Tool Edge

1. wI
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an
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Society of
Manufacturing
Engineers
1999
0 ALL RIGHTS RESERVED
MR99-167
The Effect of Tool Edge
Geometry on Workpiece
Sub-Surface Deformation and
Through-Thickness Residual
Stresses for Hard-turning of
AlSl 52100 Steel
authors
JEFFREY D. THIELE SHREYES N. MELKOTE
Engineer Assistant Professor
Caterpillar Georgia Institute of Technology
Peoria, Illinois Atlanta, Georgia
abstract
An experimental investigation was conducted to determine the effect of tool cutting
edge geometry on workpiece subsurface deformation and through-thickness resid-
ual stresses for finish hard turning of through-hardened AISI 52100 steel.
Polycrystalline cubic boron nitride (PCBN) inserts with “up-sharp” edges, edge
hones, and chamfers, were used as the cutting tools in this study. Examination of the
workpiece microstructure reveals that large edge hone tools produce substantial
sub-surface plastic flow. Examination of through-thickness residual stresses shows
that large edge hone tools produce deeper, more compressive residual stresses than
small edge hone tools or chamfered tools. Explanations for these effects are offered
based on assumed contact conditions between the tool and workpiece.
conference
NAMRC XXVII
May 25-28, 1999
Berkeley, California
terms
Hard Turning
Sub-surface
Deformation
Edge Geometry
Residual Stress
Sponsored by the
North American Manufacturing Research Institution
of the Society of Manufacturing Engineers
One SME Drive l P.O. Box 930 l Dearborn, Ml 4812
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3. HE EFFECT OF TOOL EDGE GEOMETRY ON WORKPIECE SUB-SURFACE
EF Al-ION AND THROUGH-THICKNESS RESIDUAL S
URNING OF AISI 52100 STEEL
ffrey D. Thiele and S
W. Woodruff School
Georgia Institute of Technolog
Atlanta, Georgia
ABSTRACT
An experimental investigation was conducted to determine
the effect of tool cutting cdgc geometry on workpiece suh-
surface deformation and through-thickness residual stresses
for finish hard turning of through-hardened AISI 52100
steel. Polycrystaliine cubic boron nitride (PCBN) inserts
with “up-sharp” edges, edge hones, and chamfers, were
used as the cutting tools in this study. Examination of the
workpiece microstructure reveals that large edge hone tools
produce substantial sub-surface plastic flow. Flow is not
observed when turning with small edge hone tools or
chamfered tools and the workpiece microstructure appears
random for these cases. Examination of through-thickness
residual stresses shows that large edge hone tools produce
deeper. more compressive residual stressesthan small edge
hone tools or chamfered tools. Explanations for these
effects are offered based on assumed contact conditions
between the tool and workpiece.
INTRODUCTION
Workpiece surface/sub-surface deformation and residual
stressesproduced in hard turning affect the fatigue life and
tribological properties of the machined component (Konig
et al, 1993). The extent to which these properties are
affected depends on the type and amount of sub-surface
microstructure aIterations and the state, magnitude and
through-thickness distribution of residual stresses. These
are in-turn functions of workpiece properties and
machining parameters such as cutting conditions and tool
geometry. It is therefore important to correlate machining
parameters and workpiece properties to workpiece sub-
surface characteristics and residual stress distribution for
hard turning.
Prior work on hard turning deals mostly with the effects of
cutting conditions (speed, feed. depth of cut). workpiece
hardness and tool wear on chip formation, surface finish,
residual stresses. and ‘white layer’ formation. Recent
studies on chip formation show that segmented chips
produced in hard turning are influenced by a number of
factors such as cutting speed, rake angle. tool wear. depth
of cut. and feed rare (Davies et al, 1996; Vyas and Shah,
1997). El-Wardany and Elbestaui (1998) studied material
side flow in hard turning as a function of tool nose radius,
tool wear. and feed rate.
