34 CHAPTER 3 PROMOTION OF CHIP CURL 3.1 INTRODUCTION Among the factors that affect the cutting performance during minimal fluid application, tool chip contact length plays an important role owing to its effect on friction and temperature at the tool-chip interface (Sadik et al., 1995). Low tool chip contact length can lead to lower cutting force; lower tool wear and better surface finish. Hence any mechanism that will lead to reduction in tool chip contact length can bring forth better cutting performance. As revealed in literature review, reduction in tool chip contact length can be achieved by promoting the chip curl. Chip curl radius can be reduced by the following methods. 1. By providing chip breaker on the rake face (Shaw, 1984), 2. By using restricted contact tools (De Chiffre, 1982, Sadik et al,1995), 3. By effecting better rake face lubrication (Seah et al., 1997, Tasdelen, 2008, Suresh et al., 2009), 4. By adding free machining additives to the work material (Shaw, 1984), 5. By reducing thermal conductivity of tool (Balaji et al., 1999), 6. By increasing cutting velocity, decreasing feed and depth of cut (EmreOzlu et al., 2009), and 7. By increasing work piece hardness (Luo et al., 1999). In the present investigation an attempt was made to promote chip curl by introducing a minimal high velocity pulsing slug of cutting fluid on the top side of the chip. The presence of cutting fluid at the top side of the chip is expected to cause contraction of the top surface of the chip which results in the bending of the chip away from the rake face thereby reducing the tool chip contact length. 3.2 SELECTION OF WORK MATERIAL Through hardenable AISI 4340 high strength low alloy steel was used as work material throughout the investigation. It is a general purpose steel having wide range of applications in automobile and allied industries by virtue of its through hardenability, enabling it to be used in fairly large sections (Varadarajan et
Transcript
34
CHAPTER 3
PROMOTION OF CHIP CURL
3.1 INTRODUCTION
Among the factors that affect the cutting performance during minimal fluid
application, tool chip contact length plays an important role owing to its effect on
friction and temperature at the tool-chip interface (Sadik et al., 1995). Low tool
chip contact length can lead to lower cutting force; lower tool wear and better
surface finish. Hence any mechanism that will lead to reduction in tool chip
contact length can bring forth better cutting performance. As revealed in literature
review, reduction in tool chip contact length can be achieved by promoting the
chip curl. Chip curl radius can be reduced by the following methods.
1. By providing chip breaker on the rake face (Shaw, 1984),
2. By using restricted contact tools (De Chiffre, 1982, Sadik et al,1995),
3. By effecting better rake face lubrication (Seah et al., 1997, Tasdelen,
2008, Suresh et al., 2009),
4. By adding free machining additives to the work material (Shaw, 1984),
5. By reducing thermal conductivity of tool (Balaji et al., 1999),
6. By increasing cutting velocity, decreasing feed and depth of cut
(EmreOzlu et al., 2009), and
7. By increasing work piece hardness (Luo et al., 1999).
In the present investigation an attempt was made to promote chip curl by
introducing a minimal high velocity pulsing slug of cutting fluid on the top side of
the chip. The presence of cutting fluid at the top side of the chip is expected to
cause contraction of the top surface of the chip which results in the bending of the
chip away from the rake face thereby reducing the tool chip contact length.
3.2 SELECTION OF WORK MATERIAL
Through hardenable AISI 4340 high strength low alloy steel was used as
work material throughout the investigation. It is a general purpose steel having
wide range of applications in automobile and allied industries by virtue of its
through hardenability, enabling it to be used in fairly large sections (Varadarajan et
35
al., 2002a). Considering its wide range of application in the industry this grade of
steel was considered as the work material in the present investigation. Workpiece
was through hardened followed by tempering to achieve hardness 45 HRC. Bars of
70mm diameters and 350 mm length with composition as in Table 1 were used in
the present investigation. In order to assure the required stiffness of
chuck/workpiece/cutting system, the ratio of cylindrical turning length to the initial
diameter of workpiece (L/D ratio) was approximately kept as 4.
