2. Manufacturing Operations Scheduling
Chapter 16
Manufacturing Operations Scheduling
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3. Overview Scheduling Process-Focused Manufacturing
Scheduling Product-Focused Manufacturing
Computerized Scheduling Systems
Wrap-Up: What World-Class Companies Do

4. Scheduling
Process-Focused
Manufacturing

5. Process-Focused Manufacturing
Process-focused factories are often called job shops.
A job shop’s work centers are organized around similar types of equipment or operations.
Workers and machines are flexible and can be assigned to and reassigned to many different orders.
Job shops are complex to schedule.

6. Scheduling and Shop-Floor Decisions
Master
Production
Schedule (MPS)
Product Design
and
Process Plans
Material
Requirements
Plan (MRP)
Capacity
Requirements
Plan (CRP)
Order-
Processing or
Routing Plans
Planned
Order Releases
Report
Work Center
Loading and
Overtime Plan
Assignment of
Orders to
Work Centers
Day-to-Day Scheduling and Shop-Floor Decisions

7. Pre-production Planning
Design the product in customer order
Plan the operations the product must pass through this is the routing plan
Work moves between operations on a move ticket

8. Common Shop Floor Control Activities
The production control department controls and monitors order progress through the shop.
Assigns priority to orders
Issues dispatching lists
Tracks WIP and keeps systems updated
Controls input-output between work centers
Measures efficiency, utilization, and productivity of shop

9. Shop Floor Planning and Control
Input-Output Control
Gantt Chart
Finite and Infinite Loading
Forward and Backward Scheduling

10. Input-Output Control
Input-output control identifies problems such as insufficient or excessive capacity or any issues that prevents the order from being completed on time.
Input-output control report compares planned and actual input, planned and actual output, and planned and actual WIP in each time period

11. Input-Output Control Report
Week:
Planned input: labor-hrs
Actual input: labor-hrs
Cumulative deviation
Planned output: labor-hrs
Actual output: labor-hrs
Cumulative deviation
Planned ending WIP: l-h
Actual ending WIP: l-h

12. Gantt Charts
Gantt charts are useful tools to coordinate jobs through shop; graphical summary of job status and loading of operations

13. Gantt Charts Work Centers Mon. Tue. Wed. Thu. Fri. Sat. Machining
Fabrication
Assembly
Test E F G C D E F H C D E H C D
Scheduled
Progress
Setup, Maint.

14. Assigning Jobs to Work Centers: How Many Jobs/Day/Work Center
Infinite loading
Assigns jobs to work centers without regard to capacity
Unless excessive capacity exists, long queues occur
Finite loading
Uses work center capacity to schedule orders
Popular scheduling approach
Integral part of CRP

15. Assigning Jobs to Work Centers: Which Job Gets Built First?
Forward scheduling
Jobs are given earliest available time slot in operation
excessive WIP usually results
Backward scheduling
Start with promise date and work backward through operations reviewing lead times to determine when a job has to pass through each operation
Less WIP but must have accurate lead times

16. Order-Sequencing Problems
Sequencing Rules
Criteria for Evaluating Sequencing Rules
Comparison of Sequencing Rules
Controlling Changeover Costs
Minimizing Total Production Time

17. Order-Sequencing Problems
We want to determine the sequence in which we will process a group of waiting orders at a work center.
Many different sequencing rules can be followed in setting the priorities among orders.
There are numerous criteria for evaluating the effectiveness of the sequencing rules.

18. Order-Sequencing Rules
First-Come First-Served (FCFS)
Next job to process is the one that arrived first among the waiting jobs
Shortest Processing Time (SPT)
Next job to process is the one with the shortest processing time among the waiting jobs
Earliest Due Date (EDD)
Next job to process is the one with the earliest due (promised finished) date among the waiting jobs

19. Order-Sequencing Rules
Least Slack (LS)
Next job to process is the one with the least [time to due date minus total remaining processing time] among the waiting jobs
Critical Ratio (CR)
Next job to process is the one with the least [time to due date divided by total remaining processing time] among the waiting jobs
Least Changeover Cost (LCC)
Sequence the waiting jobs such that total machine changeover cost is minimized

20. Evaluating the Effectiveness of Sequencing Rules
Average flow time - average amount of time jobs spend in shop
Average number of jobs in system -
Average job lateness - average amount of time job’s completion date exceeds its promised delivery date
Changeover cost - total cost of making machine changeovers for group of jobs

21. Experience Says: First-come-first-served
Performs poorly on most evaluation criteria
Does give customers a sense of fair play
Shortest processing time
Performs well on most evaluation criteria
But have to watch out for long-processing-time orders getting continuously pushed back
Critical ratio
Works well on average job lateness criterion
May focus too much on jobs that cannot be completed on time, causing others to be late too.