Matsumoto et al 11936) studied the effect of tiorkpiece
hardness on residual stresses for facing of hardened AISI
4340 steel. They showed that. in the absence ,~i‘
metallurgical phase transformations, the stresses become
more compressive with increase in hardness. -vtru an;:
Matsumoto (1990) correiatcd residua1 stress patterns in
tube facing of AISI 4340 steel with shear angle effects. In
general, residual stress generation depends on
thermoplastic deformation of the workpiecc (Brinksmeicr
et al 1982).
Surface integrity studies for hard turning reveal the
existence of white layers and suggest that martcnsitic white
layer formation is primarily caused by thermal effects
(Chou and Evans, 1997). Griffiths (1987) lists thermal
effects. contact pressure. mechanical deformation. and
surface chemical reaction as factors influencing white layer
formation.
Aspinwall et al (1995) investigated the effects of tool wear
on hard turning of AISI 52100 bearing steel. They also
compared the fatigue life of hard turned and ground
specimens with the same surface roughness and found that
‘white layers’ produced by hard turning correlate with
compressive residual stress (Abrao and Aspinwall, 19961.
Tonshoff et al (1995) studied the effects of tool
composition and wear on surface residual stresses and
integrity for hard turning of ASTM 51 1.5 steel. Other
studies have shown that forces and surface roughness tend
to increase with wear for hard turning (Chou and Evans.
1997; Sista et al. 1997).

4. m99-167-Z
Although these studies have provided valuable insight into
surface generation in hard turning, they do not account for
the effect of the tool cutting-edge geometry. Cutting-edge
geometry. also known as ‘edge preparation’, is significant in
finish hard turning because the undeformed chip thickness
is of the same order of magnitude as the edge geometry
iBrinksmeier et al, 1982). Therefore, most of the tool-
workpiece interaction occurs along the cutting edge.
Jang et al (1996) have considered the effects of tool edge
radius on residual stresses in turning of 304 stainless steel.
However, only surface residual stresses wcrc considered.
itlso. no attempt was made to correlate the measured
stresses with workpiece microstructure changes due to
machining. Liu and Barash (1982) suggested that an
increase in cutting-edge radius would result in a more
compressive surface residual stress. However. their results
were based on tool flank wear data and cutting edge radius
data was not presented.
This paper presents the results of an experimental study of
the effects of cutting-edge geometry on workpiece sub--
surface deformation and through-thickness residual stresses
for finish/semi-finish hard turning of AISI 52100 steel. In
particular. the effect of nominally sharp. honed, and
chamfered PCBN inserts is examined. Physical
explanations for the observed effects are given.
~~~E~IME~TAL PROCEDURE
Tool Edqe Characterization
Several types of edge preparation are available for PCBX
cutting tools used in hard turning operations. These include
“up-sharp” edges, “up-sharp” chamfers, hones, and
chamfers with edge hones applied. These representative
edges are shown schematically in Figure 1. Measurements
of “up-sharp” inserts show that small, measurable edge
hones (radii) exist at the flank-rake face interface.
Therefore, ali PCBK inserts have a finite edge radius that
may be obtained in various sizes.
ChiR
Workpiece
Honed
&Up-Sharp
FIGURE 1. TYPICAL PCBN CUTTING-EDGE
PREPARATION (GEOMETRY).
PCBN inserts with three types of edge preparation were
used in this experiment: (I) “up-sharp” edges. (2) large
edge hones (100-150 pm radius), and 13) “up-sharp”
chamfered edges nominally 11.5.urn X 17” (chamfer width
x chamfer angle). Because the “up-sharp” edges have a
finite hone, this investigation effectively involves a
comparison of the effects of edge hone size. Prior to
machining, each insert was measured using a stylus-type
instrument specifically designed for characterization of
edge geometry. Measurements were taken at five locations
around the tool nose. The average value and standard
deviation for each insert, and the corresponding nominal
values, are given in Table 1. Henceforth. all inserts arc
identified by the measured value of the hone or chamfer
corresponding to each edge preparation,
TABLE 1. NOMfNAL AND MEASURED EDGE RADIl COR
EACH TYPE OF EDGE PREPARATION
I Edge Nominal Value Measured Value
i
I
Up-SharpEdge
The inserts arc low-CBX content finishing inserts of the
same grade iKennameta1 K13050 and Valenite VC722;. and
are composed of PCBN tips brazed th: a VC subst-a~.
