The Design of Machine Cluster for Loading and ...

The Design of Machine Cluster for Loading and Unloading Slider in the Hard Disk Drive Manufacturing Amarin Wongsetti*1 , Suksan Prombanpong 2 Department of Production Engineering, Faculty of Engineering, King M ongkut’s University of Techonology Thonburi (KM UTT), Bangkok , Thailand

1 [email protected]; 2 [email protected]

Abstract. This paper is aimed at demonstrating a design of a loading and unloading station cell for transporting the Slider, one of the critical components in a hard disk drive, which is attached at the end of the head gimbal arm, for ele ctronic inspection and verification before transferring to the Head Gimbal Assembly (HGA) line. This is due to the fact that the Slider must be defect free so that a hard disk drive can perfectly perform its function as a data storage device. Therefore, each Slider must be 100% quality checked and identified as a conforming or nonconforming product and it must be done by an automated testing machine. The required yearly demand of a Slider for testing is around 160 mi llion pieces. Therefore, a design to meet this criterion is a must and must be cost-effectiveness. A machine cluster concept is proposed to design load/unload station configuration. It is found that the proposed design is consisted of four cells which is composed of 44 automated load/unload machines with only one worker to perform feeding service to these cells.

1

Introduction

A Slider used in the hard drive Head Gimbal Assemblies is o f important part in the hard disk drive manufacturing industry. Its main function is to slide up and down between plates in a hard disk drive. Th is Slider is made of wafer which is trimmed in shape and then assembled in Head Gimbal Assemblies (HGAs). Then the next step is a Head Stack Assembly (HSA) where the HGAs are stacked and assembled together and then move to the final assembly line where all co mponents are assembled in the frame of a hard disk drive. Fig. 1 shows operation flow of a hard disk drive manufacturing process. Production inspection of hard drive HGAs requires the accurate mea surement of a number of critical factors in order to meet tight tolerances. These crucial factors include pitch and ro ll angles, Slider positioning, and Slider orient ation/align ment issues. Due to these comp lex assembly factors, an HGA inspection platform must combine highly accurate and repeatable measurement capabilit ies with versatile lighting, mu ltip le magnification options, and advanced programming fun ctionality.

adfa, p. 1, 2011. © Springer-Verlag Berlin Heidelberg 2011

Fig. 1. The operation flowchart of hard disk drive manufacturing

A HGA manufacturing process starts with FOLA (Front of Line Automation ( for assembling Slider into HGA , then cleaning by deionizat ion water, LSA (Laser Stat ic Attitude Adjust Machine) for adjustment HGA Slider p itch and GL (Gram-Load) for measuring the flexib ility of HGA’s flexu re. Finally, testing is performed in HGA ET (Head Gimbal Assembly Electrical Testing) station where the good HGA will be accepted and delivered to HSA process; otherwise, it will be rejected and most of the defective HGAs are due to Slider malfunction. The flow process can be depicted in Fig. 2(a). This is due to the fact that a Slider plays an important function in a HGA; therefore, an inco ming Slider requires a tight inspection within the process as shown in Fig.2 (b). A Slider must be inspected by electrical testing (ET) and this can be a ccomplished by load/unload system module.

Fig. 2. A flow process chart of (a) HGA Process. (b) HGA process with online inspection module.

In this module, a Slider will be loaded and transferred to ET for testing and subs equently unloaded, and sorted to separate defective Sliders. Only defect free Slider will be further processed to FOLA. More details of the load/unload system module will be discussed in section 2.1. Thus the load/unload system module must be designed and the prototype was built and tested. This module can acco mplish the task at a speed of 475 p ieces per hour. However, the required yearly production demand is around 162,837,486 p ieces which is equivalent to 21,245 p ieces per hour. Thus, it is obvious that a single load/unload system module cannot satisfy the required production and

