A Report And Insight Into Machinability Testing Biology Essay

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Abstract: Machinability is a consideration in the materials selection process for automatic screw machine parts. Depending on the application, machinability may be seen in terms of tool wear rate, total power consumption, attainable surface finish. Absolute tests seek to define machinability through analytical expressions using materials properties, i.e., they imply that "machinability" is itself a derivative material property.

CHAPTER 1

INTRODUCTION

Machinability is a process where the materials selection takes place for automatic screw machine parts. The easiness with which a metal can be machined is one of the principle factors affecting a product's utility, quality and cost. [1]

Machinability is so complex subject that it cannot be defined precisely. Depending on the application, machinability may be seen in terms of tool wear rate, cutting forces, total power consumption, attainable surface finish or several other forces. Machinability - therefore depends a great deal on the viewpoint of the observer; in fact, the criteria for one application frequently conflict with those for another.

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Another difficulty is that the property we call machinability depends on the joint influences of a large number of factors, many of which are quite complex. For example, machinability is certainly closely linked to the physical and mechanical properties of the workpiece: hard, brittle metals being generally more difficult to machine than soft, ductile ones. But very ductile metals, such as pure copper, stainless steels and some aluminum alloys tend to form long stringy chips, which makes them at least trouble-some to machine.[4]

Machinability is also strongly dependent on the type and geometry of tool used, the cutting operation, the machine tool, metallurgical structure of the tool and workpiece, the cutting/cooling fluid, and the machinist's skill and experience. It is therefore not surprising that some observers have concluded that machinability simply cannot be precisely described and that, despite the considerable body of research that has been devoted to the subject, the term can have little meaning except in a loose quantitative sense.

The ability to predict machining rates, and therefore production economics, would be especially beneficial to the automatic screw machine parts industry, where high productivity is essential. A quantitative machinability index would also rationalize the materials selection decision which, for screw machine parts, is still based as much on tradition as on machinability data. The work reported here attempts to provide a means to predict machinability in terms of production rate for automatic screw machine products for several combinations of workpiece and tool materials.

CHAPTER 2

THEORY

2.1 Mechanics of Metal Cutting

Metal ahead of the cutting tool is compressed. This results in the deformation or elongation of the crystal structure-resulting in a shearing of the metal. As the process continues, the metal above the cutting edge is forced along the "chip-tool" interference zone and is moved away from the work. The determination of cutting forces necessary for deformation the work material at the shear zone is essential for several important requirements:

to estimate the power requirements of a machine tool

to estimate the straining actions that must be resisted by the machine tool components, bearings, jigs and fixtures

to evaluate the role of various parameters in cutting forces

to evaluate the performance of any new work material, tool material, environment, techniques etc. with respect to machinability (cutting forces).[2]

2.1.1 Forces of Metal Cutting

There are different kinds of forces acting during the process of machining of a workpiece of any material :

From work side

FSh- shear force:

The force which causes shear deformation to occur in the shear plane.

FN- Force normal to the shear force

The force which is normal to the shear force.

From tool side

F - Friction force at chip tool interface:

The force between the tool and chip, which resisting the flow of the chip along the rake face of the tool.

N - Force normal to rake face:

The force which is normal to the friction force. Friction coefficient: m = F/N

Other Forces

Fc - Cutting force:

The force in the direction of cutting, the same direction as the cutting speed v.

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Ft - Thrust Force

The force which is perpendicular to the cutting force.

The below figure (fig 2.1) will explain about the different kind of forces explained that is the shear force, cutting force, thrust force, friction force, normal force etc..

Figure 2.1: Forces Generated During Cutting

CHAPTER 3

FORCE DETERMINATION

3.1 Merchant's Circle

In orthogonal cutting when the chip flows along the orthogonal plane, the cutting force (resultant) and its components FC and Fáµ- remain in the orthogonal plane. The above figure schematically is showing the forces acting on a piece of continuous chip coming out from the shear zone at a constant speed. That chip is apparently in a state of equilibrium.

Figure 3.1: Merchant's Circle

The forces in the chip segment are:

From work side

FSh- shear force

FN- Force normal to the shear force

Where Fsh + FN = R

From tool side

R' = R ( equilibrium)

R= F+ N

Where F = Friction force at chip tool interface

N = Force normal to rake face

The circle(s) drawn taking R' or R as diameter which contains all the force components concerned as intercepts. The two circles with their forces are combined into one circle having all the forces contained in that as shown by the diagram called Merchant's Circle Diagram (MCD) as in figure 2.

