IE434 Lecture Notes
SPRING 2011
SCHEDULING
I. INTRODUCTION
In one’s everyday life, although the word “scheduling” is not pronounced much,
almost every action is taken by considering several important factors. These factors
can be summarized as “which actions are to be taken”, “in which sequence they are
going to be performed” and “how long each action will take”.
1.1. A Sample Daily Life Problem
Inorder to clearly visualize what scheduling process is, one can simply think of the
following case of an emergency situation.
Lets assume that you live in Dikmen and it is a Saturday morning, 10:20 am.
Suddenly you receive a call from one of your friends, who is living in İzmir, telling you
that he is waiting at AnkaMall, Akköprü, reminding that you had to meet him for
breakfast at Kocatepe Kahvecisi at 11:00 am and he has to leave at 11:30. Having
the shock of realizing that you had completely forgotton about it, you remember that
you had another promise for the day which is about taking your mother, who is living
at Ayrancı, to a doctor at 12:00 am. Which means that you and your mother must be
at Kızılay by that time. The alternative routings which you can take are as given
Figure 1.
You can not cancel your meeting with your friend, because he will be in Ankara for a
very limited time and then leave for İstanbul for a business trip. Also, you can not
postpone the visit to the doctor since it will take another week to have a new one. In
such cases, usually the problems follow one another. Your credit card is out of date.
You are free to spend 160 YTL cash which is in your pocket, and earliest time you
can leave home is 10:22.
Now, you have a real problem which is namely a “scheduling problem” having its
own constraints in terms of time and cost. You have to find a satisfactory solution for
it.
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Figure 1
The simple sequential definition of the above problem can be itemized as following;
a) Travel from home to Anka Mall (constraints: travel time<= 38min at min cost)
b) Breakfast at Anka Mall (constraints: stay time=30min, cost=30 YTL)
c) Travel from Anka Mall to mother’s home and from there to Dr (constraints :
travel time<=30min, travel cost min, Dr’s fee =100 YTL
with the assumptions that;
a) on each single route (a) thru (g) as shown in Figure 1, one can utilize either
one of the three transportation media, which are namely; Taxi, Public (Dolmuş
or Bus) and Metro (where available).
b) Appointment hours of 11:00 a.m and 12:00 a.m are sharp. Even +1min is not
allowed.
In this problem, 30 min of stay time at Anka Mall, 30 YTL of cost for breakfast and fee
of 100 YTL for Dr are constants. For this reason, one can deal with the combinations
of two phases of transportation (home-Anka Mall and Anka Mall-Mother-Dr) which
include variables.
Accordingly, let’s say that one comes up with 4 alternative ways of transportation
from home to Anka Mall as follows;
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Table 1
Table 2
Table 3
Table 4
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..... and with 2 alternatives for the routing of Anka Mall-Mother’s Home-Dr;
Table 5
Table 6
Thus, the evaluation results of the different combinations of routings can be given as
in Table 7.
Table 7
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Now, one can choose R1+R2 combination as the best one among the 8 lines of
Table 7, according to our purpose. If alternative R1C+R2A is choosen, then the
accompanying schedule will be as follows;
10:22
10:23 – 10:28
10:29 – 10:40
10:41 – 10:46
10:47 – 10:59
11:30
11:31 – 11:33
11:34 – 11:48
11:49 – 11:51
11:42 – 11:48
Departure from home
Waiting for bus or dolmuş (a1)
Transportation to Kızılay (a1 + b1)
Waiting for Metro (e1)
Transport to Anka Mall (e1)
Departure from Anka Mall
Waiting for Taxi (d1)
Transport to mother’s home (d1 + f1)
Waiting for taxi (g1)
Transport to Dr. (g1)
1.2. Surprising Resemblence
Infact, the above given example is very much alike the ones that one can come
across within the daily life of a production environment.
It is easy to transform it into a shop floor scheduling problem by making the following
analogy;








Accomplishing the overall route (Home-Anka Mall-Mother-Dr) ≡ 1 piece of
a sheet metal part,
The environment where, Home-Anka Mall phase takes place ≡ sheet metal
shop,
The environment, where Anka Mall-Dr phase takes place ≡ chemical
process shop,
Each of the individual portion of the overall routing (a thru g)≡ a production
operation,
Each type of transpotation media ≡ a different machine / work center
having specific capabilities,
Waiting time ≡ queue time,
Transport time ≡ setup+operation time for a specific machine / work center,
Total span ≡ cycle time (to complete 1 piece of the specified product).
Thus, the alternate flows of the subject sheet metal part towards its completion can
be shown as in Figure 2. Also, the associated cycle times and costs for different
alternatives can be seen in tables 8a and 8b respectively.
