Makespan - Production Planning & Scheduling Solutions Ltd.
The Effect of Sequencing Jobs on Total Lead Time
|
Table-1 |
|||
|
PROD’ |
QTY |
M1 |
M2 |
|
A |
5 |
20 |
10 |
|
B |
5 |
10 |
20 |
In order to explain the sequencing effect on the total
lead time let’s take a simple example: Two products A and B are manufactured by
two machines M1 and M2 in the order M1→M2. The quantity and process time per item (in minutes)
are given in Table-1. In order to simplify the example we assume zero setup
times and identical priority for the two batches. There are two possible
sequences A→B and B→A.
The Gantt chart of the two sequences is shown in the chart below.
We
can see that the total lead-time of sequence
B→A
at the bottom side is 160 minutes, while the total lead time of sequence
A→B, at the upper side of the chart is 210 minutes (31% more). The reason lies in the different wait/waste time
(green sections)
of Machine M2. Machine M2 can start processing an item only
after machine M1 had finished it, see
the processing schedule of any item (1, 2, 3, 4, 5) by machine M1 and then by
machine M2. In sequence A→B machine M2 is forced to wait 60 minutes: either 20 minutes for the first item and 10 minutes for each of
the other 4 items of product A (as seen in the chart) or wait for 60 minutes
and then process the five items continuously. While in sequence B→A machine M2 is forced to wait
only 10 minutes (for the first item of product B).
This simple example shows the sequencing effect on the
total lead time. The main cause is the variation of process parameters
between product A and B.
In a real shop floor environment the situation is more complicated, there are many parameters which affect the total lead time for example:
- Many work centers, each one with one machine or more.
-
The
products can have several different parameters such as process times, setup
times,
common setup etc.
- The jobs can have different quantities, delivery dates etc.
The result of poor sequencing is low efficiency of the production line, long lead times and more jobs in process. Hence the ability to accept customer orders with short delivery dates is decreasing.
