International Journal of Machine Tools & Manufacture 45 (2005) 719–726

www.elsevier.com/locate/ijmactool

Chip geometries during high-speed machining

for orthogonal cutting conditions
G. Sutter*
L.P.M.M., U.M.R. C.N.R.S. n8 7554, I.S.G.M.P., Université de Metz, Ile du Saulcy, 57045 Metz cedex 1, France
Received 21 May 2004; accepted 23 September 2004
Available online 23 November 2004

Abstract
The originality of this work consists in taking photographs of chips during the cutting process for a large range of speeds. Contrary to
methods usually used such as the quick stop in which root chips are analyzed after an abrupt interruption of the cutting, the proposed process
photographs the chip geometry during its elaboration. An original device reproducing perfectly orthogonal cutting conditions is used because
it allows a good accessibility to the zone of machining and reduces considerably the vibrations found in conventional machining tests. A large
range of cutting velocities is investigated (from 17 to 60 m/s) for a middle hard steel (French Standards XC18). The experimental measures of
the root chip geometry, more specifically the tool-chip contact length and the shear angle, are obtained from an analysis of the pictures
obtained with a numerical high-speed camera. These geometrical characteristics of chips are studied for various cutting speeds, at the three
rake angles K5, 0, C58 and for different depths of cut reaching 0.65 mm.
q 2004 Elsevier Ltd. All rights reserved.

Keywords: High-speed machining; Tool-chip contact; Orthogonal cutting; Shear angle

1. Introduction realizable without some difficulties by the study of the chips

after the end of the cutting process and it is then necessary to
Machining is a process of shaping by the removal of make hypotheses. For instance, to examine the length of
material which results in chips. The geometrical and contact between the cutting tool and the chip, or the
metallurgical characteristics of these chips are very behavior of the primary shear plane in the chip root, a
representative of the performances of the process. Indeed, snapshot of the process is desirable.
they bear witness to most of the physical and thermal With this aim, the most frequently used device is the
phenomena occurring during the machining. This explains quick stop process [1,6–15]. Those devices interrupt the
the large number of works made on chips [1–6]. However, cutting process while trying to reduce the disturbing
some important phenomena in metal cutting process are consequences on the state of the chip. The relative velocity
focused on the interaction occurring between the cutting between the cutting tool and the cutting surface must tend
edge of a tool and the workpiece. Indeed this contact zone is abruptly towards zero. This can be obtained either by
the main area of concentration of the friction and then of the accelerating the tool out of the cutting area and so from the
rise of temperature. In addition, it was established that the workpiece or by accelerating the workpiece to separate it
process of plastic deformation in the primary shear zone from the tool. Let us mention for instance Williams et al. [8]
depends upon the condition of the sliding of the chip along or Jaspers and Dautzenberg [13] who used a tool holder,
the tool face. So, it is of utmost importance to investigate which pivots about its end and rests on a brittle shear pin
closely the zone of contact along which the chip is separated during machining. With the principle of propulsion with the
explosion of powder, a projectile is fired to shear the pin.
from the remainder of the workpiece. This analysis is not
The tool is rapidly disengaged from the workpiece with a
mean acceleration of 33!104 m/s2 with experiments of
* Corresponding author. Tel.: C33 03 87 31 53 67; fax: C33 03 87 31 53 66. Williams et al. [8] and 6!105 m/s2 with experiments of
E-mail address: sutter@lpmm.univ-metz.fr. Jaspers and Dautzenberg [13].
0890-6955/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijmachtools.2004.09.018
720 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726

