process standardization through taguchi design of experiments

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Process Standardization

Process Standardization through Taguchi Design of Experiments

Lean tool - standardization eliminates variation and incorrect assumptions in the way

that work is performed. The resent work involves standardization of process parameter for a

electrical discharge machine through Taguchi design of experiment there by reducing the

variation between parts in the process. Through conducting experiments optimum levels are

identified for the materials EN8, EN353 and EN24 are explained in this present work.

Standardization is achieved through finding the optimum levels for each material and

communicating the findings through standard operating procedures (SOP). The project

starts with the study of the process (EDM), Taguchi design of experiment, regression

analyses and optimization methods.

4.1 Electrical Discharge Machining

The basic concept of electrical discharge machining is cratering out of metals

affected by the sudden stoppage of the electrical beam by the solid metal surface of the

anode. The portion of the anode facing the direct electrical pulse reaches the boiling point.

Even in the case of median long pulse the rate of temperature increase in tens of millions of

degree per second which means dealing with explosion process. The shock wave produced

spreads from the center of the explosion inside the metal and on the way crush the metal

and deforms crystal.

When a suitable unipolar (pulsed) voltage is applied across two electrodes separated

by a dielectric fluid, the later breaks down. The electrons, so liberated, are accelerated in the

presence of the electric field collide with the dielectric molecules, causing the later to be

robbed off their one or two electrons each and immunize. The process grows and multiplies

the second emission followed by an avalanche of electrons and ions. The resistant of the

dielectric layer drops as it is ionized result in ultimate break down. The electrical energy is

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discharged into the gap and multifarious action takes place electrodynamics waves set in

and travels at speed causing shock-impact and high temperature rise at the electrode

surface. The instantaneous temperature may rise as high as 1000oc causing localized

vaporization of the electrode.

+

- Dielectric

Work Piece

(anode)

Tool (Cathode)

Dielectric

reservoir

Working

tankPower

supply

Servo

head

Hand

wheel

Figure 4.1 block diagram of EDM process

The electrical discharge machining process involves finite discrete periodic sparks

between tool electrode and conductive work-electrode separated by a thin film of dielectric

that causes removal of work material.

4.1.2 High temperature theory

According to this theory, due to the bombardment of the high energetic electrons on

the electrode surface, the spot attains high temperature (100,00oc) especially with material

of low thermal conductivity. At this temperature, material at that spot instantaneously melts

and vaporizes leaving a crater on the surface.

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4.2 Taguchi Design of Experiments

Taguchi method was designed to improve the quality of product and process where

the performance depends on many factors. In laying out a test and development strategy,

simple logic will usually be sufficient to establish all possible combination of factors along

with allowable range of each of the factors involved. But the engineering projects involving

many factors, the number of possible combination is prohibitively large. To reduce the

number of experiments to a practical level, only a small set from all possibilities, is selected.

The method of selecting the limited number of experiment which produces the most

information is known as a partial factorial experiment.

Taguchi constructed a special set of general design for fractional experiments that

covers many applications. The special set of designs consists of orthogonal arrays. The use

of this orthogonal array helps to determine the least number of experiments needed to a

given set of factors. The combination of standard experimental design techniques and

analysis methods in the Taguchi approach produces consistency and reproducibility.

Taguchi experimental results is primarily to seek the answers to the following three

questions

1. What is the optimum condition?

2. Which factors contribute to the result and by how much?

3. What will be the expected result at the optimum condition?

4.2.1 Taguchis philosophy involves three central ideas

1. Products and processes should be designed so that they are robust to external

source of variability.

2. Experimental design methods are engineering tools that help this objective.

3. The operation on target is more important than conformance to specifications.

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Figure4.2 The Design of Experiment Process

4.3 Regression Analysis

Chi Square test tells us whether there is any relationship between variables but it

does not tell us, what the relationship is. Regression analysis shows us how to determine

both the nature and strength of relationship between two variables. So regression analysis

serves as a efficient model building technique.

