Design of cellular manufacturing systems—An industrial case study

Robotics & Computer-Inte#rated Manufacturino, Vol. 10, No. 1/2, pp. 147-151, 1993 Printed in Great Britain

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0736-5845/93 $5.00 + 0.00 c~' 1992 Pergamon Press Ltd

Paper DESIGN

OF CELLULAR MANUFACTURING INDUSTRIAL CASE STUDY PEIHUA

Gu*~ a n d

SYSTEMS--AN

AL M O N I D t

*Department of Mechanical Engineering,The University of Calgary, Calgary, Alberta, Canada, T2N 1N4 ~'Department of Manufacturing Engineering,Jarvis Clark Company Ltd., Burlington, Ontario, Canada, LTR 3Y8

This paper discusses the development of a cellular manufacturing system in a manufacturing company. A new clustering algorithm has been applied to the design of the system. The algorithm consists of two parts, a clusterseeking process and the minimization of bottleneck machines. Two parameters are input by the user: (a) the desired number of machine cells or part families, and (b) the minimum number of parts within each cell or part family. A number of cells have been designed for a preparation shop, a fabrication shop, and a machine shop. Both the advantages and disadvantages of the algorithm have been analyzed, especially concerning its applications to industry.

1. INTRODUCTION Cellular manufacturing is based on the concept of group technology. The fundamental idea of group technology (GT) is the similarity of part design and manufacturing processes. Two main approaches have been used to describe similarity in GT. These are coding and classification systems and production flow analysis (PFA). Since a considerable amount of preparation work is required for the implementation of any coding and classification system (e.g. all drawings must be analyzed and assigned a GT code), our company prefers the PFA method. EIMCO Jarvis Clark Company Ltd. is a mining equipment manufacturer. The main products of the company include LDH (]_oad, dump and haul) and dump trucks. The company also supplies spare parts for its products and other manufacturers' products. The production plant consists of a preparation shop, a fabrication shop, and an assembly line. In order to compete with other manufacturers, the company has had to increase production efficiency and reduce inventory and manufacturing costs, so that product prices can be kept at a competitive level. Although raw material prices, labor costs and inflation are moving higher, the company has had to adopt new manufacturing technology to remain competitive. Through a few years experience, the company decided to implement cellular manufacturing technology as a first step, which includes machine cell formation, layout, and group tooling. Only cell formation is discussed in this paper.

2. BRIEF LITERATURE SURVEY Burbidge 1 first presented an analytical approach to the formation of part families and machine cells suitable for small problems. King and Nakornchai 7 developed an iterative algorithm, called rank order clustering (ROC), which generates diagonalized groupings of the machine-component matrix. Vannelli and Kumar 9 developed a method for finding the minimal number of bottleneck cells based on graph theory. In this approach, the minimal number of bottleneck machines and/or parts is found from the minimal cut-nodes which disconnect the graph. Kusiak a utilized the p-median model for formation of part families and machine cells. An objective function is formed as the maximum of the total sum of similarities and an integer programming technique is used to find a solution. Chandrasekharan and Rajagopalan 2 developed an approach called ZODIAC (Zero-One Data Idea seed Algorithm of Clustering). The algorithm establishes whether there exists a limit for grouping efficiency, depending upon data structure and cluster membership. Gu and E1Maraghy4 reported the application of the cluster-seeking algorithm to the formation of machine cells and part families using Kmeans, revised K-means and isodata algorithms.

3. CLUSTERING ALGORITHM Group formation of cellular manufacturing systems can be viewed as identifying classes in a given set of components, based on the similarity of their manufacturing operation. Thus, the problem can be stated as: A set of components {X 1, X2 . . . . . XN} and their operation routes are known, the component families and

~:Previous address: EIMCO Jarvis Clark, Burlington, Ontario, Canada. 147

148

Robotics & Computer-Integrated Manufacturing • Volume 10, Numbers 1/2, 1993

associated machine cells are unknown. The following clustering algorithm is used to form part families and machine cells. A component is expressed as a pattern vector:

8.