An other original quick-stop mechanism design by Buda Based on the work of Okoshi and Fukui [21], Bagghi and
[15] allows to break the chip root instantly. A series of Wright [22] used a transparent single crystal of sapphire in
notches along the sample edges initiate the severance of the photo elasticity studies to determine the stress boundary
chip from the workpiece. By this way, the material removal condition during machining. Steel and brass specimens were
process is effectively frozen in time and the chip-workpiece machined orthogonally at speed of up to 75 m/mn.
interface can be sectioned for detailed examination. Previously Doyle [23] used a similar tool to make direct
However, in this method the retraction of the tool observation of the chip-tool interface. However, this method
requires no negligible time. The cutting tool remains in is confined to the use of transparent tool materials and it
contact with the workpiece until contact with the chip ends. cannot be assumed that the action at the tool-chip interface
Consequently the instantaneous conditions are altered. is the same as when using different tools.
According to Jaspers and Dautzenberg [13], at a cutting To understand chip formation without need for
speed of 4 m/s, the tool travels approximately 13.3 mm hypotheses or approximations, the most faithful is video
through the workpiece material during retraction. This quick recording. As early as 1948, Field and Merchant [24]
stop device is so considerably improved but the process followed the discontinuous chip formation by shot movies
remains, however, not instantaneous. Furthermore, at higher through a microscope under orthogonal condition at
cutting speeds (in the order of 60 m/s), as those proposed extremely slow cutting speed (about 13 mm/mn). The
here, this device would allow the tool to cover the distance speed of the movie films was 24 frames per second, so
of 3 mm in the workpiece. When comparing this value to the there were between 25 and 50 frames showing the
chip thickness, in order of about a tenth of millimeter, the formation of each segment. Later, at higher cutting
performance of this modified quick stop device seems to be speed (55 m/mn), Komanduri and Brown [6] used a
unsuitable for high speed of cutting. This set-up based on high-speed movie camera (3300 fps) combined with an
the dynamics of the quick motion of the tool away from the explosive quick-stop device to examine the mechanics of
workpiece impose to neglect the course of the tool in the chip segmentation.
workpiece during its retraction as well as the phenomena The aim of this work is an experimental analysis of the
which are associated to it. In addition these methods are chip roots by using high-speed camera. Low carbon steel is
cumbersome and time consuming. Moreover adaptation and chosen as work material in the hope that continuous chips are
adjustments of these systems on conventional machine tools obtained. Although discontinuous or segmented chips are
are not always easy due to the machine design and as a result more frequent, a continuous ‘phenomenon’ is needed to
of the inertia forces generated by the moving workpiece. validate this set-up. Orthogonal cutting condition is achieved
To remedy these problems, other techniques were for a large range of velocities (up to 60 m/s) in a ballistic set-
elaborated that allow to analyze the influence of tool-chip up developed by Sutter et al. [25]. This unusual set-up, which
contact in a indirect way. Sadik and Lindström [16] and presents a high rigidity in comparison with conventional
Fang and Jawahir [17] analyze the role of tool-chip contact machine tools, allows to get cutting conditions which remain
length by using cutting tool with restricted contact length. perfectly orthogonal and quasi steady-state after a short
To analyze the stress distribution on the rake face and to transient period. Effects of cutting conditions (feed, cutting
investigate the signification of tool-chip contact area, speed, rake angle) on the chip root characteristics are studied
Takeyama and Usui [18] used also a specially designed and compared with previous results.
tool, restricting artificially the tool-chip contact area.
Similarly to the groove-type chip breaker tools, double-
rake angle tools are used by Fang [19] to develop a slip-line 2. Experimental set-up
model accounting for the tool-chip contact on the tool
secondary rake face. 2.1. Mechanical part
The most common and easy technique used by Friedman
and Lenz [20] for measuring the length contact is Experiments in manufacturing are greatly interested in
microscopic examination of the traces left by the sliding cutting speed. Yet, when speed increases, the vibrations of
of the chip on the tool face. The tool face can be covered the machine tools disturb not only the cutting process but
with marker to enhance the trace of the contact zone also the feasibility of measures. Moreover the increase in
between chips and tool. Radwan [5] measured the chip-tool the quickness of phenomena adds some difficulties such as
contact length for aluminum alloy, by measuring the scour the reduction of the measurement time duration and the
trace on the tool face using toolmaker’s microscope. A large increasing acquisition frequency. To free cutting exper-
machining time is necessary to obtained clear traces and iments from some of the constraints found in conventional
other parameters such as wear should then be taken into machining tests, a device was specially developed to
account. It should be noted that the pressure distribution on reproduce orthogonal cutting conditions (see Fig. 1) [25].
the tool rake face is not uniform along the tool-chip The larger possible range of cutting speed varies between
interface, which implies that the limits of the contact traces 10 and 120 m/s. In the presented work, the range
are not clearly defined. of investigated cutting speed is from 17 to 60 m/s.
 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726 721

