OPTIMIZATION OF INPUT PARAMETERS OF CNC TURNING OPERATION FOR THE GIVEN COMP

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OPTIMIZATION OF INPUT PARAMETERS OF CNC TURNING OPERATION FOR THE GIVEN COMP Powered By Docstoc
					 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
                          AND TECHNOLOGY July - August
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4,(IJMET) (2013) © IAEME


ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)                                                     IJMET
Volume 4, Issue 4, July - August (2013), pp. 188-196
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       OPTIMIZATION OF INPUT PARAMETERS OF CNC TURNING
       OPERATION FOR THE GIVEN COMPONENT USING TAGUCHI
                          APPROACH

               Prabhat kumar sinha, Manas tiwari, Piyush pandey, Vijay kumar
                             Mechanical Engineering Department
          Sam Higginbottom Institute of Agriculture Technology and sciences, Allahabad


ABSTRACT

        Quality is inversely proportion to variability. In the other words as variability reduces,
Quality improves. This approach has been used in this work. In the present work variability in the
dimension of the manufactured part has been reduced. Reduction in the surface roughness as well as
tolerance is the basic aim of this work. The increase of consumer needs for quality metal cutting
related products (more precise tolerances and better product surface finish) has driven the metal
cutting industry to continuously improve quality control of metal cutting processes. Within these
metal cutting processes the turning process is one of the most fundamental cutting processes used in
the manufacturing industry. Surface finish and dimensional tolerance, are used to determine and
evaluate the quality of a product, are two of the major quality attributes of a turned product. The
project work has been carried out at Neeraj Industries, Badli Industrial Area, Delhi in which the
optimization of input parameter has been done for improvement of quality of the product in turning
operation on CNC machine. Feed Rate, Spindle speed & Depth of cut are taken as the input variables
and the dimensional tolerances and the surface roughness are taken as quality output. In the reduction
of variation of performance characteristics and quality measures, Taguchi approach is very useful in
the design of experiments. In the present work L9 Array has been used in design of experiment for
optimization of input parameters. This project attempts to introduce and thus verifies experimentally
as to how the Taguchi parameter design could be used in identifying the significant processing
parameters and optimizing the surface roughness of the turning operation.
        There are two purposes of this research .The first is to demonstrate a systematic approach of
using Taguchi parameter design of process control of individual CNC turning machine.The second is
to demonstrate the use of Taguchi parameter design in order to identify the optimum surface
roughness and dimensional tolerance performance with a particular combination of cutting
parameters in a CNC turning operation.The present work shows that the spindle speed is key factor
for minimizing the dimensional variation and feed rate is most effective input parameter for
minimizing the surface roughness.

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1- INTRODUCTION

       The project deals with the manufacturing of coupling of the juicer mixer grinder (JMG) for a
known group which is planning to introduce new juicer mixer grinder in the market. The critical
dimension is the diameter of the cylindrical projected part at the center of the coupling which has to
be fit with the other part so other important quality parameter for this product is the surface
roughness of the cylindrical part.
The process involved for manufacturing this coupling is the turning process for which CNC lathe is
used so that the close tolerances can be achieved.

TURNING OPERATION
        Turning is the removal of metal from the outer diameter of a rotating cylindrical work piece.
Turning is used to reduce the diameter of the work piece, usually to a specified dimension, and to
produce a smooth finish on the metal. Often the work piece will be turned so that adjacent sections
have different diameters.
Turning is the machining operation that produces cylindrical parts. In its basic form, it can be defined
as the machining of an external surface:
   With the work piece rotating.
   With a single-point cutting tool, and
   With the cutting tool feeding parallel to the axis of the work piece and at a distance that will
remove the outer surface of the work.




                     Figure 1.1: Adjustable parameters in turning operation

        Turning is carried on a lathe that provides the power to turn the work piece at a given
rotational speed and to feed to the cutting tool at specified rate and depth of cut. Therefore three
cutting parameters namely cutting speed, feed and depth of cut need to be determined in a turning
operation. The purpose of turning operation is to produce low surface roughness of the parts. Surface
roughness is another important factor to evaluate cutting performance. Proper selection of cutting
parameters can produce precise and lower surface roughness.