M

D(I) :

Each element of the vector is a one-dimensional attribute representing a machine operation. If a machine is required x~i = 1, otherwise x~j = O. All components are represented in an incidence matrix. The similarity between two components is measured using the Euclidean distance: D = II x , - x~ II.

(2)

j=

Determine initial cluster centers Z~(i < i < N~) using the first N~ parts: Z,(k) = X~ k = 1, i = 1, 2 . . . . . N~.

2.

F o r m part families for the N - N~ components, using the relation: X 6 S t if IIX - Zjll < ItX - Z, ll, i, j = 1, 2 . . . . . N~i :~ j

3.

(5)

where Nj is the number of parts in the jth part family. Update cluster centers by: Zj(k+

5.

(4)

where Sj is the jth part family. Discard a part family if parts in the family are fewer than the minimum Q. pre-set by the user: N~(k + 1) = Nc(k ) - 1 i f N j < Q,, j = l, 2 . . . . . N~

4.

(3)

1)=1

~ X,j--- 1,2 . . . . . N c. (6) N~ x~s~

Check convergence of iteration using the pre-set criterion: IZ~(k + 1) -- Zij(k)t < e, i = 1, 2 . . . . . N~, j = 1, 2 . . . . . M. (7)

6.

If it is satisfied, go to 7, otherwise continue. Check whether the iteration exceeds the limit L: k + 1 > L.

7.

(8)

If this is true, processing fails and ask user to adjust parameters of Q, and N~. Otherwise, k=k+ 1 and go to 2. F o r m cell configurations: C j = [Cjl , cj2 . . . . . CjM ]

j = 1, 2 . . . . . Nc (9)

where Nj

Qk = 1 if ~ xlj > 0, otherwise c~, = 0 i=l

k = 1,2 . . . . . M,

j=

1,2,

.,No.

(10)

1,2 . . . . . N c

(I1)

where Yil = 1 (ifxi2 = 1 or cij = 1), otherwise Yu = 0. 9.

10.

(12)

Assign parts to the current jth cell by: Xt6SjifD(l)=O,l=

1,2 . . . . . N - N j .

(13)

Reconfigure the other cells: Nj

The algorithm requires a user to input the parameters, the number of cells desired No, and the minimum number of parts in each family Q.. The algorithm is formulated as follows: 1.

~ Ic,~ - y.I, l = 1, 2 . . . . . N - N j, i=1

(1)

Xi = [xil, xi2 . . . . . xiM] T.

Compute distances between cells and components:

Ckj = 1 if ~ xij > 1, otherwise Ckj = 0, k :~ j (14) l=1

(j = current chosen cell). 11.

Check convergence of cell re-configuration and the minimization of bottleneck machines ISj(k + 1) - Si(k)l < e j = 1, 2 . . . . . N~.

(15)

If the above criterion is satisfied, the process terminates; otherwise, go to 7. Users can control the clustering analysis by manipulating two parameters, the desired number of cells to be formed, and the minimum number of parts allowed in each cell. In general, users first decide the desired number of cells N c. The minimum number of parts for each part family Q, can be determined by: O, <

N . Nc

(16)

Q, is an important parameter for the formation of machine cells, and it is generally recommended that a few Q,s be tried to compare the clustering results, when N c is first determined. An optimization method has been developed to help the user to determine these parameters. 5 An example shown in Fig. 1 consists of 43 components and 16 machines. A solution given in Fig. 2 has been found, which consists of five cells and part families. 4. AN I N D U S T R I A L CASE S T U D Y 4.1. Data preparation In theory, all parts produced in the company should be included in an incidence matrix for formation of machine cells. The company is making various models of machines which have hundreds and thousands of parts, which would be impractical and time-consuming in the stage of forming the machines-component incidence matrix. A huge number of routings need to be reviewed to retrieve process information and this is not feasible in terms of manpower. Besides, it will be extremely difficult for results analysis. Alternatively, a typical model of the products (machines) is chosen as an initial data for cell grouping and the final cells designed will be again analyzed with a number of supplementary parts, based on our knowledge of the

