Push and Pull Production Systems

Transcription

1 Push and Pull Production Systems The Key Difference Between Push and Pull You say yes. I say no. You say stop. and I say go, go, go! Push Systems: schedule work releases based on demand. inherently due-date driven control release rate, observe WIP level Pull Systems: authorize work releases based on system status. inherently rate driven control WIP level, observe throughput The Beatles 1 Push vs. Pull Mechanics What Pull is Not! (Exogenous) Schedule PUSH Job Production Process Push systems do not limit WIP in the system. Job PULL (Endogenous) Void Production Process Pull systems deliberately establish a limit on WIP. Make-to-Order: MRP with firm orders on MPS is make-to-order. But it does not limit WIP and is therefore a push system. Make-to-: Pull systems do replenish inventory voids. But obs can be associated with customer orders. Forecast Free: Toyota s classic system made cars to forecasts. Use of takt times or production smoothing often involves production without firm orders (and hence forecasts). 3 4 Push and Pull Examples Push and Pull Line Schematics Are the following systems essentially push or essentially pull? Kinko s copy shop: Soda vending machine: Pure MRP system: Doctor s office: Supermarket (goods on shelves): Tandem line with finite interstation buffers: Runway at O Hare during peak periods: Order entry server at Amazon.com: Pure Push (MRP) Pure Pull (Kanban) CONWIP 5 Authorization Signals Full Containers 6

2 Pulling with Kanban Inventory/Order Interface Outbound stockpoint Completed parts with cards enter outbound stockpoint. Outbound stockpoint Concept: Make-to-stock and make-to-order can be used in same system. Dividing point is called the inventory/order interface. This is sometimes called the push/pull interface, but since WIP could be limited or unlimited in both segments, this is not a strictly accurate term. Benefit: eliminate entire portion of cycle time seen by customers by building to stock. Production cards When stock is removed, place production card in hold box. Production card authorizes start of work. Implementation: kanban late customization (postponement) 7 8 Example Custom Taco Production Line Example Quick Taco Production Line Make -to- Make -to-order Make -to-order Make -to- Refrigerator Cooking Assembly Packaging Sales Refrigerator Cooking Assembly Packaging Sales Warming Table Notes: I/O interface can differ by time of day (or season). I/O interface can differ by product Example IBM Panel Plant Example HP DesketSupply Chain Original Line U.S. DC Treater Treater Prepreg, Copper Revised Line Prepreg, Copper Lamination Lamination Machining Circuitize Drilling Copper Plate process that gives boards personality Procoat Machining Core Circuitize Drilling Copper Blanks Plate Sizing, Test Procoat Sizing, Test Notes: Moving I/O interface closer to customer shortens leadtime seen by customer. Small number of core blanks presents opportunity to make them to stock. Integrated Circuit Manufacturing Printed Circuit Assembly & Test Final Assembly and Test European DC Far East DC Notes: I/O interface located in markets to achieve quick response to customers Delayed differentiation of products (power supplies for different countries) enables pooling of safety stocks 11 1

3 Conclusions Advantages of Pull Systems Basic Tradeoff: responsiveness vs. inventory (time vs. money) moving PPI closer to customer increases responsiveness and (usually) inventory Optimal Position of : need for responsiveness cost of carrying inventory product diversification Levers: product design (postponement) process design (quick response manufacturing) Low Unit Cost: low inventory reduced space little rework High External Quality: high internal quality pressure for good quality promotion of good quality (e.g., defect detection) Good Service: short cycle times steady, predictable output stream Flexibility: avoids committing obs too early encourages floating capacity The Magic of Pull Pull Benefits Achieved by WIP Cap Pulling Everywhere? You don t never make nothin and send it no place. Somebody has to come get it. No! It s the WIP Cap: Kanban WIP cannot exceed number of cards WIP explosions are impossible Hall 1983 WIP 15 Reduces Costs: prevents WIP explosions reduces average WIP reduces engineering changes Improves Quality: pressure for higher quality improved defect detection improved communication Improves Service: reduces cycle time variability pressure to reduce sources of process variability promotes shorter lead times and better on-time performance Maintains Flexibility: avoids early release (like air traffic control) less direct congestion less reliance on forecasts promotes floating capacity 16 CONWIP CONWIP Controller Assumptions: 1. Single routing. WIP measured in units Mechanics: allow next ob to enter line each time a ob leaves (i.e., maintain a WIP level of m obs in the line at all times). Modeling: MRP looks like an open queueing network CONWIP looks like a closed queueingnetwork Kanban looks like a closed queueing network with blocking Indicator Lights R G PC Work Backlog PN Quant Workstations PC LAN 17 18

