j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1810–1815

journal homepage: www.elsevier.com/locate/jmatprotec

Comparison of alternative approaches of single point

incremental forming processes

A. Petek a,∗ , B. Jurisevic b , K. Kuzman a , M. Junkar a

a Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, 1000 Ljubljana, Slovenia
b Akrapovic d.d., Malo Hudo 8a, 1295 Ivancna Gorica, Slovenia

a r t i c l e i n f o a b s t r a c t

Article history: Incremental sheet metal forming is becoming an attractive technology for fast prototyping
Received 8 October 2007 and small batch production of sheet metal parts. The majority of investigations are focused
Received in revised form on the use of a rigid tool to incrementally form the sheet metal into a final shape. An inter-
11 April 2008 esting alternative is to substitute the rigid tool with a high velocity water jet (WJ). The
Accepted 20 April 2008 comparison between using a rigid tool and a WJ shows that each method has its advantages
and disadvantages. This investigation is aimed to identify the most influential parame-
ters affecting the forming process through experimental comparison of the two observed
Keywords: methods. Technological windows based on non-dimensional values and relevant process
Forming parameters like force on the rigid tool and water pressure were defined.
Water jet machining © 2008 Elsevier B.V. All rights reserved.
Technological windows

1. Introduction by Iseki (2001) and Jurisevic et al. (2003) instead of a rigid tool
have been considered as well.
Modern production requires manufacturing processes, which The aim of this investigation is to compare rigid tool sin-
are flexible and adaptable in order to fulfil the requirements of gle point incremental forming (RTSPIF) and water jet single
new products. An important aspect of developing new prod- point incremental forming (WJSPIF). In case of RTSPIF the main
ucts is the possibility to produce functional prototypes in a tool is a rod-shaped punch with a smooth hemispherical head,
time and cost effective way. In case of sheet metal products, while in case of WJSPIF the main tool is a high velocity WJ as
incremental sheet metal forming represents a reliable fab- shown in Fig. 1.
rication method for prototypes and small batch production Both processes are compared by means of technological
as described by Jeswiet et al. (2005). Incremental sheet metal windows, which show the optimal operating area depending
forming is a process in which a simple geometry tool moves on workpiece material properties and part geometry require-
along an arbitrary trajectory over a sheet metal workpiece. In ments.
this way locally controlled plastic deformations are inserted A similar comparison has been carried out by Jurisevic et
until the target geometry is achieved. The majority of investi- al. (2004), in which both processes were evaluated qualita-
gations in this field are focused on the use of a rigid tool, as for tively, without any experimental verification. The results of
instance the investigations made by Jeswiet et al. (2005) and that study showed some advantages of using a WJ as the main
Petek et al. (2005). Nevertheless, alternative principles like the tool. The tool workpiece interface conditions, equipment costs
use of a laser described by Geiger and Vollertsen (1993) and and process flexibility are better comparing to the case when
Kim and Na (2003) or a high velocity water jet (WJ) reported a rigid tool is applied. On the other hand the use of a rigid tool

∗
Corresponding author. Tel.: +386 1 4771 736; fax: +386 1 2518 567.
E-mail addresses: ales.petek@fs.uni-lj.si, marta.ilesic@fs.uni-lj.si (A. Petek).
0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2008.04.033
 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1810–1815 1811

Fig. 1 – The main tool in RTSPIF and WJSPIF.

