IE434 Lecture Notes
SPRING 2007
SCHEDULING
III. CONTRIBUTION OF KEY TOPICS TO SCHEDULING
Objective of this section is to emphasise the importance of some key topics which
become very important in real life applications in which a schedular is involved or
affected in some way or another.
3.1. Development of “Standard Operation Times”
The terminology of “standard time” is believed to be self explanatory. In this case,
what we are talking about is the “standard operation time per piece” which have to be
spent for a shop order of a specified part number when it is processed at a work
center. Thus, the sum of all “standard operation times per piece” in the routing of a
shop order multiplied by the number of pieces on it will be equivalent to its “standard
work content” in terms of man or machine hours (Figure 32).
Figure 32
A schedular must have information or at least an idea of “standard work content”s
of shop orders to be loaded to a work center in order to come up with respective
“remaining work content” values associated with each respective shop order.
Thus, it will be possible to use specific techniques to determine the sequence of
loading. Such techniques will be discussed in later sections of the lecture notes.
One last thing we should say about “standard operation times” is that, in majority of
real life cases, it is usually difficult to find them especially in small and medium scale
companies. In large scale companies, they are usually obtained by treating the
“actual operation time”s with some factors which are found to be appropriate.
Sometimes, the operation times which are standardized in this way, are assigned to
the work center itself. In such cases, it is assumed that all the part numbers passing
through that specific work center will have exactly the same “standard operation
time”. This approach may arise some questions. Forexample (Figure 33), is it
acceptable to assign the same standard operation time to two different part numbers
at a drilling work center, when one of them requires one drilling of one hole while the
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Figure 33
other one needs drilling of three holes? Even the differences in the size or shape of
the holes or the type of the material itself (aliminum or steel) cause differences in
operation times.
For the above stated reasons, development of reliable “standard operation time”s is a
very important subject for realistic scheduling especially in job shops.
3.2. The concept of “Earned Hours”
In the previous section, the “remaining work content” of a shop order was mentioned.
In this section, we are going to tell some specific facts about the “earned hours”,
because, inorder to determine the “remaining work content”, one has to know the
“earned hour” value of a specific job at any point in time(Figure 34).
“Earned Hours” is the measure of how much of the work is completed in standard
terms. Number of actual hours spent is rarely equal to earned hours. In practice
there is a wide range of factors causing this fact.
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Figure 34
Determination of earned hour values in flow shops (either flow assembly work or flow
fabrication work) is somewhat straight forward. On the contrary it is difficult to make
exact determinations in job shop environments. Such environments may require
additional tasks of mapping the routing of each part number in terms of
manufacturing progress steps. An example of such a mapping of routing can be seen
in Figure 35.
Figure 35
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Obviously, the approach given in Figure 35 requires extensive computations by a
specifically designed software in the existince of hundreds of shop orders floating in
job shops of a manufacturing company.
3.3. The concept of “Group Technology” (GT)
The central theory of group technology states that; various situations that require
decisions can be grouped together based on preselected, commonly shared criteria,
and that the decision which applies to one situation in the group will apply to all of
them in that group.
Thus, the definition of group technology tells that; it is a technique for identifying and
bringing together related or similar parts/components or machines in a production
