Maritime University of Szczecin
Akademia Morska w Szczecinie
2010, 24(96) pp. 105–110 2010, 24(96) s. 105–110
The use of tools improving quality in the production process
Wykorzystanie narzędzi doskonalenia jakości w procesie
produkcyjnym
Bożena Skotnicka-Zasadzień
Silesian University of Technology, Faculty of Organisation and Management Institute of Production Engineering
Politechnika Śląska w Gliwicach, Wydział Organizacji i Zarządzania, Instytut Inżynierii Produkcji 41-800 Zabrze, ul. Roosevelta 26, e-mail: bozena.skotnicka@polsl.pl
Key words: quality management, mining industry, Pareto’s method, Ishikawa’s diagram, improvement
of the quality
Abstract
This article presents the use of Pareto’s method and Ishikawa’s diagram for analyzing flaws which occur in a hydraulic prop produced for the mining industry. The study has pointed out that a human’s mistake and badly matched materials are the reasons of occurred incompatibilities. Correcting measures were proposed in the conclusions.
Słowa kluczowe: doskonalenie jakości, proces produkcyjny, metoda Pareto, diagram Ishikawy, stojak
hydrauliczny
Abstrakt
W artykule przedstawiono zastosowanie metody Pareto oraz diagramu Ishikawy do analizy wad występują-cych w stojaku hydraulicznym produkowanym na potrzeby przemysłu górniczego. Analiza dokumentacji do-tyczącej reklamacji produktów wykazała, że na 286 zareklamowanych produktów, aż 234 stanowią stojaki „Valent”. Firma poniosła znaczne straty z tego tytułu, a liczba reklamacji z roku na rok była coraz większa. Badania wykazały, iż przyczyną występujących niezgodności był błąd człowieka oraz źle dobrane materiały. We wnioskach zaproponowano następujące działania korygujące: należy przeprowadzić serię szkoleń pro-gramowych dla poszczególnych pracowników, wykonać wzorce oraz zakupić bardziej zaawansowane techno-logicznie maszyny, w dokumentacji rysunkowej należy wnieść poprawki, dotyczące średnicy wewnętrznej tu-lei obejmy, celem zwiększenia obszaru tolerancji, natomiast w strukturze organizacyjnej należy ustanowić komórkę organizacyjną odpowiedzialną za stałe monitorowanie procesów produkcyjnych.
Introduction
The aim of any enterprise is a cost-effective production of products which are compatible with a specification. Each and every enterprise is inte-rested in raising the productivity of production and quality of manufactured products. Thus, what is essential is to monitor and control processes in such a way that all units fulfil the requirements, which are written down in a specification of a given product [1].
In each and every enterprise, which looks after a client, all customers’ complaints are diligently
and thoroughly examined. Most incompatibilities and mistakes in a product are created during a production process. Thus, it is crucial to analyse precisely all cycles of manufacturing process in order to avoid a complaint of ready-made product, which in turn means the increase of cost in a given enterprise [2].
The methodology of study
The Pareto–Lorenz’s analysis and Ishikawa’s diagram have been used in the study in order to detect flaws and mistakes of an analysed product.
The stages of Pareto–Lorenz’s analysis [3]: collecting information,
defining the measures quantities,
counting accumulated percentage values, drawing Pareto–Lorenz graph,
drawing up conclusions.
The Ishikawa’s method has been used to find out the reason of problems of a studied product.
The stages of analysis done by means of Ishikawa’s method are as follows [4]:
defining the most important groups of problems which have an influence of a given problem, specifying the reasons by means of Ishikawa’s
diagram,
the analysis of results.
The analysis of results
The research was conducted in the enterprise producing machines and equipment for mining industry. The friction prop, called “Valent” was the object of the analysis. This friction prop, “Valent”
is an individual element of mining lining which is used to support a roof in a mining excavation. The analysis of the technical documentation concerning product complaints has pointed out that 234 out of 286 faulty products were “Valent” props [5]. As a consequence, the company made a significant loss and a number of complaints were much higher year after year. During the audit, the auditor has pointed out that it is important to introduce indexes which will be defining quantitatively the problem of sub-assembly. On the basis of the numerical results, it will be easier to make a decision which will influ-ence on the improvement of quality of a product. Therefore, conducting such an analysis is justified not only from the economic point of view but also from the enterprise’s image point of view (Table 1). The Pareto–Lorenz’s graph (Fig. 1) was created on the basis of data included in table 1.
They help to visualize data included in the above mentioned table and they lead to draw conclusions concerning flaws. The values from the graphs were deciphered according to the Pareto–
Table 1. The juxtaposition of flaws on account of the frequency of their occurrence Tabela 1. Zestawienie wad ze względu na częstość ich występowania
Flaw
No. Kind of flaw flawed pieces Number of Number of accumulated flawed pieces Manner of flaw removal 1 Material flaw 121x8,8 (Upper prop pipe) 14 14 Exchange of a part 2 Material flaw 141x6 (Lower prop pipe) 27 41 Exchange of a part 3 Wrong value of working support of prop (Lock) 178 219 Exchange of a part
4 Damaged castellated head 13 232 Exchange of a part
5 Wrong size of a prop 2 234 Exchange of parts with wrong size
Fig. 1. Pareto-Lorenz’s graph
Rys. 1. Zestawienie wad ze względu na częstość ich występowania Flaw No. No . o f flaw ed p iec es
Lorenz’s rule which says that 20–30% of reasons cause 70–80% of effects.
