three dimensional thermal mapping of … · finite element method ... mechanical work is converted...

THREE DIMENSIONAL THERMAL MAPPING OF SINGLE POINT CUTTING TOOL

USING FINITE ELEMENT METHOD AND ITS EXPERIMENTAL VERIFICATION

GOURAB CHAKRABORTY1, SATYABRATA PODDER

2 & UTTAM ROY

3

1,2Mechanical Engineering Department, Brainware Group of Institutions, Kolkata, West Bengal, India 3Mechanical Engineering Department, National Institute of Technology, AP, Yupia, India

ABSTRACT

Over the years, metal cutting researchers have developed a number of modeling techniques among which the

Finite Element Method (FEM) has particularly become the most popular tool for simulating metal cutting processes. It is

already known that during machining, the maximum temperature generated is on the chip-tool interface. Though majority

of the heat generated is taken away by the chip, the amount of heat distributed in the work piece greatly affects the quality

of machined part. In this study, the temperature distribution in the single point cutting tool is simulated using Finite

Element Methods. For this, a special experimental set up has been fabricated where lathe turning of a single point cutting

tool was carried out in dry condition as today’s manufacturing industry would like to embrace dry machining for both

environmental and economic reasons. The temperature reading is taken using Alumel-Chromel (k type) thermocouple with

the help of a Datalogger (DT-85). This temperature reading was further used as input for 3-D transient thermal modeling of

the HSS cutting tool during machining.

KEYWORDS: Thermal Mapping, Single Point Cutting Tool, FEM

INTRODUCTION

The knowledge on the temperature distribution in tool is a matter of great importance due to the severe effects of

intense local heat generated during machining which could affect the heat treatment or artificial aging properties, hardness

and residual stresses of the material, all of which affect the fatigue life of the component.

For the past fifty years metal cutting researchers have developed many modeling techniques including analytical

techniques, slip-line solutions, empirical approaches and finite element techniques. In recent years, the finite element

method has particularly become the main tool for simulating metal cutting processes [1, 2]. Finite element models are

widely used for calculating the stress, strain, strain-rate and temperature distributions in the primary, secondary and tertiary

sub-cutting zones. In consequence, temperatures in the tool, chip and work piece, as well as cutting forces, plastic

deformation (shear angles and chip thickness), chip formation and possibly its breaking can be determined faster than using

costly and time consuming experiments. It is especially important that FEM analysis can help to investigate some thermo

dynamical effects occurring in the cutting zone which, as so far, cannot be measured directly [2]. An example for such

effects is the influence of cutting tool rake surface on the heat transfer and friction, and resulting cutting temperature

distribution in the chip and the tool. It should be noted that the majority of previous orthogonal metal cutting simulations is

devoted to uncoated carbide tools and now the opposite trend to consider single and multiply coatings has been observed

[2-4]. In fact, the first completed work focusing on the evaluation of a predictive orthogonal cutting model for coated

carbide tools with multiple coating layers using the FEM was presented by Yen et al [2]. The main finding from this

simulation was that for AISI 1045 steel the predicted steady-state interface temperatures are in good agreement with the

International Journal of Mechanical and Production

Engineering Research and Development (IJMPERD)

ISSN 2249-6890

Vol. 3, Issue 1, Mar 2013, 241-252

©TJPRC Pvt. Ltd.

242 Gourab Chakraborty, Satyabrata Podder & Uttam Roy

experimental values given in ref. [5] within 5-11% difference. During a cutting process the mechanical energy due to the

plastic deformation developed at the primary shear plane and at the chip–tool interface is converted into heat. Studies have

shown that the chip and the environment dissipate a great deal of this heat while the reminiscent is conducted both into the

work piece and into the cutting tool. However, this small quantity of heat conducted into the tool (8–10% of the total heat

rate) is enough to create high temperatures near the cutting edges, which in same cases can reach the level of 11000C [6].

As a result of the high temperatures developed at the tool surfaces critical tool wear, short tool life and poor work piece

surface integrity will generally impair productivity. This fact makes the cutting temperature field identification

fundamental on the quality of the finished product. However, direct measurements of temperatures at the tool–chip–work

piece interfaces are very difficult due to the cutting movement and the small contact areas involved. Conventional

experimental methods such as infrared pyrometer [7], embedded thermocouple [8] and tool–work piece thermocouple

usually present problems [6]. The infrared pyrometer can represent a good solution if some limitations, like sensor

resolution and chip interference near the cutting zone, are alleviated [9,10]. In the other hand the use of the tool–work piece

thermocouple are limited to tools that can conduct electricity. In addition the thermocouple does not measure the

temperature at a specific point, but an average temperature at the heat affected zone between the tool and the work piece.

