an experimental investigation into the machinability of ggg-70...

Indian Journal of Engineering & Materials Sciences

Vol. 16, April 2009, pp. 116-122

An experimental investigation into the machinability of GGG-70 grade spheroidal

graphite cast iron

Ihsan Korkuta*, Kadir Yavuz

b & Yakup Turgut

a

aTechnical Education Faculty, Gazi University, 06500, Besevler, Ankara, Turkey bAtaturk A.T.L. A.M.L. and Industry Career High-School, Sivas, Turkey

Received 22 July 2008; accepted 16 March 2009

In this study, machining tests of the bearing necks of crank shafts produced from GGG-70 grade ductile iron (DI) are

carried out. Coated cemented carbide inserts are used in the machining tests. The tests are carried out at four different

cutting speeds (250, 275, 300 and 325 m/min), three different feed rates (0.15, 0.25 and 0.30 mm/rev) and two different

depth of cuts (0.5 mm and 2 mm). The effects of these parameters on the surface roughness and the cutting force are

investigated. Increasing cutting speed is found to deteriorate the surface roughness. The best average surface roughness

values are obtained at 250 m/min which is the lowest cutting speed. Main cutting forces increased with increasing feed rate

and depth of cut.

Keywords: Ductile iron (DI), Cam shaft, Machinability, Cutting forces, Surface roughness

Crank shafts are usually made from ductile iron (DI).

Cost of crank and crank shafts made from DI is about

30% lower than those made from forged steel1-4

.

Strength, ductility and toughness of ductile iron are

better than those of flake graphite cast iron. This is

the result of spheroidal graphite morphology in the

structure of ductile iron. Due to its enhanced

properties over flake graphite cast iron, ductile iron is

used where high strength and wear resistance are

required. Machine parts, pipe, and crankshafts are

typical industrial applications of ductile iron5,6

.

However, comparing with graphite flake and

malleable cast irons, the machinability of ductile iron

is poor because of its enhanced mechanical property7.

In ductile iron, the graphite spheres are less effective

than the flake graphite in weakening the material in

the shear plane, and the flow zone material may

sometimes be extremely ductile8.

The aim of this study is to carry out machining

tests on bearing necks of crank shafts produced from

GGG-70 grade ductile iron used in automotive

industry. This paper concentrates on the influences of

cutting tool geometries, cutting speed, feed rate and

depth of cut on surface roughness and cutting forces.

Materials and Methods

For machining tests, GGG-70 grade ductile iron

crank shafts were used as the workpiece materials.

Chemical composition of the workpiece materials is

given in Table 1. The chemical composition analysis

of this material was carried out using an OBLF-

Spektrometer device.

Sections were cut out of the workpiece materials to

carry out microstructural examinations and hardness

analysis. These sections were hot mounted in Bakelite

and then ground using SiC papers. After polishing the

sections using diamond paste, they were etched in 2%

nital solution (Fig. 1).

Figure 1 shows microstructure photographs that

were taken at various magnifications from these

etched specimens using an optical microscope.

Hardness values of the workpiece specimen was

determined as 262 HB using an Instron Wolpert

hardness measuring unit employing a 10 mm diameter

ball under 3000 kg load. Machining tests were carried

out in accordance with ISO 3685 on a Johnford TC35

CNC turning centre, with a variable spindle speed of

up to 4000 rpm and a power rating of 10 kW. The

cutting parameters used are given in Table 2.

The turning tests were carried out using coated

cemented carbide cutting tool with negative and

positive geometries. A total of 48 tests were carried

out. The cutting tools used were commercial grade

inserts produced by Sandvik Coromant with the

geometries of DNMG 110408-KM and DCMT ——————

*For correspondence (E-mail: [email protected])

KORKUT et al.: MACHINABILITY OF GGG-70 GRADE SPHEROIDAL GRAPHITE CAST IRON

117

Table 2—Cutting parameters used for the tests

Cutting conditions I II III IV

Cutting speed (m/min) 250 275 300 325

Feed rate (mm/rev) 0.15 – 0.25 – 0.3

Depth of cut (mm) 0.5 - 2

Tool geometry Negative, Positive

Fig. 1—Microstructure of GGG-70 grade ductile iron used for the

machining tests (a) magnification: ×100 and (b) magnification:

×200; (etched: 2% nital)

Fig. 2—Variation of chip cross-section depending on depth of cut

and feed rate (a) a=2 mm, f=0.3 mm/rev and (b) a=0.5 mm, f=0.3

mm/rev

11T308-KM. These inserts had GC 3210 Sandvik

designation which is equivalent to K10 according to

ISO and recommended for machining ductile irons.

