Toolwear surface-qual

w
W
n
<
n
Z
Society of
Manufacturing
Engineers
2000
MROO-204
An Investigation of Tool Wear and
Surface Quality in Hard Turning
author(s)
TY G. DAWSON
THOMAS R. KURFESS
Georgia Institute of Technology
Atlanta, Georgia
abstract
This paper discusses experimental results of turning experiments on 52100 steel
hardened to 58-62 HRC. A set of five different cutting conditions was selected to
machine with five polycrystalline cubic boron nitride (PCBN) cutting tools for the
life of each tool. The objective was to determine the effect of the cutting
parameters on tool wear, changes in tool geometry, and resultant workpiece
surface quality.
conference
NAMRC XXVIII
May 24-26,200O
University of Kentucky
Lexington, Kentucky
terms
Hard Turning White Layer
CBN Surface Integrity
Tool Wear Crater Wear
Sponsored by the North American
Manufacturing Research Institution of
the Society of Manufacturing Engineers
Mllmm
One SME Drive
Dearborn. MI 48121
Phone (313) 271-1500
www.sme.org/namri
Copyright (c) 2000 Society of Manufacturing Engineers. All rights reserved.

SME TECHNICAL PAPERS
This Technical Paper may not be reproduced in whole or in part
in any form without the express written permission of the Society
of Manufacturing Engineers. By publishing this paper, SME
neither endorses any product, service or information discussed
herein, nor offers any technical advice. SME specifically
disclaims any warranty of reliability or safety of any of the
information contained herein.

AN INVESTIGATION OF TOOL WEAR AND SURFACE QUALITY
IN HARD TURNING
Ty G. Dawson
The George W. Woodruff School of Mechanical Engineering
Atlanta, GA 30332-0405
Thomas R. Kurfess
The George W. Woodruff School of Mechanical Engineering
Atlanta, GA 30332-0405
ABSTRACT
This paper discusses experimental results of turning
experiments on 52100 steel hardened to 58-62 HRC.
A set of five different cutting conditions was selected
to machine with five polyctystalline cubic boron
nitride (PCBN) cutting tools for the life of each tool.
The objective was to determine the effect of the
cutting parameters on tool wear, changes in tool
geometry, and resultant workpiece surface quality.
INTRODUCTION
Hardened steel is a material that is used extensively
for components such as gears, shafts, and bearings.
Traditionally, the process required to manufacture
such components has consisted of the following
operations: machining soft material to approximate
size and shape, heat treating the material to achieve
desired hardness, and then grinding the hardened
part to specified geometry and surface requirements.
Hard turning has the potential to replace this
sequence of operations, and thus offers
manufacturers an attractive potential replacement
operation to grinding. Some attractive features of
hard turning are increased flexibility and speed, less
expensive machine tools, decreased setup times,
and environmentally friendly machining due to the
elimination of cutting coolant for most applications.
However, there are still questions regarding the
ability of the process to generate finished surfaces
with surface roughness, residual stress profiles, and
microstructural characteristics that are comparable to
ground surfaces. Thus, more needs to be learned
about the process before hard turning is implemented
more abundantly in industry.
This research consisted of a set of machining
experiments in which hardened 52100 steel was
machined with high content CBN cutting tools. Five
tools were used to machine at five different cutting
conditions for the life of each tool. The effect of
varying cutting parameters on the rate of tool wear
was investigated, as well as changes in cutting edge
geometry resulting from crater wear. The influence of
changing conditions and cutting edge geometry on
cutting forces, surface roughness, and surface
integrity were investigated.
EXPERIMENTAL PROCEDURES
A total of six testing conditions were selected, as
shown in Table 1. However, no test passes were
made at condition #2 because the more aggressive
conditions did not result in noticeable surface
damage. Notice that condition #6 is a replication of
condition #I, but on a slightly harder workpiece. The
workpiece for conditions l-5 was hardened to 58
HRC, while the workpiece for condition #6 was
approximately 62 HRC. The cutting tool for all cutting
experiments was a high content CBN tool with a -5’
rake angle, a 0.794 mm nose radius, and a 20” edge
chamfer that was 0.102 mm wide. Cutting forces
were recorded during each test pass using a
piezoelectric dynamometer.