Due to these experimental difficulties many analytical and numerical methods solution have been employed to

predict machining temperature [11, 12]. Tay [12] has classified the methods of calculation of the temperature generated

during steady machining as: the moving heat source method, the image sources method, the finite difference method, the

semi-analytical methods and the finite element method. Due to the irregular tool geometry, the majority of numerical

techniques use finite element method [13-18].

Analytical Model of Single Point Cutting Tool

Heat generation while machining has significant influence on machining. It can increase tool wear and thereby

reducing tool life. It gives rise to thermal softening of cutting tool. It is commonly accepted that both the wear and failure

mechanisms which develop in cutting tools are predominantly influenced by temperature and it also results in modification

to the properties of work piece and tool material such as hardness. In order to predict the wear and failure characteristics of

a tool, it is necessary to quantify the temperatures which develop during the cutting operation. In machining operations,

mechanical work is converted to heat through the plastic deformation involved in chip formation and through friction

between the tool and work piece.

Considering a continuous type chip, as the cutting speed increases for a given rate of feed, the chip thickness

decreases and less shear energy is required for chip deformation so the chip is heated less from this deformation. About 80-

85% of the heat generated in shear zone. The chip-tool interface zone, where secondary plastic deformation, due to friction

between the heated chip and tool takes place. This causes a further rise in the temperature of the chip. This chip-tool

interface contributes 15-20% of heat generated. The work-tool interface zone 3, at flanks, where frictional rubbing occurs.

This area contributes 1-3% of heat generated.

Several methods have been used for measuring the temperatures generated during metal cutting operations. Tool-

Chip Thermocouple Technique is one of them. The numerical methods were successfully applied in calculating the

temperature distribution and thermal deformation in tool, chip and work piece.

Especially, the finite element and boundary element methods can deal with very complicated geometry in

machining; they have great potential to solve the problems in practice. In metal cutting, severe deformations take place in

Three Dimensional Thermal Mapping of Single Point Cutting Tool Using 243

Finite Element Method and its Experimental Verification

the vicinity of the cutting edge of the tool because of the high temperatures resulting from machining operation. These

elevated temperatures have a negative impact on tool life, and quality of surface.

The present work deals with the formulation of three-dimensional governing equation for heat conduction used to

obtain the temperature distribution on the face of the tool, to be used for obtaining the hardness at the various positions on

the tool. The eight nodes brick elements have been used for the FEM modeling.

Experimental Planning

The direct experimental approach being expensive and time consuming, to study machining processes, especially

when a wide range of parameters like tool geometry, materials, cutting conditions, etc are included; an alternative

approach, the finite-element method (FEM) is utilized to find the thermal mapping. FEM is one of the most frequently and

widely used tool to analyze and find out the results. Several finite-element techniques has been utilized for accurate and

efficient modeling of the machining process considering material specific non-linear analysis, mesh rezoning techniques,

tool wear modeling, residual stress prediction, etc.

Immense heat and high temperature generation during the machining process is taken into the consideration to

find out the cause of unsatisfactory tool life, poor surface finish and limitations on cutting speed. To analyze the effect of

work surface temperature during machining over surface roughness, a 3-Dimensional Transient Thermal Finite Element

Model has been used here with the help of commercially available FEM software ANSYS 13.0.

To perform the necessary experiments, an entire cutting process is considered that is to be simulated by FEM in

ANSYS from the initial to the transient phase. The work piece material of choice, high speed steel, is modeled as thermo

elastic-plastic, while the flow stress is considered to be a function of strain, strain-rate and temperature to represent better

the real behavior in cutting. Friction between the tool and chip is of Coulomb type with the value of 0.5.

The material as specified in table 01 has been machined in a lathe by a single point cutting tool high speed steel

having tool rake angle (degree) 00, tool clearance angle (degree) 200 and measured cutting edge radius (µm) 17-33 µm.

Table 1: High Speed Steel Material Properties

Density 8.17g/cc

Coefficient of Thermal Expansion 1.2e-005 0C-1

Specific Heat 434 J kg-1 0C-1

The schematic diagram of the experimental set up is illustrated in the figure 01. As discussed earlier, during the

time of turning a huge heat generation takes place in the tool chip interface. Most of the produced heat approximately 85%

is carried away by the chip. But rest of heat flows through the tool, hence temperature of the tool is increased. This

temperature of tool is being recorded by K- type thermocouple wires.

Type K (Chromel{90% nickel and 10% chromium}– Alumel{95% nickel, 2% manganese, 2% aluminium and 1%

silicon}) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/0C and chromel is

positive relative to alumel. The Fe-K type (J-type) thermocouple is used for sensing the temperature of the coil.

For typical metals used in thermocouples, the output voltage increases almost linearly with the temperature

difference (∆T) over a bounded range of temperatures. For precise measurements or measurements outside of the linear

temperature range, non-linearity must be corrected. The nonlinear relationship between the temperature difference (∆T)

and the output voltage (mV) of a thermocouple can be approximated by a polynomial:


(1)

The data taker DT80 range of data acquisition and logging instrument is used to measure and record the

temperature reading as recorded by thermocouples. A personal computer attached to the data logger is further used to

monitor the system.