These inserts were clamped mechanically on a rigid

tool holder.

Surface roughness measurement was carried out on

the machined surfaces using a Mahr Perthometer M1

instrument. Cutting force was measured with a Kistler

9257A three component piezoelectric dynamometer

and associated 5019 B130 charge amplifiers

connected to a PC employing Kistler Dynoware force

measurement software.

Results and Discussion

Evaluation of cutting forces

Depending on the cutting tool tip radius, radial

force (Fr) and feed force (Ff) components show varia-

tions. This is also same for the depth of cuts lower

than the cutting tool tip radius. Increasing depth of

cuts increases radial cutting force components. Radial

force components lead to the deflection of workpiece

and cutting tool and affect the rigidity negatively.

The tip radii of cutting tools used for the

machining tests were 0.8 mm. For the tests carried out

at 0.5 mm depth of cut, a depth of cut lower than the

tool tip radius was the case. This results in tapering of

the chip and affects the chip flow directions. Cutting

tool tip radius also determines the minimum depth of

cut. When depth of cut was lower than the tool tip

radius, the cutting tool tries to push away workpiece

rather than cutting and hardens the workpiece surface.

This, in turn, results in greater deformation. As the

process continues, the chip accumulates ahead of the

cutting tool and chatter takes place. This also results

in a hardened and burnished surface.

When the depth of cut was 2 mm for the tests, feed

forces obtained were larger than radial forces. How-

ever, when the depth of cut was 0.5 mm, radial forces

obtained were larger than the feed forces. This was

the same for the both tool geometries. Figure 2 shows

the variation of chip cross-section when cutting at 2

and 0.5 mm depth of cuts and 0.3 mm/rev feed rate9.

Table 1—Chemical composition of GGG-70 ductile iron (% weight)

C 3.57 S 0.018 Cu 0.151 V 0.002 Sb <0.001

Si 2.05 Cr 0.065 Al 0.012 W <0.003 Mg 0.050

Mn 0.604 Ni 0.022 Co <0.001 Pb <0.001 Fe Balance

P 0.043 Mo <0.001 Ti 0.012 Sn 0.093

INDIAN J. ENG. MATER. SCI., APRIL 2009

118

At a depth of cut lower than the cutting tool tip ra-

dius, the cutting tool acts as a round shaped insert and

tool-workpiece contact length in radial direction be-

comes larger when compared to feed direction.

Therefore, radial force becomes larger than the feed

force. This larger radial force can be attributed to

lower depth of cut. Chip cross-sectional area in-

creases with increasing depth of cut and feed rate. In

this study, all the three components of the cutting

force increased significantly with increasing chip

cross-sectional area. This situation is in agreement

with the literature.

As can be seen from Fig. 3a, main cutting force

shows a decreasing trend with increasing cutting

speed from 250 m/min up to 300 m/min, however,

further increase in cutting speed from 300 m/min to

325 m/min leads to an increasing trend of primary

cutting force. This situation is the same for all the

three feed rates.

Low cutting speed decreases chip curl radius and

increases chip thickness. Larger chip thickness

decreases the shear plane angle and this, in turn,

requires larger forces and stresses to deform the

material in the cutting zone. As the result of larger

forces and stresses, vibration and heat also increase.

Further increase in cutting speed beyond 300 m/min

also increased the primary cutting force. This

situation indicates that 325 m/min cutting speed is

high for the cutting tool and workpiece materials and

the cutting tool wears rapidly at this cutting speed.

Figure 3b shows that main cutting forces increase

with increasing cutting speed from 250 m/min up to

300 m/min. However, further increase of cutting

speed to 325 m/min leads to a decrease in main

cutting forces. This situation is the same for all the

three feed rates. The variation in main cutting forces

is opposite of that in Fig. 3a. In Fig. 3b, the lowest

main cutting forces are observed at 250 m/min. It can

be said that 0.5 mm depth of cut is not suitable for the

cutting tool tip radius and this can be a reason for this

reverse trend.

Rake angle (γ) has significant influence on both

main cutting force and radial cutting force

perpendicular to the main cutting force. Negative

cutting tools with - 6° rake angle produced greater

cutting forces than the cutting tools with 0° rake

angle. In the literature, resultant force increases with

changing rake angles from negative to positive. The

similar results were obtained from the tests. This can

be seen from Figs 3 and 4.

