Pull Production Systems Chapter 8 Outline Production control

Pull Production Systems Chapter 8

Outline • Production control systems • Pull systems and push systems • Necessary conditions for pull production systems • How to achieve pull production • Other mechanisms for signal and control • To pull or not to pull MME 492 b – Production Management © 2000 R. O. Buchal

Production control systems • Objective: to produce goods in the right quantities and at the right time to meet demand • Two approaches: push and pull MME 492 b – Production Management © 2000 R. O. Buchal

The Traditional Push System • In traditional manufacturing, an item is released for production at a specified time, with an associated due date • The item moves through a sequence of operations MME 492 b – Production Management © 2000 R. O. Buchal

The Traditional Push System • When one operation is finished, the item is “pushed” to the next operation • Finally, the product is pushed to inventory, to meet forecast demand • This is classical push-type production, as incorporated in the MRP approach MME 492 b – Production Management © 2000 R. O. Buchal

The Push Scheduling Problem • Schedule the release of jobs into the system, and control their progress, so that queue time is minimized, and jobs are completed as close to their due dates as possible MME 492 b – Production Management © 2000 R. O. Buchal

A Flow Line IN OUT MME 492 b – Production Management © 2000 R. O. Buchal

Scheduling Flow Lines • Flow lines are easy to schedule: – Identify the required cycle time based on demand – Allocate tasks for each station and balance the line – Work moves directly from one station to the next on a mechanized material handling system – Short lead time and minimum WIP inventory MME 492 b – Production Management © 2000 R. O. Buchal

Batch and Job Shop Production Route of part A shown. 1 2 3 4 5 6 Batch of part A (route: 1 - 5 - 3 - 4 - 2 - 6) Batch of part B (route: 4 - 2 - 5 - 6 - 3) MME 492 b – Production Management © 2000 R. O. Buchal

Scheduling Batch Production • Variety of jobs processed • Parts usually made in large batches • Different routing and processing requirements of each job • Number of different orders in the facility at any one time • Competition for common resources MME 492 b – Production Management © 2000 R. O. Buchal

Push Scheduling • Estimate total lead time based on processing time, move times and queue times at each machine • Schedule jobs to start based on lead time so they are completed on schedule • Schedule/sequence jobs at each machine MME 492 b – Production Management © 2000 R. O. Buchal

Variety of Jobs Necessitates. . . • Planning for the production of each job as it arrives • Scheduling its use of limited resources • Monitoring its progress through the system MME 492 b – Production Management © 2000 R. O. Buchal

Objectives in Scheduling • • • Meet customer due dates Minimize job lateness Minimize response time Minimize completion time Minimize time in the system Minimize overtime Maximize machine or labor utilization Minimize idle time Minimize work-in-process inventory MME 492 b – Production Management © 2000 R. O. Buchal

Responsibilities of Production Control Department • Loading – Check availability of material, machines & labour • Sequencing – Release work orders to shop & issue dispatch lists for individual machines • Monitoring – Maintain progress reports on each job until it is complete MME 492 b – Production Management © 2000 R. O. Buchal

Scheduling and Lead times • To meet the master production schedule, the actual lead times must match the planned lead times • The lead times depend on the quantity and mix of jobs in progress, and are hard to predict accurately MME 492 b – Production Management © 2000 R. O. Buchal

Scheduling and Lead times • Early orders increase inventory and WIP • Late orders delay production • Since production delays are very costly, planned lead times are estimated very conservatively “just in case” MME 492 b – Production Management © 2000 R. O. Buchal

Scheduling and Batch Production • Ordering and producing in batches makes accurate scheduling very difficult • Small lots and pull production achieve efficiencies of a flow line MME 492 b – Production Management © 2000 R. O. Buchal

The Pull System • The pull system focuses on the output of the system rather than the input • Finished products are “pulled” from the final operation in response to firm customer orders • This leads to a chain reaction, with each station pulling material from its preceding station MME 492 b – Production Management © 2000 R. O. Buchal