Fig. 1. Experimental set-up for high-speed orthogonal cutting.

The workpiece is fixed on a projectile and its length determined by the focal of the lens. This distance of about
determines the uncut chip thickness. The projectile is 30 mm disturbs the lighting of the process. In addition, even
launched inside a tube by an air gun. A sufficient length of for continuous chips, edge effects are present. Thus an
the tube allows to achieve constant speed for the projectile. inadequate depth of field makes unworkable photographic
At the end of this tube, a second tube aligned with the first images. Increasing the depth of field requires a reduction in
one supports two symmetrically fixed tools. The specimen lens aperture and a proportional increase in the intensity of
impacts the tools and orthogonal machining occurs. The light. In other respects, higher cutting speed reduces the
second tube leads the projectile bearing the manufactured duration of the process, for example less than 300 ms at
sample into a shock absorber. Speed and acceleration are 50 m/s. The phenomenon duration time to be filmed requires
measured by a set of photo-diodes and a time counter. The a very short exposure time (of the order of a few
design of the tool holding fixture allows three rake angles: microseconds) so a significant ‘luminous intensity’ is
K5, 0, C58. A low carbon steel is chosen as work material needed, which is assured by two flashes of high power. The
in the hope that continuous chips would be produced at these two light sources are balanced to overcome shadows. This
rake angles. requires a perfect synchronization of the trigger mechanism
with the photographic recording system. Actually the times
of response of the different electronic components require a
2.2. Optical part
post-synchronization of the trigger to guarantee an optimum
intensity of light. A global synchronization of all the parts of
High rigidity and good accessibility from this device
the photographic recording process is necessary. The start of
allow to develop this photographic recording set-up. Due to
the manufacturing corresponds to the initial moment of the
high cutting speed, the exposure times are very much reduced
protocol. An early release activates the light sources and then
and need high performances camera. To make use of the
a delay drives the beginning of the video recording.
photographic recording for the measurement of the chip
According to the cutting speeds and optical control, an
characteristics, a strong magnification and a fine definition of
aperture time is defined. This time does not exceed 5 ms for
the digital matrix are necessary. A numerical high-speed
problems of neatness. The performances of this device are
camera located near the zone of manufacturing is chosen.
shown by the possibility of measuring field temperature
However, considerable attention and practice are required to
during the process [26].
obtain high quality pictures. Sufficient depth of field,
sufficient and adequate lighting (in order to limit blur) and
maximum magnification are parameters to be considered.
The optimal conditions consist in coming to a good 3. Results and discussion
arrangement of the different adjustment parameters.
In order to limit the incertitude due to the blur and to get The present work allows experiments to be performed
more details, the magnification is about !5 by means of without some of the constraints found in devices abruptly
reverse wide-angle lens. Such a magnification imposes to interrupting the cutting of a workpiece. Figs. 2–5 present
reduce the distance between the lens and the object at a value photographic recordings obtained with our device at
722 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726

Fig. 2. Real time photographs of chip formation. (a) Cutting speed VCZ25 m/s; depth of cut t1Z0.27 mm; rake angle aZ08. (b) Cutting speed VCZ25 m/s;
depth of cut t1Z0.49 mm; rake angle aZ08.