The analysis of Pareto’s diagram shows that only one reason constitutes almost 80% of the accumulated value. That’s why, the influence of flaw no. 3 should be removed or limited to mini-mum so as to obtain the best, possible effect in creating preventive actions for “Valent” prop. Too low prop- support capacity is a consequence of the flaw no. 3 which is visible in the documentation of a complaint proceeding. The scope of searching was limited to a sub-assembly “Lock” because it is an element which is responsible for its proper functioning if support capacity and motoric characteristics are taken into account.
The identification of a reason
In order to understand the essence of the pro-blem, the construction and the idea of functioning of the studied construction element should be observed closer. The lock consists of 9 mutually cooperating with each other parts (Fig. 2): the sleeve of a clamping ring (2 items) no. 1, the extension bar of a clamping ring (2 items) no. 2, the resistance of wedge (2 items) no. 3, wedge (2 items) no. 4, and connecting plate (1 item) no. 5 [6].
1
5
2
4
3
5
1
2
4
Fig. 2. The friction prop, “Valent” SN-400: the lock – assem-bly drawing
Rys. 2. Stojak Valent SN-400: zamek – rysunek złożeniowy
The idea of functioning of the lock is based on jamming in the sleeves of a clamping ring on the upper prop by means so called “jamming” of wedges which go through the extension bars of
clamping ring and push on the resistance of wedge and in turn, causes the clamping of clamping ring on the upper prop.
The lock works as a blockade of the prop as well as a security element because it is constructed in such a way that the prop can be damaged as a consequence of a sudden crump of a rock mass. The idea of prop functioning is based on the strength of a friction and the whole mechanism has such characteristics which let the lock unlock and the upper prop slides down during sudden crumps of rock mass (Fig. 3).
The explanations to the picture:
F – strength which is needed to block the lock, PO – resistance surface.
Numbers define the order in which the following reactions of subassembly to the action of strength F occurred. 3 2 P0 1 P0 F
Fig. 3. The friction prop, “Valent” SN-400: the lock – ope-rating principle
Rys. 3. Stojak Valent SN-400: zamek – zasada działania
The production of a lock is mostly based on the parts manufactured by subcontractors. The com-pany is forced to buy ready made parts due to the fact that it does not have machines which would allow producing such parts: a wedge, a resistance of wedge, a connecting plate or an extension bar of clamping ring. When producing these elements, the following technologies are use as: a laser burning and forging. The purchase of a cutter or forging furnace is not an economically justified venture due to high purchase costs as well high cost of their operation. That’s why, it is very profitable to buy ready made parts at this level of production. Undoubtedly, it is connected with a high risk of quality change of these elements. Thus, it is so crucial to choose subcontractors carefully.
1 5 2 1 5 4 2 3 4
This is a list of technologies used in the production of the lock:
Sleeve of a clamping ring: is the only product
manufactured in the company. The steel pipes without a joint marked with high hardness and stress factor. The following technologies are used for machining:
cutting (band-saw);
turning (numerically controlled and general machine): a sleeve of an appropriate size is cut out of steel pipe of an appropriate size by means of a band saw. Then, it is treated on numerically controlled machines or general one in order to gain a proper value of inside and outside dia-meter. The end faces are also planned and sharp edges are subjected to a process of milling. The last stage is to cut a sleeve crosswise by means of a band saw.
Extension bar of a clamping ring: this element
is completely produced by subcontractors. The sheet of thickness is defined in the documentation and made of alloy steel and is used for its pro-duction. It is subjected to the process of hardening in order to achieve a target durability of this part. After its working, hardening is done by means of a laser burning and forging press. The following technologies are included in the process of extension bar production:
cutting (laser or plasma burning), forging (forging press),
hardening.
The extension bar of clamping ring after having it welded with a sleeve of clamping ring creates the clamping ring.
Resistance of wedge: this is the next element
which is completely produced by a subcontractor. The sheet of thickness defined in the documentation made of alloy steel was used to produce this element. After its working, hardening is done by means of laser burning and forging press.
The following technologies are included in the process of extension bar production:
cutting (laser or plasma burning), forging (forging press),
hardening.
Wedge: it is also made by the subcontractors. It
is made of alloy steel bar of a diameter defined in the documentation. The process of hardening is done after the process of forging. The following technologies are included in the process of extension bar production:
forging (forging press), hardening,
milling.
Connecting plate: similarly, it is completely
manufactured by subcontractors. It is also alloy steel sheet that was used in its production and whose thickness is defined in the documentation. The process of hardening is conducted after the working by means of laser burning or forging press. The following technologies are included in the process of extension bar production:
cutting (laser or plasma burning), forging (forging press),
hardening.