allows better forming accuracy and reduces the machining 2.1. Experimental setup for RTSPIF
time.
Experimental work in case of RTSPIF has been executed on
a CNC milling machine using specially adapted tooling. In
2. Principles of RTSPIF and WJSPIF
this process the rod-shaped punch with a smooth hemi-
processes
spherical head of diameter (dRT ) 1.6 mm is clamped into the
spindle of the milling machine. Because of superior tribo-
In the presented research two relatively new and innovative
logical properties the rotation of the spindle is taken into
metal forming technologies, namely RTSPIF and WJSPIF are
account (rotation speed, vR = 30 rpm). The metal plate is fixed
observed and compared. Both methods are capable of forming
and positioned on a specially designed blank holder, which is
sheet metal parts of complex unsymmetrical shape without
placed into the worktable of the milling machine. The CNC
the need of a dedicated die.
milling machine Mori Seiki with the FANUC MSC-521 con-
The main difference between RTSPIF and WJSPIF is in the
trol system for three-axis positioning and linear interpolation
forming mechanisms. During RTSPIF the position of the tool
as well as two axis circular interpolation has been used. The
against the workpiece is defined with the process kinematics.
blank holder remains in place during the entire forming pro-
Therefore, the force acting on the workpiece depends on the
cess in which the punch presses and deforms the plate by
tool path and is the result of the forming process itself. In case
inserting locally controlled plastic deformations. The tool path
of WJSPIF the situation is inverted. The available force of the
strategy is defined as increments in horizontal and vertical
tool (a high velocity WJ) is defined mainly with the water pres-
direction y and z, respectively. Phases of single point incre-
sure and nozzle characteristics (diameter and type). Therefore,
mental forming with rigid tool are shown in Fig. 2. In order to
the forming force is not a consequence of the process but is
avoid any undesired issues arising from friction between the
rather defined by the operator through the process parame-
tool and the workpiece the SYLAC Al 46 lubricant oil was used.
ters. The working principles and the most relevant process
parameters of RTSPIF and WJSPIF are presented in Fig. 2.
2.2. Experimental setup for WJSPIF

Experimental work concerning WJSPIF has been carried out

on a specially developed machine consisting of three main
components: high-pressure unit, working table and control-
ling system. The main component of the high-pressure unit
is the pump itself, a three plunger design, which is capable of
reaching water pressures up to 20 MPa and water volume flow
up to 50 l/min. Besides the pump the unit is fitted with a fil-
tering system and a 500 l water tank, which enables to reuse
the same water. During the forming process the workpiece
is placed into the working table inside the same workpiece
holder used in RTSPIF, while the forming head moves along a
predefined arbitrary trajectory. The forming head contains a
water nozzle through which the WJ is generated as the main
tool. During the investigation the forming head was positioned
Fig. 2 – Working principles and relevant process 20 mm above the workpiece, presented as the standoff dis-
parameters of RTSPIF and WJSPIF. tance (hS0 ) in Fig. 2. Process kinematics is controlled with a
1812 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1810–1815