process, in order to take advantage of their similarities by making use of the benefits
of flow production methods.
Although both MRP and Group Technology are usefull in planning an control of
multiproduct, small lot-sized job shops, they appear to be in conflict with respect to
their individual operational characteristics. When group technology is applied to the
manufacturing of parts, the emphasis is on grouping them in terms of their common
manufacturing characteristics regardless of the level of product structure they are in
or the timing of when they should be manufactured. On the other hand, MRP strictly
considers the position of respective part within the product structure hierarcy, and the
timing of fabrication to meet the required end item due date.
In other words, group technology wants similar parts manufactured at the same time
regardless of the requirement dates, and MRP wants parts to be manufactured at the
scheduled times regardless of the manufacturing impact.
At this point, one may ask the question of whether it is possible to integrate MRP and
GT. The answer will be a carefull “yes”. Especially at workstations where both the
number of unique part numbers and the volume of production is high, respective
parts, reardless of their position in the product structure, can be grouped by their
manufacturing characteristics and by the requirement quantities for fixed intervals of
time. For such a group, most probably, the production schedule (qty and timing) will
be the production schedule of the part which has the earliest due date. The rest is to
define the respective part numbers by a specific code or group number within the
MRP data base and assign ordering parameters (e.g. order policies) accordingly if
MRP software is considered to be modified for handling such parts. But, since trying
to modify the engine of MRP is a dangerious task and costly, required replenisments
can be downloaded to a PC as raw schedules for further processing and group
scheduling. The most important aspect to be remembered of this case is that,
“dynamic grouping of parts” must be performed. By “dynamic grouping” it is meant
that, each time when the machine is being loaded, the group content may be different
from the previous case. That is, if a specific load is composed of Part A, Part B, and
Part C; the following load may contain Part A, Part C, Part E and Part F. If one tries
to utilize “static grouping” (e.g. Part A-Part B-Part C always togerher), after the first
release of a specified group of shop orders, things will get much more complicated.
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Deficiencies of the “static grouping case” can be observed in the example given by
Figure 36.
Figure 36
3.4. The concept of “Flow efficiency”
Measuring flow efficiency in job shop manufacturing environment will give important
clues to the scheduler about the shop behaviour in terms of the speed of production
flow. This is important especially where jobs (shop orders) are flowing through
multiple stages (work stations).
In order to provide a clear vision of flow efficiency, the following has been
summarized from “Factory Flow Benchmarking Report, Lean Aircraft Initiative, 1996”.
“Flow Efficiency is defined as the ratio of the fabrication time (touch labour time)
to the cycle time (also called throughput or flow time). Factory Flow
Benchmarking Report (#RP96-06-61) by the Lean Aircraft Initiative defines
major components of the cycle time as; fabrication time, lot process delay,
storage delay, and transportation delay. Subject report says that; in the airframe
sector, proportion of cycle time that the product was waiting was 96 %. Also it is
indicated that, the airframe sector had flow efficiencies from 0.02 percent to 0.8
percent (Note: These values belong to 1996).
Actual labour hours per part per crew member
Flow Efficiency = (
)*100
Total cycle time the part is in the system – router queing
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Figure 37
Correlation analyses carried out had shown that, there would be higher flow
efficiencies with “lower lot sizes” and with “shorter distance travelled”. Other
findings can be listed as in below;