The performance of Ishikawa’s graph for the production process of studied subassembly
6M system was used to the analysis (Fig. 4).
Fig. 4. Ishikawa’s graph
The analysis of separate factors
Method. The incorrectly blocked lock can be
a reason for the studied problem. However, this reason is too obvious which can be identified by a user. If the prop does not work properly, it is blocked a couple of times and if the problem still occurs, the procedure of complaint starts.
This hypothesis was rejected.
Material. The wrong parameters of material,
which is used to manufacture a given part. This cause seems to be justified because the company buys materials in a large quantity, of about a couple thousands items. It is impossible to examine each and every part separately. The subcontractor always encloses one certificate which consists a couple of randomly examines parts.
The subcontractor verifies the batch if there is a suspicion that the material does not fulfil expected requirements. This hypothesis has been rejected.
The incorrect parameters of parts. This hypo-thesis has been rejected as each and every part is precisely measured before assembling, what in turn eliminates the use of faulty part.
The improper filler (wrongly matched welding wire for separate types of steel). The company has the technological documentation of welding for separate product. This eliminates the possibility of using wrong parameters as well as materials during welding treatment. This hypothesis has been rejected.
Environment. Wrong lightening over the
as-sembly position. The company systemically invests in work environment. Each and every work position has a separate lightening which is adjusted to all BHP requirements. This hypothesis has been rejected.
Measurement. Improper conditions for
con-ducting a technical acceptance measurement. Each and every position is equipped with proper measurement equipment. Each of them has a sepa-rate position for conducting measurement. This hypothesis has been rejected.
The performance of measurement is in discord with the established frequency. The frequency of measurement is set in the documentation. Each and every measurement is written down in technical acceptance card. This hypothesis has been rejected.
The performance of measurement conducted by means of not model like tools. The register of measurement tools is conducted in the company and additionally, there is a procedure concerning frequency of measurement devices calibration. What’s more, every measurement tool is checked by means of inner-company measurement standards
(legalized in certified accreditation unit) before its use. This hypothesis has been rejected.
Machine. The damage of push-pull wire feeder
in a welder. The low durability of welds can be the manifestation of it while the separation of any part could be the effect. There was no such example in the documentation concerning complaints. Thus, the hypothesis has been rejected.
Worker. The mistakes during the assembly.
During the preliminary identification of a problem, the production manager suggested that this factor had the biggest influence on the creation of incompatibilities. He proved this hypothesis with a fact that last year, the company noted a huge number of orders and so called “bottle neck” occurred which included also welding positions.
These mistakes could be the consequence of haste when making these subassemblies. So, the issue of the last identified factor, that is the haste during work occurred. It can be also observed that these two factors result from each other and they are dependent on each other. Thus, they will be treated as one factor. The hypothesis has been accepted.
Conclusions
The accepted hypothesis which says that incompatibilities which were a cause of later complaints were caused by employees’ mistakes being a consequence of haste during work. This hypothesis can be supported with a fact that the production in this company is periodic which is a consequence of lack of continuity of orders in the marketing department
Besides having put forward hypothesis, the following improvement and correcting measures were suggested in the studied enterprise.
Human resource. A series of training for each
and every worker should be separately conducted in order to improve their abilities and strengthen their awareness concerning the need for a constant professional self-improvement.
The assembly of products. In order to increase
repetitiveness of processes, patters should be done and more technologically advanced machines (semi-automation) should be bought.
As an alternative to above mentioned sugges-tions, tools which facilitate the installation of the above mentioned subassembly should be bought or created by the company itself.
Some alternations concerning the inside dia-meter of a sleeve of clamping ring in the technical sheet should be done in order to increase the scope of tolerance while a special work position should be
created which will be responsible for a constant monitoring of production process in the company. The worker on this position will have the qualifications to do measures at any time and at any position of work of all crucial products, part or subassembly.
References
1. ACZEL A.: Statystyka w zarządzaniu. Wydawnictwo
Nau-kowe PWN, Warszawa 2000.
2. GAŁAŻEWSKA I.: Statystyczna Kontrola Jakości – znaczenie
i zastosowanie. [in:] Zarządzanie Jakością w Praktyce. Wydawnictwo Wiedza i Praktyka, Warszawa 2005.
3. HAMROL A.: Zarządzanie jakością z przykładami.
Wy-dawnictwo Naukowe PWN, Warszawa 2005.
4. KACZMARCZYK R.: Analiza reklamacji produktów firmy
Hime Sp. z o.o. w oparciu o metodę QC Story [Master’s thesis under direction of B. Skotnicka-Zasadzień PhD]. Zabrze 2007.
5. SZKODA J.: Sterowanie jakością procesów produkcyjnych.
Teoria i praktyka. Wyd. Uniwersytetu Warmińsko-Mazur-skiego, Olsztyn 2005.
6. Technical documentation of the friction prop, “Valent”.
Recenzent: dr hab. inż. Ruta Leśmian-Kordas, prof. AM Akademia Morska w Szczecinie