Table 1 – Material properties of aluminium alloy 4. Results

Strength coefficient, C (MPa) 139.12
Strain hardening exponent, n 0.025 In RTSPIF the predetermined tool path of the rigid tool defines
Material anisotropy, r0 , r45 , r90 0.25, 0.62, 0.57 the forming process in vertical direction. On the other hand
Young’s modulus of elasticity, E (GPa) 70
in WJSPIF is still almost impossible to control the displace-
Density,  (kg/m3 ) 2750
ment of the workpiece material in vertical direction. In this
Initial workpiece thickness, t0 (mm) 0.23
Tensile strength, Rm (N/mm2 ) 126 case the forming tool is not rigid and the energy needed for
Yield strength, Rp (N/mm2 ) 120 the forming process depends on the water pressure, nozzle
diameter and horizontal movement, what also influence the
size of pyramid wall angle through the resulting force of the
PC using an Enhanced Machining Controller (EMC) software WJ. Therefore, the investigation was divided in two sections.
based on the G-code. In the first section the pyramid geometry were formed using
WJSPIF, where the horizontal step size was varied. This results
3. Experimental procedure in achieving different pyramid wall angles. The biggest wall
angles achieved in WJSPIF were used as the input in defining
the plan of experiments for RTSPIF, where comparable process
The material used in experiments which were repeated three times
was a 0.23-mm thick aluminium alloy plate of size 125 mm × 125 mm. parameters adapted to RTSPIF were applied as listed in Table 2.
Material properties of aluminium alloy obtained by a uniaxial tensile
test are shown in Table 1.
In order to compare the observed forming processes, the same tool
4.1. Influence of the wall angle and horizontal step on
diameter (dRT = dWJ = 1.6 mm) and workpiece holder were used in both forming
cases. During this investigation, a simple pyramidical shape has been
formed. The final geometry depends on the applied process parame- The horizontal step size and wall angle are included in this
ters. However, the base of the pyramid had a square side of 40 mm (A) contribution since they present two of the most influential
and the top of 3 mm (a) as shown in Fig. 3. process parameters affecting the forming limits chosen on
During this investigation the forming time, forming accuracy and the basis of the previous investigations made by Petek et al.
forming limits, have been observed, since they are very important (2005), where only the deformation or displacement in the
for choosing the appropriate method, equipment and optimal process
perpendicular direction to the tool motion were investigated
parameters.
using graphometric analysis. To determine the influence of the
Three of the most influential process parameters affecting the
horizontal step size (y) and the wall angle (˛) on the form-
forming time and forming accuracy are selected on the basis of the pre-
vious investigations made by Petek et al. (2005) and Petek and Kuzman
ing process, four different horizontal steps were observed in
(2006) analysing the influence of various process parameters on the WJSPIF, namely 0.4, 0.8, 1.2 and 1.6 mm and three horizontal
forming forces and deformations and Jurisevic et al. (2004) investigat- steps were observed in RTSPIF (0.2, 0.4 and 0.8 mm). Other pro-
ing the RTSPIF and WJSPIF process comparison aspects include forming cess parameters were kept unchanged according to the values
time, forming accuracy, surface finish, strain distribution, flexibility, listed in Table 2. In case of WJSPIF the size of horizontal step
tooling needed and energy consumption on the technology level from has a stronger influence on the wall angle compared to RTSPIF.
part design to part production, but without any experimental verifica- If the horizontal step increases the wall angle decreases as
tion. However, those three parameters are the traverse rate, horizontal shown in Fig. 4. The obtained results indicate that fracture
step size and wall angle by RTSPIF and the traverse rate, water pres-
occurs when the horizontal step is smaller than 0.8 mm, what
sure and horizontal step size by WJSPIF. The above-mentioned process
corresponds to the maximal wall angle of approximately 26◦
parameters also influence the forming limits, especially the horizontal
for the applied workpiece material in this research. In case
step size and wall angle. Accordingly, the forming limit will be deter-
mined for both technologies taking into account these two. of smaller horizontal step the WJ passes several times over
Table 2 shows the selected process parameters for RTSPIF and the same area of the workpiece what leads to the excessive
WJSPIF, which are based on preliminary tests and available literature. material hardening and later on to fracture occurrence. A pos-
During this study only horizontal step size was varied in WJSPIF, while sibility to enlarge the slope of the pyramid is to use higher
horizontal step size and wall angle were varied in RTSPIF. The other water pressure. Nevertheless, this is possible up to a thresh-
process parameters from Table 2 were kept unchanged. old value, where erosion starts to appear on the workpiece

Fig. 3 – Observed tool trajectory and a formed feature during the experiments.
 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1810–1815 1813

Table 2 – Applied process parameters

RTSPIF WJSPIF
◦ ◦ ◦ ◦
Wall angle, ˛ 26 , 44 , 50 , 60 Defined with pW and y
Rotation speed, vR (rpm) 30 –
Punch diameter (dRT )/water jet diameter (dWJ ) (mm) 1.6 1.6
Traverse rate, vT (mm/min) 300 300
Horizontal step size, y (mm) 0.2, 0.4, 0.8 0.4, 0.8, 1.2, 1.6
Lubricant SYLAC Al 46 –
Standoff distance, hS0 (mm) – 20
Water pressure, pW (MPa) – 10

