Performance of single Si3N4 and mixed Si3N4+PCBN wiper cutting tools applied to high speed face milling of cast iron

Performance of single Si3N4 and mixed Si3N4CPCBN wiper cutting tools

applied to high speed face milling of cast iron

Antonio Maria de Souza Jr.a, Wisley Falco Salesb, Sandro Cardoso Santosc,*,Alisson Rocha Machadod

aFiat-GM Powertrain Ltd, Betim, MG, BrazilbMechatronic Engineering, Polytechnic Institute—IPUC, Pontifical Catholic University of Minas Gerais—PUC Minas, IPUC, Belo Horizonte, MG, Brazil

cMechanical Engineering Department, Federal Technologycal Education Centre, CEFET-MG, Av. Amazonas, 7675, Nova Gameleira,

Belo Horizonte 30410-000, MG, BrazildFaculty of Mechanical Engineering, Federal University of Uberlandia, Uberlandia, MG, Brazil

Received 22 October 2003; accepted 3 August 2004

Available online 19 October 2004

Abstract

In this work two face milling cutter systems were used in high speed cutting of gray cast iron under cutting condition encountered in the

shop floor. The first system, called ‘A’, has 24 Si3N4 ceramic inserts all with square wiper edges. The second system, called ‘B’, is a mixed

tool material system, having 24 wiper inserts, 20 of them are Si3N4 intercalated by four PCBN inserts. Cutting speed (vc), depth of cut (doc)

and feed rate per tooth (fz) were kept constant. Surface roughness (Ra and Rt) and waviness (Wt), tool life (based on flank wear, VBBmax) and

burr formation (length of the burr, h) were the parameters considered to compare the two systems. System ‘B’ presented better performance

according to all parameters, although only end of life criterion based on Rt parameter has been reached.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Si3N4; PCBN; Tool wear; Wear mechanisms; Surface finishing; Burr formation

1. Introduction

Advances in milling processes technologies have been

accelerating in recent years. Designers and users of cutting

tools have worked to optimize shape and tool geometry, tool

life, processes parameters related surface finish, pro-

ductivity and finally, the most important factor, the

machining cost per piece. Until recent years it was common

to use single tool material or tool geometry in a tool holder.

This paradigm in milling processes has been overcome, with

the use of the mixed technology, that is, two or more tool

materials or tools of more than one geometry mounted in a

tool holder [1].

0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijmachtools.2004.08.006

* Corresponding author. Tel.: C55 31 3319 5208; fax: C55 31 3319

5212.

E-mail addresses: [email protected] (A.M. de

Souza Jr.), [email protected] (W.F. Sales), [email protected]

(S.C. Santos), [email protected] (A.R. Machado).

PCBN tools are used in machining operations, where

tight dimensional tolerances are required due to their high

elasticity modulus and hardness and low expansion volu-

metric coefficient. This material is recommended to

machine practically all work piece materials [2], except

low carbon steel [3]. This is because low carbon steel is

ductile enough to generate a large chip/tool contact area,

promoting strong chemical interactions and consequently

activating diffusion wear mechanisms. In the last years,

PCBN tools were used on shop floors in high speed

machining of gray cast iron, white cast iron and hardened

steels with low machining costs per piece. In these

applications they tends to be economically advantageous

[1,4,5].

Si3N4 ceramic tools are largely used to machine gray cast

iron and hardened steels. When cutting gray cast iron, high

speed machining is frequently used with low costs per

pieces.

International Journal of Machine Tools & Manufacture 45 (2005) 335–344

www.elsevier.com/locate/ijmactool

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Fig. 1. Upper view of the engine block and indication of some machining

operation details.

A.M. de Souza Jr. et al. / International Journal of Machine Tools & Manufacture 45 (2005) 335–344336

The conventional milling processes use all inserts with

the same geometry, mounted on the cutter. Normal

octagonal inserts are frequently used for roughing cuts.

For finishing cuts wiper systems offer much better

surface finishing. When machining blocks and engine heads

good surface finishing is required and wiper systems are

usually used [5].

Milling cutters with two or more tool materials or tool

geometries is a recent technology. This system uses

octagonal inserts and intercalated by wiper inserts.