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Figure 2
For the above mentioned single product, the problem was to minimize both the time
and cost of production flow. “What happens when there are 50 unique parts with
different production lot sizes ranging from 4 to 12 and all utilize the same resources?”
is a different question for which an answer must be found.
Table 8a
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Table 8b
1.3. Definitions for “Schedule” and “Scheduling”
As an introduction to the subject of “Scheduling”, some of the definitions presented in
various literature can be given as follows;
“A “schedule” may be defined as a plan with reference to the sequence of and time
allocated for each item or operation necessary to its completion.”
“Scheduling can be thought of as a process; that is, someone prepares a schedule
either for when an end item or an item will be completed or what series of activities or
jobs are to be completed during a specified time by the work center of interest.”
“Scheduling is ... a ‘Tabulated statement of detail, a timetable'. This clarifies the role
of the schedule as a defined statement of work to be done, fully sequenced and
prioritised for employment of management of the activities themselves.”
“Scheduling concerns the allocation of limited resources to tasks over time. It is a
decision making process that has as a goal of the optimization of one or more
objectives”.
With reference to the above definitions and the field applications;
One can say that a schedule is the product of a scheduling activity within the
boundaries of a defined scheduling process.
Here, a schedule, is the timetable/plan showing the prioritized sequence of
activities including the time allocated for each activity inorder to complete a
specified quantity of a product or service. Thus, one can conclude that, a
schedule has four basic elements in its self. These are; “the definition of end
result/item and respective quantity” (the answer for what and howmany),
“sequence of steps” (the routing for obtaining the end result/item), “definitions
of each step” (along the routing) and “time allocated to each step”.
Scheduling, as an activity, is the use of data and different tools or
techniques in consideration with the constraints to come up with a reasonable
schedule (plan) among several alternatives. While performing this activity, a
schedular will need specific pieces of data and information related with each
step for ontaining the defined end result in terms of resource utilization
(Figure 3).
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Figure 3
Scheduling process, can be defined as the way of doing the
business/environment of scheduling activities within a company. It defines the
associations of different functional groups and computerized system modules
with respect to flow of required data, establishment of accepted rules and
policies to be used for scheduling activities including the maintenance of data
and availability of the required tools and techniques.
As a result of this dicussion, one can observe that, in field applications there
are three basic elements within the definition of “scheduling” which need to be
clearly understood (Figure 4).
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Figure 4
Generally speaking, scheduling activity can also be considered as an
intermediate step between planning and execution steps. It is also worth
mentioning that, capability of a company’s scheduling process directly effects
its scheduling performance in creating realistic schedules, and the scheduling
performance has a significant impact on the company’s manufacturing
systems performance. As a result of this argument one can say that;





Schedule is the end product – which is “meal”.
Scheduling activity is the preparation of schedule– which is “cooking”.
Scheduling process is the environment where the scheduling activity is
performed– which is “kitchen + required materials”.
Execution of a schedule is the realization– which is “eating the served
dish of meal”.
Whether the “served dish of meal” is “eatable or not” is the main
problem in many production companies.
1.4. Understanding Levels of Scheduling Tasks in a Manufacturing
Company
In any manufacturing company, there should be a series of scheduling actions for
each specific contract signed with a specific customer.
Usually such scheduling actions start with the development of a “schedule for
production preparation” and continues with the preparation of “master production
schedule”, “requirement and replenishment schedules” calculated by an MRP
software or a similar tool, “Shop Floor Movement Schedules” calculated by a Shop
Floor Control (SFC) software or a similar tool and reaches down to the level of “final
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assembly build schedule” for assembly parts and “work center loading schedules” for
detail part production respectively.
It is important to realize that, each of the above mentioned schedules differ from one
another by their purpose and also by their nature. Different tools and techniques are
used for their construction. Also, they are continuously subject to change at any time
since some of the factors effecting the life cycle of a schedule may be out of control
(change in customer demands at end item level, energy shortage, supplier problems,
breakdowns etc).
Before going into specifics of “scheduling”, one should be able to clearly visualize
that, what kind of scheduling approach will be appropriate to use when. At this
point, it is not wrong to tell that, number of levels and kind of scheduling
approaches/strategies
are
dependent
on
the
type
of
current
“Manufacturing/Production Planning and Control” (MPC) approach of the
company. Also, the type of the MPC approach is dependent on the production
characteristics of the product which is usually defined by the complexity of the
product, nature of the required processes and amount of time required between
successive units. Figure 5 explains this argument.
