minimization of rejection rate using lean six sigma … · lean six sigma tool in medium scale...

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International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 1, January 2018, pp. 1184–1194, Article ID: IJMET_09_01_126

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=1

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

MINIMIZATION OF REJECTION RATE USING

LEAN SIX SIGMA TOOL IN MEDIUM SCALE

MANUFACTURING INDUSTRY

S. Nallusamy

Professor, Department of Mechanical Engineering,

Dr. M G R Educational and Research Institute, Chennai, Tamilnadu, India

R. Nivedha, E. Subash, V. Venkadesh, S. Vignesh and P. Vinoth kumar

UG Scholars, Department of Mechanical Engineering,

Dr. M G R Educational and Research Institute, Chennai, Tamilnadu, India

ABSTRACT

The objective of this research is the pilot implementation of six sigma DMAIC

phases to improve the rejection rate of manufacturing industry. The current global

market is focusing on zero defects that are 3.34 defectives per million parts produced.

Six Sigma places the emphasis on financial results that can be achieved through the

virtual elimination of products and process defects. Profits and industry stakeholder

value is maximized. In this research it is proposed to analyze a case study by

implementing six sigma tool for minimization of rejection rate. The defect is identified

by process flow, pareto chart, fish bone diagram and all were also adopted to analyze

the depth of the issue and to identify the critical factors which is required for

controlling and improving the overall expectations for the zero rejection rate. From

the results it was found that, rejection rate of the selected manufacturing industry is

brought to 1.2% from 5.3% through the implementation of DMAIC.

Keywords: Lean, Six Sigma, DMAIC, Rejection Rate, Wheel Cylinder

Cite this Article: S. Nallusamy, R. Nivedha, E. Subash, V. Venkadesh, S. Vignesh

and P. Vinoth kumar, Minimization of Rejection Rate Using Lean Six Sigma Tool in

Medium Scale Manufacturing Industry, International Journal of Mechanical

Engineering and Technology 9(1), 2018, pp. 1184–1194.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=1

1. INTRODUCTION

A wheel cylinder is a component in a drum brake system. It is located in each wheel and is

usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto

the shoes so as to bring them into contact with the drum and stop the vehicle with friction.

The wheel cylinders are usually connected to the shoes with small bird-beak shaped rods. On

older vehicles these may begin to leak and hinder the performance of the brakes, but are

S. Nallusamy, R. Nivedha, E. Subash, V. Venkadesh, S. Vignesh and P. Vinoth kumar


normally inexpensive and relatively easy to replace. The wheel cylinder consists of a cylinder

that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects

the piston with a brake shoe. When brake pressure is applied, the pistons are forced out

pushing the shoes into contact with the drum. Wheel cylinders must be rebuilt or replaced if

they show signs of leaking. Wheel cylinders used to be made of cast iron. However, they were

more prone to rusting and aluminium is now the preferred material. As the safety measures

are higher for the mentioned product, it includes various operations in order to satisfy all the

safety measures. So it is compulsory to fix each and every problem that arises against safety.

Humongous wastage occurs in a wheel cylinder manufacturing company, due to various

defects caused in different operations. In this project we have been used six sigma tool, to

bring down the wastage percentage and to improve the quality of the product and also for the

profit of the manufacturing company.

2. LITERATURE REVIEW

As per Toyota’s production system, lean manufacturing can be defined as ‘A systematic

approach to identifying and eliminating wastes through continuous improvement by making

the product at the pull of the customer in pursuit of perfection’. Waste can be eliminated and

lean development can be achieved by various tools such as 5S, Kanban, Kaizen, Value stream

mapping and Work standardization [1-5]. VSM is a process for analyzing the current scenario

and to design a future scenario for framing the sequence of activities that takes a product from

its beginning through to the consumer. VSM is used as a main tool to identify the

opportunities for various lean techniques. Different research articles have discussed the

various applications of VSM technique like aircraft, supply chain, logistics, healthcare,

service related industries and information flow analysis in different manufacturing industries

[6-10]. A supply chain model was to achieve success in agile supply chain where the system

should adapt to changes immediately and the stakeholder should possess knowledge about the

various stages of the supply chain and share the information at the right time to sustain the

agility in the supply chain was proposed [11-15]. Multi objective mixed-integer linear

program with total cost, total flow time, and total lost sales as key objectives was developed

for addressing production, distribution, and capacity planning of global supply chains by

considering cost, responsiveness, and customer service levels simultaneously [16-20]. A

customized Artificial Neural Network (ANN) model that allows changes for the decision-

making process was proposed. The model employed with fuzzy analytics hierarchy process