Design of cellular manufacturing systems • P. Gu and A. MONID

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company's products. The product chosen consists of 396 parts, and their process routings are analyzed in association with their drawings. A small portion of the formed machines-components matrix is shown in Fig. 3. A total of 44 machine centers is included. Each row represents a part. Zero means no corresponding process required and one represents the process required. 4.2. Cellformation The initial effort was made to form cells with machines from different shops such as the preparation shop, the fabrication shop, and the machine shop. The resulting cells will be product- or unit-based cells such as frame cells and lift arm cells. Obviously, these types of cells have certain advantages over other types of cells which are mainly based on processes, called processbased cells. A very intensive study was conducted, which included numerous computer simulations and associated analysis. The results indicated that many bottleneck machines are present and utilization of machines in the cells cannot be kept well balanced. Besides, the overhead gantry cranes are only available for certain areas and some large machinery is difficult

and costly to move (not realistic at present). The purchase of additional machinery is also not possible. Based on the above analysis, cell formation was carried out for each shop separately. This will result in a situation where parts flow from one cell to the other and even may return to the previous shop again. However, the artificial division makes the analysis of results much simpler. After numerous experimental

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Robotics & Computer-lntegrated Manufacturing • Volume 10, Numbers 1/2, 1993

150

runs, the initial results were obtained and are shown in Tables 1, 2 and 3. As discussed before, the three shops are artificially separated, based on the initial study. However, it will be ideal that some parts m a y fit well with a productbased cell, the others m a y be suitable for processbased cells. Therefore the final cell formation must be further analyzed with consultation of previous study results, process plans, material flow and loading balances such that the formed cells will be acceptable and practical. 4.3. Preparation cells Five preparation cells were formed, shown in Table 1. F o u r of the five cells require machine JB507; three T a b l e 1.

Initial cells for p r e p a r a t i o n s h o p

Cell N o .

Work center code

Work center description

1 2

KF825 ........... KF817 ........... JB507 ........... KA698 ........... KB502 ........... JB507 ........... KF826 ........... KB502 ........... JB507 . . . . . . . . . . . KC800 ........... KB502 ........... JB507 ...........

Koike CNC burn Tracer flamer burner 400 ton × 12 ft press ½in. × 10 ft shear 125 ton punch press 400 ton x 12ft press Linde CNC burn 125 ton punch press 400 ton × 12 ft press Marvel 18" x 20" saw 125 ton punch press 400 ton × 12 ft press

3 4 5

T a b l e 2.

Initial cells for f a b r i c a t i o n s h o p

Cell N o .

Work center code

Work center description

1

CA415 ........... LA416 ........... LA512 ........... LA460 ........... LA485 ........... WH130 ........... CA415 ........... LA416 ........... LA462 ........... LA513 ...........

F a b r i c a t i o n r a d i a l drill S m a l l fit-up a n d t a c k Medium part weld T a n k fit-up a n d a s s e m b l y R i m fit a n d w e l d s t a t i o n Fit a n d weld (cylinders) F a b r i c a t i o n r a d i a l drill S m a l l fit-up a n d t a c k L a r g e p a r t fit-up a n d t a c k L a r g e p a r t finish w e l d

T a b l e 3.

Initial cells for m a c h i n e s h o p

Cell No.

Work center code

Work center description

I

AD405 CA017 DA073 LH020 NA715 XB995 XC097 DA090 HH150 LH020 LH080 XB999

........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ...........

Vertical t u r r e t l a t h e M a c h i n e s h o p r a d i a l drill L a r g e b o r i n g mill VDF lathe Work bench Okuma CNC lathe Mori-Seiki machining center S m a l l b o r i n g mill Hone VDF lathe Mori-Seiki CNC lathe L o d g e & Shipley C N C l a t h e

XC900 ........... CA017 ........... XC097 ........... DA073 ........... XB995 ........... XC900 ...........

