INGENIERIA DE ABASTECIMIENTO

Gestin de compras y Proveedores

Agenda de LA SESINAnlisis ABC.Aplicacin.Criterios.Usos.Anlisis Lote ptimo de Compra.Caso de Estudio Conclusiones

Because the customer usually isnt sitting at the plant exit!

Queen Elizabeth research station in AntarcticaWhy do we have inventories?Operations and Supply Processes

DETERMINISTIC EOQINVENTORY MODELSInventory CostsHolding (or carrying) costsCosts for storage, handling, insurance, and so onSetup (or production change) costsCosts for arranging specific equipment setups, and so onOrdering costsCosts of placing an orderShortage costsCosts of running out5Independent Versus Dependent DemandIndependent demand: the demands for various items are unrelated to each otherFor example, a workstation may produce many parts that are unrelated but meet some external demand requirementDependent demand: the need for any one item is a direct result of the need for some other itemUsually a higher-level item of which it is partBasic EOQ ModelDemand for the product is constant and uniform throughout the periodLead time (time from ordering to receipt) is constantPrice per unit of product is constant Inventory holding cost is based on average inventoryOrdering or setup costs are constantAll demands for the product will be satisfiedEOQ AssumptionsKnown & constant demandKnown & constant lead timeInstantaneous receipt of materialNo quantity discountsOnly order (setup) cost & holding costNo stockoutsInventory Holding CostsHousing (building) cost6%Material handling costs3%Labor cost3%Inventory investment costs11%Pilferage, scrap, & obsolescence3%Total holding cost26%% of Category Inventory ValueEOQ Model Order QuantityAnnual CostOrder QuantityAnnual CostHolding CostEOQ Model Why Order Cost DecreasesCost is spread over more unitsExample: You need 1000 microwave ovens Purchase OrderDescriptionQty.Microwave1000Purchase OrderDescriptionQty.Microwave1Purchase OrderDescriptionQty.Microwave1Purchase OrderDescriptionQty.Microwave1Purchase OrderDescriptionQty.Microwave11 Order (Postage $ 0.35)1000 Orders (Postage $350)Order quantityOrder QuantityAnnual CostHolding CostOrder (Setup) CostEOQ Model Order QuantityAnnual CostHolding CostTotal Cost CurveOrder (Setup) CostEOQ Model Order QuantityAnnual CostHolding CostTotal Cost CurveOrder (Setup) CostOptimal Order Quantity (Q*)EOQ Model How much to orderEconomic order quantity is the amount that balances the cost of ordering with the cost of maintaining average inventoryAssumes demand and costs are relatively stable for the yearDoes not consider impact of joint ordering of multiple products

Annual Product Costs, Based on Size of the Order

D = Annual demand (units)S =Cost per order ($) C = Cost per unit ($) I =Holding cost (%)H =Holding cost ($) = I x C

Economic Order QuantityEOQ Model Equations

D = Demand per yearS = Setup (order) cost per orderH = Holding (carrying) cost d = Demand per dayL = Lead time in daysEOQ ExampleYoure a buyer for SaveMart. SaveMart needs 1000 coffee makers per year. The cost of each coffee maker is $78. Ordering cost is $100 per order. Carrying cost is 40% of per unit cost. Lead time is 5 days. SaveMart is open 365 days/yr.

What is the optimal order quantity & ROP? SaveMart EOQ

D = 1000S = $100C = $ 78 I = 40%H = C x IH = $31.20EOQ = 80 coffeemakersROP = demand over lead time = daily demand x lead time (days) = d x l

D = annual demand = 1000Days / year = 365Daily demand = 1000 / 365 = 2.74Lead time = 5 days

ROP = 2.74 x 5 = 13.7 => 14 SaveMart ROP Interest rates go up ?Order processing is automated ?Warehouse costs drop ?Competitive product is introduced ?Product is cost-reduced ?Lead time gets longer ?Minimum order quantity imposed ?

What if Basic FixedOrder Quantity Model

Place OrderLead timeReceive orderUse inventoryEjemplo: Una ferretera vende 20.000 taladros al ao. El costo anual de mantenimiento de existencias es de $5. El costo de hacer el pedido y recibir cada despacho es de $500. Por tanto, el EOQ es:

EjercicioUna compaa comercializadora adquiere de un proveedor externo cajas de chocolates belgas que distribuye en toda la meseta central del pas. La empresa espera vender aproximadamente 100,000 cajas de estos chocolates durante el ao. La demanda es relativamente constante durante el ao. El costo asociado a los pedidos es de 25 por cada uno. La poltica de costo de inventario que la empresa ha utilizado tradicionalmente es cargar el 20% del costo de compra como costo anual de conservacin de los inventarios, para cualquier artculo. El precio que se paga al proveedor por cada cada caja de chocolates es de 6.25Determine la cantidad ptima de pedido y el costo total.Supngase un tiempo de entrega de dos das, cul ser el punto de reorden? Utilice un ao de 365 das.

