Selection of Organizational Structure of Vertical Milling Centre
Final Assembly Process
Výber organizačnej štruktúry finálneho montážneho procesu
vertikálneho frezovacieho centra
This paper presents the issue of selecting the most Methodology presented in this paper is divided into two
favorable technological assembly organizational structure stages. At the first one the task of assembly line balancing is
from the point of view of productivity. The task was solved used to identify permissible assembly variants organiza-
using the modern software tools for modeling a simulation tional layout . At the second stage the most favorable
of production processes. The final part of the paper presents variant is selected observing the criteria of the productivity.
the results and conclusions from conducted analyses.
Keywords: technological assembly process, graph of se- 2 The hitherto assembly process
quence restrictions, assembly line balancing. Final assembling of milling centre was carried out in the
assembly hall. Stationary assembling was executed by spe-
Abstrakt cialized teams of mechanics and electricians (fig. 1).
Článok prezentuje problematiku výberu najhodnejšej
organizačnej štruktúry technologickej montáže z hľadiska
produktivity. Úloha bola riešená s využitím moderných soft-
vérových nástrojov pre modelovanie a simuláciu výrobných
procesov. Finálna časť príspevku prezentuje výsledky a zá-
very vykonaných analýz.
Kľúčové slová: technologický montážny proces, graf
sekvenčných obmedzení, nastavenie montážnej linky.
The assembly process is the part and most important
stage of the production process, where product's useful
characteristics are formed and which is of considerable in-
fluence on product's quality. This stage also has an effect on
the timeliness of execution of orders and ease of adjusting
production capacity to changing market situations as far as
product types and options are concerned. Increasing compe- Fig. 1 Vertical milling centre during final 24 hr testing
tition in world and local markets forces manufacturers to technological operation
ceaselessly reduce production costs. One of the areas where Obr. 1 Vertikálne frézovacie centrum počas finálneho 24
savings can be expected and found is the assembly process. hodinového testu technologickej prevádzky.
This component has a considerable share in the cost of the During the final assembly process the workers and
product and may account for more than 35% of the total teams performed the following main activities preparing a
cost [2, 3]. Thus, it should be noted that new or actual reali- complete set of parts and assembling of the drive
zing assembly process must be designed in such a way as to axes X, Y, Z, the drive of tool magazine, the main milling
ensure their productivity as far as the assembly process is spindle, the systems and control panel, the pneumatic,
concerned and harmonize product's technological require- hydraulic installations and lubricate system, the internal
ments with the production system. land external covers also assembling and connecting the
Within the technological assembly process one of the electrical installation, turning and testing, determining the
most important tasks is determining the most favorable machine tool geometry and finishing operation and
organizational layout and sequence of putting of putting preparing for the shipping.
individual elements together to create the final product. Individual and group, random timekeeping was used to
Tasks of this kind must be treated in an individual way, measure duration of repetitive, identifiable elements of the
taking into account the special characteristics of the produc- assembly process. The main objective of conducted research
tion layout, assembly work cells, types of manufactured was to determine labour consumption of each operation of
products, financial and human resources as well as several the assembly process with the precision making it possible
other factors . One of the priorities in this area is to to determine time required for its execution, keeping in
determine such an organizational layout of assembly pro- mind the organizational and technological conditions and
cess as to maximize the productivity and minimize the cost the verification of hitherto standards.
and duration of the assembly process. In order to identify Time measurements of each workstation were carried
the most advantageous solution, it is therefore necessary to out by checking duration of all operations on the assembly
use mathematical modeling and simulation. line with a stopwatch. The measurements were carried out
three times and their arithmetic mean was calculated. By
assigning duration of each operation to a particular worker at a given workstation it was possible to determine duration
of assembling operations (tab. 1). (which is quite natural) and this factor should be taken into
consideration to maintain high quality of assembly (the
Table 1 Time keeping measurements for the final assembly
calculated durations have a 10% margin of error).
of machine tool
As it has been already said, the main task of assembly
Tab. 1 Časový pomer konečej montáže obrábacieho stroja
line balancing is the optimum assignment of operations to
specific stands . Therefore, while developing the algo-
3,48 rithm it was assumed, that the following data are supplied as
15 3,84 input: pace and number of assembly line stands as well as
14 28,8 the set and durations of operations with their sequential
relation – a graph of sequence restrictions (fig. 2).
11 12,6 Development of a graph of sequence restrictions calls
3,6 for a detailed analysis of each operation, taking into account
8 8,76 the sequence and place where each component is assem-
8,76 bled. In the case under consideration the objective was to
5,76 minimize the number of stations on the assembly line.
4 8,4 The analytical approach to the problem in question was
11,95 conducted on the basis of the heuristic algorithm of assem-
1 bly line balancing detail discussed in works [1, 3, 5]. The
model can be used only for assembly lines where assembled
0 10 20 30 products do not come “back” to workstations they have
Duration of operation [hr] already “visited”.