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DIMENSIONAL ACCURACY

    Accuracy of an operation: closeness of the agreement between the actual value resulting from
    an operation and a target value of the quantity. Accuracy is a qualitative description.
    Uncertainty of an operation: parameter, associated with the result of an operation that
    characterizes the dispersion of the values that could reasonably be attributed to the quantity.

SURFACE ROUGHNESS

    It is defined as closely spaced, irregular deviations on a scale smaller than that of waviness.
    Roughness may be superimposed on waviness. Roughness is expressed in terms of its height, its
    width, and its distance on the surface along which it is measured.




                                       Fig. 1.2 Surface Texture

Waviness: It is a recurrent deviation from a flat surface, much like waves on the surface of water. It
is measured and described in terms of the space between adjacent crests of the waves (waviness
width) and height between the crests and valleys of the waves (waviness height).
Flaws: Flaws, or defects, are random irregularities, such as scratches, cracks, holes, depressions,
seams, tears, or inclusions as shown in Figure 1.2.
Lay: Lay, or directionality, is the direction of the predominant surface pattern and is usually visible
to the naked eye. Lay direction has been shown in Figure 1.2.

FACTORS AFFECTING THE QUALITY OF TURNING PROCESS:

        Whenever two machined surfaces come in contact with one another the quality of the mating
parts plays an important role in the performance and wear of the mating parts. The height, shape,
arrangement and direction of these surface irregularities on the work piece depend upon a number of
factors such as:

A) The machining variables which include
a) Cutting speed
b) Feed, and
c) Depth of cut.
d) Cutting tool wears


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(i) Depth of cut:
Increasing the depth of cut increases the cutting resistance and the amplitude of vibrations. As a
result, cutting temperature also rises. Therefore, it is expected that surface quality will deteriorate.
(ii) Feed:
Experiments show that as feed rate increases surface roughness also increases due to the increase in
cutting force and vibration.
(iii) Cutting speed:
It is found that an increase of cutting speed generally improves surface quality.
(iv) Cutting tool wears:
The irregularities of the cutting edge due to wear are reproduced on the machined surface. Apart
from that, as tool wear increases, other dynamic phenomena such as excessive vibrations will occur,
thus further deteriorating surface quality.

EXPERIMENTAL SETUP

        In this study, L9(33) orthogonal array of Taguchi experiment is selected for three parameters
(speed, cutting depth, feed rate) with three levels in optimizing the multi-objective (surface
roughness and dimensional tolerance) precision turning on an STORM-A50 CNC (Computerized
Numerical Controlled) lathe. Through the examination of surface roughness (Ra) and the calculation
of dimensional tolerance; the multiple objectives are then obtained. By using Grey Relational
Analysis (GRA), the multiple objectives can additionally be integrated and introduced as the S/N
(signal to noise) ratio into the Taguchi experiment. However, G.R.A. has not been included in the
present work. The mean effects for S/N ratios are moreover analyzed by MINITAB software to
achieve the optimum turning parameters. Through the verification results, it is shown that both
surface roughness and dimensional tolerance from present optimum parameters are greatly improved
in comparison to those from benchmark parameters.
        The precision diameter turning operation of Aluminium alloy (φ9.64 x 12 mm) work piece on
an STORM-A50 CNC lathe is arranged for the research. The TOSHIBA WTJNR2020K16 tool
holder with MITSUBISHI NX2525 insert is utilized as the cutting tool.

MATERIALS AND METHODS

        The study proposed in this research was conducted in 2008. In this study, the multi-objective
integration and parameter optimization technique for CNC turning operations on Aluminium Alloy
are proposed using Taguchi method.

TAGUCHI METHOD

        The Taguchi method is a robust design method technique, which provides a simple way to
design an efficient and cost effective experiment. In order to efficiently reduce the numbers of
conventional experimental tasks, the orthogonal array using design parameters (control factors) in
column and standard quantities (levels) in row is proposed and further adopted. The performance
measure, signal to noise ratio (S/N) proposed by Taguchi is used to obtain the optimum parameter
combinations. The larger S/N means the relation to the quality will become better. The lower quality
characteristic will be regarded as a better result when considering the smaller-the-better quality. The
related S/N ratio is defined as:




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6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 4, July - August (2013) © IAEME



                                                        (Equation:4.1)

       where, n is the number of experiments for each experimental set and yi expresses the quality
characteristic at the ith experiment. On the contrary, the larger quality characteristic will have better
result when considering the larger-the-better quality; therefore, by taking the inverse of quality
characteristic into Eq. (4.1), the related S/N ratio can also be deduced and shown in Eq. (4.2).