The inserts are triangrrlar and correspond te ASSi
ciassifiaation TNGA-332 [251 with a 0.813 mm nominai
nose radius. Thcsc inserts were used with a tooi hoidcr
with a side rake angie of -5”. a back rake angic of -5”, and a
0” lead angle (Kennametal DTGNL- 164Dj.
Workpiece material
AISI 52100 steel bars nominally 25.6 mm in diameter were
cut to 0.25 m length. The bars were heat-treated to 45 and
60 HRC. However. subsequent measurements showed that
the actual hardness values were 41 F 1.0 HRC and 57 i 0.5
HRC. Henceforth. hardness values are defined by
measured values.
Hard-Turning Process
Longitudinal turning was conducted on a rigid, super-
precision slant-bed lathe (Hardinge Conquest T42SP. 1.5
HP) at a constant surface speed of 121.9 m/m& The depth
of cut was heId constant for all tests at 0.254 mm. The
workpiece bars were held in the machine with a caller to
minimize runout and maximize rigidity. The length of cut
for each test was 20.3 mm in the axial direction.
EXPERIMENTAL DESIGN
The workpiece microstructure was examined according to
the experimental design summarized in Figure 2. A total of
21 samples representing two hardness values, three feed
rates. and three types of edge geometry were used for
scanning eIectron microscope (SEM) analysis. Through-
thickness residual stress measurements were made for the.

5. M.R99-167-3
following factor combinations: (I) 57 HRC workpiece,
121.9 urn Hone, 0.15 mm/rev feed rate (2j 57 HRC
workpiece, 25.4 pm Chamfer, 0.15 mm/rev feed rate (3)
31 HRC workpiece, 121.9 *urn Hone, 0.15 mm/rev feed
rate. (4) 4 I HRC workpiece, 22.9 pm Hone. 0.15 mm/rev
feed rare.
FIGURE 2. FACTORS AND FACTOR LEVELS.
EXAMINATION OF WORKPIECE
MICROSTRUCTURE (FLOW)
Samples were prepared for microscopic analysis using
traditional metallographic techniques. Specimens were cut
with an abrasive cut-off saw under extremely mild
condilions to avoid excessive modificakion of the
workpiecc microstructure. Samples were then encapsulated
in an epoxy mold using a mounting press. The epoxy used
for this investigation (Beuhler EpometrLI) was designed to
maximize edge retention for polishing of high hardness
materials. The samples were then polished using 320 grit
paper followed by rhree successive poiycrystalline diamond
(PCDj slurries (9um, 3pm. and lprn PCD suspensions) and
Beuhler UltraPadT”, Texmet 1000TM, and Texmet 2000T”
polishing cloths, respectively. The samples were then
briefly polished using a 0.05 ym alumina slurry. The
samples were etched using a 2% Nitai soIution for
approximately 15 seconds and were viewed using the back-
scattered electron signal on a SEM (Hitachi S800 PEG),
tvhich allowed for enhanced compositional contrast at high
magnification (3000X).
MEASUREMENT OF THROUGH-THICKNESS
RESIDUAL STRESSES
The through-thickness residual stresses were measured
using x-ray diffraction at the Timken Co. The procedure
involved etching each sampIe to remose small layers of
surface material to expose interior regions of the
workpiece. Residuai stress measurements were made for
each stage of material removal and the process was
repeated until the through-thickness stress distribution was
revealed.
Residual stress measurements were made on a TEC Model
1600 goniometer using Chromium K, radiation to scan the
{211} peak of steel. A rotating anode gcncrator operating
at 35 kV and 1.5 mA was used as the x-ray source. .A
round 3 mm collimator was used to minimize divergence of
the x-ray beam. Count times of 100 seconds were used to
ensure good counting statistics and peak shapes. Finally.
the samples were scanned at tilt angles of 0”. 18.4”. 26.6”.
33.2‘. 39.2”. and 15” and biaxial Stress analysis was used Fo
calculate residual stresses from the diffraction data (Nob an
and Cohen. 1987). Layers of material were removed in
increments of 12.7 urn using eiectrolytic polishing.