Burgmaster Machine shop radial drill Mori-Seikimachining center Large boring mill Okuma CNC lathe Burgmaster

2

3

cells need machine KB502. However, only two of each are currently available; it is not possible to form five independent cells. One alternate solution has been found based on the analysis of process plans, material flow, current machine layout, and other physical constraints such as crane availability. The solution is to form conceptual cells instead of five physically independent cells. Since the machines JB507 and KB502 are needed by several cells, these machines m a y be laid out in such a way that other machines in respective cells can link them in the direction of material flow with the m i n i m u m distances. In other words, these two types of machines are arranged so that these machines not only form cells, but also the total material handling time can be minimized for all cells. Another alternative is to form three cells. Cell 1 is kept unchanged and Cells 2, 3, 4 and 5 are combined into two physical cells. In this case, locations of the machines m a y need changes and other physical constraints are also needed to be considered as well. 4.4. Fabrication cells Three cells have been initially formed for the fabrication shop. If Cell 2 only has one work center, it is also found that all parts processed on the machine belong to hydraulic cylinders. Thus, the cell should be attached or arranged in association with other work required by cylinder manufacturing such as machining and assembly. Only two cells, therefore, will be physically laid out in the shop. After an analysis of process plans and drawings was conducted for the c o m p o n e n t s produced in the two cells, it was found that the first cell is mainly formed for small parts fit-up and welding, and rim fabrication. Logically, the small fit-up (LA416) should be close to the small parts welding centers (LA512) such that the material flow distance is minimized and the cranes can be used. Again, because of the artificial separation of the three shops, the processes of rim manufacturing must be analyzed to justify machine centers layout. The analysis results indicated that rim fit-up and welding should be grouped together with the machine (AD405) to form a rim manufacturing cell. The last cell is dedicated to large part fabrication such as frames, lift arms and buckets. The fit-up is the bottleneck process, based on the analysis of their process routers. Thus, as a further improvement for these aspects it is proposed to use a flexible fixture for their large part fitting process. Dedicated machine centers are assigned for frames, lift arms, and bucket assembly. In combination with effective tooling and redesigned parts and processes, it is expected that the bottleneck processes can be eliminated. 4.5. Machine cells The initial grouping results show that some parts are manufactured in one cell, then m o v e d to another for completion of their machining, because the duplications of some machines such as XC900 and XB995 are not available. A supplementary study of process

Design of cellular manufacturing systems • P. G u and A. MONID Table 4. Cell No.

Formed cells for preparation shop

Table 6.

151

Formed cells for machine shop

Work center code

Work center description

Cell No.

Work center code

Work center description

KF825 ........... KF817 ........... KF826 ........... KB502 ........... JB507 ........... KA698 ........... KC800 ........... KB502 ........... JB507 ...........

Koike CNC burn Tracer flame burner Linde CNC burn 125 ton punch press 400 ton x 12 ft press ½ in. x 10 ft shear Marvel 18" × 20" saw 125 ton punch press 400 ton x 12 ft press

1

AD405 ........... CA017 ........... DA073 ........... LH020 ........... NA715 ........... XB995 ........... XC097 ........... XC900 ........... DA090 ........... HH 150 ........... LH020 ........... LH080 ........... XB999 ........... XC900 ........... CA017 ........... XC097 ........... WH130 ...........

Vertical turret lathe Machine shop radial drill Large boring mill VDF lathe Work bench Okuma CNC lathe Mori-Seiki machining center Burgmaster Small boring mill Hone VDF lathe Mori-Seiki CNC lathe Lodge & Shipley CNC lathe Burgmaster Machine shop radial drill Mori-Seiki machining center Fit and weld (cylinders)

Table 5.

Formed cells for fabrication shop

Cell No.