Table 3.1 Composition of work piece material
C Ni Cr Mo Mn Si Fe
0.44 1.91 1.25 0.34 0.68 0.38 Rest
3.3 SELECTION OF CUTTING TOOL
Multicoated hard metal inserts with sculptured rake face geometry with the
specification SNMG 120408 MT TT5100 from Taegu Tec were used as cutting
tools in this investigation. The inserts have a multilayer CVD coating (TiN/MT-
TiCN/Al2O3) on a cemented carbide substrate. CVD coating consists of TiN for
reducing friction and a medium temperature CVD coating of TiCN for high
fracture toughness and good abrasive wear resistance. Figure 3.1 presents a
photograph of the turning tool insert. Table 3.2 gives the dimensions of various
elements of the insert. The specification of the tool insert is presented in Table 3.3.
The inserts were mounted on a pin and hole type tool holder having specification
PSBNR 2525 M12 (Figure 3.2). The resulting working tool geometry and basic
dimensions of insert and tool holder are presented in Table 3.4. The cutting tool
inserts and the tool holder were selected as per the recommendations of M/s.
Tageu Tec India (P) Limited who were extending their technical/material support
for this research work.
3.4 SELECTION OF CUTTING FLUID
Since the quantity of cutting fluid used is extremely small, a specially
formulated cutting fluid was employed in this investigation. The base was a
commercially available mineral oil. The formulation contained, in addition to
coolant and lubricant, additives such as surfactant, evaporator, emulsifier,
36
Figure 3.1 Photograph of the insert with its design details
Table 3.2 Dimensions of the insert
Designation L d R t
SNMG 120408 MT TT5100 11.9 mm 12.7 mm 0.8 mm 4.756 mm
Table 3.3 Specification of the insert
Specification of SNMG 120408 MT TT5100 Insert
S Type of Shape
N Clearance angle.
M Tolerance.
G Grain size
12 Cutting edge length
04 Thickness
08 Corner radius
MT Medium roughing
TT5100 Grade
Table 3.4 Working tool geometry and dimensions of insert and tool holder
Working tool geometry
Angle of inclination, λs=- 6o
Orthogonal rake angle, γo=-6o
Orthogonal clearance angle, αo=6o
Auxiliary orthogonal
Clearance angle, α’o=6o
End cutting edge angle, ϕe=15o
Principle cutting edge angle, ϕ=75o
Nose radius, r=0.8 mm
Tool holder size 25 x 25 x 14.7 mm
Cutting tool stand out 50 mm
37
Figure 3.2 Photograph of turning tool holder
stabilizer, biocide and a deodorizing agent (Varadarajan et al., 2002b). It acted as
an oil in water emulsion. Table 3.5 shows the various constituents present in the
cutting fluid formulated.
Petroleum sulphonate acts as a multifunctional additive. It can act as an
emulsifier, a rust inhibitor, a surfactant and as an EP agent. The polar nature of the
Sulphonate end of the molecule functions as a typical anionic surfactant. The tail
of the Sulphonate is made up of a hydrocarbon chain which has no charge.
Sulphonates act on the surface of oil droplets by binding at the tail. The head of the
Sulphonate has a polar charge, allowing the head to bond to water droplets. Thus
the Sulphonate can hold oil and water apart so that they can co-exist and form an
emulsion.
Ethylene glycol resists freezing due to its low freezing point and acts as a
coupling agent to increase the stability of the emulsion. The use of ethylene glycol
not only depresses the freezing point but also elevates the boiling point such that
the operating range for the heat transfer fluid is broadened on both the ends of the
temperature scale. The increase in boiling temperature is due to pure ethylene
glycol having a much higher boiling point and lower vapor pressure than pure
water. Oleic acid is an unsaturated fatty acid which is used as an emulsifying or
solubilizing agent in aerosol products. Besides serving as an agent for improving
the lubricity of the cutting fluid (agent for lowering the friction coefficient –
38
friction modifier), this compound forms an effective agent for enhancing
permeability. In water soluble cutting fluids, Triethaol Amine is used to provide
the alkalinity needed to protect the work against rusting and it acts as an anti-
oxidant. It also controls the evaporation rate of water in cutting fluid.