3.1.1 Significance of the forces displayed in the MCD

Fsh - shear force essentially required to produce or separate chip from the parent body ( work) by shear. FN - inherently exists along Fsh.

F- Friction force at the chip tool interface.

Fc - main force or power component acting in the direction of cutting velocity.

The magnitude of Fsh provides the yield shear strength of the work material under the cutting condition. The values of F and the ratio of F and N indicate interaction like friction at the chip-tool interface. The force components FC FD Ft are generally obtained by direct measurement. Again Fc helps in determining cutting power and specific energy requirement. The force components are also required to design the cutting tool and the machine tool.

Advantage of the Merchant's Circle

Easy, quick and reasonably accurate determination of several other forces from a few known forces.

Friction at chip tool interface and dynamic yield shear strength can be easily determined.

Equations relating the different forces can be easily derived.

Limitations of use of Merchant's Circle diagram

MCD is valid only for orthogonal cutting.

It is based on single shear plane theory.

It gives apparent (not actual) coefficient of friction.

Friction at chip tool interface and dynamic yield shear strength can be easily determined.

Taguchi Methods: Orthogonal Arrays

Taguchi Method of Orthogonal Arryas was developed by Dr. Taguchi of Nippon Telephones and Telegraph Company, Japan. Through the taguchi method experiments will give much reduced inconsistency for the experiment with optimal settings of control parameters. Thus the design of experiments with optimization of control parameters to obtain best results is achieved in the Taguchi Method. Orthogonal Arrays provide a set of well balanced (minimum) experiments. [5]

The general steps involved in the Taguchi Method are as follows:

Define the process aim or objective, or a particular parameter for measuring the performance of the process. This may be a cutting force, shear force, speed at which the work piece revolves, temperature, etc. The key parameter of a process may be minimum or maximum; for example, the goal may be to maximize the output of the cutting force. In simple terms, we can say that the loss function can be defined by the variation in the performance characteristic from the given target value.

Determine the parameters that are affecting the performance measure process. The factors that affect the performance measure are the parameters which can be varied or which can be changed such as feed rate, depth of cut, the speed at which the work piece revolves, etc. that can be easily controlled. The parameters should be varied and at must be specified for the number of experiments to be conducted. For example, in the experiment conducted the feed rate is varied from a low and high value of 0.25mm and 1mm. We can increase the number of experiments to be conducted by increasing the number of parameters

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Orthogonal arrays are created for indicating the number of condition for the experiments.

The experiments areaconducted in way that the effectaon the performance measure data is collected in the completed array.

The effect of the different parameters is analyzed by the performance value after the parameters are analyzed. [3]

L4 Array

Each of the vectors is statistically independent of the others.

When linearly added, the resultant is the arithmetic sum of the individual components as in Table {3.1}.

Table 3.1: L4 Array

Advantages of Taguchi Method

It mainly concentrates on the mean performance values which are close to the target value, thus the quality of a given product is improvised.

With the help of this method many solutions could be solved in the mechanical field. It can be used to identify problems in a manufacturing process from the given data.

This method helps in identifying the key parameters so that these parameters has the highest effect on the performance value.

Disadvantages of Taguchi Method

One of the main drawback of Taguchi method is that the result which is obtained do not point out which parameter has the highest effect on the performance of a particular parameter.

The Taguchi method has a difficulty in accounting for relations between parameters.

CHAPTER 4

METHODOLOGY

4.1 FLOW OF METHODOLOGY PROCESS

Figure 4.1 shows the overall process of the experiment that being conducted.

The function of this flow chart is to give guideline and information about the project.

From this flow chart, the critical part of this research can be determined. The critical

part of this research is the lathing process. This part of this experiment must be conducted very well to get the accurate data.

Raw material

Lathe machine

Setting the speed and the Cutting Force.

.

Using Taguchi method finding the apt temperature, cutting force, speed at which the work piece rotates

With the help of chip we can find depth of cut, area swept while machining

Required result is attained

Figure 4.1: Flow of Methodology Process

CHAPTER 5

EXPERIMENT

According to the flowchart in the specified in the previous chapter we conduct the experiment with the Lathe machine. In the lathe machine, turning process is what plays the role. What is turning? It is the process of machining round stock on a lathe . The commonest use is for machining work piece to various diameters.

The different lathe machines which are available at the present market are:

Tracer lathes - They are similar to the engine lathe but the tool path maybe guided by a template or programmed on a punched tape.

Swiss type Automatic Screw Machine - They are designed to produce small diameter parts, such shafts for clocks or watches.