22. Example: Sequencing Rules
Use the FCFS, SPT, and Critical Ratio rules to sequence the five jobs below. Evaluate the rules on the bases of average flow time, average number of jobs in the system, and average job lateness.
Job Processing Time Time to Promised Completion
A hours 10 hours B C D E

23. Example: Sequencing Rules
FCFS Rule A > B > C > D > E
Processing Promised Flow
Job Time Completion Time Lateness A B C D E

24. Example: Sequencing Rules
FCFS Rule Performance
Average flow time:
141/5 = 28.2 hours
Average number of jobs in the system:
141/49 = 2.88 jobs
Average job lateness:
90/5 = 18.0 hours

25. Example: Sequencing Rules
SPT Rule A > E > C > B > D
Processing Promised Flow
Job Time Completion Time Lateness A B C D E

26. Example: Sequencing Rules
SPT Rule Performance
Average flow time:
127/5 = 25.4 hours
Average number of jobs in the system:
127/49 = 2.59 jobs
Average job lateness:
76/5 = 15.2 hours

27. Example: Sequencing Rules
Critical Ratio Rule E > C > D > B > A
Processing Promised Flow
Job Time Completion Time Lateness
E (.875)
C (.889)
D (1.00)
B (1.33)
A (1.67)

28. Example: Sequencing Rules
Critical Ratio Rule Performance
Average flow time:
148/5 = 29.6 hours
Average number of jobs in the system:
148/49 = 3.02 jobs
Average job lateness:
93/5 = 18.6 hours

29. Example: Sequencing Rules
Comparison of Rule Performance
Average Average Average
Flow Number of Jobs Job
Rule Time in System Lateness
FCFS
SPT CR
SPT rule was superior for all 3 performance criteria.

30. Controlling Changeover Costs
Changeover costs - costs of changing a processing step in a production system over from one job to another
Changing machine settings
Getting job instructions
Changing material
Changing tools
Usually, jobs should be processed in a sequence that minimizes changeover costs

31. Controlling Changeover Costs
Job Sequencing Heuristic
First, select the lowest changeover cost among all changeovers (this establishes the first two jobs in the sequence)
The next job to be selected will have the lowest changeover cost among the remaining jobs that follow the previously selected job

32. Example: Minimizing Changeover Costs
Hardtimes Heat Treating Service has 5 jobs waiting to be processed at work center #11. The job-to-job changeover costs are listed below. What should the job sequence be?
Jobs That Precede
A B C D E A B C D E
Jobs
That
Follow

33. Example: Minimizing Changeover Costs
Develop a job sequence:
A follows D ($50 is the least c.o. cost)
C follows A ($92 is the least following c.o. cost)
B follows C ($69 is the least following c.o. cost)
E follows B (E is the only remaining job)
Job sequence is D – A – C – B – E
Total changeover cost = $ = $286

34. Minimizing Total Production Time
Sequencing n Jobs through Two Work Centers
When several jobs must be sequenced through two work centers, we may want to select a sequence that must hold for both work centers
Johnson’s rule can be used to find the sequence that minimizes the total production time through both work centers

35. Johnson’s Rule
1. Select the shortest processing time in either work center
2. If the shortest time is at the first work center, put the job in the first unassigned slot in the schedule. If the shortest time is at the second work center, put the job in the last unassigned slot in the schedule.
3. Eliminate the job assigned in step 2.
4. Repeat steps 1-3, filling the schedule from the front and back, until all jobs have been assigned a slot.

36. Example: Minimizing Total Production Time
It is early Saturday morning and The Finest Detail has five automobiles waiting for detailing service. Each vehicle goes through a thorough exterior wash/wax process and then an interior vacuum/shampoo/polish process.
The entire detailing crew must stay until the last vehicle is completed. If the five vehicles are sequenced so that the total processing time is minimized, when can the crew go home. They will start the first vehicle at 7:30 a.m.
Time estimates are shown on the next slide.

37. Example: Minimizing Total Production Time
Exterior Interior
Job Time (hrs.) Time (hrs.)
Cadillac
Bentley
Lexus
Porsche
Infiniti

38. Example: Minimizing Total Production Time
Johnson’s Rule
Least Work Schedule
Time Job Center Slot
Infiniti Interior 5th
1.6 Porsche Interior 4th
1.9 Lexus Exterior 1st
2.0 Cadillac Exterior 2nd
2.1 Bentley Exterior 3rd

39. Example: Minimizing Total Production Time
Exterior
Interior L C B P I
Idle
Idle L C B P I
It will take from 7:30 a.m. until 7:30 p.m. (not
allowing for breaks) to complete the five vehicles.