Table 3.5 Composition of the cutting fluid
S.No. Name of the constituent Percentage
1 Petroleum Sulphonate
(molecular weight=490 to 520) 15 %
2 Ethylene glycol
1%
3 Oleic acid
3%
4 Triethaol amine
3%
5 Alcohol Ethoxylate
2%-6%
6 Mineral oil (Paraffinic)
rest
Alcohol ethoxylate is a nonionic surfactant created by adding ethylene
oxide groups to long chain (high molecular weight) alcohols. Alcohol ethoxylates
possess greater resistance to water hardness than many other surfactants. It also
acts as a secondary emulsifier which enhances the emulsification capability of the
sulfonate. It is formulated from selected aliphatic hydrocarbons and alcohol
ethoxylates are known for their biodegradability.
Mineral oils are hydrocarbons obtained during refining of crude oil. Their
properties depend on their chain length and structure. The formulation was
developed and used successfully by Vardarajan et al. (2002b) during their
investigation on turning of hardened AISI4340 Steel. The same formulation is
being tried in the present investigation also.
3.5 MINIMAL FLUID APPLICATOR
A photograph of the minimal fluid applicator used for this
investigation is shown in Figure 3.3. Figure 3.4 shows the schematic diagram of
minimal fluid applicator. The fluid applicator consists of a fuel pump (Bosh Type)
of a four cylinder compression ignition engine which is coupled to an infinitely
39
variable electric drive. The fuel pump has a plunger with helical groove which can
rotate about its axis and the degree of rotation of plunger determines the quantity
of fluid delivered per stroke. There is a provision for rotating the plunger so that
the quantity of fluid delivered per stroke can be controlled accurately. The cutting
fluid was delivered using a standard fuel injection nozzle (Bosh make) used in
compression ignition engines with a specification DN0SD151with out any
modifications. Fuel injector with this specification had a spray angle of 0o and
gave the best performance for fluid minimization applications (Philip et al., 2001).
The plunger reciprocates as the motor rotates and delivers one pulse of cutting
fluid for each revolution through the fluid injector. The pressure of the cutting
fluid at the injector before it is delivered through the nozzle can be set at any
predefined value. The fluid coming out of the injector consists of myriads of tiny
droplets, the velocity of which depends upon the pressure set at the fluid injector
nozzle. Higher the pressure, higher will be the velocity of the individual particles.
Figure 3.3 Minimal fluid application system
40
Figure 3.4 Schematic diagram of minimal fluid applicator
Figure 3.5 Direction of fluid jets
For a given pressure at the fluid injector, a particular rate of fluid
application can be maintained irrespective of the frequency of pulsing. For
example, if the pressure at the fluid injector was maintained at P1 bar and the
injector delivers at the rate of Q ml/min at a frequency of 500 pulses/min, the same
delivery rate of Q ml/min can be maintained at a frequency of 1000 pulses/min
also. This is achieved by rotating the plunger with the helical groove in a proper
direction. The quantity delivered per pulse is equal to Q/N, where N is the
frequency of pulsing (in pulses/min). For example, the quantity of delivery per
pulse when the frequency of pulsing is 1000 pulses/min is equal to Q/1000 where
as it is equal to Q/500 when the frequency of pulsing is 500 pulses/min. Like this,
maintaining any delivery rate of cutting fluid for any frequency of pulsing is also
41
possible for a given pressure set at the injector nozzle. In short, in the fluid
application system developed it is possible to vary the pressure, frequency of
pulsing and rate of delivery independently. A specially formulated mineral oil
base cutting fluid (Varadarajan et al., 2002b) which acted as an oil in water
emulsion, as mentioned earlier was applied as a narrow pulsed slug at tool work
interface (Philip et al., 2001) and at the top side of the chip as shown in Figure 3.5.
