Incremental Sheet Forming (ISF)

Yogesh Kumar and Santosh Kumar

Abstract Incremental sheet forming (ISF) process is an advanced flexible manu-

facturing process to produce complex 3D products. In conventional forming
process, the products are produced by dedicated tools, i.e. die and punch. However,
ISF does not require any dedicated tool. A laboratory setup of single-point incre-
mental forming machine has been developed using a motion card to control the 3
servo motors for controlling individual 3-axis and spindle movement at IIT (BHU),
Varanasi. The strain distribution on the sheet over the length of deformation has
been computed. The surface quality of the products is found to be good. Simple
simulation has also been carried out.

Keywords Metal forming
Incremental forming
Dieless forming
Deforming

tool

1 Introduction

Metal forming is the backbone of modern manufacturing industry besides being a

major industry in itself. Throughout the world, hundreds of million tons of metals
go through metal forming processes every year. As much as 1520 % GDP of
industrialized nations comes from metal forming industry. Besides, it fullls a
social cause by providing job opportunities to the millions of workers. Metal
forming industry, in general, is a bulk producer of semi-nished and nished goods

Y. Kumar (&)
S. Kumar (&)

Mechanical Engineering Department, Indian Institute of Technology (BHU),
Varanasi, Uttar Pradesh 221005, India
e-mail: yogeshiitbhu@gmail.com
S. Kumar
e-mail: santoshkr.mec@gmail.com

Springer India 2015 29

R.G. Narayanan and U.S. Dixit (eds.), Advances in Material Forming and Joining,
Topics in Mining, Metallurgy and Materials Engineering,
DOI 10.1007/978-81-322-2355-9_2
30 Y. Kumar and S. Kumar

and this is one reason that it is viable to undertake large-scale research and
development projects because even a small saving per ton adds up to huge sums
(Juneja 2013).
The incremental forming process is originated from stretch forming and metal
spinning process. The ISF has been originated with partial hybridization of stretch
forming process and metal spinning processes. Thus, ISF process has combined
advantages of stretch forming processes and metal spinning process. In the later
developments in ISF process, the process is found to be capable of producing 3D
complex shapes with multiple features on it.

1.1 Elements of Incremental Forming Process

Incremental forming is a new technique for deforming sheet metals by the appli-
cation of step-by-step incremental feed to a deforming tool (DT). For the production
of parts by conventional sheet forming techniques, dedicated tools are required. The
dedicated tools are complex 3D design and thus are expensive. The design of
tooling (die and punch) for complicated shapes is very difcult and costly.
In incremental forming technique, only a DT is required for deforming the sheet
metals. There are four basic elements of an ISF process as shown in Fig. 1a sheet
metal, a blank holder, a single-point forming tool or DT, and CNC machine.
The shape of DT may be hemispherical. The design of DT is quite simple and
economical. A CNC controller having capability for controlling individual 3 axes
can be used to control the movements of the DT. The tool path needs to be
optimized and a free run is taken before performing the actual manufacturing on the
sheet metal in order to ensure the proper functioning of the machine.

Fig. 1 Basic elements of ISF

Incremental Sheet Forming (ISF) 31

Fig. 2 Single-point
incremental forming (SPIF)

1.1.1 Classication of Incremental Sheet Forming

The Incremental forming processes are broadly classied into two categories:
(i) Conventional Incremental Sheet Forming (CISF):
In CISF process, generally a sheet of metal is deformed by progressive and
localized plastic deformation using a simple hemispherical/ballpoint tool, and
this path of the DT is controlled by a CNC machine. The DT moves over the
surface of the sheet and results the nal shape. There is no other tool or
external pressure applied for deforming the sheets into the desired shape. The
conventional incremental forming process can be further classied as follows:
(a) Single-Point Incremental Forming (SPIF) also known as Negative
Dieless Forming:
In SPIF, only one tool moves over the surface of the sheet as shown in
Fig. 2.
(b) Two-Point Incremental Forming (TPIF) or Positive Dieless Forming:
In TPIF, two tools, one called DT and another one supporting tool, move
over the surface of the sheet as shown in Fig. 3.
(ii) Hybrid Incremental Sheet Forming (HISF):
HISF processes are the modied forms of conventional incremental forming.
In these processes, DT moves over the surface of sheet metals, while the
another side of surface of sheet metals is supported by pressurized hydraulic
fluid, partial die or full die to get the desire shape and size. Hybrid incremental
forming processes are further classied as follows:
(a) SPIF with Hydraulic Fluid also known as Single-Point Incremental
Hydro-Forming:
In this type of hybrid incremental forming process is different from
conventional SPIF process that a single tool moves over one side of the
surface of the sheet metals and other side of surface of sheet metals is
supported by the pressurized hydraulic fluid as shown in Fig. 4.
32 Y. Kumar and S. Kumar

