Scheduling

Managing OperationsSupply Chain ManagementInventory ManagementAggregate PlanningMaterial Requirements PlanningShort – Term SchedulingProject ManagementMaintenance & Reliability

CAPACITY PLANNING1) Facility Size2) Equipment Procurement

AGGREGATE PLANNING1) Facility Utilization2) Personnel Needs3) Subcontracting

MASTER SCHEDULE1) MRP2) Disaggregations of

Master Plan

SHORT-TERM SCHEDULING1) Work Center Loading2) Job Sequencing

Long – Term

Intermediate- Term

ST

SCHEDULING pertains to establishing the TIMING

of the use of equipment, facilities, and human activities in an organization

is the final step (in the operations decision-making hierarchy) in the transformation process before actual output occurs

Strategic Importance of Scheduling

1) LOWER COST with better asset utilization

2) FASTER DELIVERY better customer service

3) DEPENDABLE DELIVERY competitive advantage

Scheduling CriteriaThe correct scheduling technique depends on

Volume of orders Nature of operations Complexity of jobs Importance placed on each of the

following scheduling criteria:1) Minimize completion time or process times2) Maximize utilization of staff equipment and

facilities3) Minimize Inventory (WIP & FG)4) Minimize customer waiting time

SCHEDULING Involves assigning DUE DATES to

specific jobs, but many jobs compete simultaneously for the same resources

Scheduling Techniques1. Forward Scheduling – begins as soon

as the requirements are known2. Backward Scheduling – begins with

the due date and schedules the final operation first and the other job steps in reverse order

Scheduling Operations Scheduling encompasses

a) allocating workloads to specific work centers, and

b) determining the sequence in which operations are to be performed

Scheduling tasks are largely a function of the volume of system output

Scheduling in High-Volume Systems High-volume systems: characterized by

standardized equipment and activities that provide identical or highly similar operations or products as they pass through the system (often referred to as Flow Systems)

Goal: obtain a smooth rate of flow of goods through the system in order to get a high utilization of labor and equipment

Flow Shop Scheduling: examples are PCs, cars, appliances, toys, etc.

Scheduling in Intermediate-Volume Systems

Standard Intermediate Made-to- type of Volume Order of Output System (job shop)

More economical to process such items intermittently, thus intermediate volume centers periodically shift from one job to another (e.g. canned foods, baked goods, paints and cosmetics)

3 basic issues: Run size of jobs Timing of jobs Sequence in which jobs are processed

Scheduling in Low-Volume Systems Products are made to order Orders usually differ considerably in

Processing requirements Materials needed Processing time Processing sequence & setups

Job Shop Scheduling: scheduling for low-volume systems with many variations in requirements; fairly complex with 2 basic issues:

a) How to distribute workload among work centers

b) What job processing sequence to use

Production Planning & Control To run a facility in a balanced and efficient

manner PPC system should

1. Schedule incoming order without violating capacity constraints of individual work centers

2. Check the availability of tools and materials before releasing an order to a department

3. Establish due dates for each job and check progress against need dates and order lead times

4. Check work-in-progress as jobs move through the shop5. Provide feedback on plant and production activities6. Provide work efficiency statistics and monitor operator

times for payroll and labor distribution analyses.

Scheduling System Whether manual or automated, must be

accurate and relevant; Requires a production database with both planning and control files

3 types of planning files1) Item Master File contains information about each

component the firm produces or purchases2) Routing File indicates each component’s flow

through the shop3) Work-Center Master File contains information about

the work center, such as capacity and efficiency

Control files track the actual progress made against the plan for each work order

LOADING JOBS in WORK CENTERS

Loading means the assignment of jobs to work or processing centers so that costs, idle time or completion times are kept to a minimum

2 forms of loading work centers: Oriented to capacity

Technique: INPUT-OUTPUT CONTROL Related to assigning specific jobs to work

centers Approaches: GANTT-CHART

ASSIGNMENT METHOD

LOADING JOBS in WORK CENTERS

Infinite Loading – jobs are assigned to work centers 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

LOADING JOBS in WORK CENTERS

Vertical Loading – loading jobs at a work center, job by job, usually according to some priority criterion, using infinite loading

Horizontal Loading – loading each job on all work centers it will require, one at a time, according to some priority criterion, using finite loading

Input-Output Control A system that allows operations personnel

to manage facility work flows by tracking work added to a work center and its work completed.

If the work is arriving faster that it is being processed, the facility is overloaded and backlog develops Overloading causes crowding in the facility, leading to

inefficiencies and quality problems If the work is arriving at a slower rate than

jobs are being performed, the facility is underloaded and the work center may run our of work Underloading results in idle capacity and wasted

resources

Assignment Method A special class of linear programming models

that involves assigning tasks or jobs to resources (e.g. jobs to machines, contracts to bidders, people to projects, and salespeople to territories)

The objective is most often to minimize total costs or time required to perform the tasks at hand

One important characteristics of assignment problems is that only one job (or worker) is assigned to one machine (or project)

Assignment Method Each assignment problem uses a

table The numbers in the table will be the

costs or times associated with each particular assignment

The assignment method involves adding and subtracting appropriate numbers in the table in order to find the lowest opportunity cost for each assignment

Assignment Method Steps:1. Subtract the smallest number in each row from every

number in that row and then, from the resulting matrix, subtract the smallest number in each column from every number in the column. This step has the effect of reducing the numbers in the table until a series of zeros, meaning zero opportunity costs, appear. Even though the numbers change, this reduced problem is equivalent to the original one, and the same solution will be optimal.