Figure 5
In Figure 5, up-down axis points to the complexity of the products with reference
to the number of component parts (sub-parts) in the respective BOM structure(s). For
example, the body of an aircraft moving along the different work centers (scheduled
work breakdown structures) of an assembly line may have 40-50 levels in its defined
BOM structure, where as a detail part has only one level.
On the other hand, left-right axis points to the amount of time required between
successive units. In other words, it refers to the time span required to start the next
unit. Forexample, in petroleum refineries or in food industry, there is almost no time
between two successive units. Production flows in streams (the far left side of Figure
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5). Production of detail parts such as a “bushing” or “pin” are of repetitive type, since
the time between to successive units is measured in minutes. Just in time and MRP
approaches cover a wider range of products and processes. MRP is still an key
approach when the case involves management of complicated parts situation. Just in
time approach has the specific goal of standardizing the processes, reducing the
cycle times, reducing lead times, reducing inventory levels and hence moving the
nature of “uniqueness” towards to “repetitiveness”. For this reason, these two
approaches are tried to be integrated where possible.
In any case, all of the above indicated approaches operate on the flow rate of the
end items which is given by MPS. But, there are some differences between them
with respect to the tools and techniques that are used.
This relation between levels of scheduling and the MPC approach in use can be best
explained by studying the behaviour of an imaginary manufacturing company starting
from the contract award thru the serial production activities of a specific product as
described in the following example case.
1.5. CASE STUDY [Manufacturing Planning and Control (MPC) process at
Shiny Metal and Assembly Works A.B (SMAW)]
1.5.1. Getting to know our company (SMAW)
Let’s assume that, “Shiny Metal and Assembly Works A.B.” (SMAW) is a
“make to order” company (Figure 6) for which the business strategy is focused
on overtaking business offers which are potentially worth $ 50 million and
above in building structural assemblies for aerospace industry. It does not
have a product of its own, but produces for large companies which have at
least one. In the meantime, it also manufactures great majority of the detail
parts which are going to be used in the above mentioned assembly
/subassembly products. For this reason, in its long range plans, it studies for
acquiring capability of new processes and potential capacity increase
requirements by considering potential and high potential business offers.
Hence, it has a large investment in sheet metal shop, nc machine shop,
conventional machine shop, chemical processing shop and paint shop (Figure
7) all of which are job shops.
Due to the nature of the business type in which it is involved, each business
offer is considered to be a different project each having its own budget but
sharing the same manufacturing resources at the level of detail parts
production. Usually, the contract for a new project must be signed (become
firm) 6 to 12 months prior to the serial production depending on its volume of
work content. It is required for the revision / modification / adaptation of middle
range plans to the new situation. This planning stage usually covers the
allocation and operation of the resources inorder to gear up the plant for serial
production requirements. It mainly covers the manufacturing of the required
tools, the preparation of the master schedule, material requirements planning
and procurement activities for the new project. Currently SMAW has nine
projects for which the production is flowing through its shops.
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Figure 6
Figure 7
Since, most of the projects undertaken by SMAW has complex product
structures, its Manufacturing Planning and Control approach is MRP based
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(Figure8) and it has a good software architecture for manufacturing planning
and control activities (Figure 9).
Figure 8
It has a computerized management information system
Figure 9
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1.5.2. A multinational customer : FARSCAPE A.B.
Again let’s assume that, “Farscape A.B” (FARSCAPE) is a company which
designs and produces its own products (Figure 10). According to their new
strategy, they want to offload some of their workload to a subcontractor
company for a reasonable amount of price which they are willing to pay.
SMAW draws their attention, because of its reputation both in assemly and
detail parts manufacturing capabilities, and in achieving quality. So they offer a
job opportunity to SMAW which will initially cover a period of two years starting
from December 2008.
The offer covers to build a total of 1700 assemblies for two different models
(A and B) of an end product which is currently being sold at the market.
Inorder to produce 1700 end item subassemblies, SMAW should also
manufacture a total of 3350 items of detail parts, procure 2575 items of
outside production parts and procure 528 items of raw material. FARSCAPE
states that monthly production rates of Model A and Model B are 10 and 6
respectively (Table 9).
Figure 10
Model
A
B
Total
FARSCAPE's BUSINESS OFFER
Number of
Number of
Subassy
Detail Parts Monthly Rate
1175
2200
10
525
1150
6
1700
3350
Table 9
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1.5.3.What happens at SMAW upon receipt of business offer?
