0910SEM2-ME4262

ME4262/ME4262E

NATIONAL UNIVERSITY OF SINGAPORE

ME4262/ME4262E ME4262/ME4262E – AUTOMATION IN MANUFACTURING MANUFACTURING

(Semester II : AY2009/2010)

Time Allowed : 2 Hours

INSTRUCTIONS TO CANDIDATES:

1.

This examination paper contains FOUR (4) questions and comprises SIX (6)  printed  pages.

2.

Answer all FOUR (4) questions.

3.

All questions carry equal marks.

4.

This is an OPEN-BOOK EXAMINATION.

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QUESTION 1

A local electronic manufacturing company currently has a manual assembly line, occupying a floor area of 1,800m2  with an output of 65,000 units daily. It currently employs 30 manual operators and 6 supervisors over 3 shifts, 8 hours per shift, 5 days per week and 50 weeks per annum. In a recent board meeting, the company seriously considered automating the assembly line. A preliminary study of the facilities indicate that the current manual assembly line can  be replaced by a fully automated assembly line with the same output capacity, consisting of 6 automated machines, an integrated material handling system with buffer storage. The study also indicates that only 6 manual operators and 3 supervisors will be needed over 3 shifts when fully automated. Furthermore, the fully automated assembly line will occupy only 1,400m2 of floor area. The cost data are given below. All equipment costs include installation. Six automated machining centres Integrated material handling system with buffer storage Estimated life of the proposed automated line Annual maintenance cost of proposed automated assembly line Manual operator’s rate Supervisor’ salary Electricity bill for the automated assembly line Rent Revenue

$1,800,000 $ 500,000 5 years $161,100 $10/hr $ 4,500/month $ 12,000/month $ 30/m2 per month $ 1.80/unit

(a)

Assuming that the initial capital required for the investment can be secured through a  bank loan at an interest of 5% per annum, justify to the Company’s board members if the investment is cost justifiable for the proposed automated line. (14 marks)

(b)

A closer study of the above automation proposal indicates that the area of the automated assembly line can be further reduced. Indicate the final area of the automated assembly line in order for the proposal to be viable? (4 marks)

(c)

Apart from the reduction in space occupied mentioned in (b), suggest any other alternative way(s) that you may consider in making the automated assembly line  proposal viable. (4 marks)

(d)

Suggest 2 possible reasons for incorporating buffer storage in the above proposed automated assembly line? (3 marks)

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QUESTION 2

(a)

(b)

An oval shape single-directional carousel storage/retrieval system is to be designed for the storage and retrieval in a local hardware shop. The width of the carousel is 4 m and the length is yet to be determined. The speed of the carousel is given as 30 m/min. The handling time for each retrieval = 30 seconds. (i)

What is the length of the carousel needed if the designed system must be capable of making 40 retrievals per hour? What is the actual time needed per retrieval? (Note: The length of the carousel must be an integer) (11 marks)

(ii)

Given that the max spacing between carriers is 1m, design suitable number of carriers and the number of bins hung on each carrier if there are a total of 900  parts on the carousel system when full and each bin can carry 3 parts? (4 marks)

Indicate the correct answer for the following multiple-choice questions: (i) Which of the following is not a linear transfer mechanism? (a) Walking beam system (b) Geneva mechanism system (c) Power roller conveyor system (d) Chain-drive conveyor system

(10 marks)

(ii)

Which of the following statement is not true? (a) An automated guided vehicle generally uses independently operated, self-propelled vehicles. (b) Automated guided vehicles are generally used where different materials must be moved from various loading points to various unloading points. (c) Automated guided vehicles are generally applied in automation of low and medium-volume handling situation, where routing of materials is more individualized. (d) All automated guided vehicles today use the defined pathways guided system in the floor.

(iii)

The advantages of web-based training are given below except: (a) Access of training material anytime, anywhere with the use of internet (b) Allow collaborative design of parts anywhere with the use of internet (c) Cost savings in logistic and training personnel (d) Ability to monitor the users’ progress remotely.

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(iv)

Which of the following type of conveyor system provides for storage and delivery of parts: (a) Skate-wheel conveyor (b) Overhead trolley conveyor (c) Carousel storage conveyor (d) Re-circulating conveyor

(v)

The followings describe the major differences between Just-in-time/Just-incase processes except? (a) (b) (c) (d)

Less work-in-process Less machines used in the process. Reduce space and handling (Inventory) Less scrap and rework

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QUESTION 3

(a)

Indicate TRUE or FALSE for the following: (i) In the development of a generative CAPP system, a coding system is sufficient to be used as a part description format. (ii)

In feature recognition, multiple interpretation is a difficult problem to resolve. The design-by-feature approach can effectively avoid this problem.

(iii)

In the development of a variant process planning system, the standard plan for a particular part family is a process plan of one of its members.

(iv)

To convert a functional layout to a GT layout, a set of OP codes need to be defined such that each OP code represent a set of operations carried out by a  particular machine.

(v)

For a valid classification and coding system, a part must not have two different codes.

(vi)

In the application of GT, the major weakness of a coding system is that only limited details of part design information can be represented.

(vii)

In RP manufacturing, the layer thickness can be used to control the stair-case error.

(viii)

In parts manufacturing, RP has achieved limited success due to that only limited engineering materials can be used.

(ix)

In a part feeding device, the use of low-level sensor can sufficiently resolve the “stock-out” situation.

(x)

For a multi-station assembly machine, reducing the loading and unloading times has no effect on the uptime efficiency of the machine. (10 marks)

(b)

A part feeding device serves a station in a multi-station automated assembly machine. When a jam occurs at one of the stations, if its part feeding device continues to work,  briefly describe the subsequent actions of this feeding device. (6 marks)

(c)

A part feeding device serves an automated assembly machine. The ideal cycle time of the assembly machine is 6 seconds. The feed-rate of the feeder is 6 times of the cycle rate of the assembly machine. On the feed-track, two sensors (the high-level one is  placed at n2 and the low-level one at n1) are used to control the queue and prevent the stock-out situation. It is observed that (i) (ii)

When n2 =  N , it takes k   minutes for the parts to go from n1 to n2, and 0.2k minutes from n2 to n1. When n2 = N  +18, it takes 2k  minutes for the parts to go from n1 to n2.

Determine k . (9 marks)

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QUESTION 4

(a)

A single-station assembly machine performs 4 work elements to assemble 2 components to the base part. The elements are listed, together with the fractional defective rates (q) and the probability (m) that a defective component will jam the station in the table below. The average down time per jam is 200t seconds. Element 1 2 3 4

Description Add part A Fasten part A Add part B Fasten part B

Time (s) 4t  t 4t  t

q q A q B  -

m 0.5 0.5 -

It was observed that when q A = q B = 0.02, it took T  hours to produce 3600 assemblies; when q A = q B = 0.01, it took (T -2) hours to produce 3600 assemblies. When q A = q B = 0, it took 15 hours to produce 3600 assemblies.

(b)

(i)

Determine t .

(ii)

(8 marks) Determine how long it takes to produce 3600 assemblies when q A = q B = 0.03. (4 marks)

A synchronous assembly machine has 4 stations. The fractional defective rates (q) for all the stations are the same. Under the memory  control strategy, the machine  produced 960 good assemblies and 40 bad assemblies in 10 hours. Under the instantaneous  control strategy, the machine produced 1000 good assemblies in 11 hours. (i)

Obtain the ideal cycle time of the machine (T c) and the average down time per  jam (T d ). (9 marks)

(ii)

When m for all the stations is set as 0.5, how long does it take the machine to  produce 1000 good assemblies? (4 marks)

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