planning production preparation processesusing the critical ...

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42 ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 15, ISSUE 1/2017

PLANNING PRODUCTION PREPARATION PROCESSESUSING

THE CRITICAL CHAIN METHOD

Iwona LAPUNKA1, Iwona PISZ

2 and Piotr WITTBRODT

1

ABSTRACT: This paper explains the usefulness of critical chain project management for planning

production preparation processes. A synthesis of issues of project management and production

preparation processes is presented, and a conceptual model of concurrent design processes using the

critical chain method is developed. The proposed model of planning production preparation processes

enables improving the effectiveness and economic efficiency of new product development. The main

assessment indicators of efficient project management in production preparation are: shortening the

preparation time of new product development, rational use of resources, and reducing the risk of failure.

The proposed approach will be illustrated with a case study. The presented problem is one of the most

significant elements of management processes concerning the manufacture of products, particularly under

the conditions of unrepeatable and irregular production, which, to a large extent, is characteristic of small

and medium-sized enterprises (SMEs).

KEY WORDS: production preparation processes, critical chain project management, planning process,

new product development.

1 INTRODUCTION

The course of a production process and its

effectiveness depend on preparative actions

undertaken prior to commencing production, known

collectively as the production preparation process

(Chapman, 2005; Brun et al., 2008). This process

entails structural and technological preparation,

taking into account organizational (operative)

planning (Laarhoven & Zijm, 1993).

The main aims of production preparation are:

elaborating designs of new products and their

production methods, commencing production, and

the continuous improvement of products.

Production preparation is integral to the functioning

of production companies, since it comprehensively

shapes their technical and organizational level,

affecting the economic outcomes of business

activity (Toni & Meneghetti, 2000).

Contemporary production preparation systems

are characterized by a specific scope of works for

the preparation of production processes (Baraldi &

Kaminski, 2008; Pavletić & Soković, 2009). Recent

trends in this scope include (Slack, Chambers &

Johnston, 2007; Gustavsson & Wänström, 2009;

Vasant, Weber & Dieu, 2008): 1

Opole University of Technology, Faculty of

ProductionEngineering and Logistics, Ozimska 75,

45-370 Opole, Poland

2 Opole University, Faculty of Economics, Ozimska

46a, 45-058 Opole, Poland

E-mail: [email protected]; [email protected];

[email protected]

- progressing evolution of the notion of

production preparation – currently, it

comprises all activities leading to the

preparation, maintaining and termination of

production for a given product, as well as

the design of processes governing

processing, sale, and production process

supply,

- emergence of concurrent (simultaneous)

engineering (CE),

- progressing integration of works related to

structural preparation, the design of

production processes and their organization,

- increasing use of computer-aided systems

in production preparation,

- use of new optimization methods and

techniques, based on systems engineering

(project management, cost engineering,

modeling and simulation of processes, etc.),

- maintaining databases of previously

designed production processes,

- extensive use of computer methods for

production preparation, with the ensuing

changes to process design methodology,

- application of modeling and simulations of

structures, production processes and the

production system.

Typical stages of production preparation

include: elaborating technical and maintenance

requirements and preliminary design principles,

drawing up the preliminary draft and the

Pobrano z https://repo.uni.opole.pl / Downloaded from Repository of Opole University 2022-10-02

mailto:[email protected]

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ACADEMIC JOURNAL OF MANUFACTURING ENGINEERING, VOL. 15, ISSUE 1/2017 43

engineering and construction design, building and

testing a prototype, a test run and, ultimately, a

production run. At this stage the information on the

documentation. Such documentation describes the

structure of the entire product, all its parts, all data

necessary for technological preparation of

production, technical specifications for acceptance,

and the specifications of product operation,

maintenance and servicing. It can include such

design documents as: product assembly drawings,

assembly drawings of units and subunits, working

drawings, and operation and maintenance manual.