RESULTS
Workpiece Mi~rQs~r~c~~re
High-magnification analysis of the workpiece
microstructure was conducted for all samples used for
surface residual stress measurements. This analysis shoas
that large edge hone tools produce plastic ilo’ of
workpiece material in the circumferential direction for both
the 41 HRC and 57 HRC workpiece materials as shown in
Figures 3 and 4. respectively.
FIGURE 3. WORKPIECE FLOW: 121.9 pm HONE, 0.10
mm/rev FEED RATE, 41 HRC WORKPIECE.
FIGURE 4. WORKPIECE FLOW: 121.9 pm HONE. 0.15
mm/rev FEED RATE, 57 HRC WORKPIECE.
Sub-surface plastic flow of workpiece material was not
observed with either the 22.9 pm hone or the 25.4 ym

6. m99-167-4
chamfer. Representative images for these cases arc given
in Figures 5 - 7. The workpiece microstructure in each
figure is largely random and shows no evidence of
significant sub-smface flow.
Note that no attempt was made in the above SEM pictures
to highlight the presence or absence of’ ‘white layers’ in the
specimen. This is because the primary emphasis of this
paper is on sub-surface rather than surface effects. It must
also be pointed out that the bright surface layer apparent in
some of the SEM micrographs shown here does no: always
imply the presence of a ‘white layer’. Instead, in some
sampies it is solely due to contrast arising from specimen
edge rounding produced during the sample preparation
process. A detailed optical microscope was also carried out
for each specimen to determine the presence or absence of
a phase change-induced white layer. In summary, optical
microscopy shows that large edge hone tools generally
produce continuous white layers while small edge hone
tools generally produce no white layers. The reader is
referred to Thiele (1998) for details.
FIGURE 5. WORKPIECE MICROSTRUCTURE: 22.9 urn
HONE, 0.10 mm/rev FEED RATE, 41 HRC WORKPIECE.
FIGURE 6. WORKPIECE MICROSTRUCTURE: 22.9 pm
HONE, 0.05 mm/rev FEED RATE, 57 HRC WORKPIECE.
FlGURE 7. WORKPIECE MICROSTRUCTURE: 25.4 ~/rn
CHAMFER, 0.15 mm/rev FEED RATE, 57 HRC
WORKPIECE.
Throuqh-Thickness Residual Stresses
Through-thickness residual stress distributions arc plotted
in Figures 8-1 I. These figures show that stresses in the
axial and hoop directions produced by large cdgc hone
tools are deeper and more compressive than stresses
produced by - sharp tools. This verifies the
relationships presented in Figures 3 - 7.
-1630
-1800
-12: PrnltrmHW I
- - t - -25 4 mm Chsmmj
-2000 1
FIGURE 8. HOOP RESIDUAL STRESS. 57 HRC
WORKPIECE, 0.15 mm/rev FEED RATE.
flow
FIGURE 9. AXIAL RESIDUAL STRESS. 57 HRC
WORKPIECE, 0.15 mm/rev FEED RATE.

7. X299-167-5
D-wth fwl
100
c .._ ,
-100 f- 20 40 ,:.I 6C 80 : 0
,' I
f -2cr_
r
g -cjoc
E
iij -4x
::
g -532
-6’3^1
-73G
-8X I J
FIGURE 70. HOOP RESIDUAL STRESS. 41 HRC
WORKPIECE. 0.15 mm/rev FEED RATE.
102/
0
-1C"
8 +--..* j
23 40 :i 60 123
2 -2co
r
y) -333
:
g 4ccI
I
‘2 -500
-630
-700
FIGURE 11. AXIAL RESIDUAL STRESS. 41 HRC
WORKPIECE, 0.15 mm/rev FEED RATE.
A valid though-thickness residual stress distribution must
satisfy force equilibrium. Examination of Figures 8-11
shows that the through-thickness residual stressesapproach
tcnsian but do not always reach tension. This is significant
because the residual stresses must become tensile at some
depth in the sample to satisfy static equilibrium. It is
assumed here that residual stresses become tensile at
increasingly larger depths. Therefore, through-thickness
measurements were not made once the distribution reached
zero residual stress. Also. because each distribution is
decaying, it is assumed that the tensile region is distributed
over an extremely large depth below the surface such that
force equilibrium is satisfied.