Turret Lathe - they are made of two turrets, on at the tailstock end of the machine and the other mounted on the cross slide. Each of the tools on either of the turrets can be indexed and locked into position and then brought up to the stock for the desired cut.

Single Spindle Automatics - They are turning machines whose movements are directed by gears and cams. It is used for bar or individual pieces held in a chuck.

Multispindle Automatic Lathes - They are built similar to the single spindle automatics but are equipped with four, six or eight spindles. [8]

The lathe machine used is single spindle automatic machine. In this lathe we use the 4 jaw chuck where we have to adjust the work piece in such way that they are to be tightened and loosed till the work piece is aligned parallel to the surface bed.

The Lathe machine is shown below (fig 5.1):

Figure 5.1: Lathe Machine Used for Machining

Figure 5.2: Four Jaw Chuck (Work piece holder)

In the 4 jaw chuck we have can have high precised value compared to the 3 jaw chuck which is commonly used [9]. These 4 jaw chucks can hold both rectangular and cylindrical work piece. (fig 5.2)

The work piece made of Aluminum (Al), cylindrical in shape which has a tensile strength of 70 MPA is machined with the different speed, feed rate which is the amount that the tool advances per revolution of the work piece and depth of cut to measure the performance by using the taguchi method.

In Taguchi method at first we randomly assign experimental conditions will with high probability create a near optimal design.

Now, design of the possible conditions is created as given in the Table (5.1) given below.

 

X

Y

Z

1

1

1

1

2

1

2

2

3

1

3

3

4

2

1

2

5

2

2

3

6

2

3

1

7

3

1

3

8

3

2

1

9

3

3

2

Table 5.1: Orthogonal Array

From the Table (5.1) we assign 1 to be the speed at which the work piece is expected to revolve, 2 is the feed rate which is to be varied at different speed and finally, 3 to be the depth of cut at which the work piece is to be machined.

From the Table (5.1) we analyze the different conditions at which the Aluminium work piece has to be machined at different conditions.

The table given below is the different speed, feed rate and the depth of cut to be analyzed so as to find the optimal performance value. The values given in the Table (5.2) are the ones to be varied with respect to the Table (5.1) above

 

 

1

2

3

Speed

160

360

560

Feed Rate

0.25

0.5

1

Depth Of Cut

0.5

1

1.5

Table 5.2: Variable Parameters at which Work piece to be Machined

After machining the work piece at different experimental condition, the machined chips at different conditions are taken for further calculation and experimentation.

The figure (fig 5.3) below shows the chips at different speed, feed rate and depth of cut according to the Taguchi Table (5.1) prepared at variable conditions.

Speed - 160

Feed rate - 1

Depth of cut - 1.5

Speed - 160

Feed rate - 0.25

Depth of cut - 0.5

Speed - 160

Feed rate - 0.5

Depth of cut - 1

Speed - 560

Feed rate - 0.25

Depth of cut - 1.5

Speed - 560

Feed rate - 0.5

Depth of cut - 0.5

Speed - 560

Feed rate - 1

Depth of cut - 1

Speed - 360

Feed rate - 0.25

Depth of cut - 1

Speed - 360

Feed rate - 0.5

Depth of cut - 1.5

Speed - 360

Feed rate - 1

Depth of cut - 0.5

Figure 5.3: Different Chips Machined at Different Conditions

With the chips machined with different conditions, the thickness is taken into account by using a micrometer. For all the nine readings we get variable thickness. The thickness over here talks about the actual thickness.

For example we take the 2nd reading into consideration

At first we find the Cutting Ratio,

Cutting Ratio (r) - t / tc ………………………………………………….(1)

where, t - depth of cut

tc - chip thickness

r = 1 / 0.96 mm

r = 0.81 mm

Now using the cutting ratio we have relation

sin Ф = r ……………………………………………..(2)

cos (Ф-α)

where, Ф is the shear plane angle

α is the rake angle of the tool

Assuming that the rake angle (α) is zero for all the nine different conditons

So the (2) equation becomes

tan Ф = r …………………………………………………….(3)

Therefore,

Ф = tan-1 (r) …………………………………………………….(4)

Ф = tan-1 (0.81)

Ф = 39.11°

Using the shear plane angle we can find now the friction angle (β)

We have the relation that,

Ф = 45° + α - β …………………………………………………(5)

2 2

Where, β is the Friction Angle

39.11 = 45 - β

2

β = 11.77°

Therefore the Friction Angle is 11.77°

Our main aim of this project is to find the different forces acting on the work piece. So using the above values we can find the Shear Force (Fs) acting on the work piece.

We have the relation

Fs = Ʈs * As …………………………………………….........(6)

Where, Ʈs is the shear strength of the Aluminium metal work piece.