Total assembly time: 167,25 [hr]; Results of the conducted analysis using the LINGO v9
Serial layout of 10 assembly station; arithmetic mean of line software for computer aided modelling and simulations for
cycle: 20,82 [hr] chosen configuration of assembly stations are presented in
tab. 2. Also the model of assembly line for the most favo-
3 Assembly line balancing rable configuration for 15 serial-parallel assembly worksta-
The task of assembly line balancing was first formulated tions – 6R3-10R3 was prepared and tested using the Show-
by Salveson in 1955 . Assembly line balancing consists Flow software (fig. 3). The results are presented in tab. 3
of appropriate assigning of all operations (actions), belon- and fig. 4.
ging to a given set (process), to specific assembly stands.
The technological assembly process at individual assem-
bly stations must be organized in such a way as to make the
duration of operation one – t1 equal to the duration of opera-
tion two – t2 and equal to the durations of the following
operations – tn, i.e. equal to the cycle of the line tk.
Practically, for technological and organizational reasons
the identity of durations of individual operations can be
only approximated. In practice, the minimum duration of Fig. 2 Precedence constraint graph for the vertical milling
the assembly line pace (i.e. the time between completion of centre 
two subsequent products) is limited by the duration of the Obr. 2 Graf poradia obmedzení pre vertikálne
longest assembly operation, which constitutes the so called frézovacie centrum 
bottleneck of the line.
Assembly line balancing consists of the optimum assign- Table 2 Listing of results of simulation for various
ment of all operations (tasks) from a given set (process) to configuration of the final assembly line
individual workstations of the assembly line, which ensures Tab. 2 Prehľad výsledkov simulácií pre rôzne konfigurácie
that [1, 3–6]: finálnej montážne linky
− the sequence resulting from the precedence constraint Productive of
graph is observed, Signify of assembly line model assembly line
(arrangement assembly workstations) [units per 2
− the duration of tasks performed at any workstation is shifts]
shorter or equal to the assigned duration of the line cycle, 5 linear 12,46
− operations performed at a given station belong to the same 6 linear 14,39
the so called zone. 7 linear 14,95
8 linear 14,73
4 Modelling and simulation of the 9 linear 16,16
assembly process 10 linear 16,67
The team performed sixteen main operations to assem- 12 serial-parallel workstations –8R2-13R2 23,26
ble the milling centre. The calculated durations of each 13 serial-parallel workstations – 1R2-8R2-
operation were assigned to one or a team of assembly line 13R2
workers and then to a specified stand. One should stress that 12 serial-parallel workstations – 1R2-8R2-
these durations are not (and cannot be) the same (due to a
15 serial-parallel workstations – 6R3-10R3 40/60
turnover of employees, i.e. shift work). Each assembly line
worker has his own speed of execution of each operation 16 serial-parallel workstations – 6R3-11R3 40/60
Table 3 Assignment of operations to assembly stations using 5 Summary
the heuristic algorithm
Tab. 3 Priradenie operácií montážnej stanici s využitím The developed heuristic algorithm implemented in
heuristického algoritmu. LINGO v9 make it possible to quickly calculate the most
advantageous number of stands, including the minimum
1 2 3 4 5 6
number of work cells. Once these numbers are determined, it
assembly cell becomes possible to calculate the optimum pace duration.
1 1 1 1 1 3
This will assist the management of assembly department, as
stations per cell in case of changing orders (number of assembled milling
Assign 3, 4, centres), the number of workers on the assembly line can be
1 2 6 7 8
operations 5 changed easily.
Total duration 6,3 One can say that nowadays with competition and econo-
6,21 6,71 6,57 4,32 8
per station [hr] 7 mic factors becoming increasingly important, reduction of
Number of 7 8 9 10 11 the total assembly cycle (as many as 14,37 hr – using the
assembly cell serial-parallel layout of assembly stations) is a considerable
Number of 1 1 1 3 1 achievement. The author are convinced that the method
stations per cell guarantees full flexibility in adjusting assembly lines to
Assign 9, 10 11, 12 13 14 15, 16 demand reflected by incoming orders of number of machine
operations tools - heuristic algorithmization of assembly line balancing
Total duration 6,52 6,48 5,51 8 6,28
of milling centre - will become more and more common and
per station [hr] popular method of rationalization of production processes in
everyday manufacturing practice.
Serial-parallel layout of assembly stations
(total 15 cells);
Production level: 40 units per month; Arithmetic
mean of line cycle: 6,45 [hr]
PhD Eng. Olaf Ciszak
Poznań University of Technology
Institute of Mechanical Technology
Division of Technology Planning
Piotrowo 3 Street, PL 61-138 Poznań
tel. +48 (61) 66-52-162, fax. +48 (61) 66-52-200
Fig. 3 Simulation model 6R3-10R3 final assembly line References
of vertical milling centre prepared at ShowFlow software
Obr. 3 Simulačný model 6R3-10R3 finálnej montážnej linky  CISZAK, Olaf, ŻUREK, Jan: Balancing of vertical
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Fig. 4 Graphic representation of duration of vertical balansowania linii montażowej. Zeszyty Naukowe Poli-
milling centre assembling operations for layout 6R3-10R3 techniki Rzeszowskiej, 1992, ISSN 0209-2689.
Obr. 4 Grafická prezentácia trvania montážnych operácií
pre usporiadanie 6R3-10R3