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Figure 4.3 Procedure for building the regression model

4.3.1Least square method

A good fit is required for minimize the error and to establish the regression line as a

mathematical model. The least square method statistically proves it efficiency by maintaining

the lowest error. Least square method fits the data distribution on a line with minimum error.

Estimating Equation Describing Relationship among three variables

= Estimate value corresponding to the dependant variable.

a = Y- intercepts

X1, X2 = Values of the dependant variables

b1, b2 = Slope associated with X1, X2 respectively

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Normal equation

Solving the normal provides the value of the Y intercepts and the slopes of the variables.

Using calculated data dependant variable is estimated.

4.3 Methodology

The methodology adopted to carry out the project is by Taguchis design of

experiment technique. Taguchi has suggested a new approach for design of experiments,

which identifies the nature of parameters by conducting minimum number of experiments

and which is extensively applicable in manufacturing industries. Taguchis approach will give

a clear idea about the most influencing parameters on surface finish. The objective of the

research is an experimental investigation to find optimization of machining parameters of

EDM machine for machining steel material EN-353 using Taguchis DOE. Another point

made to optimize the machining parameters for EDM is to achieve maximum surface finish.

The parameters considered are pulse on time, Pulse off time, Peak current and gap voltage.

Then the Regression aids us to build the mathematical model and the model is used to

generate optimized solution using steepest descent method.

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4.3.1 Experimental Setup

An EDM machine Electronica PSR 35 is used for conducting the

experiment. The work material electrode and the other machining conditions are as follows:

1. Work piece steel material En-353 (Anode).

2. Work piece height is 30 mm.

3. Hardness of the work piece material is 50HRC.

4. Electrode copper round rod of 50 mm diameter (Cathode)

5. Average voltage gap 45 to 55 volts by adjusting servo feed setting during machining.

6. Di- electric temperature is 22 to 25C.

7. Flushing pressure of die-electric fluid is 13 to 15 kg/cm2.

Four machining parameter such as pulse on-time, pulse off-time, peak current and

gap voltage are considered as controlling factors and each parameter has three levels.

4.3.2 Experimental Result

In this experimental study, 4 factors and 3 levels are used and L9 orthogonal array

suitable for this experimental design was selected. The completed design with description of

factors and their levels and the orthogonal array are shown in Table 4.1.

Table 4.1 Design factors and their levels

Column Factors Units Level -1 Level -2 Level -3

1 Pulse on

time

-sec 25 50 75

2 Pulse off

time

-sec 10 15 20

3 Peak current Amps 3 5 74 Gap voltage Volts 45 50 55

Note: Interactions between the factors are not considered.

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Table 4.2 L9 Orthogonal array used for the experiment

Trial 1 2 3 4Trial 1 1 1 1 1

Trial 2 1 2 2 2

Trial 3 1 3 3 3

Trial 4 2 1 2 3

Trial 5 2 2 3 1Trial 6 2 3 1 2

Trial 7 3 1 3 2Trial -8 3 2 1 3

Trial 9 3 3 2 1

Objective: Determine best machining parameters in EDM.

Characteristics- Smaller is better.

Table 4.3 Original observation and their average

Trial /

Repetitions

R1 R2 R3 R4 Average

1 2.6 2.7 2.5 2.6 2.6

2 1.9 2.0 2.1 2.0 2.03 2.5 2.3 2.7 2.5 2.5

4 2.8 3.0 3.1 2.7 2.9

5 3.4 3.6 3.7 3.3 3.56 3.1 3.3 3.3 3.1 3.2

7 3.3 3.7 3.3 3.3 3.48 2.8 2.9 3.1 3.2 3.0

9 3.1 3.2 3.4 3.5 3.3

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Quality characteristics: Smaller is better.

Result: Up to 4 repetitious shown.