(FAHP) for determining the relative weights of the attributes used posteriorly in the ANN

model with back propagation learning [21-24]. The inventory management practices of

various companies and institutions were studied and compared with necessary suggestions for

improvement using the ABC analysis inventory management method. The ABC analysis was

found to be useful to most of the companies already in usage of this tool either manually or

with an enterprise resource planning system [25-29]. Projects were undertaken to identify

areas in the process where extra expenses did exist and introduced appropriate measurement

system, which reduced expenses on production times. The variables influencing the chosen

characteristics and then optimized the process in a robust and repeatable way were

represented and focused on what six sigma is today and its roots in both Japan and in the west

and what Six-Sigma offers the world today [30-34]. Six sigma is a lean tool to improve the

business policy which makes an attempt to develop the maximum efficiency and effectiveness

of all the production processes to fulfill the customer requirements [35-38]. By implementing

Six sigma Define, Measure, Analyze, Improve, Control (DMAIC) a revolutionary approach

with its statistical tools and techniques in different processes, quality along with the monetary

savings can be achieved. A distinct methodology for integrating lean manufacturing and Six

Sigma philosophies in manufacturing facilities were highlighted [39-42]. Based on the above

Minimization of Rejection Rate Using Lean Six Sigma Tool in Medium Scale Manufacturing

Industry


literature a study was made on implementation of six sigma using DMAIC methodology in

automotive spare parts manufacturing industry cited at Chennai.

3. PROBLEM DEFINITION AND METHODOLOGY

An industrial process includes a number of repetitive tasks, the most grotesque example being

the production of a room in high volume. A room is compliant if it meets a number of criteria.

However, all the exhibits can’t be strictly identical. One of major concern of quality

management is how to master the conditions of production so that there is as little waste, the

least possible customer dissatisfaction. Hence, the manufacturing industry does more work on

wheel cylinder model of RBI, when compared to other models like GS, HA, AH, S101, LCI,

MY15, etc. RBI model wheel cylinders are used in majority of the cars. So, the selected

manufacturing company faces more rejections in RBI model comparatively. So, we are

concentrating on the rejection percentage of RBI model to bring down the overall rejection

rate of the industry. The methodology adopted is expected to cover over all possible causes of

the problems (or) issues. If the methodology used for solving the problem is not

comprehensive enough, the solutions obtained in completion may not be correct and the

problem is likely to resurface faster or later also. To increase the production rate through

minimizing the rejection rate of components at automotive spare parts manufacturing

industry, the methodology adopted using Six Sigma DMAIC a five stage improvement cycle

is given as a flow chart in Figure 1.

Figure 1 Methodology Flow Chart



4. DATA COLLECTION AND ANALYSIS

4.1. DMAIC Chart

To implement Six-Sigma, it must follow DMAIC approach step-by step is shown in Figure 2.

This approach is briefly described for the concerned organization. Lack of proper analysis

may lead to the process to a wrong way, which will deviate from the main function of

improvement. Every successful work goes on some specific sequence. This work also

completes some specific step. After completing each successful step, it is necessary to move

next step. Methodologically the total process of the work is divided into two basic stages,

measurements and improvements. The DMAIC is a basic component of Six-Sigma

methodology- a better way to improve work process by eliminating the defects rate in the

final product. Six sigma is the statistical breakthrough tool which improves the existing

performance level of business strategy and controls the variations in the processing stage to

less than the actual rejection rate.

Figure 2 DMAIC Approach using Six Sigma

4.2. D for Define Phase

In the define phase, a Six sigma team refines its problem statement and goals, identifies the

factors which are critical to quality. This also ensures the business goal, priorities and

expectations. This phase also helps us to clarify the issue of the project, in this first step, it is

necessary to focus on the process that generates the product and the map in order to be

familiar. As the major safety dimension (i.e.) Bore diameter of the RBI model wheel cylinder

lies between 17.767 ~ 17.793 mm, the work pieces which does not satisfies the given

dimensions are made to be rejected. The specific allowances for bore diameter may be within

+0.013 microns. We can easily able to notice that, the bore machining operation is highlighted

in the given process flow chart shown in Figure 3.


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Figure 3 Process Flow Chart

Hence, it is defined that majority of the rejection takes place in the boring operation.

Accordingly, bore line mark and Bore undersize are the major defects which are the reason

for increase in rejection. As the safety measures are concerned for the customer’s satisfaction

these defects must be rectified for good quality assurance. It is necessary to analyse these

defects in order to control the overall rejection rate of the manufacturing company.

4.3. M for Measure Phase

Measure phase is a step of collecting data on measurable parameters of the process. The

objective is to determine what is able to provide the process in question namely its sigma.