The receipt of the proposal, is the start of negotiations among SMAW and
FARSCAPE, as well as the start of preliminary manufacturing planning
activities at SMAW which will yield to the preparation of bid proposal. These
initial activities are concentrated around the following topics;









Request of additional technical information from FARSCAPE, which will
enable SMAW to calculate some quantitative results (Table 10) and use
in initial studies,
The quality specifications which must be satisfied both for subassemblies
and detail parts,
Determination of types of parts. Rough analysis of required
manufacturing processes and individual manufacturing routings of the
parts to be produced,
Determination of tooling requirements,
Analysis of capacity requirements in order to fullfill the stated monthly
rates of production. Determination of whether additional investment in
terms of manpower and/or machine/equipment is required,
Determination of raw and standard material requirements,
Determination of parts which will be procured from outside suppliers,
Calculation of estimated price of each individual end item which will be
delivered to FARSCAPE.
And the preparation of “Schedule for Production Preparation”.
Proposal+Planning Information obtained by studying FARSCAPE's Tech Documents
Model
A
B
Total
OP parts
(items)
1850
725
2575
Raw Material
(items)
528
528
Number of
S/A Tools
Number of
Detail Tools
925
2680
480
1650
1405
4330
Tooling
Material (items)
45
45
Table 10
After completing all the relevant studies, SMAW documents its proposal and
submits to FARSCAPE.
One of the most important attachments of this document is the “Schedule for
Production Preparation”. It shows which actions will be taken, what
resources will be utilized, and estimated start and completion times for each
activity until the shipment of first serial production batch. Thus, it will be one of
the most powerfull tools of proposal negotiations. In a way, it reflects SMAW’s
tecnical capabilities, relations with subcontractors, limitations and willingness
to overtake the business offer. An example of major portions of such a
schedule can be seen in Figures 11.a, 11.b and 11.c.
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Schedule for Production Preparation is infact a project management tool
prepared by using CPM/PERT methods and usually presented in GANTT
Chart format.
Figure 11a
Figure 11b
Figure 11c
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1.5.4. What happens at SMAW after the FARSCAPE contract is signed?
After the contract award is accomplished, a second schedule is prepared by
SMAW. It is called Master Production Schedule (MPS). Here, MPS is the
“anticipated completion/delivery schedule” for 1700 end items
(subassemblies) which are going to be delivered to FARSCAPE.
Since the forwarder (transport) company is a subcontractor of FARSCAPE,
SMAW uses “the shipment dates” as the “delivery dates” in this MPS (Figure
12).
At this level, based on the information provided by MPS, the Assembly Area /
Shop of SMAW (Figure 12) reorganizes the required portion of itself with
respect to the rate of production which it must achieve. Also, a Final
Assembly Build Schedule is prepared. But, for the other areas/shops, which
will produce the detail parts, more clarification of part flow quantities are
needed. Thus, it will be easier to answer the questions related with, whatwhen-where-how-by whom aspects of flow of parts through these shops. The
same thing is also valid for other groups such as tooling, subcontracting and
material procurement departments inorder to supply both fabrication and
assembly areas.
Figure 12
Fortunately, SMAW has an MRP II system in place. Upon the finalization of
BOM structures of 1700 subassembly part numbers, FARSCAPE MPS is
entered into the MRP subsystem enabling it to calculate the net requirements
and to prepare “requirement” (Rq) and “replenishment” (Rp) schedules for
a total of 8153 items/part numbers (Figures 13a and 13b).
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Figure 13a
Figure 13b
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In the mean time, SMAW has some problems
a) MRP software considers that there is an unlimited resource
availability:
This is the initial point where, SMAW has to solve its first “scheduling
problem” if it can. The problem is related with the “replenishment
schedules”. This is mainly because of the main characteristic of an MRP
Module. It always considers that there is an unlimited resource
availiblity. But when there is a flow of nine more projects through the
detail part manufacturing shops, the situation usually ends up in a
nightmare. This means that “calculation of net requirement quantities” are
perfect, but “when they are really needed” is blurred.
b) Conflict in the designed behaviour of two integrated modules : MRP
and Shop Floor Control (SFC) modules:
There is another important problem in Production Management System
(MRP II) of SMAW which causes conflicts at the shop floor level and it is
directly related with shop order release dates and work center loading.
Main cause of this problem is inherent in the designed behaviour of MRP
and SFC modules. Although both of them are using backward scheduling,
their calculation methodology for shop order start dates are different.
MRP uses the production lead time values as specified for the respective
part number in BOM module, but, SFC uses average move & queue times
and process times of the work centers along the routing of a part number.
This situation results in mismatched manufacturing order release and start
dates for the same shop order of a make part number within two integrated
modules of MRP II (Figure 14).
Figure 14
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c) Lacking capability of MRP Engine: It can not handle “fractional
quantities”.