In case of RTSPIF the forming force is directly related to

the process parameters. However, the forming force is a con-
sequence of the forming process itself and enables to detect
the beginning of deformation and fracture area as reported by
Petek et al. (2005). Hence, it follows that it could be useful to
define the technological window for RTSPIF according to the
relative tool diameter () and forming force in vertical direction
(FZ = FRT ), as shown in Fig. 5.
The shape of the technological window presented in Fig. 5
is defined on the basis of numerous experimental results for
three various materials (Al3003, t0 = 1.23 mm; DC05, t0 = 1 mm;
aluminium alloy presented in Section 3). With the variation of
t0 and dRT the various non-dimensional values  and form-
ing forces are observed and inserted into the diagram. In
general, technological window presents two different areas,
which are limited with the specimen fracture and area of no
Fig. 4 – Forming limits for RTSPIF and WJSPIF depending on plastic deformation on the specimen, respectively. The first
the horizontal step (y) and wall angle (˛). one describes the area where the sheet thickness (t0 ) is con-
stant and the second one presents the area where the tool
diameter (dRT ) is constant. It is worth pointing out that with
surface. In case of RTSPIF higher slope of the pyramid (˛ = 44◦ ) the proportional enlargement or reduction of the values t0 and
could be reached before fracture. This could be attributed to dRT (in order to keep the relative tool diameter  constant) the
better energy efficiency of the rigid tool. However, the maximal areas move up and down, as schematically presented with
wall angle obtained with rigid tool applying the horizontal step arrows in Fig. 5. Finally, regarding to selected relative tool
size y = 0.2 mm is 60◦ . On the other hand, larger horizontal diameter () the cross-section of the areas defines the techno-
step sizes in both processes have an influence on the appear- logical window for RTSPIF, in which the part could be formed.
ance of tool traces as can be observed in Fig. 4 and are usually In order to evaluate the influence of various process parame-
unacceptable. ters in case of both presented processes it is very convenient to
observe the formability as a main criteria, which is defined as
the case when plastic deformations are inserted into the work-
5. Definition of technological windows for
RTSPIF and WJSPIF

The determination of technological windows is very important

especially in cases when the definition of safety forming area
is required as function of the most relevant process param-
eters. Therefore, it is very convenient to define the relative
tool diameter (), as a non-dimensional value, which includes
the characteristic of the main tool and specimen thickness.
In this way the size effect of the observed process is taken
into account. The relative tool diameter is defined as the ratio
between the forming tool diameter (water jet, dWJ ; rigid tool,
dRT ) and the sheet thickness (t0 ) as defined with the following
equation:

dWJ dRT
= = (1)
t0 t0 Fig. 5 – Technological window defined for RTSPIF.
1814 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1810–1815