The system can supply a pulsing slug of cutting fluid at four locations in the same
machine tool or to four separate machine tools simultaneously.
3.6 MEASUREMENT OF PROCESS PARAMETERS
Cutting force was measured using a Kistler piezoelectric dynamometer of
type 9257B. It consists of a multichannel charge amplifier (Type:5070A) as well
as a data acquisition and analysis system (DynoWare). This multi component
dynamometer facilitates dynamic and quasi-static measurement of the three
orthogonal components of cutting force. The assembly of Kistler dynamometer
and data analysis and display units is shown in Figure 3.6 and 3.7 respectively.
Figure 3.6 Photograph of Kistler dynamometer (Type 9257B)
42
Figure 3.7 Kistler-data analysis and display system
Figure 3.8 Photograph of surface roughness tester
The surface roughness was measured using a stylus type surface roughness
tester TR100 developed by the TIME with a cut off distance of 0.8mm. Its main
features are high accuracy, wide range of application, simple operation, and stable
performance. When the sensor driven by a driver is making a linear uniform
motion along the test surface, the contact stylus which is perpendicular with the
work surface moves up and down with the work surface. Its motion is converted
into electric signals, which are amplified, filtered and transformed into digital
43
signals through an analog to digital converter. The signals are then processed by
the CPU into Ra and Rz values before being displayed on the screen. Photograph of
surface roughness tester is shown in Figure 3.8. The average roughness (Ra) is the
area between the roughness profile and its mean line, or the integral of the absolute
value of the roughness profile height over the evaluation length. Ra averages all
peaks and valleys of the roughness profile, and then neutralizes the few outlying
points so that the extreme points have no significant impact on the final results. In
the present investigations, Ra was selected to express the surface roughness.
Figure 3.9 Photograph of tool makers’ microscope
Average flank wear and tool chip contact length were measured using a tool
makers’ microscope (Metzer make) with a least count of 0.005 mm. The
photograph of Metzer tool makers’ microscope is shown in Figure 3.9. When the
relief face of a cutting tool rubs against the workpiece, flank wear occurs on this
face and this type of tool wear is caused by an abrasion mechanism and it
progresses gradually. Flank wear impairs the accuracy of the parts machined
44
because it causes deflection of the cutting tool. Flank wear is usually maximum at
the extremities of the cutting edge and in the central zone the wear land it is fairly
uniform. Flank wear land width (VBB) shown in Figure 3.10 is the criterion of tool
life according to the ISO 3685 (1993) standard. When the wear patterns formed on
relief face of cutting tool are regular, VBB =0.3 mm is the criterion of tool life, and
if the wear patterns formed on relief face of cutting tool are not regular, VBB
max=0.6 mm is considered as the criterion of tool life.
Figure 3.10 (a) Top view of crater wear and nose profile and (b) flank wear land
and notch wear of cutting tool based on ISO 3685 (ISO, 1993).
Nose area of cutting tool is where the nose wear (VBC) occurs. When severe
nose wear is formed catastrophic tool failure can occur which will bring the life of
the tool to a premature end. Due to the limitation of the existing methods, which
cannot measure the nose wear fast and accurately, only few studies have been
carried out in the past to investigate the effect of nose wear. The reason why VBB
is often used is that it can be measured in a fairly objective way while nose wear is
difficult to quantify. Fortunately time history of each of these types of wear is
45
similar. In this study average flank wear land width (VBB) was considered and
measured for quantifying tool wear.