Fig. 3—Main cutting forces (Fc) obtained when machining with negative cutting tool against cutting speed at (a) 2 mm and (b) 0.5 mm

depth of cuts

Fig. 4—Main cutting forces (Fc) obtained when machining with positive cutting tool against cutting speed at (a) 2 mm and (b) 0.5 mm

depth of cuts


119

Approach angles (λ) of the negative and positive

cutting tools used for the machining tests were - 7°

and 0°, respectively. Approach angle directly

influences chip flow direction. When approach angle

is positive, chip flows towards the cutting tool.

However, when it is negative, chip flows towards the

workpiece and a decrease in cutting edge length is

observed. In this case, tool edge can also shift below

the workpiece axis. Negative approach angle

increases cutting edge strength but in this case the

tool encounters difficulty in penetrating the

workpiece.

It is seen from Fig. 4a that main cutting forces

increase when cutting speed is raised to 275 m/min

from 250 m/min and then decrease when cutting

speed is increased to 300 m/min and finally increase

again when cutting speed is increased to 325 m/min.

Main cutting forces exhibit a fluctuating trend for all

the feed rates. The lowest cutting forces with positive

cutting tool were also obtained at 300 m/min cutting

speed as in the case of negative cutting tool.

Therefore, it can be said that 300 m/min is the

optimum cutting speed in terms of main cutting force

for the both cutting tool geometries at 2 m/min depth

of cut.

In Fig. 4b, it is seen that main cutting forces

decrease with increasing cutting speed for the three

feed rates at 0.5 mm depth of cut. Positive cutting

tool geometry eliminates the risk of BUE formation.

0° inclination angle of positive cutting tool decreases

the radial cutting forces and the constant relief angle

around the cutting tool nose radius with strong cutting

edge form. This cutting tool geometry is suitable for

machining long and thin parts sensitive to vibration

and also suitable for general turning applications

where the cutting conditions vary. Positive geometry

provides low cutting forces. When positive and

negative rake angles are compared, negative rake

angle increases cutting forces while it decreases loads

on machined surfaces.

It should be noted that cutting tool nose radii of

more than 0.8 mm allow higher feed rates and if there

is vibration tendency, smaller nose radii should be

selected. Vibration tendency is a result of cutting

forces. Vibration cannot only stem from deflections

of cutting tools and workpiece but also variations in

cutting and material conditions can result in vibration.

BUE formation also leads to variations in cutting

forces and to vibrations. Importance of chip formation

geometry, providing suitable chip break, using a

cutting tool of positive rake angle and selection of

high cutting speed have all positive influence on

cutting forces. It is seen from the curves in Figs 5 and

6 that increasing feed rates and depth of cut increases

main cutting forces. This increase is seen for the both

tool geometries used in the tests.

Chip cross-sectional area product of feed rate and

depth of cut is the most important factor in

determining main cutting force. Figures 5 and 6 show

that main cutting force increases as depth of cut and

feed rate increase. Increasing chip cross-sectional

area with increasing feed rate and depth cut is the

reason for this increase.

Evaluation of surface roughness

Surface roughness measurements were made three

times at different places on the machined surfaces

after machining with each tool and the averages of

these three measurements were taken. Figures 7-10

give the surface roughness measurement results.

When Figs 7 and 8 are examined, variations of

average surface roughness values depending on 0.15

mm/rev, 0.25 mm/rev and 0.30 mm/rev of feed rates

at different cutting speeds are seen. When cutting

speed is increased, average surface roughness values

Fig. 5—Main cutting forces (Fc) obtained when machining with negative cutting tool against feed rate at (a) 2 mm and (b) 0.5 mm depth

of cuts


120

Fig. 6—Main cutting forces (Fc) obtained when machining with positive cutting tool against feed rate at (a) 2 mm and (b) 0.5 mm depth

of cuts

Fig. 7—Surface roughness (Ra) obtained by machining with negative cutting tool against cutting speed at (a) 2 mm and (b) 0.5 mm depth

of cuts

Fig. 8—Surface roughness (Ra) obtained by machining with positive cutting tool against cutting speed at (a) 2 mm and (b) 0.5 mm depth

of cuts

Fig. 9—Surface roughness (Ra) obtained by machining with negative cutting tool against feed rate at (a) 2 mm and (b) 0.5 mm depth of

cuts


121

also increase. Differently, average surface roughness

initially decreases with increasing cutting speed and

then increases with further increase in cutting speed

when machining with negative cutting tool at 0.5 mm

depth of cut and 300 m/min cutting speed. Increasing

surface roughness with increasing cutting speed is in

disagreement with literature. However, quite long

workpiece was considered to lead to excessive

vibration and deflections. As the result of vibration

and deflections, chatter causing a poor surface quality

occurs. In order to prevent this, workpiece should be

supported suitably during machining.