As is the area of the shear.

Now to find the find the area of the shear

As = b * t ……………………………………………….......(7)

sin Ф

where, b is the Width of cut which also means the feed rate

t is the Depth of cut

As = 0.5 * 1

sin (39.111)

As = 0.792 mm2

The Shear Strength of Aluminium is 70 MPa

Substitute the value of shear strength and the shear area to equation (6)

Fs = 70 * 0.792

Fs = 55.484 N/m2

The aim is to find the remaining forces like, the Cutting force, normal force, Normal force acting along the shear plane, Friction force, Thrust force etc…. We use the Merchant Circle Diagram to find the remaining force.

To find the remaining force using merchant circle we must follow the following steps:

Construct a horizontal line "AB"

Draw an angle Ф (which is the shear plane angle) from the horizontal line "AB". From the point "A" construct the shear force (Fs) at the length with which we calculated using the shear strength and the area of shear.

Draw another line "R" at an angle of β - α (that is the difference between the friction angle and the rake angle)

Drop a perpendicular line from Fs (Shear Force). The perpendicular line intersects with the line "R". The length from the point from where the perpendicular line is dropped to the point where the line intersects is the Normal Force at the shear plane (Fn)

Taking the line "R" as the diameter draw a circle.

Now from the point "A" drops a perpendicular line to the circle. And it meets at a point "C". The length obtained is the Friction Force (F).

From the point "C" draw a perpendicular line "N" which is the Normal Force.

Draw another perpendicular line from the line "N". The perpendicular line gives us the Thrust Force (Ft).

Measure the different forces.

Taking the above steps into account, the diagram shows about the Merchant Circle Diagram where we find the different forces. (fig 5.4)

Figure 5.4: Merchant Circle Diagram

Using the above Merchant Circle Diagram, the forces are found out for all the experimental conditions. The Table (5.3) below gives the information about the different forces acting, thickness etc… at different runs.

 

Thickness

Shear Force

Cutting Force

Thrust Force

Friction Force

Normal Force

Normal Force

 

t

Fs

Fc

Ft

F

N

Fn

1

0.84

17.113

31.3

17.7

17.7

31.3

32.4

2

1.23

55.4

85.1

16.7

16.7

85.1

68.6

3

1.67

69.8

107.2

13.7

13.7

107.3

79.5

4

1.45

30.8

48

17.5

17.5

48

41.7

5

1.72

79.8

124.4

18.3

18.3

124.4

96.5

6

0.86

69.7

121.7

68.7

68.7

121.7

120

7

1.63

50.1

90

46.5

46.5

90

88.4

8

0.7

30.1

50

16.5

16.5

50

43.6

9

1.3

114.8

172.2

46.3

46.3

172.2

141.4

Table 5.3: Different Forces obtained using Merchant Circle Diagram

Now we have the different forces mentioned we take only the cutting force to find out the optimal operating condition by the taguchi analysis. ,

In this array, it can be seen that any number of repeated observations (trials) may be used. Ti,j represents the different trials with i = experiment number and j = trial number. It should be noted that the Taguchi method allows for the use of a noise matrix including external factors affecting the process outcome rather than repeated trials.

 

X1

X2

X3

Fc1

Fc2

Fc3

1

1

1

1

31.3

34.3

28.3

2

1

2

2

85.1

88.1

82.1

3

1

3

3

107.2

110.2

104.2

4

2

1

2

48

51

45

5

2

2

3

124.4

127.4

121.4

6

2

3

1

121.7

124.7

118.7

7

3

1

3

90

93

87

8

3

2

1

50

53

47

9

3

3

2

172.2

175.2

169.2

Table 5.4: Cutting Force the key parameter for Performance Measuring

So, by creating an array it can be seen that any number of repeated observations (trials) may be used. Now to find the signal noise ratio or Sn ratio we have a relation to minimize

So using this relation we find the Sn ratio value of all the conditions upto to the nineth run.

For example: Taking the Run 2

SN = -10 * log (85.12 + 88.12 + 82.12)

3

SN = -10 * log ( 7248.01)

SN = -10 * 3.8602

SN = - 38.602

In the similar way we find the remaining Sn values. And thereby, we get the below Table (5.5) after calculating all the values of Sn

 

X1

X2

X3

Fc1

Fc2

Fc3

Sn

1

1

1

1

31.3

34.3

28.3

-29.93

2

1

2

2

85.1

88.1

82.1

-38.6

3

1

3

3

107.2

110.2

104.2

-40.6

4

2

1

2

48

51

45

-38.4

5

2

2

3

124.4

127.4

121.4

-41.89

6

2

3

1

121.7

124.7

118.7

-41.7

7

3

1

3

90

93

87

-39.08

8

3

2

1

50

53

47

-33.98

9

3

3

2

172.2

175.2

169.2

44.72

Table 5.5: Signal Noise ratio

To find the optimal condition for the cutting force we have to find the SNA1, SNA2, SNA3, SNB1, SNB2, SNB3, SNC1, SNC2, SNC3.

