Post on 20-Jan-2018
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Inventory Inventory Management: Management:
Economic Order Economic Order Quantity, JIT, and Quantity, JIT, and
the Theory of the Theory of ConstraintsConstraints
Prepared by Douglas Cloud
Pepperdine University
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1. Describe the just-in-case inventory management model.
2. Discuss just-in-time (JIT) inventory management.
3. Explain the basic concepts of constrained optimization.
4. Define the theory of constraints, and tell how it can be used to manage inventory.
ObjectivesObjectives
After studying this After studying this chapter, you should chapter, you should
be able to:be able to:
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Just-in-Case Inventory Just-in-Case Inventory ManagementManagement
Three types of inventory costs can be readily identified with inventory:
(1) The cost of acquiring inventory.
(2) The cost of holding inventory.
(3) The cost of not having inventory on hand when needed.
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1. Ordering Costs: The costs of placing and receiving an order.
Examples: Clerical costs, documents, insurance for shipment, and unloading.
2. Carrying Costs: The costs of carrying inventory.Examples: Insurance, inventory taxes, obsolescence,
opportunity cost of capital tied up in inventory, and storage.
Economic Order QuantityEconomic Order Quantity
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3. Stock-Out Costs: The costs of not having sufficient inventory.
Examples: Lost sales, costs of expediting (extra setup, transportation, etc.) and the costs of interrupted production.
4. Setup Costs: The costs of preparing equipment and facilities so they can be used to produce a particular product or component.
Examples: Setup labor, lost income (from idled facilities), and test runs.
Economic Order QuantityEconomic Order Quantity
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1. To balance ordering or setup costs and carrying costs2. Demand uncertainty3. Machine failure4. Defective parts5. Unavailable parts6. Late delivery of parts7. Unreliable production processes8. To take advantage of discounts9. To hedge against future price increases
Traditional Reasons for Traditional Reasons for Carrying InventoryCarrying Inventory
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Economic Order QuantityEconomic Order Quantity
TC = PD/Q + CQ/2
The total ordering (or setup) and carrying cost
The cost of placing and receiving an
order (or the cost of setting up a
production run)
The known annual demand
The number of units ordered each time an order is placed (or the lot size for
production)
The cost of carrying one unit of stock for
one year
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An EOQ IllustrationAn EOQ Illustration
EOQ = 2PD/C
D = 25,000 units
Q = 500 units
P = $40 per order
C = $2 per unit
EOQ = (2 x 25,000 x $40) / $2
EOQ = 1,000,000
EOQ = 1000 units
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Reorder point = Rate of usage x Lead time
Example: Assume that the average rate of usage is 4 units per day for a component. Assume also that the time required to place and receive an order is 10 days. What is the reorder point?
Reorder point = 4 x 10 = 40 units
Thus, an order should be placed when inventory drops to 40 units.
When to Order or ProduceWhen to Order or Produce
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The Reorder PointThe Reorder Point
Reorder point = Rate of usage x Lead time
1000
800
600
400
200
0
2 4 6 8 10 12 14 16 18 20
Reorder Point
(EOQ)
Days
Inventory (units)
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Demand Uncertainty and ReorderingDemand Uncertainty and ReorderingTo avoid running out of parts, organizations often choose to carry safety stock. Safety stock is extra inventory carried to serve as insurance against fluctuations in demand.
Example: If the maximum usage of the VCR part is 120 units per day, the average usage is 100 units per day, and the lead time is four days, the safety stock is 80.
Maximum usage 120Average usage -100Difference 20Lead time x 4Safety stock 80
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Example Involving SetupsExample Involving Setups
Expedition Company manufactures edgers. The manager is trying to determine the size of the
production run. The controller has supplied the following information:
Average demand for edgers: 720 per dayMaximum demand for edgers: 780 per dayAnnual demand for edgers: 180,000Unit carrying cost: $4Setup cost: $10,000Lead time: 22 days
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Example Involving SetupsExample Involving Setups
EOQ = 2PD/C
EOQ = (2 x 180,000 x $10,000)/$4
EOQ = 900,000,000
EOQ = 30,000 edgers
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Example Involving SetupsExample Involving SetupsSafety stock:
Maximum usage 780Average usage 720Difference 60Lead time x 22Safety stock 1,320
Reorder point = (Average usage x Lead time) + Safety stockReorder point = (720 x 22) + 1,320Reorder point = 17,160 edgers
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JIT reduces the costs of acquiring inventory to insignificant levels by:
1. Drastically reducing setup time
2. Using long-term contracts for outside purchases
Carrying costs are reduced to insignificant levels by reducing inventories to insignificant levels.
