Scheduling

Definition of scheduling

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

• In the decision-making hierarchy, scheduling decisions are the final step in the transformation process before actual output occurs.

• Effective scheduling can yield– Cost savings

– Increases in productivity

Volume

• Scheduling tasks are largely a function of the volume of system output.– High-volume systems– Intermediate-volume systems– Low-volume systems

High-Volume Systems

• Flow system: High-volume system with Standardized equipment and activities

• Flow-shop scheduling: Scheduling for high-volume flow system

• Goal: a smooth rate of flow of goods or customers through the system to get a high utilization of workforce and equipment

• Highly repetitive nature system leads to: – …the determination of many loading and sequence decisions during the design

of the system,– …highly specialized: tools and equipnment, arrangement, division of labour– automatization

Balancing the line

• Allocating the required tasks to workstationsso that they satisfy technical (sequencing) constraints and are balanced with respect to equal work times among stations.

• Goal: maximum utilization and highest possible output rate

Hindrances of highly specialized jobs

• Discontent of workers:– Too simple tasks: monotonous, boring– Give rise to atigue, absenteesm, turnover– Reduce productivity

High-Volume Success Factors

• Process and product design: manufacturability• Preventive maintenance: to minimize disruption of

the flow of work• Rapid repair when breakdown occurs• Optimal product mixes: linear programming• Minimization of quality problems: to minimize

disruption • Reliability and timing of supplies: to avoid shortages

(a and high carrying costs)

Intermediate-Volume Systems

• Outputs are between standardized high-volume systems and made-to-order job shops– Usually standard outputs

– Not continuous but intermittent production (periodical shifts from one job to another)

– Run size (large), timing, and sequence of jobs

• Economic run size:

• MRP approach

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Setup costs

• Depend on the similarity of products• Complex sequencing problem: different setup

costs for every combination

• Off-line setups, modular set-ups, flexible equipment

Scheduling in Low-Volume Systems

• Products made to order not to inventory• Orders can be very different• Job-shop scheduling: Scheduling for low-volume

systems with many variations in requirements• Schedules cannot be made prior to actual job

order• Loading - assignment of jobs to process centres

and to various machines in the centres• Sequencing - determining the order in which

jobs will be processed

Aims of loading

• Find arrangement to minimize:– Processing and setup costs– Idle time among work centres and machines– Job completition time

Gantt load charts

• Visual aid• Depicts the loading and iddle times for a

group of machines or departments• Trial-and-error schedule development

WorkCenter

Mon. Tues. Wed. Thurs. Fri.

1 Job 3 Job 42 Job 3 Job 73 Job 1 Job 6 Job 74 Job 10

• Infinite loading: jobs are assigned without regard to the capacity of work centres. This can lead to over- and underloads.

• Finite loading: takes into account the work center capacity and job processing times. Need frequent updating.

• Forward scheduling: scheduling ahead from a point in time. ‘How long will it take to complete this job?’

• Backward scheduling: scheduling backward from due date. ‘When is the latest job can be started?’

Loading

Gantt schedule chart

• Shows the orders or jobs in progress and whether they are on schedule – horizontal axis: time, – vertical axis: jobs in progress

Sequencing

• Sequencing: Determine the order in which jobs at a work center will be processed (and the order in which jobs are processed at individual workstations within work centers). Crucial if work centres are heavily loaded.

• Workstation: An area where one person works, usually with special equipment, on a specialized job.

Sequencing

• Priority rules: Simple heuristics used to select the order in which jobs will be processed.

• Job time: Time needed for setup and processing of a job.

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

Top Priority

Effectiveness of a given sequence• Job flow time: the length of time a job is at a particular

workstation or work center.Processing time + waiting

• Job lateness: the length of time the job completition date is expected to eyceed the date the job was due or promised to a customer.Actual completition time – due date

• Makespan: total time needed to complete a group of jobs from the beginning of the first job to the completition of the last job

• Average number of jobs:total flow time / makespan

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