The flow efficiency, at its best, will vary inversely with the lot size (Max
attainable flow efficiency = 1 / lot size).

It has been detected that, there were no flow efficiencies above 2 % if the
parts travelled more than 2000 feet (609.6 m).

It has been found that, the process type layout had a great deal to do
with the flow efficiency. Job shop layouts did not achieve above 0.1 %
flow efficiencies while flow shops, cells or dedicated lines were able to
achieve values as high as 18.7 % flow efficiencies.

The median values for routing queuing (mnfg approval time) ranged from
11% to 32% of the cycle time. Maximum value was 83 %.

The distances travelled by the parts found to be averaging from 2416
feet (736 mt) to 5023 feet (1531 mt).

Process control steps ranged from a low of 9.3 % to a high of 81.4 %
and averaged from 44 % to 55 %. The predominant process control
method (at about 80 %) was process verification consisting mostly of
manual inspection.”
An analysis had been carried out at Turkish Aerospace Industries in 1999, about
fabricated flat sheet metal parts. This study was covering a total of 599 shop
orders (4867 pieces) for which “flow efficiency” & “cycle times” were calculated.
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Obtained results were as following;
Scheduled Flow Efficiency
= 0.36 %
Actual Flow Efficiency
= 0.60 %
Attainable Max Actual Flow Efficiency = 30 %
Scheduled Cycle Time = 66 M-days.
Actual Cycle Time
= 70 M-days.
However, there is a trap in dealing with flow efficiency values, from which a schedular
must avoid. This can be better explained by the following example;
 Let’s assume that, shop order F200200 is released for manufacturing of 6
pieces of part number 353535-35.
 Again, let’s assume that,
o Only one operator worked on the shop order at each work station it
passed through.
o The scheduled cycle time for shop order is 10 M-days.
o The total standard work content (std touch labour) for shop order is 150
minutes.
o The standard router queue time is 5 M-days.
o The actual router queue time is realized as 7 M-days.
o The actual cycle time for shop order is realized as 15 M-days.
o The total actual touch labour for shop order is realized as 270 minutes.
 Therefore the calculation for realized flow efficiency will yield following result;
FE = {[(270/6)/1]/[[(15-7)*7,5]*60]}*100 = (45/3600)*100 = 1,25% and
Max FE = 1/6 = 16,7 %
 Where the scheduled flow efficiency for the case was as follows;
FE = {[(150/6)/1]/[[(10-5)*7,5]*60]}*100 = (25/2250)*100 = 1,11 % , and Max
FE has the same value of 16,7 %.
Above example suggests us, “the longer touch labour hours on the shop order” tends
to show “higher flow efficiencies”. So, especially during the setups of machinery or
where man power is utilized to perform the required operation, if the job progress is
slower than some target value, flow efficiency value might increase artificially. We
can conclude that, in the above case, either something is wrong with the
demonstrated performance of the labour, machinery, equipment or with the standard
values we are using. A good schedular would have good guess of what was
happening at the shop floor.
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3.5. Activity Grouping in large Assembly Work Stations
In large assembly lines where the end item moves through a series of work
stations, each station corresponds to a specific “work breakdown structure” (as
discussed in section 2.3.2). In this way, the whole end item is represented by a
series of “work breakdown structure”s each of which is a subject of master
scheduling. That is, there is a separate master schedule for each “work
breakdown structure” where each master schedule is sequentially dependent
on the previous one in the series (Figure 38). For this reason they are called
“scheduled work breakdown structures” (SWBS).
Figure 38
Within each SWBS, there are a number of assembly operations some of which
are also sequentially dependent to others. Usually, the time required to
complete the whole tasks for an SWBS may take several weeks. Naturally, the
respective assembly operations require detail and subassembly parts together
with some materials which are fabricated in the same facility or procured from
outside.
Figure 38 shows an example assembly line for such an end item and its
breakdown into SWBSs. It also shows breakdown of the work content of
sample SWBS which is named SWBSC2 in terms of the sequential operations
to be completed. Let’s assume that the make span assigned to SWBSC2 is 45
M-days and the part requirements of the 14 operations are as in Figure 39.
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Part Requirements for Operations of SWBSC2
# of required
OPERATION
required qty
make items
1
2
3
4
5
6
7
8
9
10
11
12
13
14
220
170
125
130
450
210
180
280
190
175
375
230
195
345
480
512
200
150
970
230
190
420
225
210
420
520
380
460
TOTALS
3275
5367
Figure 39
In MRP database, the independent requirement for SWBSC2 will normally
have due date as M-Day 4545, and the operation part numbers will have
process time of 45 M-Days and, lets say, 2 M-Days as stock time which is
going to be used as a part of move time from SWBSC2 to SWBSAC1.
Exploding MRP only with this information will result in a necessity to complete
all 5367 pieces just before M-Day 4500 which is the scheduled start date of
SWBSC2. All of the Rp schedules of respective details and subassemblies will
be calculated accordingly. At the time of ordering these parts to fabrication
areas, there will be a sudden jump in the work load and most probably
temporary bottlenecks will be created.
Activity grouping in such assembly lines may prove to be useful in the
operation of job shops by distributing the work load as evenly as possible
through out the time. This grouping is about the grouping of the operations
within an SWBS by considering their interdepencies. Thus, it becomes
possible to distribute the requirement dates of detail parts over the make span
of the respective SWBS. In the example of Figure 38, 14 operations of
SWBSC2 are divided into three groups for which the respective requirement
dates are M-Day 4500 (for 2532 pieces), M-Day 4515 (for 1365 pieces) and
M-Day 4530 (for 1470 pieces) respectively. The next step to do is to treat the
process times of 14 operations as follows;