Table 4.4 Main effect

Column Factors Level -1 Level -2 Level -3 L2 L11 Pulse on

time

2.366 3.2 3.233 .834

2 Pulse off

time

2.966 2.833 3 -0.134

3 Peak current 2.933 2.733 3.133 -0.24 Gap voltage 3.133 2.866 2.799 -0.267

755025

3.2

3.0

2.8

2.6

2.4

201510

753

3.2

3.0

2.8

2.6

2.4

555045

PULSE ON TIME PULSE OFF TIME

PEAK CURRENT GAP VOLTAGE

Data Means

Figure 4.4 mean effect chart (EN353)Table 4.5 ANOVA table(353)

Column Factors Dof

( f )

Sum of

squares

Variance

(V)

F

ratio

Percent

P%

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(S) ( F )

1 Pulse on

time

2 1.446 0.723 - 75.343

2 Pulse off

time

2 0.046 0.023 - 2.43

3 Peak

current

2 0.239 0.119 - 12.499

4 Gap voltage 2 0.186 0.093 - 9.721

Other error 0Total 8 1.92 100

Table 4.6 Optimum conditions and performance

Column Factors Level

description

Level Contribution

1 Pulse on time 25 1 -.567

2 Pulse off time 15 2 -.101

3 Peak current 5 2 -.2

4 Gap voltage 55 3 -0.134

Contribution from all the factors (Total) - 1.002

Current grand average of performance 2.933

Expected result at optimum condition 1.931

Nine work pieces EN-353 round rod diameter 45mm top surface were machined in

EDM by using electrode copper diameter 50mm. Each sample was measured for surface

roughness by using TAYLOR-HABSON Surtonic 3 surface roughness measuring

equipment. Since longer life was desirable, the quality characteristic applicable in this case

was Smaller is better. The main effect was shown in (Table 4.4). The study of the main

effects indicates some interaction between the factors.

Percentage contribution for EN-353 materials was shown in (Table 4.5) and from this

the significant factor, which is having more influence for surface finish was obtained. In this

experiment, the result of the contribution in the order of Pulse on time (75.34%), Peak

current (12.5%), Gap voltage (9.72%) and pulse off time (2.43%).

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The purpose of the ANOVA is to investigate which parameters significantly affect the

performance characteristics. The result of the ANOVA (Table4.5) indicates that pulse on-

time, Peak current, Gap voltage, and Pulse off time are the significant machining parameters

and the percentage contribution in the order of 75.343%, 12.499%, 9.721%, and 2.43%. In

the ANOVA table, the factor assigned column has the smallest sum of squares and hence

the least influence. It is proved from the ANOVA (Table 4.5), the factor assigned to the

column (2) has the smallest sum of squares (0.046) and hence the least influence.

Confirmation test was conducted at the following levels and the factors and its surface

roughness value is measured and found as 2.1.

Table 4.7 Optimum conditions Rafor EN-353 material

Trial No Pulse on time Pulse off time Peak current Gap voltage

1 25 15 5 55Confirmation experiment result 2.1 micron.

Expected result at optimum condition 1.931 micron.

A key component of Taguchis philosophy is reduction of variability. It often

requires that each quality as a target or nominal value. The objective is to reduce the

variability around this target. In this experiment the difference between the results is very

minimum such as 0.169 micron. In this experimental value, Taguchis philosophy is satisfied.

4.3 Regression analysis

Regression analysis was carried out using Minitab 15 the results are summarized

below

The regression equation isRa = 3.43 + 0.0173 PULSE ON TIME + 0.0033 PULSE OFF TIME

+ 0.0500 PEAK CURRENT - 0.0333 GAP VOLTAGE

S = 0.375832 R-Sq = 70.6% R-Sq(adj) = 41.1%

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4.6 Standard operating parameter

To serve the purpose of standardization the optimum levels are communicated to the

shop floor through work instruction by standard operating procedure. The list of standard

operating parameter are presented in the (Table 4.8).

Table 4.8 Standard operating parameters

material\Parameters Peak Current Pulse on time Gap voltagePulse oftime

EN 8 3 25 50

EN 24 3 25 45

EN 353 5 25 55

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process standardization through taguchi design of experiments

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