40. Scheduling
Product-Focused
Manufacturing

41. Product-Focused Scheduling
Two general types of product-focused production:
Batch - large batches of several standardized products produced
Continuous - few products produced continuously.... minimal changeovers

42. Scheduling Decisions
If products are produced in batches on the same production lines:
How large should production lot size be for each product?
When should machine changeovers be scheduled?
If products are produced to a delivery schedule:
At any point in time, how many products should have passed each operation if time deliveries are to be on schedule?

43. Batch Scheduling EOQ for Production Lot Size
How many units of a single product should be included in each production lot to minimize annual inventory carrying cost and annual machine changeover cost?

44. Example: EOQ for Production Lots
CPC, Inc. produces four standard electronic assemblies on a produce-to-stock basis. The annual demand, setup cost, carrying cost, demand rate, and production rate for each assembly are shown on the next slide.
a) What is the economic production lot size for each assembly?
b) What percentage of the production lot of power units is being used during its production run?
c) For the power unit, how much time will pass between production setups?

45. Example: EOQ for Production Lots
Annual Setup Carry Demand Prod.
Demand Cost Cost Rate Rate
Power Unit 5,000 $1,200 $
Converter 10,
Equalizer 12, ,
Transformer 6,

46. Example: EOQ for Production Lots
Economic Production Lot Sizes

47. Example: EOQ for Production Lots
% of Power Units Used During Production
d/p = 20/200 = or 10%
Time Between Setups for Power Units
EOQ/d = 1,490.7/20 = days

48. Batch Scheduling Limitations of EOQ Production Lot Size
Uses annual “ballpark” estimates of demand and production rates, not the most current estimates
Not a comprehensive scheduling technique – only considers a single product at a time
Multiple products usually share the same scarce production capacity

49. Batch Scheduling Run-Out Method
Attempts to use the total production capacity available to produce just enough of each product so that if all production stops, inventory of each product runs out at the same time

50. Example: Run-Out Method
QuadCycle, Inc. assembles, in batches, four bicycle models on the same assembly line. The production manager must develop an assembly schedule for March.
There are 1,000 hours available per month for bicycle assembly work. Using the run-out method and the pertinent data shown on the next slide, develop an assembly schedule for March.

51. Example: Run-Out Method
Assembly March April
Inventory Time Forec. Forec. On-Hand Required Demand Demand
Bicycle (Units) (Hr/Unit) (Units) (Units)
Razer
Splicer
Tracker ,500 1,500
HiLander

52. Example: Run-Out Method
Convert inventory and forecast into assembly hours
Assemb. March March
Invent. Time Forec. Invent. Forec.
On-Hand Req’d. Dem. On-Hand Dem.
Bicycle (Units) (Hr/Unit) (Units) (Hours) (Hours)
Razer
Splicer
Tracker ,
HiLander
Total 470 1,250
(1) (2) (3) (4) (5)
(1) x (2)
(2) x (4)

53. Example: Run-Out Method
Compute aggregate run-out time in months
Aggregate Run-out Time =
= [(Total Inventory On-Hand in Hours)
+ (Total Assembly Hours Available per Month)
- (March’s Forecasted Demand in Hours)]
/ (April’s Forecasted Demand in Hours)
= ( , ,250)/1,250 = months

54. Example: Run-Out Method
Develop March’s Production Schedule
March’s March’s
Desired Desired Assembly
Ending End.Inv. Required Time Inventory & Forec. Production Allocated
Bicycle (Units) (Units) (Units) (Hours)
Razer
Splicer 158 1,
Tracker 264 1,764 1,
HiLander
999.8
(6) (7) (8) (9)
(3) x .176
(3) + (6)
(7) - (1)
(8) x (2)

55. Computerized Scheduling
Develops detailed schedules for each work center indicating starting and ending times
Develops departmental schedules
Generates modified schedules as orders move
Many packages available.... select one most appropriate for your business

56. Wrap-Up: World-Class Practice
In process-focused factories:
MRP II refined.... promises are met, shop loading is near optimal, costs are low, quality is high
In product-focused factories:
EOQ for standard parts containers, this sets S, lot sizes are lower, inventories slashed, customer service improved
Scheduling is integral part of a computer information system

57. End of Chapter 16