(b) TPIF with Partial Die:

In this type of hybrid incremental forming process, a single tool moves
over one side of the surface of the sheet metals and other side of surface
of sheet metals is supported by a partial die to get the desire impression as
shown in Fig. 5.
(c) TPIF with Full Die:
In this type of hybrid incremental forming process, a single tool moves
over one side of the surface of the sheet metals and other side of surface
of sheet metals is supported by a full die to get the desire shape and size
as shown in Fig. 6.

Fig. 3 Two-point
incremental forming (TPIF)

Fig. 4 Single-point
incremental hydro-forming
(SPIHF)
Incremental Sheet Forming (ISF) 33

Fig. 5 Two-point
incremental forming with
partial die (TPIFPD)

Fig. 6 Two-point
incremental forming with full
die (TPIFFD)

1.1.2 Advantages and Disadvantages of ISF

The advantages and disadvantages of SPIF are as follows

A. Advantages:
Useable parts can be formed directly from CAD data with a minimum of spe-
cialized tooling. These can be either rapid prototypes or small volume produc-
tion runs.
The process does not require either positive or negative dies; hence, it is dieless.
However, it does need a backing plate to create a clear change of angle at the
sheet surface.
Changes in part design sizes can be easily and quickly accommodated, giving a
high degree of flexibility.
Making metal rapid prototypes is normally difcult, but easy with this process.
The small plastic zone and incremental nature of the process contribute to
increased formability, making it easier to deform low formability sheet.
34 Y. Kumar and S. Kumar

A conventional CNC milling machine or lathe can be used for this process.
The size of the part is limited only by the size of the machine. Forces do not
increase because the contact zone and incremental step size remain small.
The surface nish of the part can be improved.
The operation is quiet and relatively noise free.
B. Disadvantages:
The major disadvantage is the forming time is much longer than competitive
processes such as deep drawing.
As a result, the process is limited to small-size batch production.
The forming of right angles cannot be done in one step, but requires a multi-step
process.

1.1.3 Applications of ISF

There are a number of areas, where high precision of the products is required for the
accuracy of the performance. Areas of products can manufacture by ISF are as
follows:
Aerospace Industry: Instrument panel, body panel, passenger seat cover, etc.;
Automobile: Door inner/outer panel, hood panel, engine cover, etc.;
High customized products: Denture plate, ankle support, metal helmet, etc.;
Cellular phones;
IC lead frames;
Electronics;
Health care;
Miniature fasteners;
Hard disk drives;
Products of national security and defense;
Automobiles; and
Sensors.

1.1.4 Basic Terminology in Incremental Sheet Forming

The typical terminology in ISF can be understood from the terminologies used in
the incremental sheet hydro-forming (ISHF) as shown in Fig. 7, with notations
below:
Incremental Sheet Forming (ISF) 35

Fig. 7 Basic terminology in deformed part in ISHF

ti Initial sheet thickness:

tf Final sheet thickness:
Dz Incremental step  down size:
u Draw angle or forming angle:

The incremental step-down size (step size, z) affects the machine time and the
surface quality. Feed rate is the speed the forming tool moves around the mill bed.
The angle between the un-deformed sheet metal and the deformed sheet metal is
dened as forming angle () as shown in Fig. 7. The forming angle can be used as a
measure of material formability. The maximum angle (max) is the greatest angle
formed in a shape without any failures. The forming angle is set within CAD
software (Ham and Jeswiet 2007).

