Learning Objectives of the Article:
- Define and explain KANBAN. Where and how
KANBANS are used.
- What are advantages and disadvantages of
using KANBAN system?
A Kanban system is a means to achieve
just in time (JIT)
production. It works on the basis that each process on a production line
pulls just the number and type of components the process requires, at just
the right time. The mechanism used is a Kanban card. This is usually a
physical card but other devices can be used Two types of such cards are
A withdrawal Kanban:
kind and quantity of product which a manufacturing process should withdraw
from a preceding process. This card (illustrated below) shows that the
preceding process which makes this part is forging, and the person carrying
this card from the subsequent process must go to position B-2 of the forging
department to withdraw drive pinions. Each box of drive pinions contain 20
units and the shape of box is "B". This Kanban is the 4th of 8 issued. The
item back number is an abbreviation of the item.
Example of Withdrawal
A production ordering
Specifies the kind and quantity of the product which the preceding process
must produce. The one illustrated below shows that the machining process
SB-8 must produce the crankshaft for the car type X50BC-150. The crankshaft
produced should be placed at store F26-18. The production ordering Kanban is
often called an in-process or simply a production Kanban.
Example of Production Kanban
Each process (area, cell) on the
production line has two KANBANS "post-boxes", one for withdrawal and one for
production ordering KANBANS. At regular intervals a worker takes withdrawal
KANBANS that have accumulated in his process post-box, and any empty
pallets, to the location where finished parts (components, assemblies) from
the preceding process are stored. Each full pallet has attached to it one or
more production ordering KANBANS which he removes and puts in the
appropriate post-box belonging to the process that produced the parts. The
worker now attaches a withdrawal Kanban to the pallet and takes it back to
his own process area. When this new pallet begins to be used, its withdrawal
Kanban is put back into the withdrawal post-box. At each process on the
line, production ordering KANBANS are periodically removed from their
post-box and used to define what parts and quantities to produce, next.
KANBANS help simply planning and
to fine tune production to meet changing customer demand of up to + or -
10%. The system requires planned monthly and weekly production schedules.
KANBANS simplify day to day flexibility, and changes to the production
schedule need only to be given to the final assembly process and will then
automatically work their way back up the line. Kanban systems can be
tightened by removing cards or by reducing the number of parts on pallet.
The effect will be to speed the flow through the process and hence reduce
lead time. However it also makes the system more vulnerable to breakdowns
and other causes of dislocation. By identifying the areas within the line
that are causing disruption, efforts can be made to improve them. Thus the
overall efficiency of the line is raised by tackling the key points.
Other types of Kanban also used
are supplier kanbans -to withdraw goods from external suppliers, and two types
of Signal Kanban, which are inserted near the bottom of a stack of item.
These automatically initiate production of batch produced items when the
stock reaches a preset order level.
In a single card KANBAN system,
parts are produced and bought according to a daily schedule, and deliveries
to the user are controlled by a "conveyancing" (withdrawal) KANBAN. In
effect, the single card system is a push system for production coupled with
a pull system for delivery to the point of use. Single card KANBAN system
controls deliveries very tightly, so that the using work center never has
more than a container or two of parts and and the stock point serving the
work center is eliminated. Single-card systems work well in companies in
which it is relatively easy to associate the required quantity and timing of
component parts with the schedule of end products. These are usually
companies with a relatively small range of end products, or products which
are not subject to rapid, unexpected changes in demand levels.
- Low costs associated with the transfer of
- Provides quick response to changes
- Delegates responsibility to line workers
- It is a simple technique not involving
computers so its cost is low.
- Lead times are reduced.
- It is less effective in shared-resource
situations. Suppose the upstream station made several parts. Then a
request to make more of the part needed by the downstream station will
have to wait if other parts have to be made. A buffer is needed to ensure
the downstream station doesn't run out meanwhile. And, because each part
needs a separate signaling card, the system becomes more complex than if
the resources were dedicated.
- Surges in mix or demand cause problems
because KANBAN assumes stable repetitive production plans. It is less
suited to industries where mix and volumes fluctuate.
- KANBAN in itself doesn't eliminate
variability, so unpredictable and lengthy down times could disrupt the
system; poor quality in terms of scrap and rework also affect its good
- KANBAN systems are not suited for
manufacturing environments with short production runs, highly variable
product demand, poor quality products, and a multitude of product types.
- A breakdown in the KANBAN system can
result in the entire line shutting down.
- The throughput of a KANBAN system is not
managed but is instead a result of controlled WIP and known cycle times.