In the metal cutting process, after the chip is formed in the shear zone, it
slides over the rake face of the tool until it leaves away from the tool. The distance
from the tool tip to where it leaves the tool is called tool chip contact length as
shown in Figure 3.11. Total tool-chip contact length consists of sticking region
adjoining the cutting edge where sticking and adhering taking place followed by
sliding region in which the chip slides over the tool rake face until it leaves the
tool. Tool chip contact length affects chip form and chip ratio, surface finish, tool
temperature, cutting force, power consumption and tool life during metal cutting
(Sadik et al., 1995). Tool-chip contact length is one of the most important factors
governing tool performance owing to its effect on the flank wear and tool
temperature. The microscopic examination of cutting tool inserts used in metal
cutting processes clearly shows superficial marks left on the cutting tool rake
surface. Hence, in this work, tool-chip contact length was estimated by measuring
the length of the rubbing marks on the insert rake face after the machining tests
with the aid of tool maker’s microscope.
Figure 3.11 Tool chip contact length
46
Tool work thermocouple technique is widely used for measuring cutting
temperature. But this technique requires a reliable method of calibration, which is
difficult to accomplish especially when the tool is in the form of an insert.
Frequent short-circuiting by the chip complicates the measurement of thermo
e.m.f. The cutting temperature was measured using an extrapolative prediction
technique (Varadarajan et al., 2000) based on Finite Element Analysis. Two
standard K type thermocouples were planted at the interface between the cutting
tool inset and the holder symmetrically. The bottom of the insert and the two sides
were insulated for thermal isolation of the insert from the tool holder as shown in
Figure 3.12.
Figure 3.12 Location of thermocouples (Extrapolative prediction of cutting
temperature during turning, Varadarajan et al., 2000)
The temperature as indicated by the two thermocouples was measured
simultaneously 60 seconds after the commencement of cutting to achieve a steady
state condition in the insert. A correlation was developed between the nodal
temperature and the average temperature of the tool tip using a finite element
model which was validated using tool work thermo couple technique (Varadarajan
et al., 2000). This method provided a fairly accurate method of predicting cutting
47
temperature since the nodal temperatures are measured using standard
thermocouples. All measurements were repeated three times, and the average of
these three measurements was taken as the final value of tool wear, surface
roughness, and cutting force.
Figure 3.13 Photograph of experimental setup
A Kirloskar Turn Master- 35 all geared lathe was used for this research
work and the photograph of experimental setup is shown in Figure 3.13. The
specifications of lathe are as under.
Distance between centers (max) : 800 mm
Height of center : 175mm
Motor : 3 hp/2.2 Kw
Speed : 0 - 1500 rpm
Feed rate : 0 – 2mm\rev
Depth of cut : 0 – 1.25 mm
Feed drive : 1 hp DC motor
48
3.7 EFFECT OF AUXILIARY PULSING SLUG OF CUTTING FLUID
3.7.1 Experimentation
Photograph of the experimental set up is shown in Figure 3.14. A specially
formulated mineral oil base cutting fluid (Varadarajan et al., 2002b) which acted
as an oil in water emulsion, was applied as a narrow pulsing slug at tool work
interface (Philip et al., 2001) and at the top side of the chip as shown in Figure
3.15. An eight run experiment was designed based on Taguchi’s Technique
(Lochner and Matar, 1990) and the design matrix is shown in Table 3.6. The
process parameters such as rate of fluid application, frequency of pulsing,
composition and direction of fluid application were varied at two levels as shown
in Table 3.7. For the auxiliary jet, the rate of fluid application was kept at 2ml/min,
the frequency of pulsing at 600 pulses/min and the composition of the cutting fluid
was kept as 10% oil and the rest water (Philip et al., 2001). The cutting velocity
was maintained at 80m/min, feed at 0.1mm/rev, the depth of cut at 1.25mm and the
pressure at fluid injector at 80 bar (Varadarajan et al., 2002a). Cutting experiments
were conducted with three replications.
Figure 3.14 Experimental set up
49
Figure 3.15 Direction of fluid application
The results are presented in Table 3.8. The set of levels of input parameters
