Operations Scheduling

Scheduling in aProcess-Focused Environment

3MTSU

Terminology Job shop - a functional organization whose departments or

work centers are organized around particular types of operations

Order - quantity of a unique part number Routing - network of processes or work centers through

which the order must pass before it is completed Schedule - allocate resources to be used to complete each

job, when, and for what period of time. Often performed in two phases - loading of jobs and sequencing of jobs

4MTSU

Possible Objectives ofJob-Shop Scheduling

Meet promise or due dates Minimize lead time or make span Minimize setup time or cost Minimize work-in-process inventory Maximize resource utilization

5MTSU

Elements of the Job-Shop Scheduling Problem

Job arrival patterns Number and variety of machines in the shop Ratio of workers to machines in the shop Flow pattern of jobs through the shop Priority rules or methods for allocating jobs

to workers or machines Schedule evaluation criteria

6MTSU

Shop-Floor Control Activities Assigning a priority to each order Issuing dispatching lists to each work center

– Orders to be produced– Order priorities– Order due date

Keeping the work-in-process inventory updated Providing input-output control Measuring the efficiency, utilization and productivity

7MTSU

Scheduling andShop-Floor DecisionsMPS

MRP CRP

Plannedorder release

report

Work centerloading and

overtime plan

Routing plans

Assignment oforders to

work centers

Design andprocessing plans

1. Set priorities of orders2. Assign resources to orders3. Shop-floor control-dispatching

8MTSU

Some Approaches to Developing Job Shop Schedules

Priority rules Forward scheduling Backward scheduling Finite and infinite scheduling

9MTSU

Examples ofSequencing (Priority) Rules

First-come, first served (FCFS)– sequence is based on the order jobs arrive

Shortest processing time (SPT)– job with the shortest processing time among waiting

jobs is next Earliest due date (EDD)

– job with the earliest due date among waiting time is next

more

10MTSU

Rules (continued) Start date

– Due date minus normal lead time Slack time remaining (STR)

– due date minus processing time remaining Slack time per operation Critical ratio (CR)

– job with the least critical ratio (time to due date divided by total remaining production time) among the waiting jobs is next

11MTSU

Forward Scheduling

Jobs are assigned to the earliest unassigned time slots in work centers

Excessive in-process inventories usually result because jobs tend to sit and wait for their next work-center assignments

12MTSU

Backward Scheduling

Starting with an order’s promised delivery date, schedulers work backward through each stage of production using lead times to determine when the order should pass through each stage

An order is assigned to the latest possible time slot in work centers to allow the promised delivery date to be met

13MTSU

Finite and Infinite Loading

Infinite loading approach– jobs are assigned to work centers without

considering the capacities of the work centers Finite loading approach

– capacities of work centers are allocated among a list of jobs

– no more work is scheduled to a work center during any hour than the capacity of the work center

14MTSU

Possible Criteria forEvaluating a Sequence

Average flow time– average amount of time jobs spend in the

system Average number of jobs in the system Average job lateness

– average amount of time that a job’s completion date exceeds its promised delivery date

Shop-Floor Control

16MTSU

Gantt Charts

A plan of resource commitments by each work center to orders is plotted on the chart

Actual performance is then plotted on the same chart to identify deviations from the plan

This type of chart assists the manger in coordinating the orderly flow of jobs between work centers

17MTSU

Input-Output Control

The amount of work flowing to a work center is compared to the amount flowing out

Identifies such problems as insufficient capacity, excessive capacity, and production difficulties

For example if too much is flowing into the work center compared to its capacity, then excessive WIP inventory precedes the work center

18MTSU

Minimizing Total ProductionTime For a Special Case

Sequencing n jobs through two work centers1 Select the shortest processing time in either

work center2 If this time is on the first work center, place that

job in the first open slot3 If this time is on the last work center, place that

job in the last open slot4 Eliminate the job assigned and go back to Step 1

Synchronous Manufacturing

20MTSU

Synchronous Manufacturing and the Theory of Constraints

The Goal of the firm

The hockey-stick phenomenon

Performance measurement

Capacity and flow issues

21MTSU

The Hockey Stick Phenomenon

The end-of-period rush!