Necessary conditions for pull production systems • • • Planning and control at the shop floor Produce to meet demand Inventory reduction Preventive maintenance Quality assurance Short setups and small lots Plant layout facilitating smooth flow Uniform production plans Cooperative work attitudes MME 492 b – Production Management © 2000 R. O. Buchal

Flow Production • On a mechanized flow line, all operations are paced by the line speed • No operation can produce more than is needed by the next downstream operation • It is impossible for WIP to build up MME 492 b – Production Management © 2000 R. O. Buchal

Turning Batch Production into Flow Production • Production in large, irregular batches results in very uneven flow and poor performance • Flow production efficiency can be achieved through small-lot production and uniform production schedules MME 492 b – Production Management © 2000 R. O. Buchal

The Kanban System • The Kanban system uses simple cards to strictly control production • The basic idea is that no station is permitted to produce more than is immediately required by the succeeding station • This simple idea prevents the buildup of inventory by making a group of multipurpose machines act like a flow line • No computer is required! MME 492 b – Production Management © 2000 R. O. Buchal

Kanban Production Control System • Kanban card indicates standard quantity of production • Derived from two-bin inventory system • Kanban maintains discipline of pull production • Production kanban authorizes production • Withdrawal kanban authorizes movement of goods MME 492 b – Production Management © 2000 R. O. Buchal

A Sample Kanban Part no. : 7412 Description: Slip rings From : Machining M-2 MME 492 b – Production Management © 2000 R. O. Buchal Box capacity 25 Box Type A Issue No. 3/5 To: Assembly A-4

The Origin Of Kanban a. Two-bin inventory system Bin 1 Reorder Card Bin 2 b. Kanban Inventory System Kanban Q-R R Q = order quantity R = reorder point = demand during lead time Key realization: Why do we need bin 1 at all? It is just excess inventory! MME 492 b – Production Management © 2000 R. O. Buchal

Dual Kanbans P X X X W X P X W Container withdrawal kanban Flow of work P Container with production kanban Flow of kanban MME 492 b – Production Management © 2000 R. O. Buchal

Types Of Kanbans • Conveyance (withdrawal) kanbans – Authorize movement of material between stations • Production kanbans – Authorize production at an operation • Signal Kanban – triangular kanban used to signal production at the previous workstation • Material Kanban – used to order material in advance of a process • Supplier Kanbans – rotate between the factory and suppliers MME 492 b – Production Management © 2000 R. O. Buchal

Determining Number Of Kanbans where N = number of kanbans or containers d = average demand over some time period L = lead time to produce parts S = safety stock C = container size MME 492 b – Production Management © 2000 R. O. Buchal

Kanban Calculation Example Problem statement: d = 150 bottles per hour d. L = (150)(0. 5) = 75 7. 5 C = 25 bottles L = 30 minutes = 0. 5 hours S = 10% d. L = 10% x 75 = Solution: Round up to 4 (allow some slack) or down to 3 (force improvement) MME 492 b – Production Management © 2000 R. O. Buchal

Bucket Brigade Analogy • Buckets are kanbans • Lead time is time required for bucket to make one round trip (e. g, the red bucket) • At the fire, a full bucket arrives just as the previous one is emptied MME 492 b – Production Management © 2000 R. O. Buchal

Other Points to remember • Demand cannot be greater than capacity of slowest machine • To adapt to increasing demand, lean systems are designed with excess capacity, and are run at far below 100% utilization MME 492 b – Production Management © 2000 R. O. Buchal

Other mechanisms for signal and control • • Wheeled carts Kanban squares Golf balls Electronic kanban Clothespin clips Milk run Kanban sequence board MME 492 b – Production Management © 2000 R. O. Buchal

Kanban Squares X X X Flow of work Flow of information MME 492 b – Production Management © 2000 R. O. Buchal X

To pull or not to pull Production requires: • Continuous, stable demand • Uniform (level) production schedules • Short setup times • Limited product variety • Continuous flow MME 492 b – Production Management © 2000 R. O. Buchal