Although, they show satisfactory results with respect to

surface roughness the main reason for their choice is related

to low machining costs, presented when compared to a

system having only wiper tools [5].

With the need of better efficiency and performance of

automotive engines and in order to be competitive car

manufacturers have introduced in their engines a new

metallic cylinder head junction. This provides a greater

rigidity in the junction between the cylinder head and the

block, with practically no deformation during lifetime and

keeping the integrity of the junction. This reduces premature

wear, which is usually observed in conventional head

junctions, due to high load, and pressure variations that

occur during the thermal cycles. The use of metallic head

junctions demands better surface finishing (roughness

parameters Ra and Rt, and waviness Wt) and reduction or

elimination of burr formation to allow a good assembling of

the cylinder head onto the cylinder block.

Burrs are defined as ‘undesirable projections of material

beyond the edge of the workpiece due to plastic deformation

during machining’ [6]. The burrs generated in milling

processes are extremely undesirable because they present

accident risks to the operators in the assembly lines.

Furthermore, they can hinder contacts between surfaces,

compromising or damaging the desirable precision in

assembled parts and most critical of all is that burrs can

become detached and contaminate the lubrication and

cooling lines, in the case of internal combustion engines.

Burr removal is an expensive time consuming operation.

Thus, the knowledge of the phenomenon of burr

formation is of great importance because the milling

process can be controlled, particularly the cutting par-

ameters such as cutting speed (vc), feed rate (f ), depth of cut

(doc), tool geometry, and approach angle, cr, so that burr

formation can be minimized and, in some cases eliminated.

With respect to wear, both flank and crater affect the burr

formation process. The dimensions of the burr tend to

increase with increasing wear levels. This is more critical

after a certain amount of wear has been reached [7]. In the

final stage of the process of burr formation a breakout can

occur, particularly in brittle workpiece materials such as

gray cast iron that can lead to scrap the part.

In the production line both burr and breakout must be

avoided because serious problem will happen lately at the

serial assembly line. Occurrence of either will usually lead

to the loss of the part. More critical is when this

phenomenon happens on the last machining operation

after a reasonable amount of money had already been

aggregated into the process.

The objective of this work is to compare the performance

of two milling cutter systems and to study the formation of

burrs at the exit edge of the workpiece considering also and

the surface quality and their relations with tool wear. Both

systems have 24 wiper inserts. System ‘A’ is composed by

Si3N4 ceramic tools while the system ‘B’ has 20 Si3N4 tools

intercalated by four PCBN tools. The tests were all

performed in the production line, using the same serial

machinery for engine production. The cutting speed (vc), the

depth of cut (doc) and feed rate per tooth (fz), were kept

constant.

2. Experimental work

2.1. Machining process

The top surfaces of engine blocks of car engines as

shown in Fig. 1 were face milled. This surface is

characterized by a complex system of tool entrance and

exit to and from the workpiece due to the inherent geometry

of the block.

The engine block material was GH 190 UNI gray cast

iron, with the following chemical composition: 3.2–3.5%C;

2.0–2.5%Si; 0.2%Cr; 0.15%S; 0.10%P and an average

hardness 200 HV.

The experiments were carried out on a transfer line

machine during the engine block production of a car

manufacturing industry. The milling machine with a Siemens

840D electronic control unit was driven by a tri-phasic

asynchronous engine and has a GR ISO130 mono-mandrill.

A constant spindle speed of 1270 rpm (vcZ1000 m/min) was

always used. All the tests were performed during a normal

production regime. Feed per tooth (fz) and depth of cut (doc)

were 0.06 mm/tooth and 0.3 mm, respectively.

Fig. 1 also shows schematically the milling cutter

position with its rotation and feed directions, and

Fig. 2. Positions of PCBN inserts on mill that characterize system B.

A.M. de Souza Jr. et al. / International Journal of Machine Tools & Manufacture 45 (2005) 335–344 337

the region, where the maximum burr length was always

observed.

2.2. Cutting tools

Two different cutting tool assemblies, named system ‘A’

and system ‘B’ were evaluated in this investigation. Both

systems used milling cutters of 250 mm diameter.

System ‘A’ had 24 Si3N4 wiper inserts. These inserts had

the following ISO designation: SNEN 1504ADTR with T

05!15 chanfer edge.