different cutting speeds and for rake angles from K5, 0, C In addition the different contrasts on the picture require
58. Four cutting speeds are used 17, 25, 40 and 60 m/s. The uncertainty percentages depending on the analyzed zone.
workpiece material is a low carbon steel to ensure a These percentages on the measurement are: 4% of chip
continuous chip in all cases. Measurements of the root chip thickness (t1 and t2), 7% of contact lengths and shear angles.
geometry are carried out with the software that drives the Error bars translate this vagueness on Figs. 7 and 8. The
camera and allows to strengthen only certain contrasts. The evolution of this contact for the different uncut chip
orientation of light sources allows to distinguish the tip of thickness is plotted in Fig. 7. These results are compared
the tool and so to confirm the depth of cut. with linear evolutions proposed by different authors
[12,27–31]. The equations of the length LC normalized by
the uncut chip thickness are resumed in Table 1.
3.1. Length of tool-chip contact
The evolutions suggested by Lee and Schaffer [28] and
Abuladze [30], are close and the results are superimposed.
The tool-chip contact length LC, as shown in Fig. 6, is an
Our experimental results are correlated with a coefficient of
important parameter in the cutting process [16,17,27]. In our
determination of 0.68 and our linear equation is:
experimental work, this length is measured on the
photographic recordings see Figs. 2–5, obtained during the
process in real time. The lack of definition due to blurring LC t
Z 1:92 2 K 0:09 (1)
of the pictures imposes uncertainties on the measurements. t1 t1

Fig. 3. Real time photographs of chip formation. (a) Cutting speed VCZ40 m/s; depth of cut t1Z0.24 mm; rake angle aZ08. (b) Cutting speed VCZ40 m/s;
depth of cut t1Z0.65 mm; rake angle aZ08.
 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726 723

Fig. 4. Real time photographs of chip formation. (a) Cutting speed VCZ17 m/s; depth of cut t1Z0.56 mm; rake angle aZC58. (b) Cutting speed VCZ25 m/s;
depth of cut t1Z0.24 mm; rake angle aZC58. (c) Cutting speed VCZ60 m/s; depth of cut t1Z0.56 mm; rake angle aZC58. (d) Cutting speed VCZ17 m/s;
depth of cut t1Z0.14 mm; rake angle aZC58.

Fig. 5. Real time photographs of chip formation. (a) Cutting speed VCZ25 m/s; depth of cut t1Z0.42 mm; rake angle aZK58. (b) Cutting speed VCZ40 m/s;
depth of cut t1Z0.34 mm; rake angle aZK58.
724 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726

Fig. 6. Geometry of orthogonal cutting.

This linear evolution Eq. (1) is close to the solution

proposed by Toropov [27] and Kato et al. [12]
and corroborates that LC tends near to zero for a decreasing
ratio t2/t1. The effect of the uncut chip thickness t1 on the
length LC (see Fig. 7) confirms that for an increasing value
of the uncut chip thickness t1, the contact length LC
increases. A similar tendency was observed by Marinov [29]
and Sadik and Lindström [16] but the linear evolution (of LC
versus t1) is not accepted unanimously. Results obtained by
these authors are plotted in comparison with our exper- Fig. 8. The uncut chip thickness t1 in function on the chip-tool contact
imental data (see Fig. 8a). For a wide range of uncut chip length LC. (a) For a cutting speed VC varying from 25 to 60 m/s. (b) For
three different cutting speeds (VCZ25, 40 and 60 m/s).
thickness the linear tendency seems to be verified. However,
for t1 of about 0.35 mm, the effect of the depth of cut seems
to be not so pronounced. The cutting speed effect on the [28] and Kato et al. [12], all these solutions seem to be
relationship between LC and t1 is showed as insignificant in acceptable approximations of the contact length evolution,
Fig. 8b for the cutting speeds from 25 to 60 m/s (for in particular for this range of uncut chip thickness.
example VCZ25 m/s; t1Z0.29 mm; LCZ0.70 mm and Due to the large dispersion in the results of LC/t1 on
VCZ60 m/s; t1Z0.29 mm; LCZ0.73 mm). Fig. 9, the influence of the rake angle varying for from K5
On the other hand, using the margin of error on the to 58 is not obvious. Indeed the respective linear equations,
results and due to the feeble variation between the models plotted in Fig. 9, prove that an increasing rake angle results
proposed by Poletika [31], Abuladze [30], Lee and shaffer in an augmentation in the slope of the linear approximation.
The tool-chip contact length decreases with an increasing
rake angle. The solution of Lee and Shaffer [28] taking into
account the rake angle sensitivity follows a like tendency.
For a rake angle of 08, Fig. 10 shows a decreasing of the
ratio t2/t1 with decreasing uncut chip thickness. For higher
depth of cut the ratio t2/t1 tends to the unit. A trend curve
plotted by a power law with a broken line in Fig. 10
illustrates this observation.