Setup and Carrying Costs: The JIT Approach
JIT Inventory ManagementJIT Inventory Management
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JIT Inventory ManagementJIT Inventory Management
Due-Date Performance: The JIT SolutionLead times are reduced so that the company can meet requested delivery dates and to respond quickly to customer demand.
Lead times are reduced by: reducing setup times
improving quality
using cellular manufacturing
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JIT Inventory ManagementJIT Inventory Management
Avoidance of Shutdown: The JIT Approach Total preventive maintenance to reduce
machine failures
Total quality control to reduce defective parts
The use of the Kanban system is also essential
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What is the Kanban System?What is the Kanban System?
A card system is used to monitor work in process A withdrawal Kanban
A production Kanban
A vendor KanbanThe Kanban system The Kanban system
is responsible for is responsible for ensuring that the ensuring that the
necessary products necessary products are produced in the are produced in the necessary quantities necessary quantities
at the necessary time.at the necessary time.
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Withdrawal Kanban
Item no.__________________________ Preceding Process
Item name_____________________________________
Computer Type_________________________________
Box Capacity_____________________Subsequent Process
Box Type_______________________________________________
15670T07
Circuit Board
TR6547 PC
CB Assembly
8
C Final Assembly
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Production Kanban
Item no.__________________________ Preceding Process
Item name_____________________________________
Computer Type___________________
Box Capacity_____________________Box Type______________________________
15670T07
Circuit Board
TR6547 PC
CB Assembly
8
C
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Vendor Kanban
Item no.____________________Name of Receiving Company
Item name_____________________________________
Box Capacity___________________
Box Type___________________ _ _Time to Deliver__________________________________________
Name of Supplier_________________________________________
15670T07
Circuit Board
8
Electro PC
A8:30 A.M., 12:30 P.M., 2:30 P.M.
Receiving Gate
75
Gerry Supply
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The Kanban Process
WithdrawalWithdrawalStoreStore
CB CB StoresStores
Lot with P-Kanban
ProductionOrdering Post
(6) Signal
CB Assembly
Remove(4) P-KanbanAttach toPost
(5) AttachW-Kanban
(1) Remove W-Kanban Attach to Post
Withdrawal Post
(2), (3)
(7)
Final Assembly
(1)
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Discounts and Price Increases: JIT Purchasing Versus Holding Inventories Careful vendor selection Long-term contracts with vendors
Prices are stipulated (usually producing a significant savings)
Quality is stipulated The number of orders placed are reduced
JIT Inventory ManagementJIT Inventory Management
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JIT LimitationsJIT Limitations• Patience in implications
is needed.
• Time is required.
• JIT may cause lost sales and stressed workers.
• Production may be interrupted due to an absence of inventory.
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Linear ProgrammingLinear Programming
The unit contribution margins are $300 and $600 for X and Y, respectively.
Z = $300X + $600 Y
Total contribution margin
This equation is called the objective function, the function to be optimized.
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Linear ProgrammingLinear Programming
Internal constraints:X + Y 80X + 3Y
1202X + Y 90
External constraints:X 60
Y 100
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Linear ProgrammingLinear Programming
X + Y 80X + 3Y 1202X + Y 90
X 60Y 100X 0Y 0
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Multiple Constrained ResourceMultiple Constrained Resource
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20 40 60 80 100 120 140
Y 100
X 60
X + Y 80
X + 3Y 120
2X + Y 90
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Multiple Constrained ResourceMultiple Constrained Resource
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20 40 60 80 100 120 140
A
BC
D
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Linear ProgrammingLinear Programming
Corner Point X-Value Y-Value Z = $300X + $600Y
A 0 0 $ 0B 0 40 24,000C 30 30 27,000D 45 0 13,500
C is the optical solution!
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Throughput Inventory Operating expenses
Three Measures of Systems Performance:
Theory of Constraints
(Sales revenue – Unit-level variable expenses)/Time
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1. Identify an organization’s constraints.
2. Exploit the binding constraints.
3. Subordinate everything else to the decisions made in Step 2.
4. Elevate the organization’s binding constraints.
5. Repeat the process as a new constraint emerges to limit output.
Five-Step Method for Improving Performance
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The Drum-Buffer-Rope SystemThe Drum-Buffer-Rope System
Materials
Process AProcess A
Process CProcess C
Rope
Drummer ProcessDrummer Process
Time Buffer
Final ProcessFinal Process
Finished Goods
Initial ProcessInitial Process
Process BProcess B
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New Constraint Set: Schaller Co.New Constraint Set: Schaller Co.
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20 40 60 80 100 120 140 160 200 220 240
Y 100
X 60
X + Y 80
2X + Y 90
X + 3Y 240
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New Constraint Set: Schaller Co.New Constraint Set: Schaller Co.
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20 40 60 80 100 120 140 160 200 220 240
A
BC
D
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End ofEnd of
ChapterChapter
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