Comparison between Figures 3-7 and Figures S-11 shows
that samples displaying significant sub-surface flow have
larger compressive (more negative) residual stress than
samples that lack sub-surface flow. This can be explained
by considering the stress state produced by the tool as it
slides across the workpiece during cutting. This is shown
schematically in Figure 12.
T;Tl*n~y ,I
) l/--F- /i
Wcrkomee
:‘) COMPRESSiON Ej TENSION
$ b-j& jrq+s,--. +.
5:
k--S-+
y
(3) EOUILIBRIUM
s2 = s 26; T 26,
FlGURE 12. FORMATION OF MECHANICALLY-INDUCED
RESIDUAL STRESS IN MACHINING.
As shown in Figure 17, a compressive stress Geld (labeled
C) is created in the workpiece ahead of the tooi (labeled Tj
and a tensile stress field is created behind the tool. This
stress condiiion has been verified by prior research (Vu
and Matsumoto. 1990). The workciece surface clement
first experiences compressive stress and resulting
compressive plastic deformation. denoted by & in Figure
12. step 1. The clement then experiences rensile stress and
corresponding tensile plastic deformation. denoted bq’cS,in
step 2. EquiIibrium after stress cycling is based on the
equation given in step 3. If SC>?&, the unconstrained
surface element after machining bS2) wilI be smaIIei than
the interior element (I=%. To achieve geometric
compatibility and force equilibrium with the intcrinr
element. the surface element is placed in residual tension.
Conversely, if 6&T. the unconstrained surface element
will be larger than the interior element. In this case. the
surface element is placed in residual compression.
The above explanation can be extended to the effect of
edge hone on residual stress. Specifically: large edge hone
tools result in increased frictional interaction between the
tool and workpiece. This is shown schematically in Figure
13 where the interaction caused by the large edge hone too1
(L2) is larger than the interaction caused by the small edge
hone tool iL7). Machining with a large edge hone tool
causes an increase in the tensile stress field behind the tool
(T2) relative to the small edge hone tool iTI) due to the
increased interaction length. The increase in the tensile
field behind the tool produces larger compressive (more
negative) residual stresses, as shown in Figure 13.
FIGURE 13. MODIFICATION OF STRESS DlSTRlBUTlON
CAUSED BY LARGE EDGE HONE TOOL.

8. YR99-167-6
These explanations account for the increase in compressive
residual stress associated with large edge hone tools shown
in Fi,nures S-l 1. in addition, they also account for the flou
observed in Figures 3-I.. Flow resulting from machining
with large edge hone tools is a manifestation of increased
interaction along the cutting-edge and represents eiongation
oi‘ the workpiece material produced by the tensile stress
behind the tool.
CONCLUSiON
The effect of cutting-edge geometry or ‘edge preparation’
on worh+ece sub-surface deformation and through-
thickness residual stressdistribution in hard turning of AN
52100 steel was investigated in this paper. Specifically. the
effects of ‘up-sharp’, honed. and chamfered PCBN inserts
were experimentally studied.
The results show that large edge hone tools produce
measurable sub-surface plastic flow. Sub-surface flow is
not observed in the case of small edge hone or chamfered
tools. Flow of the workpiece sub-surface is associated with
deep. compressive through-thickness residual stresses.
Therefore, it is concluded that large edge hone tools
produce deeper. more compressive residual stresses than
small edge hone or chamfered tools for finish hard turning
of AISI 52100 steel.
The authors would Biketo thank Dr. Yoichi Matsumoto of
Timken Research for measurement of through-thickness
residual stresses. The authors would also like to thank
Kenneth Niebauer and Jim Kasperik of Kennametal, Inc.,
and Kenny Lanxton of Valenite. Inc.. for the donation of
CBN cutting tools. tool holders. and time and resources in
measuring the edge geometry of the inserts. Finally. Dr.
John McGraw of Carpenter Technologies is acknowledged
for providing the workpiece material used in the study.
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