operations scheduling. scheduling in a process-focused environment
TRANSCRIPT
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