Production control for dependent demand items

Transcription

1 Lecture 13: MRP 184

2 Production control for dependent demand items Dependent demand items: Example: Forklift: each order requires 4 wheels, 4 tires, 1 seat, 1 steering wheel, etc. Methods: Push Pull OPT (drum-buffer-rope methods) 185

3 Push System Customer orders and forecasts are fed into beginning of line Forecasts CUSTOMER Raw materials Manufacturing Facility Product 186

4 Pull System A production system driven by actual consumption and controlled by synchronized replenishment signals. Orders for product pulled from end of line, rest of line then responds to replace removed product Supplier order CUSTOMER Raw materials Manufacturing Facility Product 187

5 Push System Customer orders and forecasts are fed into beginning of line Forecasts CUSTOMER Raw materials Manufacturing Facility Product 188

6 Pull System Orders for product pulled from end of line, rest of line then responds to replace removed product Supplier order CUSTOMER Raw materials Manufacturing Facility Product 189

7 MRP / MRPII MRP: Material Requirements Planning MRPII: Manufacturing Resource Planning Typical Push system: explodes out demand for final product into orders for raw materials and subassemblies. began in 1960 s as computerized approach to materials planning (MRP) Later expanded (MRPII), which included: master production scheduling capacity requirements planning purchasing forecasting financial modules 190

8 Background Popularity stems to the MRP crusade in 70 s by APICS (American Production and Inventory Control Society) Focus: convince industry that MRP was integrated decision support system for management of the total manufacturing business. Promoted by APICS, consultants, and computer industry Recently: MRP has come under fire: oversold and poorly implemented in practice, often leads to large WIP, large lead time 191

9 Bill of Materials (BOM) MRP core based on processing of bill of materials Bill of materials is product structure: what is needed Example: 4-legged stool bill of materials Stool Leg (x4) Seat Frame Cushion Raw Frame Cushion Material 192

10 MRP Requires: Master Production Plan (orders for finished products with due dates) On-hand inventories Bill of materials Current Status of purchased and manufactured items Replenishment rules by item lead time order quantity safety stock scrap allowance, etc. 193

11 MRP is calculating sizing and timing of net requirements for components/subassemblies Time is divided into time buckets Given gross requirements for top level: Projected Inventory = Previous Inventory + Scheduled Receipts - Gross Requirements Determine net requirements based on projected inventory Shift back by lead time to determine when to order net reqmnts Determine Gross Requirement for components by BOM, then repeat steps above for components 194

12 Example: Bill of Materials for Stool A (4 legged) and Stool B (3 legged) Master Production Schedule: Other info: lead times (weeks) current inventory Stool A: 2 10 Stool B: 2 30 Leg: 2 40 (20 more due week 3) etc.. 195

13 MRP Example: 196

14 Summary of basic method: take final orders (gross reqmnts) determine net requirements (after inventory, sched recpts) from lead time, determine when to begin order from bill of materials, determine gross reqmnts of components 197

15 What happens if the schedule changes? Two approaches: Regenerative redo everything over from scratch done infrequently Problems: Net Change MRP As changes occur, only recalculate modified parts done daily or on-line Problems: 198

16 Other issues of MRP Time buckets: bucket is fixed time slot, and system has fixed # of buckets (planning horizon) typically buckets are 1 week note that there is no explicit indication as to whether event occurs at beginning or end of the bucket Safety Stocks: excess quantities of inventory maintained to cover unexpected fluctuations in supply or demand. Just-in-case inventory 2 methods always order stock to keep inventory above minimum just hide safety stock from MRP system (don t include in inventory) 199

17 Net change and regenerative are both top-down planning Bottom-up planning is where human operator takes over rush orders, special exceptions, scheduling Firm-planned orders: allows over-ride of MRP rules override lot size or lead time Pegged Requirements: allows operator to see what final order is associated with given component orders (retrace the MRP steps) 200