TABLE 1. EXPERIMENTAL CUTTING CONDITIONS
Cutting condition Radial depth of Cutting speed Feed rate
1 0.203 mm 182.9 m/min 0.152 mm/rev
2 0.203 mm 91.4 timin 0.076 mm/rev
3 0.203 mm 91.4 timin 0.152 m&rev
4 0.203 mm 1d2.9 nVmin 0.076 mm/rev
5 0.508 mm 182.9 timin 0.152 mm/rev
6 0.203 mm 182.9 timin 0.152 mm/rev
TOOL WEAR
Tool wear is an important consideration if hard
turning is to be a viable replacement for grinding
operations, particularly because cutting materials for
hard turning are relatively expensive. The
advantages of increased flexibility and reduced cycle
times could easily be offset by excessive costs due to
premature tool failure. Therefore, tool life must be
considered in addition to surface integrity and
geometric capability to determine optimal machining
conditions.
Tool wear has typically been monitored by observing
the increasingly worn flat on the flank of the tool with
an optical microscope. However, other mechanisms
of tool wear affect tool life, cutting forces, and the
quality of the machined surface. Crater wear, for
example, can dramatically change the cutting edge
geometry. This is particularly true for hard turning,
where depths of cut are small. At depths typical of
hard turning, cutting tool edge preparation has a
dramatic effect on the performance of the tool as
shown by Thiele (1998), and crater wear can
significantly change the nominal edge geometry.
Although difficult to quantify, a Zygo New View 200
microscope allowed monitoring of crater wear
progression, as shown in Figures l-3 for cutting
condition #l. These figures clearly show that cutting
took place primarily along the chamfered edge of the
cutting tool, and that the increase in the crater size
had a definite effect on the resulting cutting edge
geometry.
83.36351
urn
11.26063
0.53
0.00 mm 0.70
FIGURE 2. CRATER WEAR AFTER 10 PASSES
78.42251
urn
19.79782
0.53
0.00 mm 0.70
FIGURE 3. CRATER WEAR AFTER 20 PASSES
As mentioned, a more typical method of quantifying
tool wear has been to observe the increasing size of
a worn flat on the flank of the cutting tool. Figures 4
and 5 demonstrate this worn portion of the tool near
the cutting edge. Notice the large flat worn on the
nose of the tool after 40 passes.
0.03 mm 0.65
FIGURE 4. CU-ITING EDGE OF A NEW TOOL
0.00 mm 0.70
FIGURE 1. ClJlTING EDGE CONDITION OF NEW TOOL

3
urn
2.24317
0.53
0.00 mm 0.65
FIGURE 5. FLANK WEAR AFTER 40 PASSES
An interesting relationship was observed between the
maximum flank land and the volume of material that
had been removed with the cutting tool. This power-
law relationship is seen in Figures 6 and 7. The
relationship indicates an initial stage of rapid wear,
with a continual decrease in the wear rate. However,
this relationship was developed merely on
observation, and further testing will be performed to
confirm or refute its validity.
Condition X1 Flank Wear Progression
FIGURE 6. FLANK WEAR PROGRESS
Interestingly, Equations (1) and (2) show that the
exponent of the relationship is identical for condition
#l and condition #3, with only the coefficient varying.
This seems to indicate that the exponent is a function
of the wear mechanism between the tool and
workpiece, while the coefficient is a reflection of the
cutting condition. If the relationship holds up to
further testing, it will provide a very powerful method
for determining the condition of a cutting tool based
solely on the amount of material that the tool has
removed.
y = 34.gx0’41(Condition #l) (1)
y = 72.5x0’41(Condition #3) (2)
Because the CBN tools that are typically used for
hard turning applications are expensive, maximum
usage of the tools are required from an economic
standpoint. However, tools obviously cannot be used
until failure. Therefore, prediction of tool failure or
tool monitoring is required to allow maximum tool
usage. The tools used in these experiments failed
catastrophically with a maximum flank land in the
range of 150-200 pm, as shown in Table 2. Please
note that these numbers represent the last flank
measurements that were made prior to failure.
Measurement intervals were not close enough to
obtain a representative flank measurement for
conditions #5 and #6 just prior to failure. However,
the data for the other three conditions indicate that a
predictive model of flank wear based on material
removal would provide a very simple method for
scheduling tool changes to maximize tool life while
preventing tool failure on the machine.
TABLE 2. TOOL LIFE DATA
Condition Material Removed Flank Wear
# Volume (mmA3) VB W-N
1 8500 180
3 37600 155
4 17000 170
5 1000 60
6 5100 90
SURFACE QUALITY
Ultimately, hard turning must produce high quality
surfaces if it is to compete with grinding as a finishing
operation for hard materials. These high quality
surfaces must achieve acceptable surface finish
while avoiding significant changes in the
microstructure of the material.
FIGURE 7. FLANK WEAR PROGRESSION