                                                            (Equation:4.2)

        In this study, the overall relational rating using GRA for multiple precision CNC machining
objectives is introduced to the Taguchi experiment as the S/N ratio. Therefore, it is judged as the
quality of larger the best. In addition to the S/N ratio, a statistical analysis of variance (ANOVA)
[Wang and Lan, 2008] is to be employed to indicate the impact of process parameters. In this way,
the optimal levels of process parameters can be estimated.

EXPERIMENTAL SET UP

       The precision diameter turning operation of Aluminium alloy (φ9.64 x 12 mm) work piece on
an STORM-A50 CNC lathe is arranged for the research. The TOSHIBA WTJNR2020K16 tool
holder with MITSUBISHI NX2525 insert is utilized as the cutting tool. The relevant specification for
the coupling of the JMG

CONSTRUCTION OF ORTHOGONAL ARRAY

       In this study, three turning parameters (Cutting Speed, Feed Rate, and Depth of Cut) with
three different levels (Table 4.1) are experimentally constructed for the machining operation. In
Table 4.1, the three levels of cutting depth, feed rate and speed are identified from the machining
handbook suggested by the tool manufacturer. The orthogonal array is then selected to perform the
nine sets of machining experiments. The parameter levels for the experiments are illustrated in
Table 4.2.

PARAMETER TABLES AND LEVELS

            TABLE 4.1. PROCESS LEVELS AND PARAMETER DESIGNATION

     Parameter Designation          Process Parameters        Level 1        Level 2       Level 3
               A                    Cutting speed(rpm)         1000           1300          1600
                                                                A3             A2            A1
                B                   Feed rate(mm/rev)          0.02           0.03          0.04
                                                                B3             B2            B1
                C                   Depth of cut(mm)           0.25           0.30          0.35
                                                                C3             C2            C1


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             TABLE 4.2: ORTHOGONAL ARRAY FOR 3 PARAMETER LEVELS

                                                        Parameter
             Experiments

                                 A(Speed)         B(Cutting Depth)         C(Feed Rate)

                   1                Level 1            Level 1                Level 1
                   2                Level 1            Level 2                Level 2
                   3                Level 1            Level 3                Level 3
                   4                Level 2            Level 1                Level 2
                   5                Level 2            Level 2                Level 3
                   6                Level 2            Level 3                Level 1
                   7                Level 3            Level 1                Level 3
                   8                Level 3            Level 2                Level 1
                   9                Level 3            Level 3                Level 2

OBSERVATIONS AND RESULTS: As discussed in section 4.1 & 4.2 three levels of each input
parameters Speed, Feed rate and depth of cut are taken and the experimental layout of three
parameters using the L9 orthogonal array is formed as shown in Table 5.1.

     TABLE-5.1 EXPERIMENTAL LAYOUT USING AN L-9 ORTHOGONAL ARRAY


     Exp.No.                                   PROCESS PARAMETER LEVELS
                             A                          B                            C
                        Speed (r.p.m.)          Feed rate(mm/rev)     Depth of cut (mm)
         1                   1600                       0.04                         0.35
         2                   1600                       0.03                         0.3
         3                   1600                       0.02                         0.25
         4                   1300                       0.04                         0.25
         5                   1300                       0.03                         0.35
         6                   1300                       0.02                         0.3
         7                   1000                       0.04                         0.3
         8                   1000                       0.03                         0.25
         9                   1000                       0.02                         0.35

       Nine experiments are conduced for the above mentioned nine sets of parameters (speed, feed
rate & depth of cut) and in each experiment 20 numbers of pieces are made and are checked with air
gauge for dimensional tolerance and for surface roughness the pieces are tested in spectro-testing lab.
The average value of dimensional tolerance and surface roughness in microns are listed in table 5.2.