Table 1
Formulation of the non-dimensional parameter LC/t1 used in Fig. 7

Authors Equation
pffiffi
Lee and Shaffer [28] LC 2
t1 Z sin f sinð45CfKaÞ
Kato [12]; Toropov [27] LCZ2t1
 
Poletika [31] LC t2
t1 Z 2:05 t1 K 0:55
 0:1 h i
Abuladze [30] LC t2 t2 1
t1 Z t1 t1 ð1K tan aÞC cos a
Fig. 7. Solutions for the tool-chip contact length proposed by different  
Marinov [29] LC t2
authors [12,27–31], compared to the experimental results obtained in the t1 Z 1:61 t1 K 0:28
present work (rake angle aZ08).
 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726 725

Fig. 9. Evolution of the non-dimensional parameter LC/t1 for three rake

angles: (B) aZ08 (C) aZC58; (K) aZK58. Fig. 11. Effect of the depth of cut t1 on the shear angle f. Rake angle aZ08,
cutting speed VC varying from 0 to 60 m/s.
3.2. Shear angle
At zero rake angle, the shear angle depends directly on the
chip thickness ratio through the following equation:
The optical observations made possible by our set-up

allow us also to measure another important parameter that is t
the shear angle. The formation of chips involves shearing of fc Z arctan 1 (3)
t2
the workpiece material in a plane extending from the tool
edge to the position where the chip leaves the work surface. So, referring to Fig. 10, the decreasing ratio t2/t1 with
The angle formed between the direction of movement of the increasing uncut chip thickness reproduces the inverted
workpiece and the shear plane defines the shear angle f relationship on Fig. 11, plotting the shear angle against the
(cf. Fig. 6). It is agreed that the shearing action for the work depth of cut. The solid line and the broken line plotted in
material occurs in an extended area close to this plane. The Fig. 11 are power laws corresponding, respectively, to
shear angle measurements proposed here are geometrical experimental and calculated results. The shear angle
values defined previously. The experimental results of the evolution given by Merchant’s equation for steel is:
angle f in Fig. 11, are compared with the calculated shear p
2 Kl Ca
angle fc given by the usual relationship: fM Z ; l Z 0:704VCK0:248 (4)
2
t1
! Fig. 12 shows the increasing shear angle for four increasing
t2 cos a
fc Z arctan (2) cutting speeds. This law underestimates our experimental
1 K tt12 sin a results. The increase of shear angle with tool rake angle is in
accordance with the principle of minimum rate of work. The
increasing shear angle reduces the primary shear area
and thus the specific shear energy. At higher rake angle,

Fig. 10. Effect of the uncut chip thickness t1 on the normalized chip
thickness value t2/t1. Rake angle aZ08, cutting speed VC varying from 0 to Fig. 12. Evolution of the shear angle f versus the cutting speed VC. Rake
60 m/s. angle aZC5; 0; K58; uncut chip thickness t1Z0.25 mm.
726 G. Sutter / International Journal of Machine Tools & Manufacture 45 (2005) 719–726

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