4
Surface finishes were investigated for all cutting
conditions and compared to theoretical roughness
values, which are based solely on the geometry of
the tool and its feed. For finishing operations, the
centerline average roughness /?a, and peak-to-valley
roughness Rt are approximated by Equations (3) and
(4), where R is the tool corner radius and f is the tool
feed per revolution. Based on a tool corner radius of
0.794 mm, the theoretical roughness values are
shown in Table 3.
f"R, =- (3)
32.R
f'R, =- (4)
8.R
TABLE 3. THEORETICAL ROUGHNESS
CALCULATIONS
Feed (mm/rev) Ra (pm) Rt (pm)
0.1524 0.914 3.658
0.0762 0.229 0.914
Experimental roughness values were obtained by
both a Zygo New View 200 microscope and a Rank
Taylor Hobson Form Talysurf Mark I contact
profilometer. Representative finished surfaces are
shown in Figures 8 and 9, and experimental
roughness measurements are listed in Table 4.
16.20819
urn
8.28901
0.53
14.33189
urn
8.99736
0.53
0.00 mm 0.70
FIGURE 9. SURFACE GENERATED AT A FEED OF
0.076 MM
Experimental surface roughness matched theoretical
values well for higher feeds, but decreased below
theoretical values once the tool wore-yielding an
improved surface. This is attributed to an effective
flattening of the corner radius, which results from an
increase in the worn flat on the flank of the tool. The
lower feed condition produced a finish significantly
worse than expected, but this has been observed in
past research and explained as an increase in
plowing action at low feeds (Thiele 1998). Overall,
the results indicate that hard turning is capable of
producing finished surfaces acceptable for many
applications. However, there is significant deviation
from expected values at low feeds, which
unfortunately prevents achieving desired roughness
by selecting appropriate tool radius and feeds.
r I I I# I I I I I B 0.00
0.00 mm 0.70
FIGURE 8. SURFACE GENERATED AT A FEED OF
0.152 MM

TABLE 4. SURFACE ROUGHNESS MEASUREMENTS
Condition 1
CuttingPass# Ra(v) Rq (pm) Rt(pm)
10 1.03 1.19 5.06
30 0.90 1.02 3.97
50 0.94 1.09 4.47
70 0.61 0.72 3.44
Condition 3
CuttingPass# Ra(p) Rq(pm) Rt@rn)
10 0.84 0.98 4.22
30 0.80 0.96 3.94
50 0.42 0.51 2.80
80 0.42 0.51 2.49
110 0.34 0.44 2.61
140 0.44 0.55 2.95
170 0.39 0.48 2.69
190 0.33 0.43 2.50
207 0.33 0.43 2.40
Condition 4
CuttingPass# Ra(v) Rq(pm) Rt(pm)
20 0.37 0.46 2.32
40 0.38 0.47 2.38
Condition 5
CuttingPass# Ra(v) RqOIJTI)Rt(pm)
8 0.60 0.74 3.96
Condition 6
CuttingPass# Ra0.m) Rq(pm) Rt(km)
20 0.61 0.71 2.67
A final requirement on finish turned surfaces is
acceptable surface integrity-meaning that no
significant change in the microstructure is caused by
the machining process, and that no residual tensile
stresses are present at the machined surface. Many
previous researchers in hard turning have observed
thermal damage in the form of rehardened “white
layers” and overtempered subsurface layers. An
example of this damage on an EDM (electric
discharge machining) surface is shown in Figure 10.
However, no evidence of significant damage was
found in these experiments, as indicated by a
representative surface shown in Figure Il. This
does not conclude that residual stress profiles were
acceptable, as these were not investigated.
Nevertheless, the results support the claim that hard
turning is capable of producing undamaged surfaces
with roughness values acceptable for many
applications. This deviation from previous research
results can possibly be explained by the difference in
thermal conductivity of the high content CBN tool
compared to low content CBN tools typically used for
finishing operations. The increased CBN content and
cobalt binder resulted in a thermal conductivity nearly
double that of most low content CBN tools (which
typically have ceramic binder materials), leading to a
reduction in the amount of heat going into the
5
workpiece, and thus elimination of thermal damage
on the surface.
FIGURE 10. EDM SURFACE (400X)
FIGURE 11. TURNED SURFACE (400X)
CONCLUSIONS
Because cutting edge geometry has been
investigated in past research and found to affect
cutting forces, residual stresses, and surface
integrity, it was desired to show how initial edge
geometry changed during the life of a cutting tool.
These changes due to crater wear were observed
successfully for all cutting conditions. Information
about edge geometry changes should prove
important in future research where a predictive model
is developed that will require detailed edge geometry
information as a critical model input. Additionally, a
relationship was found between flank wear and the
volume of material removed. This relationship will be
investigated further, and could result in an empirical
method for monitoring tool condition and failure.
Experimental surfaces matched theoretical
roughness calculations very well at higher feeds, and
actually improved with increasing tool wear. This

