ME352 - Production Control Strategies

Question 1

Common production control strategies

Answer 1

Push system
Pull system
Pond-draining systems
Focusing on bottlenecks (drum, buffer, rope)

Question 2

Push systems

Answer 2

 Use information about customers, suppliers, and production to
manage material flows
 Flows of materials are planned and controlled by a series of
production schedules that state when batches of each particular item
should come out of each stage of production
 Can result in great reductions of raw-materials inventories and in
greater worker and process utilisation when compared to pond
draining systems

Question 3

Pull systems

Answer 3

 Look only at the next stage of production and determine what is
needed there, and produce only that item
 Raw material and parts are pulled from the start (e.g. raw parts store) of the system toward the end (e.g. finished goods store) where they become completed products
 Successful implementation requires much preparation

Question 4

Push vs Pull systems

Answer 4

Push (make all we can just in case)
* Production Approximation
* Anticipated Usage
* Large Lots
* High Inventories
* Waste
* Management by Firefighting
* Poor Communication

Pull (make what’s needed when
we need it)
* Production Precision
* Actual Consumption
* Small Lots
* Low Inventories
* Waste Reduction
* Management by Sight
* Better Communication

Question 5

Pond-draining systems

Answer 5

 Emphasis on holding inventories (reservoirs) of materials to support production
 Little information passes through the system
 As the level of inventory is drawn down, orders are placed with the
supplying operation to replenish inventory
 May lead to excessive inventories and is rather inflexible in its ability to respond to customer needs

Question 6

Focusing on bottlenecks

Answer 6

 Bottleneck Operations
 Impede production because they have less capacity than upstream or downstream stages
 Work arrives faster than it can be completed
 Are binding capacity constraints that control the capacity of the system

Drum-Buffer-Rope
 The drum is the physical constraint of the plant: the work centre or
machine or operation that limits the ability of the entire system to
produce more.
 The rest of the plant follows the beat of the drum.
 We make sure the drum has work and that anything the drum has
processed does not get wasted.

 Drum
 Bottleneck, beating to set the pace of production for the rest of the system

 Buffer
 Inventory, placed in front of the bottleneck to ensure it is always kept busy
 Determines output or throughput of the system

 Rope
 Communication signal, tells processes upstream when they should begin
production

Question 7

Three pull strategies

Answer 7

 Just in time Systems
 Lean Manufacturing
 Kanban control systems

Question 8

Classic push system

Answer 8

MRP - material requirements planning

Question 9

JIT

Answer 9

The basic mechanism is that production
at one level only happens when initiated by a request from the higher
level. That is, units are pulled through the system by request.

Question 10

Comparison of MRP vs JIT

Answer 10

 Main Advantage of MRP over JIT: MRP takes forecasts for end product demand into account. In an environment in which substantial variation of sales are anticipated (and can be forecast accurately) MRP has a substantial advantage.
 Main Advantage of JIT over MRP: JIT reduces inventories to a
minimum. In addition to saving direct inventory carrying costs, there
are substantial side benefits, such as forcing improvement in quality
and plant efficiency

Also
MRP adv
Allows managers to manage
Can lead to economies of scale
in purchasing and production
Allows for the planning and completion
of complex assemblies as sub
components are delivered only on
scheduled need
MRP disadv
Can lead to large inventories
Can generate large quantities of
scrap before errors are discovered
Requires care to maintain effective
product flow
Requires maintenance of large and
complex databases

JIT adv
Limited and known Final Inventory
Workers only consume their time &
raw materials on what is actually
needed
Quality MUST be High – each piece
has a definite place to go – else
immediate feedback is given

JIT disadv
Every job is a ‘High Stress’ Rush order
Balanced systems MUST be in place
Setup times will greatly affect
throughput
Any problem will lead to unhappy
customers (either internal or external)

Question 11

Advantages and disadvantages of JIT systems

Answer 11

Small Work-in-process inventories
Advantages:
1. Decreases inventory costs
2. Improves efficiency
3. Reveals quality problems
Disadvantages:
1. May resutl in increased worker idle time
2. May result in decreased throughput rate

Question 12

2 types of kanban cards

Answer 12

Production kanban
Conveyance kanban

Question 13

Features of kanban system

Answer 13

Kanban
Information Flow System
Advantages
1. Efficient tracking of lots.
2. Inexpensive implementation of JIT.
3. Achieves desired level of WIP – based on Number of Kanbans in
the system.
Disadvantages
1. Slow to react to changes in demand.
2. Ignores predicted demand patterns (beyond two months or so).

Question 14

CONWIP systems

Answer 14

CONWIP – Constant work-in-process
Similar to Kanban system in goal of maintaining constant level of work
in-process.
Differs from Kanban in that all WIP is aggregated and treated as a whole.

Question 15

Technique to determine N for CONWIP system

Answer 15

 Identify bottleneck station.
 Gradually increase N (release faster than withdrawals occur) until input queue before bottleneck is rarely empty.

Question 16

CONWIP vs Kanban

Answer 16

 CONWIP systems immediately react to increases in customer
demand. As customers increase the rate at which products are
withdrawn, the high demand products are released into the system.
 CONWIP systems do not need the setup and maintenance of Kanban cards and containers for each part type.
 CONWIP systems react well for highly variable part mix demand
while maintaining relatively constant levels of inventory.

 CONWIP systems require a means for maintaining the backlog list.
 CONWIP systems require ample storage space before machines
because although WIP as a whole is constant, WIP is not controlled
within each workcentre.
 Over time, as demand changes, unwanted WIP may “clog up” the
system and count towards the overall WIP, thus making the system
less responsive. To resolve, periodically “cleanse” the system of the
unused WIP.