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                           TABLE 5.2: EXPERIMENTAL RESULTS
                             Factor                                    Results
   EXP.                                                       DIMENSIONAL
   NO.        SPEED          FEED (B)     DEPTH OF           TOLERANCE (X)*       SURFACE
             (A)(R.P.M)     (MM/REV)      CUT (C)              (MICRONS)        ROUGHNESS
                                           (MM)                               (Y)** (MICRONS)
     1          1600           0.04          0.35              1.192263                  1.6
     2          1600           0.03           0.3                1.311                   1.74
     3          1600           0.02          0.25                1.157                   1.66
     4          1300           0.04          0.25                1.564                   1.38
     5          1300           0.03          0.35                 1.72                   1.7
     6          1300           0.02           0.3               2.10862                  1.33
     7          1000           0.04           0.3               1.28565                  1.43
     8          1000           0.03          0.25               1.3048                   1.75
     9          1000           0.02          0.35                1.85                    1.35

    * These Dimensional variations were obtained by Air Gauge in the company premises itself
    using 20 samples per reading.
    ** These Surface roughness values were obtained using the RA values obtained from the Lab
    Reports of Spectro Test Labs which is an ISO-9001:2000 and ISO-14001:2004 certified
    organisation and works in accordance to Protocol: IS-3073-1967. A sample of 3 pieces per
    reading was supplied to the aforesaid lab for testing purpose.

ANALYSIS OF RESULTS

         In the Taguchi method the results of the experiments are analyzed to achieve one or more of
the following three objectives:
    a) To establish the best or the optimum condition for a product or a process.
    b) To estimate the contribution of individual factors.
    c) To estimate the response under the optimum conditions.
         Studying the main effects of each of the factors identifies the optimum condition. The process
involves minor arithmetic manipulation of the numerical result and usually can be done with the help
of a simple calculator. The main effects indicate the general trend of the influence of the factors.
Knowing the characteristic i.e. whether a higher or lower value produces the preferred result, the
levels of the factors, which are expected to produce the best results, can be predicted.
         The knowledge of the contribution of individual factors is the key to deciding the nature of
the control to be established on a production process. The analysis of variance (ANOVA) is the
statistical treatment most commonly applied to the results of the experiment to determine the percent
contribution of each factor. Study of the ANOVA table for a given analysis helps to determine which
of the factors need control and which do not.
         In this study, an L9 orthogonal array with four columns and nine rows was used. This array
has eight degree of freedom and it can handle three design parameters. Each parameter is assigned to
a column, nine parameters combination being available. Therefore only nine parameters are required
to study the entire parameter space using the L9 orthogonal array.


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CONCLUSION

        It is found that the parameter design of the Taguchi method provides a simple, systematic and
efficient methodology for the optimization of process parameters. Based on the results obtained in
this study, the following can be concluded:

  a) The percentage contribution of cutting speed is 57.2%, feed rate is 23.4%, depth of cut is
     12.7% and that of error is 6.7% for minimum dimensional tolerance.
  b) The percentage contribution of the cutting speed is maximum i.e. 57.2 % for obtaining the
     minimum value of the dimensional variation.
  c) The optimum combination of the parameters and their levels for obtaining minimum
     dimensional variation is A1B1C3 (spindle speed = , feed = , depth of cut = ).
  d) The percentage contribution of cutting speed is 26.5 %, feed rate is 60.6 %, depth of cut is
     5.7% and that of error is 7.2% for minimum value of surface roughness.
  e) The percentage contribution of the feed rate is maximum i.e. 60.6 % for obtaining the
     minimum value of the surface roughness.
  f) The optimum combination of the parameters and their levels for obtaining minimum surface
     roughness is A2B3C2 (spindle speed = , feed = , depth of cut = ) .
  g) Out of the above two combinations the surface roughness and dimensional tolerance were
     found to be the minimum at A2B3C2 with the RA value of 0.91 microns and standard deviation
     of 1.15 microns.
  h) The initial values of surface roughness and standard deviation for dimensional variation that
     were obtained by the operator without the application of Taguchi technique were 1.98 microns
     and 2.10862 microns respectively.
     The final values of surface roughness and standard deviation for dimensional variation that
were obtained using the optimal parameters as suggested in the project work are 0.91 microns and
1.15 microns respectively.
     Thus, it can be safely concluded that the output quality conditions (Surface Roughness and
Dimensional Tolerance or standard deviation for dimensional variation in our case) are greatly
advanced by the application of Taguchi technique. Also, the final results are in total conformance
with the customer expectations. Hence, one can very well conclude that the project work is
successfully completed.

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