During this stage, it is important to focus on critical parameters for the quality, that is, those

whose influence on the result is the largest. There are many measuring instruments like air

gauge, coordinate measuring machine (CMM), etc. which are normally used in many

industries. Because of the reason that the wheel cylinder has the critical dimension, air gauge

is used for measurement. Certain sample work pieces undergoes air gauge measuring for

regular shifts. In order to identify the severity of rejection rate, Pareto chart was constructed

as shown in Figure 4. The Pareto chart reveals the, actual rejection rate of the company for the

time period of one month of December 2017. As per the measure phase the average rejection

rate is 5.34%. Hence, the rejection rate must be decreased to meet the increase in production

rate, higher quality and customer expectation.



Figure 4 Actual Rejection Rate

4.4. A for Analyze Phase

In analyse phase, the identification of the root cause, makes an impact on the rejection rate of

the work piece. In this regard, various subcritical factors were illustrated by cause and effect

diagram as shown in Figure 5. Consequently, the brainstorming sessions were conducted to

identify the major critical factors that make an impact on the rejection rate. From Figure 5, it

is clear that the major causes are bore line mark and bore under size and the minor causes are

highlighted by the red boxes. Hence, in the brainstorming session the solutions for rectifying

the defects are discussed seriously. It was identified that, the reasons for bore line mark are

reamer tip wear out, tool life not monitored for re-tipping. The cause for bore ovality or bore

under size is due to the fixture clearance between the block and part. These main reasons are

convectively analysed to make improvements to reduce the rejection rate.

Figure 5 Cause and Effect Diagram


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4.5. I for Improve Phase

The improve phase concentrates on embarking the current system configuration, which

enlightens the minimization of rejection level. This stage was functioned through a strong

brain storming session with all the team members and experts of the department. The whole

team discussed the current status of the system. After fierce investigation, the consensus

reveals that, the permanent corrective measures to avoid bore line mark is to replace the tool

before the last 2% of the overall tool life, proper and regular monitoring of tool, creating

awareness among the workers. To avoid bore under size, butting the unwanted blocks, bore

diameter was checked for both sides of the work piece, proper instructions were given to the

operators and patrol inspectors about bore under size. Implementing these ideas these two

major defects should be eradicated, which affects the overall production rate of the industry.

A Pareto chart was prepared after implementing the ideas for the improvements to avoid the

major effects which are shown in Figure 6. The results of Pareto chart reveal the minimized

rejection rate of the company for time period of January 2018. As per the results from the

analyze phase it was found that the average rejection rate in 1.2% and the graph clearly

depicts that the rejection rate decreases gradually. Hence the rejection rate meets the increase

in production, higher quality and customer expectation.

4.6. C for Control Phase

In control phase, comparison between times of preventive works before and after using six

sigma tool and also the gains have been achieved by the team personnel through refined

process that yield maximum remunerations. In improve phase, the wheel cylinder

manufacturing industry implemented the optimal solution to achieve continuous improvement

on minimization of rejection rate. At this phase various supervisory activities are designed for

all the term members and it took lots of suggestions from the top management also.

Consequently the overall of rejection rate of the manufacturing company is enhanced by

analyzing and identifying the critical defects of the work piece. Further improvement in

overall minimization of rejection rate can also be done by analyzing and identifying various

defects in other operations on the work piece and other models too. Now, the rejection rate of

the industry is brought to about 1.2% from 5.3%. If the bore diameter of the wheel cylinder is

maintained between the dimensions and the analyzed defects are rectified continuously, the

rejection rate of the manufacturing industry could be improved more and may be taken to zero

as well.

Figure 6 Pareto Chart on Minimized Rejection Rate



5. CONCLUSION

Successful implementation and growing organizational interest in six sigma method have

been exploding in last few years factors influencing successful six sigma projects include

management involvement and organizational commitment, project management and control

skills, cultural change, and continuous training understanding the key features, obstacles, and

shortcomings of six sigma provides opportunities to practitioners for better implement six

sigma projects. However, integrating the data-driven, structured six sigma process into

organizations still has room for improvement. It integrates the lesson learned from successful

six sigma projects and considers further improvements to the six sigma approach. Based on

the analysis and results the following conclusions were arrived.

• Our conceptual frame work purpose new insights to the managers in an organization

that typically interacts with education and quality.

• To achieve this, Six Sigma approach with the proactive methodology of DMAIC is

applied.

• The overall rejection rate is reduced from 5.3% to 1.2% after implementation of Six

sigma.

• The delivery schedule of the product is met at right time.

• A positive step towards zero defects with customer satisfaction.

• Regular monitoring and controlling is adopted.

Further improvement in overall minimization of rejection rate can also be done by

analyzing and identifying various defects in other operations on the work piece and other

models too.

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