The technological progress plan and the plan

for technical and organizational activities constitute

the basic elements of production preparation. Such

plans outline tasks and means for achieving product

competitiveness through decreasing production

costs, ensuring the required ergonomics, safety,

environmental friendliness, quality, reliability, as

well as decreasing the costs of operation for

endusers. Designing production preparation

processes is necessary for planning the course of

activities, labor intensity of works, their cost, as

well as the subdivision of tasks among contractors.

The subject matter of this paper reflects the

emerging awareness in industrial practice of the

need for planning production processes in

environments concerned with single-piece and

lowvolume production, characterized by the lack of

repeatability and regularity. The efficient use of

companies’ production capacities under the

conditions of increasing competitiveness,

dynamically changing sales market, increasingly

shorter order and supply periods for the offered

goods, has become essential over the last years.

Despite this fact, relatively few research papers

tackle the issues of project management with regard

to developing new products, preparing for and

commencing production.

The aim of this work is to investigate the

planning of production preparation processes using

the critical chain method. The analysis was carried

out for the project of production preparation of a

special-purpose milling with use concurrent

engineering model (Fig. 1). Concurrent engineering

is a work model based on the parallelization of tasks

(i.e. performing tasks concurrently), which is

sometimes called simultaneous engineering or

integrated product development (IPD). It refers to

an approach used in product development in which

functions of design engineering, manufacturing

engineering and other functions are integrated to

reduce the elapsed time required to bring a new

product to the market. The elaboration and

application of the model of planning production

preparation processes for a new product in

accordance with the project approach entails the

improvement of effectiveness and economic

efficiency in the functioning of a company. The

main indicators of efficient project management in

production preparation are: shortening the

preparation time of new product development,

rational use of resources, and reducing the risk of

failure.

The presented problem is one of the most

significant elements of management processes

concerning the manufacture of products,

particularly under the conditions of unrepeatable

and irregular production, which, to a large extent, is

characteristic of small and medium-sized

enterprises (SMEs).

Figure 1. Model of CE processes for a special-purpose milling machine with the use of CCPM

Source: elaboration based on (Pająk, 2006)


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2 RESEARCH SUBJECT AND

METHODS

2.1 Research environment and subject

matter

The data for our case study were obtained from

a company specializing in large-scale, usually

single-piece, production. The data were collected

during the introduction of the product into the

market, therefore, the production order has the

features of a project according to the classical PMI

definition (one-time, intentional, separate, limited

and structurally distinct) (PMI Standards

Committee, 2013).

Exploratory research using a case study was

performed in an electromechanical company based

in south-western Poland. The company specializes

in the production of specialist machines for the

machining of wheel sets of rail vehicles, large

vertical boring mills, processing lines, and

machining centers. Said company carries out

singlepiece production, i.e. the products are

manufactured to customers’ individual orders. Each

product requires a separate technical design,

comprising technological and design solutions, with

the aim of guaranteeing the desired quality of the

finished product. The design serves as the basis for

manufacturing the product according to customers’

expectations and in compliance with legal

requirements.

We investigate a technical design for the

production preparation of a special-purpose milling

machine using gun drills to drill holes in crankshafts

(Fig. 2). The process involves deep-hole drilling

using gun drills to drill two or three ø 4.1-mm holes

at an angle of ca. 45°. The production project of the

machine was planned for 13 months. This period

includes the elaboration of complete technical

design documentation, which will subsequently be

employed in the building of the special-purpose

machine. In addition, mechanical, electric,

hydraulic, and pneumatic documentation will be

elaborated. The prepared specifications will enable

the picking of design elements. The machine will be

manufactured and assembled in a machine factory

in south-western Poland. Subsequently, it will

undergo assessments and machining tests. Test

results will be followed by implementing the

machine for use in the company of the customer for

the purpose of carrying out an industrial program of

product range production.