mu ltip le ones are inevitable and must be designed with cost effective as much as po ssible which is the main purpose of this paper. Single manufacturing cells (SM C) in form of machine cluster concept will be a pplied to determine a number of required machines and optimal nu mber of workers to serve the clusters [1]. A nu mber of related literatures in this field are investigated and will be briefly described. Pro mbanpong and Seenpipat presented a worker assignment design for machine cluster in the manufacturing cell [2]. The optimu m nu mber of workers is determined to service a machine cluster. Pro mbanpong et al. applied single manufacturing concept for a determination of nu mber of A GVs (Automated Gu ided Vehicle) in a flexible manufacturing cell and verified a result by a simu lation PROMODEL software program [3]. Hu and Zhou utilized mathematical model to optimize mach ine configuration and job scheduling strategy in semiconductor indu stry [4]. Part icle Swarm Optimization (PSO) technique is also proposed to utilize in mach ine cell layout and design [5, 6]. Tariq and Ghafoor applied hybrid genetic alg orith m in single mach ine cells design [7]. Nguyen and Takakuwa applied simu lation technique for manufacturing line design in auto mobile industry [8]. Pierreva and Plaquin applied Group Machines into Manufacturing Cells for improving material flow and utilized the Genetic Algorith ms (GA) for determining the optimal solution [9]. Xing and Marwala applied swarm intelligence on the intra-cell machine re-layout (ICM RL) problem for finding a better solution [10]. Lozano and Gimnez proposed Tabu search algorith m for searching the optimal nu mber of machine per cell on the cellular manufacturing design process [11].

2

Load/unload System Cell Design

As mentioned earlier a single load/unload system module which its production rate is 475 pieces per hour will not be ab le to satisfy the required production of 21,245 pie ces per hour. Thus, a multiple nu mber of load/unload systems must be designed under various constraints such as machine availability, utilization, and limited space. Machine availability is a measurement of machine readiness for work. If a machine is malfunction and needs a repair, a percentage of machine availab ility will be less than 100 and lo wer productivity is ensued. Thus , the machine availability factor must be taken into account in design process. Likewise, mach ine utilization is another perfo rmance measurement in terms of obtained output compared with ideal one. If machine has no idle time, it should be able to produce output according to the design calculation. Ho wever, in practice there may be some problems i.e. short of incoming work parts, wait for worker to attend the machine, or quality p roblem occurring during production and cause machine idle. As a result, a lower productivity can be expected and therefore, machine utilization must also be included in the design process.

2.1

Load/unload system module

The prototype of load/unload system was designed and it is composed of 3 main sy stems named system 1 to system 3. The system 1 is basically designed to load and unload Bola tray which is used to position a Carrier where a Slider is attached to du ring electrical testing. A figure of Bola tray, Carrier, Slider, and Slider tray is shown in Fig.3 (a),(b ),(c) and (d) respectively. The system is designed for Carrier replacement at the end of Carrier life which has been used for 3000 running cycles.

Fig. 3. (a) Bola tray (b) Carrier (c) Slider (d) Slider tray

This system ensures proper functions of the Carrier. The system 3 is dispensing area for loading and unloading o f a Slider t ray. The load/unload system module is 1.22 m. in length and 0.92 m. in width as shown in Fig. 4.

Fig. 4. The load/unload Slider system module

The working mechanism of the load/unload Slider module in Fig. 4 can be e xplained as the following. The mechanism begins with the Slider wh ich has already been processed from ET and it is mounted to the Carrier which is positione d in Bola tray will be fed into the stacker (A). The stacker will transfer the mentioned Bo la trays to dock 1 (B1) and dock 2 (B2) located in the system 1. Then, each Carrier will be transferred to nesting position (C) for d ismounting the Slider and will be sorted in the sorting area (D). At the same time the untested Slider fro m a Slider tray in a dispensing area will be mounted to the Carrier already positioned in the nesting area (C) and this Carrier will be ready for returning to ET for testing. 2.2

Cell layout design

In the cell design process, the most important factor is that the cell must be capable of transferring enough Sliders for electrical testing which is at the production rate of 21, 245 p ieces per hour. Since the availab le space is around 340 square meter or 20 m. by 17 m. Thus, it is plausible to configure as inline layout where a series of load/unload system modules are consecutively positioned next to each other to fo rm a cell of m machines and other cells can be created to form n cells as shown in Fig. 5.

Fig. 5. Cell configuration as an inline layout

Each module will be linked by a ro ller conveyor where a Bo la tray can be tran sported along the cell. Forming as a cell and linked by a conveyor will substantially reduce a number of stacker to only two stackers per cell i.e. feeding and receiving as shown in Fig. 6. This will be cost effectiveness since it is not necessary to install each stacker for each load/unload system module. However, an intellig ent system for t raffic control must be installed to prevent a collision. Since the cycle time of each sy stem module is constant and identical, the traffic strategy is designed in such a way that loading and unloading of Bola trays to and from the docking area of all modules will be simultaneously performed. Therefore, all Bo la trays on the conveyor will co n-

currently move and stop. As a consequent traffic pattern becomes much easier to d esign and implement.