Period1 2 3 4

Output($)

22MTSU

Goldratt Proclaims

The goal of a firm is to make money.

23MTSU

Performance MeasurementFinancial

Net profit– an absolute measurement in dollars

Return on investment– a relative measure based on investment

Cash flow– a survival measurement

24MTSU

Performance MeasurementOperational

1. Throughput– the rate at which money is generated by the system

through sales 2. Inventory

– all the money that the system has invested in purchasing things it intends to sell

3. Operating expenses– all the money that the system spends to turn

inventory into throughput

25MTSU

Productivity Does not guarantee profitability

– Has throughput increased?

– Has inventory decreased?

– Have operational expenses decreased?

PRODUCTIVITY IS ALL THE ACTIONS THAT BRING A COMPANY CLOSER TO ITS GOALS

26MTSU

Unbalanced Capacity Earlier, we discussed balancing assembly lines.

– The goal was constant cycle time across all stations

Synchronous manufacturing views constant workstation capacity as a bad decision.

Why is this the case?

27MTSU

The Statistics of Dependent Events

Rather than balancing capacities, the flow of product through the system should be balanced

Process Time (B)Process Time (A)

28MTSU

Some Capacity Related Terminology

Capacity– Available time for production

Bottleneck– Capacity is less than demand placed on resource

Nonbottleneck– Capacity is greater than demand placed on resource

Capacity-constrained resource (CCR)– Capacity is close to demand placed on resource

29MTSU

What’s Going to Happen?

X Y Market

Case A

X YBottleneck Nonbottleneck

Demand/month 200 units 200 unitsProcess time/unit 1 hour 45 minsAvail. time/month 200 hours 200 hours

30MTSU

What’s Going to Happen?

Y X Market

Case B

X YBottleneck Nonbottleneck

Demand/month 200 units 200 unitsProcess time/unit 1 hour 45 minsAvail. time/month 200 hours 200 hours

Y X Market

Case B

31MTSU

What’s Going to Happen?

X Y

Assembly

MarketCase C

X YBottleneck Nonbottleneck

Demand/month 200 units 200 unitsProcess time/unit 1 hour 45 minsAvail. time/month 200 hours 200 hours

32MTSU

What’s Going to Happen?

X Y

Market Market

Case D

X YBottleneck Nonbottleneck

Demand/month 200 units 200 unitsProcess time/unit 1 hour 45 minsAvail. time/month 200 hours 200 hours

33MTSU

Saving Time

Bottleneck Nonbottleneck

What are the consequences of saving time at each process?

34MTSU

Drum, Buffer, Rope

A B C D E F

Bottleneck (drum)

Inventorybuffer

(time buffer)Communication

(rope)

Market

35MTSU

Quality Implications More tolerant than JIT systems

– Excess capacity throughout system

Except for the bottleneck– Quality control needed before bottleneck

36MTSU

Inventory Cost MeasurementDollar Days

A measurement of the value of inventory and the time it stays within an area

Dollar Days = (value of inventory)(number of days within a department)

Example

37MTSU

Benefits from Dollar Day Measurement

Marketing– discourages holding large amounts of finished goods

inventory Purchasing

– discourages placing large purchase orders that on the surface appear to take advantage of quantity discounts

Manufacturing– discourage large work in process and producing

earlier than needed

38MTSU

Comparing Synchronous Manufacturing to MRP

MRP uses backward scheduling

Synchronous manufacturing uses forward scheduling

39MTSU

Comparing Synchronous Manufacturing to JIT

JIT is limited to repetitive manufacturing

JIT requires a stable production level

JIT does not allow very much flexibility in the products produced

40MTSU

Comparing Synchronous Manufacturing to JIT

JIT still requires work in process when used with kanban so that there is "something to pull."

Vendors need to be located nearby because the system depends on smaller, more frequent deliveries

41MTSU