The aim of the experiment is to collect temperature from 3 different points of the high speed steel tool at the time

of turning a mild steel bar.

Figure 1: Schematic Diagram of Experimental Set-Up for Tool Temperature

Figure 2: Experimental Set Up of Lathe Turning Operation

In figure 2 the experimental set up has been shown. A mild steel bar is held in the 3 jaw chuck of the lathe

machine. Turning operation is done by the high speed steel tool. 3 thermocouples are attached in the shank face of the

cutting tool at different points. The thermocouples are further connected into the slots of data logger DT85. The data logger

is further connected to the computer, where the temperature readings are cataloged with a 1 second interval. In figure 2

and figure 3 the views are elaborated. Here three k-type thermocouples are attached on the shank portion of the single point

cutting tool. The data logger is basically current sensitive device; hence the meter reading will be dependent on the e.m.f.

generated by tool-work-thermocouple. The thermoelectric power of the circuit is usually small and estimated by calibrating

the circuit against a reference thermocouple (Alumel-Cromel K type). The tool-work thermocouple technique is the best

method for measuring the average chip-tool interface temperature during metal cutting. The benefits of using the tool-work

thermocouple are its ease of implementation and its low cost as compared to other thermocouples.



Figure 3: K Type Thermocouples Attached with Tool at the Time of Turning

Experimental Results

The turning operation of high speed steel cutting tool with 00 rake angle used to operate a turning operation on

Mild steel round bar under following conditions.

Table 2: Cutting Conditions

Velocity v 720 rpm

Depth of Cut t 2mm

Feedrate f 0.3m/rev

The temperatures were recorded from the shank surface by the k type thermocouple at 2.5 mm distance from the

rake face. The position of the thermocouples in the shank face of the single point cutting tool is shown below in the figure

6.1. The temperature readings were recorded as below in the table 3.

Figure 4: Thermocouple Position in the Shank of the Single Point Cutting Tool

Table 3: Average Temperature Reading of Thermocouples

Steps Time

[s]

Temperature [°C] Cutting

Speed

Feed

Rate

Thermocouple

Reading 1

Thermocouple

Reading 2

Thermocouple

Reading 3 (rpm) (m/rev)

1 0 22 22 22 720 0.3

1 22 22 22.7 720 0.3

2 2 22.1 22.59 24.43 720 0.3

3 3 22.7 23.1 27.38 720 0.3

4 4 23.9 23.8 33.01 720 0.3

5 5 24.09 24.37 34.77 720 0.3


Table 3:Contd.,

6 6 25.7 25.9 35.64 720 0.3

7 7 26.48 26.79 39.1 720 0.3

8 8 26.64 27.86 43.49 720 0.3

9 8.9 29.7 30.01 46.9 720 0.3

10 9 30.01 30.7 48.3 720 0.3

11 9.1 31.2 31.46 49.9 720 0.3

12 9.2 32.28 32.9 51.8 720 0.3

13 9.3 32.84 33.7 53.57 720 0.3

14 9.4 33.58 34 54.02 720 0.3

15 9.5 32.01 34.91 54.94 720 0.3

16 9.6 32.67 35.98 55.49 720 0.3

17 9.7 33.06 36.99 56.04 720 0.3

18 9.8 34.01 38.7 57.18 720 0.3

19 9.9 35.99 39.1 59 720 0.3

20 10 36.06 39.07 59.13 720 0.3

The temperatures of the thermocouple positions in three different shank positions are plotted in the graph, which

is shown in the figure 6.2. The temperature plotting indicates that the thermocouple close to the rake surface shows the

highest temperature pick up.

Figure 5: Temperature at Various Times at Three Different Points of the Shank Face

Results Obtained from FEM Software (ANSYS)

Temperature Distribution

Figure 6 shows the thermal distribution in ANSYS. It shows the highest temperature at tool rake surface i.e. 3220

C, as conduction and convection both take place during the cutting, the temperature decreases through the shank face.

Figure 6: The Temperature Distribution of the Single Point Cutting Tool



Heat Flux

In figure 7 the heat flux vectors are shown. The heat flux vectors are flowing from the chip tool interface to the

rest of the tool. The ansys temperature distribution shows that the bottom part of the shank is at 220C, so at the bottom part

of shank the heat flux vectors are in reverse direction.

Figure 7: The Heat Flux Vectors Flowing from the Tool Chip Interface

In figure 8 the total heat flux is shown by ANSYS. Here it is also clear that the maximum heat flux is present at

the chip tool interface and the rest of the heat flux flows through the tool.