The lowest average surface roughness values were

obtained from the tests carried out at the lowest

cutting speed (250 m/min). While the average surface

roughness values were low at 250 m/min cutting

speed, it was seen that average surface roughness

values was affected negatively with increasing cutting

speed. Variations of average surface roughness values

depending on feed rate at 250, 275, 300 and 325

m/min cutting speeds are given in Figs 9 and 10 with

curves. It is seen from these curves that increasing

feed rate increased the surface roughness values at all

the cutting speeds.

It is seen from the curves that surface roughness

increase is more obvious when the feed rate is

increased from 0.15 mm/rev to 0.25 mm/rev. This

increase in surface roughness with increasing feed

rate is in agreement with the reported studies10

.

If the curves in Figs 9 and 10 are examined by

taking into consideration surface roughness

depending on depth of cut, it is seen that surfaces

obtained with negative cutting tool at 0.15 mm/rev

feed rate and 0.5 mm depth of cut are better than

those obtained at 2 mm depth of cut at the same feed

rate. However, 2 mm depth of cut results in better

surfaces than 0.5 mm depth of cut when positive

cutting tool is used.

Conclusions

The following conclusions can be drawn from the

present study:

Chip cross-sectional area which is the product of

feed rate and depth of cut is the most important factor

affecting cutting forces. With increasing feed rate,

main cutting forces for the both tool geometries

increased. When the depth of cut 2 mm, the lowest

cutting force for the both tool geometries was

obtained at 300 m/min cutting speed. When the

cutting speed was increased, only machining with

positive cutting tool at 0.5 depth of cut showed a

continuous decrease in cutting forces. It was

determined that feed rate was the most important

factor on surface roughness. At all the cutting speeds

for the both tool geometries, increasing feed rate

increase the average surface roughness. When feed

rate was increased from 0.15 mm/rev to 0.25 mm/rev,

the obtained surface roughness values increased by

100%. Similarly, 300% increase in surface roughness

values were observed when feed rate was increased

from 0.15 mm/rev to 0.30 mm/rev. Increase in cutting

speed deteriorates the surface roughness, the best

average surface roughness values were obtained at

250 m/min which was the lowest cutting speed.

Apart from 0.15 mm/rev feed rate, the depth of cut

did not have any meaningful effect on average

surface roughness. The factors having effect on

surface roughness were determined in order of

importance as follows: feed rate, cutting speed and

depth of cut.

Acknowledgement

The authors would like to acknowledge Gazi

Fig. 10—Surface roughness (Ra) obtained by machining with positive cutting tool against feed rate at (a) 2 mm and (b) 0.5 mm depth of

cuts


122

University Scientific Research Projects (41/2006–01)

for the financial support.

References

1 Lin S C, Lui T S, Chen L H & Song J M, Metall Mater

Trans, 33(A) (2002) 2623.

2 Lessiter M J, Engineered cast components for the automotive

industry, Engineered Casting Solutions, Fall, (2000) 37.

3 Bosnjak B, Radulovic B, Pop-Tonev K & Asanoviv V,

J Mater Eng Perform, 10(2) (2001) 203.

4 Thomas T R, Rough surface (Longman Group Limited,

London and New York), 1982, 12-18.

5 Groover M P, Fundamentals of modern manufacturing-

materials process and systems, (Prentice-Hall), 1996,

128-257.

6 Kalpakjian S, Manufacturing process for engineering

materials, 2nd ed. (Edison-Wesley), 1991, 12-191.

7 Seker U & Hasirci H, J Mater Process Technol, 173 (2006)

260–268.

8 Trent E M, Metal cutting (London, Tanner Ltd), 1984, 1-86.

9 Yavuz K, An experimental investigation into the

machinability of GGG-70 grade spheroidal graphite cast

iron cam shafts, M.Sc. Thesis, Gazi University Institute of

Science and Technology, Ankara, Turkey, (2006).

10 Yan B H, Huang F Y & Chow H M, J Mater Process

Technol, 54 (1995) 341-347.

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