The tool system ‘B’, had 24 inserts, 20 Si3N4 OPHN

0504 ZZN-A27 geometry and four PCBN wiper inserts

OPHX 0504ZZR-A27 geometry. The PCBN inserts were

positioned intercalating each five Si3N4 inserts, as shown in

Fig. 2.

The clamping system used a support pin for the tools and

a mechanism with axial adjustments for the wiper tools.

The adjustment of the tools was performed on an

Ingersoll table, which a flat tip stylus. For milling cutter

Fig. 3. General view of Si3N4 cutting to

of system ‘A’ all the 24 wiper inserts have individual axial

adjustment. The maximum admissible axial deviation was

30 mm. The milling cutter of system ‘B’ had admissible

maximum axial deviation of 40 mm among the Si3N4 inserts,

and 20 mm among the PCBN inserts. The PCBN inserts

were positioned 50 mm above the highest Si3N4 insert.

2.3. Measurement of tool wear, surface texture and burr size

A Wild-Heerbrugg optical microscope model 117.775

and an Omis Mini Optical Measurement Inspection System

were used for tool wear measurements. Analysis of system

‘B’ tools was restricted to PCBN wiper tools because it

greater responsibility to surface finish of machined parts.

The worn surfaces of the tools were analysed within a

Phillips scanning electronic microscope with objective to

identify predominant shapes and wear mechanisms. Surface

roughness and waviness measurements were performed by a

Taylor Hobson MK3 profilometer. Cut-off length of 0.8 mm

was adopted.

The length of the burr was measured with a diamond

stylus gauge (20 mm resolution). Five measurements were

taken and the mean value considered. Lee et al. [8] was also

utilized a similar procedure.

2.4. End of tool life criterion

End of tool life criterion based on tool flank wear

(VBBmaxZ0.7 mm) was adopted, and in order to comply

with requirements from the design of the engine block to

avoid workpiece rejection other criteria were also con-

sidered based on surface roughness parameters (maximum

roughness average, RaZ1.6 mm; maximum roughness

height, RtZ10 mm) and burr dimensions (maximum burr

length, hZ1.8 mm).

In each test the roughness parameters Ra and Rt,

maximum flank wear VBBmax and burr length (h) were

ol shows flank and chatter wear.

Fig. 4. Detail shows some wear mechanisms such as micro-cracks, plastic deformation and abrasion.


measured after machining the first five workpieces. Next, up

to the 100th engine block machined the intervals of

measurements of theses parameters was enhanced to 10

workpieces. From the 100th to the 800th engine block

machined the intervals of measurements was fixed in 100

workpieces and after the 800th engine block the interval was

increased to 200 workpieces machined until one of the tool

life rejection criteria was reached.

3. Results and discussion

Results obtained are presented and discussed in this

section, which is divided into two parts. First part presents a

sequence of scanning electronic microscope photographs of

worn cutting edges of wiper tools that will contribute for the

discussion of the results of tool wear behavior, surface

texture and burr size presented in the second part.

Fig. 5. Detail shows micro-cra

3.1. Scanning electronic microscope images

of worn surfaces

Worn surfaces of Si3N4 tools are shown in Figs. 3–6.

Fig. 3 shows the general aspects of the worn surface, that

allow identify flank wear as the main wear form. Smooth

aspect of the topography of worn region can be also

observed. Occurrence of this appearance may be attributed

to adhesion of workpiece material, diffusive wear [9] or

abrasion due the action of harder particles over softer

materials [10]. Amplifications of worn region are shown in

Figs. 4–6 that permit identify the presence of cracks in

different regions of worn surface. Propagation of cracks

causes chipping of some Si3N4 cutting tools that occurs

during the tests. Smooth topography and flow of material

over the worn surface indicates occurrence of adhesion.

High chemical affinity of tool and workpiece material

components also justifies the hypothesis of adhesion.

cks and abrasive wear.

Fig. 6. Detail of worn surface of Si3N4 cutting tools shows micro-cracks.