Figure 8: Total Directional Heat Flux Amount in the Cutting Tool

Normal Stress

Due to the forces present in turning operation a stress is developed in the single point cutting tool. In figure 9

ANSYS shows the normal stress developed along x- axis.

Figure 9: The Normal Stress Evolved During Cutting along X Axis


Total Deformation

Due to high amount of heat is generated in the chip tool interface, a deformation due to heat is certain. Ansys

predicts the deformation in the single point cutting tool. Figure 10 shows the deformation in the cutting tool.

Figure 10: Total Deformation of the Cutting Tool as Analyzed in ANSYS

DISCUSSIONS

The relationship between machining parameters and temperature generated at tool work piece interface has been

calculated using the empirical relations as given analytical model with the help of a commercial software ANSYS. The

reason behind the generation of high temperature at the contact zone of tool edge radius is mainly because of the extrusion

of work material and friction between tool edge radius and work piece.

After that, the fresh cut chip will flow into second deformation zone, where it will be continued to be extruded

and will come in more contact with the tool, resulting in a greater friction. Therefore, the temperature in chip is generally

higher than other contact zones, and its average temperature in normal machining conditions is high enough like 625˚C

approximately.

As the experimentation has been carried out in dry condition i.e. without the application of cutting fluid, the heat

dissipation is considerably less.

To understand the nature of temperature rise in a particular node, the temperatures of global maxima as well as of

global minima for that particular node are to be plotted against iterations.

Figure 11: Time vs. Temperature - Global Maximum



Figure 12: Time vs. Temperature - Global Minimum

It could be noted that, a global maximum is the highest value of the function within the entire domain; there can

only be one global maximum whereas a global minimum is the lowest value of the function within the entire domain and

there can only be one global minimum.

In contrast to the global minima, a local minimum value of a function is the lowest value of a function within a

very small interval; there can be more than one local minimum within the domain of the domain whereas a local maximum

value of a function is the highest value of a function within a very small interval and there can be more than one local

maximum within a domain.

In some cases, the global maximum of a function is the same as one of the local maximums of the function, and

the global minimum of a function is the same as one of the local minimums of the function.

Global maximums and global minimums can be found at the endpoints of a closed domain; local maximums and

local minimums cannot be found at the endpoints of a closed domain.

CONCLUSIONS

During a cutting process the mechanical energy due to the plastic deformation developed at the primary shear

plane and at the chip–tool interface is converted into heat. Studies have shown that the chip and the environment dissipate a

great deal of this heat while the reminiscent is conducted both into the work piece and into the cutting tool. However, this

small quantity of heat conducted into the tool (8–10% of the total heat rate) is enough to create high temperatures near the

cutting edges, which in some cases can reach the level of 11000C. As a result of the high temperatures developed at the tool

surfaces critical tool wear, short tool life and poor work piece surface integrity will generally impair productivity. This fact

makes the cutting temperature field identification fundamental on the quality of the finished product. However, direct

measurements of temperatures at the tool–chip–work piece interfaces are very difficult due to the cutting movement and

the small contact areas involved.

Conventional experimental methods such as infrared pyrometer, embedded thermocouple and tool–work piece

thermocouple usually present problems. The infrared pyrometer can represent a good solution if some limitations, like

sensor resolution and chip interference near the cutting zone, are alleviated. In the other hand the use of the tool–work

piece thermocouple are limited to tools that can conduct electricity. In addition the thermocouple does not measure the

temperature at a specific point, but an average temperature at the heat affected zone between the tool and the work piece.


Due to these experimental difficulties many analytical and numerical methods solution have been employed to

predict machining temperature. Tay has classified the methods of calculation of the temperature generated during steady

machining as: the moving heat source method, the image sources method, the finite difference method, the semi-analytical

methods and the finite element method. Due to the irregular tool geometry, the majority of numerical techniques use finite

element method. One thing that the models (unsteady or steady state) have in common is that the generated heat source

must be known or defined a priori considering the basic machining parameters such as rake angle, cutting speed and feed

rate.

Some method also assume that all the heat generated during machining appeared in two regions (primary shear

plane and interface) and they can be obtained from the product of the frictional forces along the rake face and along the

shear plane by the shearing and chip velocity, respectively. Unfortunately in real machining processes the heat flux

generated at the chip–tool interface are unknown and strongly dependent on the cutting condition and on the types of work-

piece and cutting tool used.

The temperature readings recorded at the shank by the thermocouples (fig 04) is mentioned in the table 03. The

readings were recorded between 360 c and 590 c. in figure 06 the ANSYS temperature distribution is shown. The shank

position where thermocouples were attached, in ANSYS thermal distribution, it shows the temperature range as 220c to

550c. By comparing both the experimental result and ANSYS thermal distribution, it can be concluded that both the results

are showing good agreement with one another.

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three dimensional thermal mapping of … · finite element method ... mechanical work is converted...

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