Figure 2. A schematic depiction of the machine using gun drills to drill holes in crankshafts


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2.2 Critical chain method

Critical Chain Project Management (CCPM)

belongs to the class of schedule network analysis

techniques (Herroelen & Leus, 2001). These

methods are the most commonly used approaches

for planning and controlling project execution. Not

only do they provide management with quantitative

information necessary for decision-making, but also

focus management’s attention on time factors,

means and labor that can increase the effectiveness

of project execution.

CCPM aids project management, particularly

under the conditions of uncertainty with regard to

the duration of individual tasks. Compared with

methods elaborated in the 1950s, such as the

Critical Path Method (CPM) and Program

Evaluation and Review Technique (PERT), CCPM

is considered to be a major improvement (Shurrab,

2015; Watson, Blackstone & Gardiner, 2007). New

approach to project planning and execution

proposed by E. M. Goldratt (1997) raises great

interest in professional literature (Herroelen, Leus

& Demeulemeester, 2002; Leach, 2005; Lechler,

Ronen & Stohr, 2005; Rand, 2000; Raz, Barnes &

Dvir, 2003; Steyn, 2000; Steyn, 2002).

The CCPM method attempts limiting of the

“Parkinson’s law” impact, which suggests that

regardless of the actual work amount, the task

execution time always takes a planned extent, and it

may even exceed it. Restrictive Goldratt concept

assumptions shorten the planned execution time of

subsequent activities, motivate people for hard work

and for meeting strictly determined deadline. As a

result the planners approach the task execution time

evaluation more optimistically. Additionally, as

schedule is established in restrictive manner, with

very short planned task execution times, without

hidden time reserves and without possibility to

delay with the commencement of each task, the so

called “student syndrome” is being levelled i.e. the

tendency to delay works commencement till the last

possible moment. Possible delays during the tasks

execution may be recovered by application of

intentionally designed safety buffers, which does

not influence the project’s delay (Stratton, 2009).

Critical chain method indicates a new method

of project management (Tukel, Rom & Eksioglu,

2006). Instead of adding safety margins to

individual tasks, common “buffers” are created,

included in the project strategic places,

concentrating in particular on the protection of the

project completion deadline and not on the

individual tasks timely execution (Luong & Ohsato,

2008). The CC/BM (Critical Chain/Buffer

Management) method proposes introduction of

additional time reserve at the end of critical chain of

a project i.e. the so called project buffer (PB) and

including buffers for the activities beyond the chain

of the so called feeding buffers (FB). Timely

execution of a project is secured by the application

of the project buffers. Introduction of additional

feeding buffers in places where activities from

beyond the critical chain link with this chain

protects from the interference in critical activities

execution. Due to the fact that the execution of

individual processes in critical chain may involve

various resources, the uninterrupted execution of

the processes is conditioned by the resources

availability not only within the planned deadlines.

Indicating earlier demand for critical resources is

facilitated additionally by the resources buffers

(RB) which are available at earlier deadlines (in

advance) for the critical chain activities. Critical

chain is the resource-constrained critical path when

using aggressive durations. At unlimited resources,

the critical chain definition is covered with the

definition of critical path.

There are various software implementations of

critical chain. Some of those available are listed

here: Agile-CC by AdeptTracker, Aurora-CCPM by

Stottler Henke, BeingManagement3 by Novaces,

cc-(M)Pulse by Spherical Angle, CCPM+ by

Advanced Projects, Concerto by Realization

Technologies, Exepron by Exepron, Lynx by Adato,

ProChain Project Scheduling by ProChain

Solutions, Inc., PS8/PSNext by Sciforma.

In each case, the general idea behind the

calculation is the use of a probability density

function for the timely completion of a task, termed

curve β (Fig. 3). It is assumed that a “conservative

(pessimistic)” estimate, i.e. ca. 90% certainty of

completing the task on time is twice as long as the

“aggressive (optimistic)” estimate corresponding to

the median (50% probability).

Figure 3. Estimating task duration using the

probability density function β


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Even though it is, in principle, possible to

determine the buffer size B using rules of addition

for random variables, other approaches are usually

applied in practice (Tukel, Rom & Eksioglu, 2006).