Caso de Estudio:Agenda de LA SESINOPTIMAL PRODUCTION LOT SIZE

29EOQ Modeling Assumptions1. Production is instantaneous there is no capacity constraint and the entire lot is produced simultaneously. 2. Delivery is immediate there is no time lag between production and availability to satisfy demand.3. Demand is deterministic there is no uncertainty about the quantity or timing of demand.4. Demand is constant over time in fact, it can be represented as a straight line, so that if annual demand is 365 units this translates into a daily demand of one unit. 5. A production run incurs a fixed setup cost regardless of the size of the lot or the status of the factory, the setup cost is constant.6. Products can be analyzed singly either there is only a single product or conditions exist that ensure separability of products.30Notation EPL ModelD demand rate (units per year)

Pproduction rate (units per year), where P>D

c unit production cost, not counting setup or inventory costs (dollars per unit)

A fixed or setup cost to place an order (dollars)

hholding cost (dollars per year); if the holding cost consists entirely of interest on money tied up in inventory, then h = ic where i is an annual interest rate.

Qthe unknown size of the production lot size 31Inventory vs Time in EPL ModelInventoryTime-DP-DProduction run of Q takes Q/P time units(P-D)(Q/P)(P-D)(Q/P)/232Solution to EPL ModelAnnual Cost Function:

Solution (by taking derivative and setting equal to zero):

setupholdingproduction

tends to EOQ as P

otherwise larger than EOQ because replenishment takes longer33The Key Insight of EOQThere is a tradeoff between lot size and inventory

Order Frequency:

Inventory Investment:

34EOQ Tradeoff Curve

EXAMPLE-- Farah CosmeticsProduction Capacity 1000 tubes/hr.Daily Demand 1680 tubesProduction cost $0.50/tube (C = 0.50)Set-up cost $150 per set-up (CO = 150)Holding Cost rate: 40% (Ch = .4(.50) = .20)Working Days: 365What is the optimal production lot size and its Total Cost?OPTIMAL PRODUCTION LOT SIZE

Since demand is 1680 per day and the production rate is 1000 per hour:D = 1680(365) = 613,200 P = 1000(24)(365) = 8,760,000

OTHER QUANTITESLength of a Production run = Q*/PLength of a Production cycle = Q*/D# of Production runs/yr. = D/Q*37Reorder PointsEOQ answers the how much questionThe reorder point (ROP) tells when to orderROP =Lead time for a new order in daysDemand per day= d x Ld = DNumber of working days in a yearReorder Point CurveQ*ROP (units)Inventory level (units)Time (days)Figure 12.5Lead time = LSlope = units/day = dReorder Point ExampleDemand = 8,000 iPods per year250 working day yearLead time for orders is 3 working daysROP = d x Ld = DNumber of working days in a year= 8,000/250 = 32 units= 32 units per day x 3 days = 96 unitsQuantity Discount ModelsReduced prices are often available when larger quantities are purchasedTrade-off is between reduced product cost and increased holding costTotal cost = Setup cost + Holding cost + Product costTC = S + H + PDDQQ2Quantity Discount ModelsDiscount NumberDiscount QuantityDiscount (%)Discount Price (P)10 to 999no discount$5.0021,000 to 1,9994$4.8032,000 and over5$4.75Table 12.2A typical quantity discount scheduleQuantity Discount ModelsFor each discount, calculate Q*If Q* for a discount doesnt qualify, choose the smallest possible order size to get the discountCompute the total cost for each Q* or adjusted value from Step 2Select the Q* that gives the lowest total costSteps in analyzing a quantity discountQuantity Discount Models1,0002,000Total cost $0Order quantityQ* for discount 2 is below the allowable range at point a and must be adjusted upward to 1,000 units at point bab1st price break2nd price breakTotal cost curve for discount 1Total cost curve for discount 2Total cost curve for discount 3Figure 12.7Quantity Discount ExampleCalculate Q* for every discountQ* =2DSIPQ1* = = 700 cars/order2(5,000)(49)(.2)(5.00)Q2* = = 714 cars/order2(5,000)(49)(.2)(4.80)Q3* = = 718 cars/order2(5,000)(49)(.2)(4.75)Quantity Discount ExampleCalculate Q* for every discountQ* =2DSIPQ1* = = 700 cars/order2(5,000)(49)(.2)(5.00)Q2* = = 714 cars/order2(5,000)(49)(.2)(4.80)Q3* = = 718 cars/order2(5,000)(49)(.2)(4.75)1,000 adjusted2,000 adjustedQuantity Discount ExampleDiscount NumberUnit PriceOrder QuantityAnnual Product CostAnnual Ordering CostAnnual Holding CostTotal1$5.00700$25,000$350$350$25,7002$4.801,000$24,000$245$480$24,7253$4.752,000$23.750$122.50$950$24,822.50Table 12.3Choose the price and quantity that gives the lowest total costBuy 1,000 units at $4.80 per unitEn una fabrica de jugos se tiene:H = .14, CO = 12, D = 6240/aoAsumiendo que se aplican este plan de descuentos: Quantity Unit Ordered Cost