Sequencing is the order of tasks to be done in chain. Hence the next task is started once the previous one is completed.
Scheduling, on the other hand is the process in which people are assigned to time to accomplish different tasks.
It improves the delivery performance and reduces the manufacturing time and cost.
Scheduling
• Scheduling: The allocation of resources over time to accomplish specific tasks.
• Demand scheduling: A type of scheduling whereby customers are assigned to a definite time for order fulfillment.
• Workforce scheduling: A type of scheduling that determines when employees work.
• Operations scheduling: A type of scheduling in which jobs are assigned to workstations or employees are assigned to jobs for specified time periods.
Operations scheduling is critical to the success of an organization; however, it can be a very complicated task. Effective schedules are needed to meet promised customer delivery dates or inventory targets.
It covers the following areas in particular:
- assign job to a particular work center/ machine
- time of assignment of job and completion
- allocation of resources like manpower and materials
- time sequence of operations
- feedback and control function to take care of deviations
Objectives of Operations Scheduling
• Making efficient use of the labour.
• Making best possible use of the equipments that are available for the use.
• Increasing the profit.
• Increasing the output.
• Improving the service level.
Objectives of Operations Scheduling
• Maximizing the delivery performance i.e. meeting the delivery dates.
• Minimizing the inventory.
• Reducing the manufacturing time.
• Minimizing the production costs.
• Minimizing the worker costs.
Functions of Operations Scheduling
• Allocation of the resources.
• Shop floor control.
• Making maximum use of the plant at minimum possible cost.
• Ensure that the needs of the manpower are optimum.
• Determination of the sequence of the jobs.
Functions of Operations Scheduling
• Specifying the start and the end time for each job (actively scheduled).
• Getting quick feedback from the shops regarding the delays and the various interruptions.
• Possess up – to – date information for the availability of the materials, expected delivery dates etc.
• Possess up – to – date data on the machine regarding its breakdown, servicing etc.
Types of Scheduling
Types of Operations Scheduling are as follows:
- Forward operations scheduling –
• Classified on the basis of the time.
• All the activities are scheduled from the date of the planned order release.
• First task of the job is scheduled.
• Its subsequent task is scheduled on the scheduled completion of the first task.
• Like this, accordingly all the tasks of the job are scheduled. - Backward operations scheduling –
• Also classified on the basis of the time.
• Activities are scheduled from the date or the planned receipt date.
• The last activity is scheduled first.
• Time of the start of the last task is considered as the time for the start of the previous activity.
Sequencing
• Prioritize jobs assigned to a resource
• If no order specified use first-come first-served (FCFS)
• Other Sequencing Rules
• FCFS – first-come, first-served
• LCFS – last come, first served • DDATE – earliest due date
• CUSTPR – highest customer priority • SETUP – similar required setups
• SLACK – smallest slack • CR – smallest critical ratio
• SPT – shortest processing time • LPT – longest processing time
Sequencing Jobs
• Operations schedules are short-term plans designed to implement the sales and operations plan
• An operation with divergent flows is often called a job shop
– Low-to medium-volume production
– Utilizes job or batch processes
– The front office would be the equivalent for a service provider
– Difficult to schedule because of the variability in job routings and the continual introduction of new jobs to be processed
• An operation with line flow is often called a flow shop
– Medium- to high-volume production
– Utilizes line or continuous flow processes
– The back office would be the equivalent for a service provider
– Tasks are easier to schedule because the jobs have a common flow pattern through the system
Priority Sequencing Rules
• First-come, first-served (FCFS)
• Earliest due date (EDD)
• Critical ratio (CR)
CR = [(Due date) – (Today’s date)]/Total shop time remaining
A ratio of less than 1.0 implies that the job is behind schedule
A ratio greater than 1.0 implies the job is ahead of schedule
The job with the lowest CR is scheduled next
• Shortest processing time (SPT)
• Slack per remaining operations (S/RO)
The job with the lowest S/RO is scheduled next
S/RO = [(Due date –Today’s date)–Total shop time remaining] / Number of operations remaining
Johnson’s Rule
Johnson’s Rules a technique that can be used to minimize the completion time for a group of jobs that are to be processed on two machines or at two successive work centers.
Objectives of Johnson’s Rule
The Objectives of Johnson’s Rule are:
Conditions for Johnson’s Rule
In order for the technique to be used, several conditions must be satisfied:
| Job time(including setup and processing) must be known and constant for each job at each work center. |
| Job times must be independent of the job sequence. |
| All jobs must follow the same two-setup work sequence. |
| Job priorities cannot be used. |
Steps Involved In Johnson’s Rule
Minimizes makespan when scheduling a group of jobs on two workstations
Step 1: Scan the processing time at each workstation and find the shortest processing time among the jobs not yet scheduled. If two or more jobs are tied, choose one job arbitrarily.
Step 2: If the shortest processing time is on workstation 1, schedule the corresponding job as early as possible. If the shortest processing time is on workstation 2, schedule the corresponding job as late as possible.
Step 3: Eliminate the last job scheduled for further consideration. Repeat steps 1 and 2 until all jobs have been scheduled