the interface between the WJ and the workpiece exceeds the

threshold value defined with the workpiece material proper-
ties. Furthermore, this upper limit does not depend on the
sheet thickness. The other limit, which defines the area where
no plastic deformation are inserted in the workpiece material
depend on the workpiece material properties and its thick-
ness.
Both technological windows presented in Figs. 5 and 6 are
generic without any values on the axis, because actual val-
ues depend on the workpiece material properties. The aim
of showing those two technological windows is to show the
influence of process parameters to the forming process. Once
that all mechanisms of both compared processes will be bet-
ter understand, technological windows for various workpiece
Fig. 6 – Technological window for WJSPIF. material will be accurately defined, what will help the operator
to set optimal process parameters.
Nevertheless, it is very difficult to predict a process in
piece material without allowing defects like cracks, wrinkling, which the tool attributes are not completely known and under-
surface damage, etc. The optimal formability is obtained when stand. In case of WJSPIF the tool is a high velocity WJ, which
the relative tool diameter is at its optimal value (OPT = 10). This has been experimentally and numerically observed in pre-
value is defined on the basis of recommended tool diameter, vious investigations made by Jurisevic et al. (2006b). The
which is defined according to previous experience reported by numerical simulation is based on the continuity equation
Matsubara (2001) and graphometric analysis based on max- and Navier–Stokes equation for uncompressible fluid flow as
imal principle strains measuring, whereas tool diameter and shown in the following equations, respectively:
material thickness were changed according to design of exper-
iments. The technological window for RTSPIF, shown in Fig. 5, ( · v̄i )i = 0 (2)
presents the technological window in case of optimal forma-
bility. This information can be used as a reference for setting 0 · v̄˙ i = 0 · fi − p̄i +  · vi,jj − 0 · (vi · vj ) (3)
j
optimal process parameter (dRT , t0 and other process parame-
ters).
Simulation results are in good agreement with the experi-
The technological window is also limited with the highest
ments, what represents a useful tool in future developments
value of the relative tool diameter (max ). This value depends
in this field.
on the size of the part, sheet thickness and tool diameter
and usually differs from case to case. However, it defines
the boundary between incremental forming, where the tool 6. Conclusions
deforms the sheet metal with locally controlled plastic defor-
mations, and other forming processes like deep drawing, The presented work is based on experimental observation
stretching, etc. of two incremental sheet metal forming processes, where
Besides all mentioned, the position and size of the tech- the main difference is in the utilized tool. In one case the
nological window (when t0 = constant and dRT = constant) main tool is a rod-shaped punch with a smooth hemispherical
depends also on the type of the workpiece material. As for head, what is addressed as RTSPIF. This process is compared
instance, high strength steel requires higher forming forces in with WJISMF, where the main tool is a high velocity WJ. Both
contrast to aluminium alloys. This influences the vertical posi- processes are characterized with technological windows that
tion of the technological window for RTSPIF. On the other hand show the optimal operational area according to the workpiece
some types of aluminium alloys have better incremental form- properties (sheet thickness and material properties).
ing ability compared to high strength steel, what influences Experimental work has been carried out on both observed
the size of the technological window. processes. In this respect forming of a simple pyramidi-
In order to better evaluate the WJSPIF process, techno- cal shape out of 0.23 mm thick aluminium sheet has been
logical windows have been introduced in this case as well observed. The diagram in Fig. 4 shows the forming results as a
(Jurisevic et al., 2006a). The influence of the water pressure function of the horizontal step and wall angle. It is very inter-
(pW ) as the most relevant process parameter on the forma- esting to observe, that the two processes are complementary
bility is observed through the relative jet diameter (). The according to the optimal range of horizontal step and wall
technological window for WJSPIF shown in Fig. 6 is like the angle. From the investigations made it could be concluded
one for RTSPIF limited by two areas, one the top where dam- that, RTSPIF is more appropriate in cases of bigger wall angles
age occurs on the workpiece and the other on the bottom and smaller horizontal steps, while WJSPIF performs better at
where no plastic deformation is inserted in the workpiece larger horizontal steps and smaller wall angles.
material. A major difference between RTSPIF and WJSPIF is in the
As shown in Fig. 6 the technological window is defined on process controlling principle. In RTSPIF the main process
the top by the water pressure at which erosion starts to take parameter is the tool kinematics, which defines the loads
place on the workpiece surface. In this case the pressure at acting on the workpiece. If they exceed the threshold value,
 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1810–1815 1815

fracture of the workpiece occurs. During WJSPIF the main pro- Jurisevic, B., Junkar, M., Jadhav, S., Kleiner, M., Kuzman, K., 2004.
cess parameter is the water pressure, which defines the loads Incremental sheet metal forming process with a water jet and
on the workpiece. The standoff distance has also an influence rigid tool. In: Proceedings of the 17th International
Conference on Water Jetting - BHR, pp. 71–81.
on the WJ attributes at the interface with the workpiece. Ongo-
Jurisevic, B., Kuzman, K., Junkar, M., 2006a. Water jetting
ing investigations show that the water pressure is far more technology: an alternative in incremental sheet metal
relevant to the process outcome. forming. Int. J. Adv. Manuf. Technol. 31 (1–2),
Observations from previous comparison study between 18–23.
RTSPIF and WJSPIF reported by Jurisevic et al. (2004) have Jurisevic, B., Sajn, V., Junkar, M., Kosel, F., 2006b. Experimental
been confirmed. RTSPIF enables higher process accuracy and and numerical study of the tool in water jet incremental sheet
shorter machining time compared to WJSPIF. metal forming. In: Proceedings of the 6th International
Conference on Integrated Design and Manufacturing in
Mechanical Engineering, 11 pp.
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