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
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 1 st, 2 nd, 3 rd,etc.
15 Machine Shop with 6 jobs waiting Use SPT (shortest processing time) Determine sequence of jobs Job NumberJob Time (setup & run) A3 days B7 days C6 days D4 days E2 day F5 days
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 Job Number Job Time (setup & run) Due Date (days from now) Remaining job time at other WC Critical Ratio A3 days15615/(3+6) B7 days208 C6 days305 D4 days203 E2 day227 F5 days205
18 Completion Data Job Number Compet- ion Date Due Date (days from now) Lateness (days) Tardiness (days) A515-100 B2720+77 C2030-100 D920-110 E222-200 F1420-60 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:____________ Job #Job Time (setup & run) Due Date -days from now Remaining job time at other WC Job FlowJob Lateness Job Tardiness CR A3 days156 B7 days208 C6 days305 D4 days203 E2 day227 F5 days205 AVE 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 centers Johnson’s rule All jobs are processed through 2 work centers sequentially 1.List the jobs and processing times 2.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 Metal JobActivity 1 Metal Cutting Activity 2 Deburr & Grind A12 B35 C24 D54 E42