Well-known computer programs for scheduling

according to the critical chain method also do

without using accurate estimates. Below we present

hree simplified methods for buffer sizing

(elaborated on the basis of empirical studies)

(Stępień, 2010):

1) BI – setting the path buffer size to half of the

sum of the differences between the

durations t0.9 (conservative estimates) and

t0.5 (aggressive estimates) of the tasks

comprising the path,

2) BII – setting the path buffer size to the

square root of the sum of squares of the

differences between the durations t0.9 and t0.5

of the tasks comprising the path. The above

path buffer sizing rules are far simpler than

those resulting from random variable

theory. Moreover, the buffer sizes BI and BII

are significantly smaller compared to those

obtained using the exact procedure,

3) BIII – in crude calculations, or when

conservative estimates t0.9 are unknown, the

buffer size can be set to the half of the sum

of the aggressive estimates. Calculated

buffer sizes should be adjusted based on

experience and intuition. The buffer sizing

methods given above should be viewed as

initial guesses. Paths of tasks which use

well-understood processes do not require

large buffers, since they are more

predictable, and the associated uncertainty

is smaller. Conversely, paths containing

“experimental” tasks (characterized by

greater uncertainty) will require larger

buffers.

3 ANALYSIS AND RESULTS

Table 1 compares different tasks in the

production preparation of a special-purpose milling

machine, showing their durations and predecessors.

Durations of all scheduled tasks were estimated

with, respectively, 50% (t0.5) and 90% (t0.9)

probability of timely completion.

Table 1. List of tasks, their durations and predecessors in the project


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3.1 Critical chain identification

Figure 4 presents a relationship network of

production preparation tasks for a special-purpose

milling machine, obtained following resource leveling.

Durations of all tasks use aggressive estimates. Non-

critical tasks are executed as late as possible (ALAP).

3.2 Placement of buffers

Setting the durations of all scheduled tasks in

accordance with the median estimate requires an

additional, focused “protection” of paths with

buffers. Consequently, the subsequent scheduling

stage consists in placing suitably sized buffers in

appropriate locations.

The pivotal buffer is the so-called project

buffer (PB) protecting the most eminent path – the

project critical chain. The aim of a project buffer is

to ensure that the declared project execution time

will be kept with a high (90%) probability.

In the analyzed example, the critical chain

comprises 30 tasks. The second, simplified, method

(expression 1) of buffer sizing (BI) was employed

in relationship network.

The project buffer protects only the tasks in the

critical chain. Non-critical paths require protection

as well, as their tasks may be delayed, potentially

delaying the entire project. This is because

noncritical (ALAP-type) tasks are “introduced” into

the critical chain in a way that is strictly localized

(as points on the time axis). This makes it necessary

to add buffers at the ends of non-critical paths,

known as feeding buffers (FB).

The sizing of FBs is carried out in the same

manner as for the project buffer. The analyzed

example requires 14 FBs, sized according to

expressions (cf. Table 2):


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Figure 4. Relationship network, highlighting the critical chain

where:

t0.9 – conservative estimate of i task duration,

t0.5 – aggressive estimate of i task duration,

k – number of the tasks comprising the path.

Single-task paths merit particular attention. We

“reinstate” the previously removed safety reserve in

tasks N, Z, AF and BA. Thus, those single-task

feeds will be completed on time with a probability

of 90%. Therefore, for the purposes of planning the

conservative estimates t0.9 can be immediately

employed for single-task paths. Table 2 presents the

results obtained for buffers of non-critical paths and

for the project critical chain.

Since FBs are added in locations where

noncritical paths merge into the critical chain,

adding a feeding buffer can potentially shift a non-

critical path so much to the left as to exhaust the

entire reserve for that path reserve. This is only the

case with non-critical paths, which separate out of

the critical chain and merge into it at a later

temporal location. We assume that if an FB forces

another non-critical path to the left of day zero, the

critical chain will not change, since it is identified

before buffers are added.