Fig. 6. Cell design for load/unload system module

2.3

Number of load/unload system modules in a cell

A determination of number of system modules in a cell can be determined from (1).

Total number of modules 

WLtotal

(1)

AT Where WLtotal is a total workload and AT is availab le t ime to acco mp lish the work. Hence WLtotal can be calculated from (2). WLtotal = W Ls + WLst + W Lcr

(2)

Where WLs is scheduled workload, W Lst is setup workload and W Lcr is Carrier replacement workload. In this case, scheduled workload, setup time and Carrier replacement time in a year is 325,676.07, 60.83, 150.77 hours in a year. Substitute in (2) we obtain:

WLtotal = 325, 676.07 + 60.83 +150.77 = 325,887.67 AT is available time per year which can be calculated from (3). AT = Tw x A x U

(3)

Where Tw is working time in a year, A is machine availability and U is utilizat ion. In this case working time (T w ) in a year is 7,665; A is 0.9903 and U is 0.9997. Substitute in (3) AT is equal to 7,588.37 hours per year. Thus substitute a total number of modules are 42.9 modules. Since the available space is 340 square meter wh ich is 20 by 17 meter. The module is 1.22 meter long and margin between modules is 2.5 meter; therefore, we can co mfortably design 11 modules in a cell. As a result, we need a total of 4 cells as shown in Fig. 7.

Fig. 7. The final layout machine cluster in the cell layout

2.4

Determination of optimum number of workers

In one cycle a worker is required to attend the designed cell by loading Bola trays to the feeding stacker at the beginning of the cell, loading Slider tray o f each machine in the cell and unloading Bo la trays fro m the receiving stacker at the end of the cell. If the system module cycle time is longer than that of worker service time, the worker is able to attend another module and perform the repetit ive task as mentioned. Howe ver, the bottom line is that the system module must not experience idle t ime; otherwise the production rate will be lowered than expected which is not a desirable situation. Therefore, it is possible to allow some idle time to a worker. Thus it is impo rtant to determine a number of system modules that one worker can attend to and this can accomplish from (4) as the following.

N  Maximum interger 

T T

m

s

 Ts   Tr 

(4)

Where N= nu mber o f modules that one worker can attend to, T m is machine or module cycle time, T s is worker service time and T r is worker repositioning time. In this this case Tm, Tr and Ts is 1,344, 1.511, 6.872 seconds respectively. Thus, substitute these parameters in (4)

N  Maximum interger 

 1344  6.872

 6.872  1,511

N ≤ 161.14 Thus, N is equal to 161 modules. Meaning that only one worker is enough to attend 161 modules; therefore, this designed cell cluster which is accommodated only 44 modules can be attended by only one worker. Thus, in this case a worker will work around 6.15 minutes and enjoys idle time around 975.15 seconds or it is equivalent to 16 minutes before coming back to repeat the task. As a result, a utility worker in the production line can be assigned to perform this task and can do other tasks during free time. The walking route of a worker during serving cluster is shown in Fig. 8. The worker will start fro m the first module in cell 1 and fo llow the path until the last mo dule in cell 4 is served.

Fig. 8. Path of service worker in the cluster.

3

Conclusion and Discussion

In the HGA assembly for hard disk d rive manufacturing industry, The Slider plays an important role to the functional of a hard drive. Thus it is impo rtant to inspect the Slider before assembly. Thus the automated load/unload system is designed for launching the Slider for electrical testing. This module requires mount and dismount the Slider to the Carrier and the prototype module is designed. It requires around 43 modules to satisfy annual production demand. Thus, a single manufacturing cell in form of machine cluster concept is implemented. It is found that with the available space one cell can acco mmodate up to 11 modules and therefore 4 cells are designed. In order to reduce a number of stackers which is required to d ispense a Bola tray, a roller conveyor is used to link all modules within a cell. Th is is due to the fact that each module still requires a worker to serve the Slider tray at each cycle and it is found that one worker is enough to support up to 116 modules . The single manufacturing cell concept is useful for determination of required work cell to satisfy production demand. It is important to include machine availability, defect rate, utilization in the calculat ion since these factors has an adverse effect to the production. Once a number of machines were determined, a machine cluster concept should be attempted to reduce labor cost or perhaps any common devices can reduce hardware installat ion cost. It should be noted that the equation (4) listed in this paper is very useful to calcu late a nu mber o f machines attended by one worker. Although it looks quite simple, only a few people recognize and hardly use it. This project is currently under imp lementation with a speculated budget of 10 mi llion US dollars.

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