6
was not the case at low feeds, where the surface was
significantly worse than theoretical values. This
difference has been explained as an increase in the
plowing action at low feed cutting conditions.
Finally, surface integrity was examined in an attempt
to understand what cutting parameters and tool
condition lead to significant thermal damage on the
surface. However, there appears to have been no
significant damage on any turned surface, even at
aggressive machining .conditions with worn cutting
tools. A possible explanation for this is a thermal
conductivity for the high CBN content tools that were
used, which was nearly double that of low CBN
content tools that have typically been used in hard
turning research due to increased tool life at finishing
conditions. Further testing will be performed to
investigate this deviation from previous research
results and determine if the conductivity of the tool is
in fact the variable allowing surfaces void of thermal
damage to be machined.
SELECTED REFERENCES
Abrao, A. M. and Aspinwall, D. K.; “The Surface
Integrity of Turned and Ground Hardened
Bearing Steel,” Wear, Vol. (196) pp. (279-284);
1996. .
Chou, Y., and Barash, M.; “Review on Hard Turning
and CBN Cutting Tools,” SME Technical Paper,
Proceedings of the Ist International Machining
and Grinding Conference, MR95-214 pp. (951-
962); 1995.
Chou, Y., and Evans, C. J.; “Microstructural Effects
in Precision Hard Turning,” ASME
Manufacturing Engineering Division, Vol. (4) pp.
(237-242); 1996.
Davies, M. A., Chou, Y., and Evans, C. J.; “On Chip
Morphology, Tool Wear and Cutting Mechanics
in Finish Hard Turning,” Anna/s of the CIRP, Vol.
(45) pp. (77-82); 1996.
Davies, M. A., Evans, C. J., and Harper, K. K.; “Chip
Segmentation in Machining AISI 52100 Steel,”
Proc. ASPE, pp. (235-238); 1995.
Konig, W., Klinger, M., and Link, R.; “Machining Hard
Materials with Geometrically Defined Cutting
Edges-Field of Applications and Limitations,”
Anna/s of the CIRP, Vol. (39) No. (1) pp. (61-
64); 1990.
Matsumoto, Y., Narutaki, N.; “Tool Workpiece
Interaction in Precision Hard Turning”, finer
Points, Vol. (8) No. (4) pp. (14-16); 1996.
Shaw, M. C.; “Chip Formation in the Machining of
Hardened Steel,” Anna/s of the CIRP, Vol. (42)
No. (1) pp. (29-33); 1993.
Thiele, Jeffrey D.; An Investigation of Surface
Generation Mechanisms for Finish Hard Turning
of A/S/ 52700 Steel; Master’s Thesis, Georgia
Institute of Technology; 1998.
Tonshoff, H. K., and Hetz, F.; “Surface Integrity of
Difficult to Machine Materials,” 2nd IMEC
Session /I, pp. (120-l 36); 1986.
Tonshoff, H. K., Wobker, H. G., and Brandt, D.; “Hard
Turning-Influence on the Workpiece
Properties,” Transactions of NAMRVSME, Vol.
(23) pp. (215-220); 1995.
Tonshoff, H. K., Wobker, H. G., and Brandt, D.; “Tool
Wear and Surface Integrity in Hard Turning,”
Production Engineering, Vol. (3) No. (1) pp. (19-
24); 1996.
Konig, W., Berktold, A., and Kock, K. F.; “Turning
versus Grinding-A comparison of Surface
Integrity Aspects and Attainable Accuracies,”
Anna/s of the CIRP, Vol. (42) No. (1) pp. (39-
43); 1993.

Toolwear surface-qual

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Toolwear surface-qual

Similar to Toolwear surface-qual (20)

More from Dave Davidson

More from Dave Davidson (20)

Recently uploaded

Recently uploaded (20)

Toolwear surface-qual