Figs. 5 and 6 show the flow of workpiece material over

the tool flank surface. Figs. 4 and 6 show details of cracks

present at the worn area of the tool. Smooth surfaces near

the cracks and surfaces that present abrasion marks can be

observed. The gray cast iron workpiece material has in its

composition silicon carbide, SiC, with high hardness

(2500 HV). According to Refs. [11,12], the presence of

hard particles in the tribological system can promote wear

and the relationship between workpiece/abrasive hardness

materials is an important effect on the wear regime. For

hardness ratio less than 0.6 the so-called soft abrasion will

occur while for ratio more than 0.6 the so-called hard

abrasion regime will take place. Large system damage is

encountered when hard abrasion is dominated. The

tribological system evaluated presents Si3N4 tool material

Fig. 7. General view of PCBN cut

with average hardness of 1600 HV and the hardness ratio is

around 0.64, which situates on the hard abrasion regime.

Fig. 7F shows cracks that propagate in several directions.

The aspects of worn region of Si3N4 justify the tendency of

catastrophic failure of the tools due chipping. Other wear

mechanisms were reported by Silva et al. [13] and Vleugels

and Van Der Biest [14]. The later showed the incompat-

ibility of the use the of Si3N4 tools at high cutting speed and

dry machining of steels because of the high interface

temperatures and the chemical compatibility between the

materials involved in the tribological system which

encourage chemical wear which is dominant under those

evaluated cutting conditions.

Aspects of the worn surfaces of PCBN cutting tools are

shown in Figs. 7–9, those are characterized by presence of

ting tools shows flank wear.

Fig. 8. Detail shows some wear mechanisms such as micro-cracks, plastic deformation and adhesion of the work piece material.


cracks running to the cutting edge. The presence of abrasion

marks is also observed in these figures. This tribological

system involves silicon carbides and a PCBN (4000 HV)

and the hardness ratio workpiece/abrasive material is

around 0.6. Therefore, the wear regime is a transition

between soft and hard abrasion, but it is clear the presence

of the abrasive wear. Aspects the worn surface suggest that

thermal and mechanical cyclic load was the main factor to

influence the wear of the cutting tools and fracture

toughness resistance should be the main property required

when machining with PCBN cutting tools. It was observed a

tendency of the workpiece material to adhere into the

cracks, hypothesis confirmed by EDX analysis.

The results indicated the presence of Fe and Si, elements

of the workpiece material. These portions contribute to

cracks growth and propagation.

Fig. 9. Detail shows evidences wear mechanisms

Gastel et al. [15] has studied the performance of PCBN

tools on grey cast iron and compact graphite iron turning.

They showed that oxidation of the tools and interdiffusion of

constituting chemical elements between tool and workpiece

were the dominant wear mechanisms.

3.2. Tool wear, surface texture and burr size

Behavior of maximum flank wear of cutting tool during

engine block production is shown in Fig. 10. System ‘A’

was able to produce 1600 parts, while system ‘B’ could

produce 2577 blocks. Tools that compose system ‘A’

showed relatively low wear until 1400 parts were produced

and after which a rapid growth was observed. System ‘B’

tools tended to present a uniform increase of flank wear

during all the tool life.

such as abrasion and plastic deformation.

Fig. 10. Evaluation of flank wear (VBBmax) as a function of number of blocks machined.


The type of wear observed on cutting tools can explain

the behavior of the maximum flank wear curves. Si3N4

cutting tools presented a tendency to chip or break as a

consequence of formation and propagation of cracks, as

shown in Figs. 4–6. The occurrence of chipping implies in

an abrupt change on tool geometry, which can be reflected

in rapid increase in flank wear, as shown in Fig. 10. On the

other hand, PCBN cutting tools presented a gradual increase

of flank wear, which is compatible with wear aspects

observed in these tools. Formation and growth of cracks

were gradually observed, fact that implies in gradual

increase of flank wear.

Surface average roughnesses obtained by the two

systems are shown in Fig. 11. Both systems presented the

same behavior throughout the tool life. The exception of this

is the last engine block machined with system ‘B’, when the

Ra parameter was much higher than the end of tool life

criterion RaZ1.6 mm.

The PCBN and Si3N4 cutting tools presented different

wear patterns, as shown by the different surface profiles of

the machined surfaces. To complement the study of surface

Fig. 11. Roughness (Ra) as a function

texture two other parameters like maximum peak to valley

distance (Rt) and waviness (Wt) were also measured as a

function of the number of engine blocks machined. Results

for Rt are presented in Fig. 12.