The third kind of problem, which may directly hit the shop floor scheduling
and increase the cost of operation at SMAW, is directly related with lack of
a specific capability in MRP engine. It can not handle fractional quantities.
During the normal explosion process, when creating planned
replenishments, MRP associates an order bill to each of them. Then, if the
respective order release date is within the “allocation window”, it allocates
material for the items listed in this order bill by calculating the required
quantities of each.
Since it can not handle fractional quantities, for some type of raw materials
(such as sheet metal or bar stock etc) allocation may result yielding one of
three possible cases : a) Allocating more than required qty, b) Allocating
less than required qty, and c) Rarely allocating the required qty.
As a result of this fact, it is very much likely to face problems with respect
to the quantities related with raw material. In such a case, although
required raw material may be available at some place in the form of a
portion of sheet metal (remnant case), the possibility of not loading a
specific shop order to a work center is high due to imaginary shortage of
raw material. Or on the contrary, excess material will be issued from the
warehouse creating more remnants. An example of such a remnant case
can be seen in Figure 15.
Figure 15
The above indicated factors, will create a mess in material planning and work
center loading impacting “the scheduling performance”.
MRP planners of SMAW who release the shop orders are well aware of the
above problems, but the fact is that they can not explain it to neither
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management nor to production people. The daily life in a manufacturing
company has its own unique rules and sometimes it is not so easy to remove
or correct them...
Anyway, having the FARSCAPE MPS input, MRP explodes through the
product structures, and prepares the Rq and Rp schedules on the basis of net
requirements, and signals the respective MRP planner for the release of first
shop orders for the detail parts. This action initiates addition of shop orders to
work center backlogs in sheet metal manufacturing, nc machining and
conventional machining areas. The composition of the backlog of a respective
work center may include shop orders of several projects at any given time.
This means, these backlogs contain a product mix at that moment, content of
which, is subject to change throught out time. Thus, there is a real problem of
sequencing and completing the jobs of a product mix within allocated time
limits. Somebody has to schedule and re-schedule the loading of respective
machine(s) within the work center. This is the lowest level of scheduling at
SMAW.
1.5.5. Summary of the “Levels of Scheduling” tasks at SMAW :
As it was previously mentioned, scheduling activity is an intermediate step
between planning and execution. In other words, it is a quantified and
sequenced prescription of where, when and within which time frame to
execute the planned activities.
Going back to the subject case of SMAW, the summary of the schedules
prepared at various sections of the legal MPC approach / system (Figure 16)
in use can be given as follows;
a) Schedule for Production Preparation is the first schedule and has
been prepared by utilizing the CPM-PERT approach. Each activity in this
schedule can be thought as a one time production of a complex product
for which it takes several months to complete.
b) Master Production Schedule is the second schedule and has been
prepared by considering the negotiated delivery dates and delivery
quantities of the respective end items after an in depth analysis of
capacity requirements including all active projects. It is the anticipated
build schedule where FARSCAPE project is included with monthly net
delivery dates and quantities.
c) MRP created Rq and Rp Scledules form the third level schedules for
each one of the low level coded component parts. They are created by
the use of product structures of the end items, on hand and on order
quantities and various ordering codes and parameters defined by
respective functional groups. At SMAW, these schedules are to be
considered as raw schedules because of the previously mentioned
reasons. The first main reason was that, MRP does not consider on
capacity constraints. And the second main problem was the
inconsistency of MRP and SFC calculated shop order start times.
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d) SFC created Schedules for each released Shop Order are the fourth
level of scheduling in SMAW’s MPC approach. It keeps the MRP
calculated order completion date as it is, and performs back scheduling
along the routing of the specified part number considering the order
quantitiy together with avarage work center M/Q and processing times.
e) Final Assembly Build Schedule is the fifth level schedule and has been
prepared by considering MRP based need dates. The assigned
assembly work centers will be used only for FARSCAPE project. It is
batch production in nature. End products, are assigned to specific work
centers and will be produced in batches. The time required to complete
one piece ranges from 1,5 hrs to 32 hrs with respect to the complexity
and size of the respective part number. Manual manipulation of MRP
orders are in place to distribute the production of monthly delivery
quantities to the days and weeks of the respective month.
Work Center Scheduling at Detail Part Manufacturing Areas is also
considered to be fifth level of scheduling. It already exists and is being
done first by loading the work centers in the order of “critical ratio”
calculated by SFC and then by giving manual priorities to shop orders
which are told to be hot by general foremen of different assembly areas
(Figure 17). That is, manufacturing control meetings are the key decision
making points. Both the variety and volume of the parts are high.
FARSCAPE is one of the projects out of 10.
Figure 16
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Figure 17
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