4 DISCUSSION AND CONCLUSIONS

A relationship network in the operative plan

for the production preparation of a special-purpose

milling machine was analyzed. For the preliminary

schedule, we performed resource leveling,

employed ALAP scheduling for non-critical tasks,

and adopted aggressive estimates for the durations

of all tasks, which ultimately allowed us to obtain a

shorter estimate for the duration of the entire

project.


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Table 2. Sizes of feeding buffers and of the project buffer

Following the introduction and sizing of PB

and FB, it transpired (in line with Goldratt’s

assumptions) that the estimated duration of the

entire project has become significantly shorter. The

initial duration estimate was 193 days. Once the

critical chain method was employed, this reduced to

152 days when estimating the durations of all tasks

at the level equal to the median. This means the

duration of the entire project was reduced by ca.

21%, corresponding to a significant acceleration of

works, compared with the initial estimate of the

implementation date.

Our analysis leads to the following

conclusions:

- setting task durations using a probability

lower than 0.9 allows to create a time

reserve that can be used for project buffers

and feeding buffers,

- the resultant size of the buffers depends on

how task durations are shortened and on the

adopted probability of completing the entire

project,

- in the analyzed schedule, the probability of

meeting the project deadline when

assuming task durations equal to t0.5, and

the calculated values of buffers, is 0.873 for

BI,

- the duration of the task path is markedly

shortened (by ca. 21%), while maintaining a

similar level of probability of meeting the

declared deadline, - the probability of

completing the entire project on time can be

improved by increasing the size of the final

project buffer,

- adopting task durations using a lower

probability threshold yields a schedule that

assumes a quicker pace of works (more

aggressive solutions) with larger buffers,

and vice versa,

- the possibility of selecting variant schedules

corresponding to a slower (less aggressive)

or quicker (more aggressive) pace of works

can be adjusted by adjusting the balance

between the sum of durations of tasks (the

adopted probability corresponding to

chosen duration), and the sizes of buffers,

- project management of production

preparation using the critical chain method

comes down to controlling the state of the

shared path buffer, rather than the more

demanding requirement to control the state

of execution of individual tasks.

Together with engineers’ increasing interest in

rapid design and manufacture tools and the

increasingly common application of integrated IT

systems, the nature and quality of conducting

manufacturing operations changes. Similarly, the

approach to risk inherent in such operations

undergoes a change. The use of modern computer

techniques aids designers in dealing with complex

and laborious design and manufacturing tasks,

resulting not merely in the shortening of the product

manufacturing cycle, but also in the mitigation of a

variety of risks that are common in production. The

above observations are particularly pertinent to


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modern design techniques, prototyping, tool

manufacturing and carrying out production.

Works undertaken during product design and

elaborating the technology for its production require

making a number of decisions that ultimately

translate into the final product with specific

functional properties. Those works can be and,

progressively more often, are performed with

computer aid. To ensure proper current business

activity of a company one must also focus on the

decision level of production preparation,

particularly in the area of structural and

technological product preparation. A departure from

traditional sequential design towards concurrent

design necessitates a search for effective methods of

planning production preparation. The project

approach and planning methods dedicated to project

management have become an inherent part of

operative plans of production preparation. The

application of critical chain project management not

only shortens production preparation of a new

product, but also enables the rational use of

resources and reduces the risk of project failure. For

the studied case of a special-purpose milling

machine, a critical chain was identified, and buffers

were appropriately sized and placed, yielding a

new, shorter duration of the production preparation

stage, while maintaining a similar level of

probability of meeting the declared deadline.

We note that the need for shortening

production preparation time demands undertaking

actions that aim to automate the individual stages of

product development. This can be achieved through

computer-aided integration of data obtained from

modeling products, product production processes,

and production means themselves. Above all, taking

advantage of extensive databases enables the

efficient creation and management of design

documentation and all the documents necessary

during production.

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