System ‘A’ tends to produce higher values of Rt

parameter during all the tool lives. Engine blocks milled

by system ‘B’ tend to present relative uniform values of Rt,

while system ‘A’ sowed crescent values. According to the

results shown in Fig. 12, both systems exceeded the end of

tool life criterion, RtZ10 mm.

Results of waviness parameter Wt are shown in Fig. 13.

As observed for Rt parameter, system ‘A’ presented

higher values of Wt than system ‘B’ practically throughout

the tool life. The wear of the PCBN tools was smaller than

the wear of the Si3N4 tools. This fact is implied by the

smaller cutting effort when machining with the former

cutting tools, and consequently smaller deformation and

vibration, which results in better surface finish.

Burr height as a function of the number of blocks

machined is shown in Fig. 14. The ceramic cutting tools

tend to produce higher burrs and this difference increases

of number of blocks machined.

Fig. 12. Maximum peak to valley distance (Rt) as a function of number of the blocks machined.


when the number of parts machined increases. Again the

higher wear of the ceramics compared to the PCBN tools is

responsible for these results.

According to Olvera and Barrow [16], the thickness

and/or the length can be used to characterize the burr but the

former is it’s the main dimension. The thicker burrs are the

more difficult to be broken during a deburring operation.

However, the measurement of its thickness is more difficult

and laborious because metallographic preparation is usually

required. On the other hand, the length of the burr can be

easily measured, allowing a large number of measurements

in a short period of time, and it can also be used for its

characterization [17]. In the present work it was thus

decided to measure the length of the burr instead of its

thickness since the entire test were carried out in the

production line.

Work by Olvera and Barrow [16] in face milling AISI

1040 steel bars at a cutting speed of 142 m/min, investigated

the influence of the depth of cut (doc) and the nose radii on

the burr length. They founded that the burr length increases

Fig. 13. Waviness parameter (Wt) as a functi

when doc and nose radius increases until a value, called

critical value, from which it starts to decrease. These

transition points encountered were docZ0.5, 0.8 and

0.9 mm for the tool nose radii of 0.4, 0.8 and 1.2 mm,

respectively. Although in the present investigation the work

material is cast iron and the cutting speed (1000 m/min) is

far higher than that used by Olvera and Barrow [16], which

might imply here the development of higher cutting

temperatures [9] the transition point might also be present.

In this work the tool nose radius was 0.4 mm and the depth

of cut was 0.3 mm and they did not vary, and therefore the

transition point (or critical value) could not be determined.

However, the influence of the wear or the burr length is

clear. The burr length increases when the wear increases.

This is in agreement with the mathematical model

presented by Hashimura et al. [18] that indicates an increase

in the burr sizes with increasing in the cutting section area.

During all the tests no breakout phenomenon was

observed. This means that the amount of wear was not

enough to increase the plastic deformation zones to an

on of the number of blocks machined.

Fig. 14. Burr length (h) as a function of the number of motor blocks machined.


extension necessary to cause rupture of the negative shear

plane [18–21] that would culminate in the breakout.

4. Conclusions

The conclusions of this study can be summarized as

follows:

–
Milling cutting system ‘B’ with four PCBN wiper cutting
tools presented better performance compared with

system ‘A’ regardless all parameters considered;

–
Among all the end of tool life criteria adopted, only the
roughness parameter Rt was reached, which makes this

the most critical index for product quality;

–
Si3N4 and PCBN cutting tools presented different
dominant wear patterns and the other parameters studied

can be related to them;

–
Concerning to tool materials, the dominant wear
mechanisms for Si3N4 were micro-cracks, plastic

deformation and abrasion while for PCBN were micro-

cracks, adhesion and abrasion;

–
Both systems presented satisfactory performance, gen-
erating surfaces with topography parameters consider-

ably below the limit established for the process.

Acknowledgements

The authors would like to thank Fiat-GM Powertrain Ltd

and FAPEMIG (Fundacao de Amparo a Pesquisa do Estado

de Minas Gerais), for technical and financial support.

