Activity Scheduling and Activity Scheduling and ControlControl

Activity Scheduling & ControlActivity Scheduling & Control

• Input: MRP Manufacturing Orders (MO’s) Subject to Capacity Constraints, Not Purchase Orders (PO’s)

• Plans Hit the Real World (the Shop Floor)

Activity Scheduling & ControlActivity Scheduling & Control• Planning

- Ensure Resources Available- Schedule Order Start & Completion Dates

• Implementation- Gather Information Needed to Make Item- Dispatch (Release) Orders to Floor

• Control (Feedback)- Rank Orders in Priority Sequence- Track Actual Performance vs. Plans- Monitor WIP, Queues, Lead Time, Etc.- Report Out – Input/Output Reports (p. 175)

Data Requirements - PlanningData Requirements - Planning• Item Master File:

Part Numbers, Description, Lead Time, On Hand, On Order, Quantity Assigned to Work Center, Lot Sizes

• Product Structure or Bill of Materials File

• Routing File:Series of Operations Along with Resources Needed, Set Up, Run Times, Lead Times

•Work Center Master FileWork Center Number, Capacity, Shifts, Machine and Labor Hours Available, Efficiency, Utilization, Queue Time, Alternate Centers

Data Requirements - ControlData Requirements - Control

• Shop Order Master File (Each Order):Shop Order Number, Order Quantity, Quantity Completed, Quantity Scrapped, Material Issued, Due Dates, Priority, Balance Due, Costs

• Shop Order Detail File (Each Operation):Operation Number, Setup Hours, Run Hours, Quantity Completed, Quantity Scrapped, Due Date or Lead Time Remaining

Types of Manufacturing/Service Types of Manufacturing/Service SystemsSystems

• Intermittent (Job Shops)- Traditional- Work Cells

• Flow (Assembly Lines)

• Projects (SOM 466)

Job ShopsJob Shops• Different Items Can be Produced at the

Same Job Center – Requires Worker and Machine Flexibility

• Design and Fabrication of Item Determines Job Shop Routing

• Capacity Required at Each Work Center Depends on Mix of Products. Hard to Predict

• Queues (Inventory) Results When Schedule Assigns too Many Jobs to Work Center – Throughput Times Generally Long

Job Shop ExampleJob Shop Example

Work CellsWork Cells• Intermediate to Job Shops & Assembly

Lines

• Group of Machines to Make Set of Parts

• Minimizes Distances, Related Handling Costs

• Minimizes Inventories

Work Cell ExampleWork Cell Example

Work Cell ExampleWork Cell Example

Scheduling in Job ShopsScheduling in Job Shops and Work Cells and Work Cells

• Forward Scheduling – Left to Right – Earliest Dates

• Backwards Scheduling – Right to Left – Latest Dates

• Infinite Loading – No Capacity Limits

• Finite Loading – Capacity Limits

• Operations Overlapping

• Operations Splitting

Scheduling in Job Shops and Work Scheduling in Job Shops and Work Cells: Shop Floor ControlCells: Shop Floor Control

Job Routing

Red A(1), B (1), C (1)Green C (1), A (2)Purple B (1), A (1)

Work Center 1 PM 2 PM 3 PM 4 PMA        

B        

C        

PR

RRG

G G P GanttChart

Bottlenecks in Job ShopsBottlenecks in Job Shops and Work Cells and Work Cells

• Establish Inventory Buffer Before Bottleneck

• Control Rate of Material Feed at Gateway Operation

• Try to Increase Capacity of Bottleneck

• Adjust Loads – Alternate Work Centers or Subcontract

• Last Resort – Change Schedule

• Use Goldratt’s “Theory of Constraints” (pp. 170-172)

Sequencing Rules in Job Shops Sequencing Rules in Job Shops and Work Cellsand Work Cells

• First – Come, First – Served (FCFS)

• Earliest Due Date (EDD)

• Earliest Operation Due Date (ODD)

• Shortest Process Time (SPT)

• Critical Ratio (CR) or Slack Rule

CR = (Due Date – Present Date) / Lead Time Remaining

Lowest CR Has Priority. CR < 1: Behind Schedule

Flow ManufacturingFlow ManufacturingAssembly LinesAssembly Lines

• Allows Mass Production

• Repetitive Production of Standardized Products

• Standard Flow Minimizes Inventories

• Routings are Fixed. Work Centers Depend on Routing.

• Capacity Fixed by the Assembly Line

Assembly Line ExampleAssembly Line Example

Scheduling Assembly Lines:Scheduling Assembly Lines:Assembly Line BalanceAssembly Line Balance

Definitions:

Task or Operation – Smallest Definable Unit of Work (Circle)

Station – Work Area (Square)

Cycle Time – Time Allowed to Do a Cycle of Work 1/(Production Rate)

Assembly Line Balance:Assembly Line Balance:Assign Tasks to StationsAssign Tasks to Stations

Example: Problem 3.6Example: Problem 3.6Task Follows Time (Sec.) A --- 25 B A 25 C A 20 D A 10 E B,C,D 15 F B,C,D 20 G E,F 10

Cycle Time = 35 Seconds

Assembly Line BalanceAssembly Line Balance

Theoretical Minimum Stations = Sum of Task Times / Cycle Time = 125 / 35 ≈ 4

Idle Time = (Stations X Cycle Time) - Sum of Task Times = (4 X 35) – 125 = 15

Assembly Line Balance:Assembly Line Balance:Assign Tasks to StationsAssign Tasks to Stations

A D B C E F G

Idle: 0 + 10 + 0 + 5 = 15

Assembly Line Balance Assembly Line Balance NightmareNightmare

Task Time(s) Cycle Time

Solutions:

- Divide Task

- Decrease Task time

- Increase Cycle Time

- Parallel Stations