First Group (6 operations) ; Process time : 45 M-days (no change).
Second Group (4 operations); Change Process times from 45 to 30.
Third Group (4 operations); Change Process times from 45 to 15.
The rest will be performed by MRP software yielding to a smoother shop order
release activity and in this way, a smoother loading of shops.
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3.6. Takt Time and Cycle Time concepsts : What do they mean to a
Schedular?
Both “takt time” and “cycle time” are the powerfull tools in understanding the
shop floor. Continuous observation of these values gives a good idea about
the behaviour pattern of a work center or of a specific shop itself. Variances in
the respective values may occur from period to period due to some expected
or unexpected reasons such as “over loading”, “machine breakdown”,
“unavailability of a tool”, “inexperienced worker”, “seasonal high or low
temperatures”, or even “the announcement of raise % in monthly salaries”.
In addition to understanding the shop floor, sometimes, decision making
process may state “which time value is to be used as a basis for scheduling
purposes : takt time or cycle time or standard time for a process”.
3.6.1. What is “Takt Time” ?
"Takt" is the German word for the baton that an orchestra conductor waves at
the musicians, to regulate the speed at which they play. In manufacturing
terms, takt time is a calculated value.
Takt time is the speed at which parts must be manufactured in order to satisfy
demand. It is simple enough to calculate:
 Determine the daily demand (order) volume. Lets say we have orders for
215 arm chairs per day.
 Determine the number of working minutes in a day. Let's say that we have
an eight-hour day, with 30 minutes for lunch and two 10-minute breaks. This
means we have:
(8 x 60)-30-10-10=430
minutes, or 430 minutes in a working day.
 Divide the number of minutes by the number of products needed. In our
current example, the calculation would be 430÷215, which equals 2. This
means that one unit must be manufactured every other minute in order to
meet demand. Takt time is 2 minutes.
Here what we have to clearly indicate is that, “takt time is the goal”. It must be
reached to satisfy demand.
In the mean time we also need to mention that there are certain points which
one has to keep in mind when talking about “takt time”. These points can be
clarified as follows;
 “Takt time” does not consider the terms “capacity” or “hours per piece”. It
simply tells “the lenth of time at which one unit of product is to be completed”
to meet the demand volume for the specified period. If takt time is less than
the process time of one unit, the ultimate goal should be to achieve some
process improvements which will shorten the lead time. In practice, usually
the first thing which is considered in such situations is to provide additional
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capacity by using overtime, which most probably will not yield to desired
results.
 Work centers for which takt time will be the measure of performance, should
be arranged in a way to process “part families”. This means that, the parts
to be processed in respective work center should be similar with respect to
operational characteristics such as, process requirements, material type,
material thickness, dimensions etc.
 In a production environment which is going to be monitored by takt time, the
work content or product mix and part routings, should remain same as much
as possible.
 Again in such environments, all of the shop orders must be “workable”.
3.6.2. What is “Cycle Time” ?
The second type of time is "cycle" time. This is a measured value, not a
calculated value as takt time is. In other words, you must go out to the floor,
and measure the time it actually does take to manufacture the product.
When making time observations, it is important to measure both the total cycle
time for each operator (how long the job takes from beginning to end), and the
time of each of the component tasks that make up the cycle. The cycle cannot
be improved without a detailed understanding of what makes it up, and often it
is possible to reassign component tasks to rebalance the operation.
After making the observations, one can draw out an Operator Loading Bar
Chart to graphically express what is going on in previous example (Figure 40).
The horizontal axis on this bar chart represents operators, whose times are
indicated by the use of stacked bars along the vertical axis, which is the time
axis. Draw a thick line across the chart to represent the takt time (two minutes,
in this case); this makes it easy to tell at a glance whether any individual
operator is working within takt time, or exceeding it.
Let us suppose that the six operators are producing one arm chair in two
minutes, but that their cycle times look like as in Figure 40.
Figure 40
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Arm chairs cannot be produced any faster than the slowest operator works, so
in this case, we get one arm chair every two minutes, but the six operators are
working for a total of 8 minutes to do it.
Again assume that, we observe enough waste in the work cycle to set a team
goal for reducing the total cycle time from 8 to 6 minutes. To determine the
staffing of this line, we divide the new total cycle by the takt time: 6 ÷ 2 = 3
operators. Three operators would be sufficient, and the new bar chart would
look like as in Figure 41.
Figure 41
Thus we can assign three freed up operators to some other value added
tasks.
A similar “Load Chart” is given in Figure 42 which is from a real life example of
what has been achieved in assembly of flight deck panel production.
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Figure 42
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