Work center code

Work center description

1

CA415 LA416 LA512 LA460 LA485 CA415 LA416 LA462 LA513

Fabrication radial drill Small fit-up and tack Medium part weld Tank fit-up and assembly Rim fit and weld station Fabrication radial drill Small fit-up and tack Large part fit-up and tack Large part finish weld

........... ........... ........... ........... ........... ........... ........... ........... ...........

routers for the components produced in the cells was carried out, based on both the analysis results and our product knowledge. An entire cylinder cell (productbased cell) will be formed in association with fabrication, preparation and assembly operations. Further justification was also conducted with a large number of cylinders produced last year to justify the plan. However, some parts process plans must be modified so that their inter-cell movements can be minimized. Comparing Cell 1 and Cell 3, it is clear that they have most of their machines in common. It is more realistic that the two cells are joined together in one cell. Since only one XC900 is available and it seems not possible to add another XC900 in the near future, the XC900 machine forms an independent cell or is located in a convenient place for both the machine cell and cylinder cell. All of the cells finally formed are shown in Tables 4, 5 and 6. 4.6. Advantages and drawbacks of the algorithm The advantages of the algorithm include ease of use and flexibility with which users can generate various cell configurations for a given problem by changing the two input parameters Q, and N c. This provides an opportunity to resolve conflict between the formed cells and the existing production environment. The disadvantages are non-convergency of clustering and inconsistency of results. These problems are caused mainly by arbitrary determination of initial cluster centers. For example, the initial clustering centers are, in fact, the first N c components in the 0-1

2 (cylinder)

incidence matrix. If the first N c components are similar, this may result in only one cell being formed. This can be resolved by artificially choosing more different parts. Inconsistency means that the final results may be different when the initial cluster centers are changed. These problems have been tackled by Gu 3 and Gu and Norrie. 6 5. C O N C L U S I O N S This paper presents the development of manufacturing cells for a manufacturing company. A clustering algorithm was applied to the design process using an IBM PC 386 computer. The algorithm is flexible and easy to use. The disadvantages of the algorithm have been identified and further research is being carried out at the University of Calgary. REFERENCES 1. Burbidge, J. L.: P r o d u c t i o n flow analysis. The Production Engineer 139-152, A p r i l / M a y 1971. 2. C h a n d r a s e k h a r a n , M. P., R a j a g o p a l a n , R.: Z O D I A C - - a n a l g o r i t h m for c o n c u r r e n t f o r m a t i o n o f part-families a n d machine-cells. Int. J. Production Res. 25(6): 835-850, 1987. 3. Gu, P." D e s i g n o f cellular m a n u f a c t u r i n g systems: a heuristic a p p r o a c h . 1991 A S M E C o m p u t e r s in E n g i n e e r ing C o n f e r e n c e , 1991 (submitted). 4. Gu, P., E l M a r a g h y , H. A." F o r m a t i o n o f m a n u f a c t u r i n g cells b y cluster-seeking a l g o r i t h m s . J. Mech. Working Technol. 20: 403-413, 1989. 5. Gu, P., E l M a r a g h y , H. A.: O p t i m u m design o f m a n u f a c t u r i n g cells. J. Manufacturing Systems (submitted). 6. Gu, P., N o r r i e , D. H.: A r o b u s t a p p r o a c h to the design o f m a n u f a c t u r i n g cells. Int. J. Production Res. ( s u b m i t t e d ) . 7. K i n g , J. R., N a k o r n c h a i , V.: M a c h i n e c o m p o n e n t g r o u p f o r m a t i o n in g r o u p t e c h n o l o g y - - r e v i e w a n d extension. Int. J. Production Res. 20: 117-133, 1982. 8. K u s i a k , A.: T h e generalized g r o u p t e c h n o l o g y c o n c e p t . Int. J. Production Res. 25(4): 561-569, 1987. 9. Vannelli, A., K u m a r , K. R." A m e t h o d for finding m i n i m a l bottle-neck-cells for g r o u p i n g p a r t - m a c h i n e families. Int. J. Production Res. 24(2): 387-400, 1986.