References

[1] D.C. Correa, W.F. Sales, S.C. Santos, E.S. Palma, P.S. Neto,

J.R. Ferreira, The use of PCBN tools for machining bimetallic

bearings, Industrial Diamond Review Issue 1 (2003).

[2] B. Mills, Recent developments in cutting tool materials, Journal of

Materials Processing Technology 56 (1996) 16–23.

[3] A.M. Abrao, The machining of annealed and hardened steels using

advanced ceramic cutting tools, PhD Thesis, University of Birming-

ham, UK, 1995.

[4] M.W. Cook, P.K. Bossom, Trends and recent developments in the

material manufacturing and cutting tool application of polycrystalline

diamond and polycrystalline cubic boron nitride, International Journal

of Refractory Metals and Hard Materials (2000) 147–152.

[5] A.M. Souza Jr., W.F. Sales, High speed machining of motor blocks

with PCBN and mixed ceramic inserts, Industrial Diamond Review

Issue 4 (2002).

[6] ASTME, Tool wear in the cutting of thin-gauge steel sheets, American

Society of Tool and Manufacturing Engineers—ASTME Research

Report no. 22, 1959.

[7] G.L. Chern, D.A. Dornfeld, Burr/breakout model development and

experimental verification, Journal of Engineering Materials and

Technology 118 (1996) 201–206.

[8] M. Lee, J.G. Horne, D. Tabor, The mechanism of notch formation

at the depth of cut line of ceramic tools machining nickel

base superalloys, Proceedings of the International Conference on

Wear Materials, ASME, Darborn, MI, USA, 16–18 April 1979,

pp. 460–469.

[9] E.M. Trent, Metal Cutting, third ed., Butterworths–Heinemann,

London, 1991, 273 pp. ISBN 0-7506-1068-9.

[10] C. Yen-Qian, D. Xiang-G, W. Fu-Xing, C. Qi-Gong, Z. Zhang-Xiao,

On wear mechanism of SIALON and metal in dry sliding, Wear 137

(1990) 175–186.

[11] I.M. Hutchings, Tribology: Friction and Wear of Engineering

Materials, CRC Press, Boca Raton, USA, 1992, p. 273.

[12] K.H. Zum Gahr, Microstruture and Wear of Materials, Elsevier,

Amsterdam, 1987.

[13] R.F. Silva, F.J. Oliveira, F.P. Castro, J.M. Vieira, Modeling of

chemical wear in ferrous alloys/silicon nitride contacts during high

speed cutting, Acta Materials 46 (7) (1998) 2501–2507.

[14] J. Vleugels, O. Van Der Biest, Chemical wear mechanisms of

innovative ceramic cutting tools in the machining of steel, Wear 225–

229 (1999) 285–294.

[15] M. Gastel, C. Konetschny, U. Reuter, C. Fasel, H. Schulz, R. Riedel,

H.M. Ortner, Investigation of the wear mechanism of cubic boron

nitride tools used for machining of compacted graphite iron and grey

cast iron, International Journal of Refractory Metals and Hard

Materials 18 (2000) 287–296.


[16] O. Olvera, G. Barrow, Influence of exit angle and tool nose geometry

on burr formation in face milling operations, Proceedings of the

Institution of Mechanical Engineers 212 (part B) (1998) 59–72.

[17] L.K. Gillespie, P.T. Blotter, The formation and properties of

machining burrs, Transactions of the ASME (1976) 66–74.

[18] M. Hashimura, Y.P. Chang, D. Dornfeld, Analysis of burr formation

mechanism in orthogonal cutting, Journal of Manufacturing Science

and Engineering 121 (1999) 1–7.

[19] A.J. Pekelharing, The exit failure uninterrupted cutting, Annals of the

CIRP 27 (1) (1978) 5–10.

[20] S.L. Ko, D.A. Dornfeld, Analysis of fracture in burr formation at the

exit stage of metal cutting, Journal of Materials Processing

Technology 58 (1996) 189–200.

[21] S.L. Ko, D.A. Dornfeld, Burr formation and fracture in oblique

cutting, Journal of Materials Processing Technology 63 (1996)

24–36.

Documents

Performance of single Si3N4 and mixed Si3N4+PCBN wiper cutting tools applied to high speed face milling of cast iron