CONWIP (A pull alternative to kanban principle) Main resources : Mark Spearman, David Woodruff and Wallace Hopp Northwestern University, Evanson, Illinois, USA Diagrams, modifications, structures and editing (J.Skorkovský,KPH) CONWIP diagrams: EuroLean+ Methodologies used for effective production control •Based on PULL principle –JIT –Kanban (tool to support JIT) –Zero inventory •Kanban is used mostly for repetitive manufacturing) •Based on PUSH principle –MRP (MRP-II) •Based on both principles (push and pull) –CONWIP (Constant Work-In-Process) greatly reduced inventory levels and production lead times Demand Demand Components Components =buffer =machine centre Kanban signals (card) PUSH and PULL •PUSH : production jobs (production orders) are scheduled (MRP and MRP-II) –often not feasible plans are generated and problems are often detected too late –used fixed lead times=LT (see next slide) does not depend on capacity utilization –Mind you, that production is a random process and estimation of LT is very pessimistic – – • •PULL : production jobs (production orders) starts are triggered by the completion of another job. •In other words : It authorizes releases of the jobs) Black box=production t=start of the job t+LT=end time of the job (where LT=constant) Kanban rules To Do Working=In Progess Done Kanban rules (signal card) PWP Technical infrastructure Training preparation Remote training To Do Working=In Progess Done To Do Working=In Progess Done PWP Remote training To Do Working=In Progess Done To Do Working=In Progess Done PWP WIP level WIP level WIP level Too high a WIP setting leads to poor multitasking and increasing the value of the lead time (reduction of Throughput) Technical infrastructure Training preparation Remote training Technical infrastructure Training preparation Remote training Technical infrastructure Training preparation PWP Flow time and Lead time •Flow time (known also as a „Cycle time“) Job is released time Job is completed Typically random time (highly variable) •Lead time (constant used for planning ) Job scheduling (MRP-MRP-II) JOB 1 JOB N WIP level FGI level Flow times FGI – finished good inventory Controlling parameters • Operation 1 (Job1) Authorization to start Op 22 Controlling parameter DOWNSTREAM UPSTREAM Operation 2 (Job1) Operation 3 (Job1) Components for Job 3 needed… 1 (kanban = card) 2 Components for Op 3 produced by Op 2 and supplied to Op 2 successor (pulled) 1 Production =kanban (production card) The number of cards in the system determines the WIP levels in the plant KANBAN Job=Work order=Production order KANBAN Trade-offs •Too many Kanban cards-> To much WIP and long Cycle times • •Too few Kanban cards->lower throughput and vulnerability to demand • Little´s low – not for AOMA •Little´s law : Cycle Time=WIP/Throughput – see basic explanation further ahead – more detailed explanantion : Factory Physics (W.J.Hopp and M.L.Spearman) JIT •Kanban is not JIT (JIT is a manufacturing philosophy) •JIT encompasses : –Kanban (card system transferring singnals) –total quality control (TQM) – e.g. scrap loss not tolerated…. –setup time reduction –worker participation !!!! –lean production (low level of waste) •Advantages of JIT philosophy : –reduced WIP (Work In Process) –shorter flow times (cycle times) –lower production costs –greater customer responsiveness –reduces setup times – PUSH and PULL are not mutually exclusive approaches and other statements… •Push and Pull can be combined •MRP is considered to be more applicable than kanban •MRP is in almost any discrete part production •Kanban(JIT,pull) – superior results if applicable •Kanban(JIT,pull) – is difficult to use if : –Jobs with short production runs –Significant setup times (numerically controlled machines) –Remarkable Scrap losses –Unpredictable fluctuation in demand – • • PUSH and PULL and the types of the queueing networks •Push : open queueing network •Pull : closed queueing network •Push : schedule Throughput and measure (observe) WIP • • T WIP • PULL : setup WIP and measure (observe) Throughput Advantage of PULL over PUSH (home