4 CONWIP vs. Pure Push Push/Pull Laws: A CONWIP system has the following advantages over an equivalent pure push system: 1) Observability: WIP is observable; capacity is not. ) Efficiency:A CONWIP system requires less WIP on average to attain a given level of throughput. 3) Robustness: A profit function of the form Profit pth - hwip is more sensitive to errors in TH than WIP. CONWIP Efficiency Example Equipment Data: 5 machines in tandem, all with capacity of one part/hr (uth t e TH) exponential (moderate variability) process times w w CONWIP System: TH( w) rb w+ W 1 w u TH Pure Push System: w( TH ) u 1 TH PWC formula 5 M/M/1 queues 19 0 CONWIP Efficiency Example (cont.) CONWIP Robustness Example How much WIP is required for push to match TH attained by CONWIP system with WIPw? w 5( w/( w + 4)) 5w w w+ 4 1 ( w/( w+ 4)) 4 In this example, WIP is always 5% higher for same TH in push than in CONWIP In general, the increase won t always be 5%, but it will alwa ys take more WIP to get same TH under push than under pull. Profit Function: CONWIP: Profit pth hw w Profit(w) p hw w + 4 5TH Push: Profit(TH) pth h 1 TH need to find optimal WIP level need to find optimal TH level (i.e., release rate) Key Question: what happens when we don t choose optimum values (as we never will)? 1 CONWIP vs. Pure Push Comparisons Modeling CONWIP with Mean-Value Analysis CONWIP Robustness Optimum Efficiency Notation: u ( w) utilization of station in CONWIP line with WIPlevelw cycletimeat station in CONWIPline with WIPlevelw Profit Push TH ( w) throughput cycletimeof CONWIPline with WIPlevelw with WIP levelw WIP ( w) averagewiplevelat station in CONWIPline with WIPlevelw n 1 of CONWIP line % 0.00% 40.00% 60.00% 80.00% % 10.00% % Control as Percent of Optimal 3 Basic Approach: Compute performance measures for increasing w assuming ob arriving to line sees other obs distributed according to average behavior with w-1obs. 4

5 Mean-Value Analysis Formulas Starting with WIP (0)0 and TH(0)0, compute for w1,, t e ( ) CT ( w) [ ce ( ) 1] TH ( w 1) + [ WIP ( w 1) + 1] te ( ) 1 w TH ( w) WIP ( w) TH ( w) n 5 Computing Inputs for MVA MEASURE: STATION: Natural Process Time (hr) t Natural Process CV c Number of Machines m MTTF (hr) m f MTTR (hr) m r 1 Availability A Effective Process Time (failures only) t e ' Eff Process CV (failures only) c e ' Jobs Between Setups N s Setup Time (hr) t s Setup Time CV c s Eff Process Time (failures+setups) t e Eff Station Rate r e Eff Process Time Var (failures+setups) se Eff Process CV (failures+setups) c e r b T t CstW W TH CT TH CT 6 Output of MVA Using MVA to Evaluate Line Performance TH CT TH CT T w Actual Actual CW CT1(w) CT(w) CT 3(w) CT 4(w) CT5(w) TH WIP Actual Best Case Worst Case PWC 7 8 Implementing Pull Capacity Buffers Pull is Rigid: replenishing stocks quickly (ust in time) requires level mix, volume, sequence. JIT Practices: Support Rigidity: production smoothing/mix stabilization Mitigate Rigidity in Production System capacity buffers setup reduction flexible labor facility layout product design (postponement, etc.) Mitigate Rigidity in Organization TQM vendor management, etc. this is the genius of pull! 9 Motivation: facilitate rapid replenishments with minimal WIP Benefits: Protection against quota shortfalls Regular flow allows matching against customer demands Can be more economical in long run than WIP buffers in push systems Techniques: Planned underutilization (e.g., use u 75% in aggregate planning) Two shifting: Schedule dummy obs to allow quick response to hot obs 30

6 Setup Reduction Flexible Labor Motivation: Small lot sequences not feasible with large setups. Internal vs. External Setups: External performed while machine is still running Internal performed while machine is down Approach: 1. Separate the internal setup from the external setup.. Convert as much internal setup as possible to external setup. 3. Eliminate the adustment process. 4. Abolish the setup itself (e.g., uniform product design, combined production, parallel machines). Cross-Trained Workers: float where needed appreciate line-wide perspective provide more heads per problem area Shared Tasks: can be done by adacent stations reduces variability in tasks, and hence line stoppages/quality problems work can float to workers, or workers can float to work 31 3 Cellular Layout Focused Factories Advantages: Better flow control Improved material handling (smaller transfer batches) Ease of communication (e.g., for floating labor) Challenges: May require duplicate equipment Product to cell assignment Inbound Outbound Pareto Analysis: Small percentage of sku s represent large percentage of volume Large percentage of sku s represent little volume but much complexity Dedicated Lines: for families of high runners Saw Lathe Mill Drill few setups Saw Mill Drill Paint can use pull effectively Grind Mill Drill Paint Weld Grind Lathe Drill Job Shop Environment: for low runners Saw Grind Paint many setups poorer performance, but only Lathe on smaller portion of business Mill may need to use push Drill Stores Stores Assembly Assembly Warehouse Warehouse Push/Pull Takeaways Magic of Pull: the WIP cap MTS/MTO Hybrids: locating the I/O interface Logistical Benefits of Pull: observability efficiency robustness (this is the key one) Overcoming Rigidity of Pull: capacity buffers setup reduction flexible labor facility layout, etc. 35