For every Signal Noise ratios at different condition are:

From X1,

SNA1 = SN1 + SN2 +SN3

3

= - 29.93 - 38.6 - 40.6 = - 36.37

3

SNA2 = SN4 + SN5 +SN6

3

= - 38.4 - 41.89 - 41.7 = 40.663

3

SNA3 = SN7 + SN8 +SN9

3

= - 39.08 - 33.98 - 44.72 = - 39.26

From X2,

SNB1 = SN1 + SN4 +SN7

3

= - 29.93 - 38.40 - 39.08 = - 35.803

3

SNB2 = SN2 + SN5 +SN8

3

= - 38.6 - 41.89 - 33.98 = - 38.15

3

SNB3 = SN3 + SN6 +SN9

3

= - 40.60 - 41.7 - 44.72 = - 42.33

3

From X3,

SNC1 = SN1 + SN6 +SN8

3

= - 29.93 - 41.7 - 33.98 = - 35.203

3

SNC2 = SN2 + SN4 +SN9

3

= - 38.6 - 38.4 - 44.72 = - 40.57

3

SNC3 = SN3 + SN5 +SN7

3

= - 40.6 - 41.89 - 39.08 = - 40.52

3

From the above readings we prepare graphs which helps us to find the optimal conditions.

A1

A3

A2

Figure 5.5: Graph SN versus SNA

From this graph (fig 5.5) we can see that the value of SNA1 is the optimal condition where we would get better performance compared to the remaining values of SNA2 and SNA3. The value of SNA is compared to the speed at which the work piece will revolve.

For this experiment the speed 160 rpm is the apt among the other speeds which are 360 and 560.

B3

B1

B2

Figure 5.6: Graph SN versus SNB

From the above graph (fig 5.6) we can study that the SNB1 is more optimal compared to the remaining values of SNB2 and SNB3. And over here we have the feed rate which is the amount that the tool advances per revolution of the work piece.

As the feed rate suitable for this experiment is 0.25 mm, which is least feed rate given for machining the work piece.

C3

C2

C1

Figure 5.7: Graph SN versus SNC

From this graph (fig 5.7) we can say that the SNC1 is more compatible compared to the values of SNC2 and SNC3. Over here we can see that the most apt depth of cut is SNC1.

The depth of cut value obtained is 0.5 mm. So, we can see that the lesser depth of cut more is the cutting force performance.

From the experiment conducted with the machined chips we can see that the most optimal solution for the experiment to get the best performance of the cutting force is

The speed at which the work piece revolves should be 160rpm.

The feed rate which is the amount at which the tool advances per revolution of the work piece should be 0.25 mm.

The depth of cut which the thickness at which work piece should be machined should be 0.5 mm.

The Table (5.6) shows the different suitable speed , feed rate and the depth of cut to get the optimum performance.

 

Speed

Feed Rate

Depth of Cut

Rank 1

160

0.25

0.5

Rank 2

560

0.5

1.5

Rank 3

360

1

1

Table 5.6: The Optimum conditions where the Performance is high

CHAPTER 5

CONCLUSIONS

5.1 CONCLUSIONS

Study of machinability testing. Study of different cutting forces on the tool, workpiece and other forces. With this experiment conducted it was possible to find out the optimum condition, that is to get the best performance with better finishing and quality from the set of readings which varies at different speed, feed rate and depth of cut. With the help of Merchant Circle Diagram and Taguchi Method a worker is able to select the best speed required for the work piece to revolve at the best feed rate and depth of cut. So it becomes easier for the manufacturer to produce the product with finishing, which in turn brings out the best quality of the product manufactured.

5.2 FUTURE SCOPE

At present most tool path planning and programming programs for sculputured parts machining focus on accurate production of a particular surface. Modest attention is given to optimization of the machining parameters to achieve maximum productivity. The enhanced cutting force model requires few cutting tests, provides fast and accurate predictions, and chains the future upgrading. So machinability testing proves out to be one of the most reliable methods to get a customer satisfactory product with better finishing with optimized condition which provides the ease in manufacturing.

CHAPTER 6