study not for MPH-AOMA) •PUSH : WIP and Throughput fluctuations – result in violation of the assumption, that CycleTimes (CT) and therefore Lead Times (LT) are constant ! •WIP is easier to optimize than Throughput (T) •Little´s low : • Average CT=Average WIP/Average T – meaning that CT cannot be constant but vary with WIP and T . • •Pull is easy to manage : why ? -> WIP is easier to control than capacities needed to appropriately release work in push system . The problem is estimation of these capacities as exact as possible, • WIP CT TH 1 8 0,125 2 8 0,250 3 8 0,375 4 8 0,500 5 10 0,500 6 12 0,500 7 14 0,500 8 16 0,500 9 18 0,500 10 20 0,500 11 22 0,500 12 24 0,500 Little´s Law (home study- not for MPH_AOMA) WIP WIP CONstant Work In Process = CONWIP •System having benefits of a PULL and can be used in variety of manufacturing environment • •CONWIP : generalized form of Kanban • •CONWIP relies on signals (electronic, paper • cards…) CONstant Work In Process = CONWIP •Kanban: card is used to signal production of a specific part • •CONWIP : card is assigned to production line and are not part number specific CONWIP Configuration For this course only red framed scenario will be considered. It is a system controoled by one loop (feedback) CONWIP 7 6 5 4 3 2 1 BOM of the final product (7) Mcbeth-Bentel-And-Margedant-s-Universal-Boring-Machine Fig-224-Horizontal-Milling-Machine-Column-Type-Courtesy-of ANd9GcQmRqL9z7X2WkUWokNnwp1Lh5nGDiAAaDT-7HGYwP--re4QaRc&t=1&usg=__TS3yjJ9DN9CP2_ONFIOOIW7wTsE= ANd9GcSX_4hzOd5f4NzHKqCdeClidgVm0GdpqMZlHV_ALf908wWv3ao&t=1&usg=__DiQcbsKVPH-ZNDBEGhSlFpCcIr8= Container A Baglog list 1 : 6 pc 2 : 6 pc 3 : 0 pc 4 : 0 pc 6 : 4 pc 5 : 0 pc 7 : 0 pc ANd9GcSX_4hzOd5f4NzHKqCdeClidgVm0GdpqMZlHV_ALf908wWv3ao&t=1&usg=__DiQcbsKVPH-ZNDBEGhSlFpCcIr8= Container A Baglog list 1 : 0 pc 2 : 0 pc 3 : 0 pc 4 : 0 pc 6 : 4 pc 5 : 2 ks 7 : 0 ks 2x 2x 3x 3x Baglog list 1 : 6 pc 2 : 6 pc 3 : 8 pc 4 : 8 pc 6 : 0 pc 5 : 0 pc 7 : 0 pc Container A ANd9GcSX_4hzOd5f4NzHKqCdeClidgVm0GdpqMZlHV_ALf908wWv3ao&t=1&usg=__DiQcbsKVPH-ZNDBEGhSlFpCcIr8= 4x 2x ANd9GcSX_4hzOd5f4NzHKqCdeClidgVm0GdpqMZlHV_ALf908wWv3ao&t=1&usg=__DiQcbsKVPH-ZNDBEGhSlFpCcIr8= Container A Baglog list 1 : 0 pc 2 : 0 pc 3 : 0 pc 4 : 0 pc 6 : 0 pc 5 : 0 ks 7 : 1 ks Container C Container B Queue (First in system first served=FSFS) System Entry Time=SET SET=8:00 SET=10:00 SET=12:00 SET=14:00 maintaining of BLL is responsibility of inventory control staff parts parts cards One loop (card based-pull based) I. Resource: EuroLean+ We have in our production line all in all 4 kanban cards When FG is consumed one card is free One loop (card based – pull based) II. Resource: EuroLean+ CONWIP –different throughput times I Resource: EuroLean+ CONWIP –different throughput times II. Resource: EuroLean+ CONWIP –different throughput times III. Resource: EuroLean+ CONWIP characteritics Resource: EuroLean+ CONWIP-air traffic control AIR-HeathrowQueue-001 Originating airport Destination airport (air above airport) airport_19537t If heavy air traffic, departing planes should be held on the ground at the originating airport rather than control flying aircrafts in the air above destination airport as a holding pattern The results : greater safety and lower fuel consumption CONWIP-Theory of Constraints •Balance the flow and not the capacity • •Operation of the CONWIP line is regulated by the bottleneck resource • •If we have sufficient demand, the correct number of the cards will maintain just enough WIP to keep bottleneck busy • • • • Thanks for your attention • (next few slides are not part of this course ) Utilization, Bottleneck rate and Raw process time (cycle time) Example WS1 WS2 WS3 WS4 Little´s Law •WIP=TH x CT ,where TH=throughput and CT=cycle time WIP CT TH 1 8 0,125 2 8 0,250 3 8 0,375 4 8 0,500 5 10 0,500 6 12 0,500 7 14 0,500 8 16 0,500 9 18 0,500 10 20 0,500 11 22 0,500 12 24 0,500 Little´s Law Conwip Mcbeth-Bentel-And-Margedant-s-Universal-Boring-Machine Fig-224-Horizontal-Milling-Machine-Column-Type-Courtesy-of 7 4 3 2 1 6 5 4 3 2 1 4 3 2 1 6 6 5 5 7 7 4 3 4 3 2 2 1 1 Buffer 3,4 1,2