2. Draw the minimum number of vertical and horizontal straight lines necessary to cover all zeros in the table. If the number of lines equals either the number of rows or the number of columns in the table, then an optimal assignment can be made (see step 4). If the number of lines is less than the number of rows or columns, proceed to step 3.

Assignment Method Steps:3. Subtract the smallest number not covered by a line from

every other uncovered number. Add the same number to any number(s) lying at the intersection of any two lines. Do not change the value of the numbers which are covered by only one line. Return to step 2 and continue until an optimal assignment is possible.

4. Optimal assignments will always be at zero locations in the table. One systematic way of making a valid assignment is first to select a row or column that contains only one zero square. An assignment can be made to that square and then draw lines through its row and column. From the uncovered rows and columns, choose another row or column in which there is only one zero square. Make that assignment and continue the procedure until assigning each person or machine to one task.

SEQUENCING JOBS in WORK CENTERS

Scheduling provides a basis for assigning jobs to work centers.

Loading is a capacity-control technique that highlights overloads and underloads.

Sequencing specifies the order in which jobs should be done at each center

PRIORITY RULES for dispatching jobs to work centers are rules that are used to determine the sequence of jobs in process-oriented facilities.

Priority Rules FCFS: First Come, First Served. The first

job to arrive at a work center is processed first.

SPT: Shortest Processing Time. The shortest jobs are handled first and completed

EDD: Earliest Due Date. The job with the earliest due date is selected first

LPT: Longest Processing time. The longer, bigger jobs are often very important and are selected first.

Priority Rules - Assumptions The ser of jobs is known; no new jobs arrive

after processing begins; and no jobs are cancelled

Setup time is independent of processing sequence

Setup time is deterministic Processing times are deterministic rather

than variable There will be or interruptions in processing

such as machine breakdowns, accidents, or worker illness

Performance Measures in judging the effectiveness of any given sequence

Job Flow Time – length of time a job is at a particular workstation or work center (Average Flow Time = total flow time number of jobs)

Job Lateness – length of time the job completion date is expected to exceed the date the job was due or promised to a customer. It is equal to the difference with compliance times that exceed due dates, with zeros assigned to jobs that are early (average tardiness = sum of the differences from due dates no. of jobs)

Performance Measures in judging the effectiveness of any given sequence Makespan – total time needed to

complete a group of jobs; length of times between the start of the first job in the group and the completion of the last job in the group (makespan = sum of the given processing times of all jobs)

Utilization – Total job work (processing) sum of total flow time or makespan

Average Number of Jobs - computed as total flow time makespan (this ratio reflects the average work-in-process inventory)

Critical Ratio A sequencing rule that is an index number

computed by dividing the time remaining until due date by the work time remaining

As opposed to the priority rules, critical ratio is dynamic and easily updated. It tends to perform better than FCFS, SPT, EDD, or LPT on the average job-lateness criterion.

CR gives priority to jobs that must be done to keep shipping on schedule.

Formula:CR = Time remaining = Due date - Today’s date

Workdays remaining Work(lead)time remaining

Critical Ratio A job with a low critical ratio (less

than 1.0) is one that is falling behind schedule.

If CR is exactly equal to 1.0, the job is on schedule.

A CR greater than 1.0 means the job is ahead of schedule and has some slack.

Sequencing N Jobs on Two Machines: Johnson’s Rule

The next step in complexity is the case in which N jobs (where N is 2 or more) must go through two different machines or work centers in the same order. This is called the N/2 problem.

Johnson’s Rule: minimizes the processing time for sequencing a group of jobs through two work centers; also minimizes total idle time on the machines; involves 4 steps:

1. All jobs are to be listed, and the time that each requires on a machine to be shown

2. Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first. If the shortest time lies the second machine, schedule the job last. Ties in activity times can be broken arbitrarily.

3. Once a job is scheduled, eliminate it.4. Apply steps 2 and 3 to the remaining jobs, working toward the

center of the sequence.

THEORY of CONSTRAINTS The body of knowledge that deals with

anything that limits an organization’s ability to achieve its goals

Managers need to identify the operations (bottlenecks) that constrain output because it is throughput – that is, units processed through the facility and sold – that makes the difference.

Constraints can be physical (such as personnel availability, raw materials or supplies) or non-physical (such as procedures, morale, training)

THEORY of CONSTRAINTS Avoided scheduling complexities and

focused on bottleneck operations (i.e., those for which there was insufficient capacity – in effect, a work center with zero idle time)

Output of the system is limited by the output of the bottleneck operation(s)

Idle time of nonbottleneck operations was not a factor in overall productivity of the system, as long as the bottleneck operations were used effectively.

THEORY of CONSTRAINTS “drum-buffer-rope” technique

Drum = is the schedule that sets the pace of production, the goal of which is to schedule to make maximum use of bottleneck resources

Buffer = refers to potentially constraining resources outside of the bottleneck; its role is to keep inventory from piling up on either side of the bottleneck operation

Rope = represents the synchronizing of the sequence of operations to insure effective use of the bottleneck operations; the goal is to avoid costly and time-consuming multiple setups, particularly of capacity of constrained resources, so they do not become bottlenecks too.

THEORY of CONSTRAINTS system of varying “batch” sizes to

achieve the greatest output of bottleneck operations Process Batch – denotes the basic lot size for a

job Transfer Batch – denotes a portion of the basic

lot that could be used during production to facilitate utilization of bottleneck operations

In effect, a lot could be split into two or more parts.Splitting a large lot at one or more operations preceding a

bottleneck operation would reduce the waiting time of the bottleneck operation.