1. There is a direct equivalence between work flow and cash flow. 2. The effectiveness of any job shop should be measured by speed of flow through the shop. 3. Schedule jobs as a string, with process steps back-to-back. 4. A job once started should not be interrupted.
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Principles of Job Shop Scheduling (Continued) 5. Speed of flow is most efficiently achieved by focusing on bottleneck work centers and jobs. 6. Reschedule every day. 7. Obtain feedback each day on jobs that are not completed at each work center. 8. Match work center input information to what the worker can actually do.
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Principles of Job Shop Scheduling (Continued) 9. When seeking improvement in output, look for incompatibility between engineering design and process execution. 10. Certainty of standards, routings, and so forth is not possible in a job shop, but always work towards achieving it.
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Shop-Floor Control System A system for utilizing data from the shop floor as well as data processing files to maintain and communicate status information on shop orders and work centers Functions 1. Assigning priority of each shop order. 2. Maintaining work-in-process quantity information. 3. Conveying shop-order status information to the office. 4. Providing actual output data for capacity control purposes. 5. Providing quantity by location and by shop order for WIP inventory and accounting purposes. 6. Providing measurement of efficiency, utilization, and productivity of manpower and machines.
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill >>>>Data Integrity (accuracy, timeliness) Gantt Charts Daily Dispatch Status and exception reports (exhibit 15.7b) Input/output control report (exhibit 15.7c) Tools for Shop Floor Control
© The McGraw-Hill Companies, Inc., 1998 Irwin/McGraw-Hill Master scheduling Material requirements planning Order scheduling Weekly workforce and customer scheduling Daily workforce and customer scheduling Process planning Strategic capacity planning Sales and operations (aggregate) planning Long range Intermediate range Short range Manufacturing Services Exhibit 13.1 Sales planAggregate operations plan Forecasting & demand management
McGraw-Hill/Irwin © 2006 The McGraw-Hill Companies, Inc., All Rights Reserved. 1.
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