1.2 Literature Review

Single-point incremental forming (SPIF) is a sheet metal forming process that

allows manufacturing components without development of complex tools in
comparison with stamping process (Thibaud et al. 2012). ISF process depends
strongly on the forming tool path which influences greatly the part geometry and
sheet thickness distribution (Azaouzi and Lebaal 2012).
There are several issues that need separate discussion as peoples around the
world are trying to optimize the ISF process the governing process parameters. The
concerned issues (formability, surface quality, geometric accuracy, forming forces,
etc.) are discussed briefly.
36 Y. Kumar and S. Kumar

Kim and Park (2002) identied that the deformation pattern in ISF is very similar
to spinning process and the sheet metals deform by shear-dominant deformation
like shear spinning. They also found that the formability is improved when a ball
tool of a particular size is used with a small feed rate and a little friction. The
formability differs according to the direction of the tool movement because of the
plane anisotropy (Kim and Park 2002). Several investigations have been done with
emphasis on assessing and improving the formability in this forming method. Kim
and Yang proposed the double-forming technique to improve formability, assuming
that only shear deformation occurs in the material (Kim and Yang 2000). The ball
tool is more effective than the hemispherical head tool in terms of formability. Little
friction at the tool and sheet interface helps to improve the formability. Also, the
formability increases as the feed rate decreases (Kim and Park 2002).
Filice et al. proposed the two characteristics of deformation in this forming
method. One is the deformation pattern. While the tool moves straight on a hori-
zontal plane, the deformation that occurs at the starting and ending points of the
straight line is biaxial stretching. The deformation that occurs between these points
is plane-strain stretching. As the curvature of the tool movement increases, the
deformation turns more into biaxial stretching. The other characteristic is the
formability of the deformation. As shown in Fig. 8, the forming limit curve, which
depicts the formability in the major and minor strain space, is expressed as a straight
line with a negative slope. Especially, for an aluminum sheet, the formability can be
quantied as a scalar number of (major + minor). It is noted that formability is the
greatest under plane-strain stretching, during which the minor strain is zero.
Therefore, a greater deformation of a sheet metal can be achieved in the ISF (Kim
and Park 2002). The ISF is characterized by a local stretching deformation
mechanics which determines a forming limit curve quite different from the

Fig. 8 A comparative plot of

FLC in conventional forming
and incremental forming
Incremental Sheet Forming (ISF) 37

traditional one and such FLC has a linear shape with a negative slope in the positive
minor side of FLD (Filice et al. 2002).
The depth and diameter have no effect the likelihood of forming a part. Also, the
material thickness, tool size, and the interaction between material thickness and tool
size have a signicant effect on maximum forming angle (Ham and Jeswiet 2006).
The ball tool is more effective than the hemispherical head tool in terms of form-
ability. The little friction at tool/sheet interface helps to improve the formability.
They also found that formability increases with decreasing feed rate (Kim and Park
2002). The formability is enhanced while deforming the metal sheets in warm
conditions, and the role of tool diameter is negligible as compared to the influence
of temperature and tool depth (Ambrogio et al. 2008). The deformation mechanisms
of both SPIF and TPIF are increasing stretching and shear in the radialaxial plane
(perpendicular to the tool direction) and shear in the tool direction (Jackson and
Allwood 2009). Hussain et al. (2007) found that the formability increases as the
radius of curvature decreases. Ham and Jeswiet (2006) formalized the two designs
of experiments for the forming parameters critical in SPIF and the degree to which
they affect formability.
Ambrigio et al. investigate that in hot ISF, the quality of bottom surface is better
as compared to the one in contact with the punch. This is due both the lower
temperature reached on the bottom side that reduces oxidation phenomenon and the
absence of mechanical actions on the sheet. Anyway because of low sheet thick-
ness, the thermal gradient is very low and this reduces the difference between the
two sheet sides (Ambrogio et al. 2012).
Attananio et al. optimized of the tool path in two-point sheet incremental
forming, with a full die in a particular asymmetric sheet incremental forming
conguration. They carried out the experimental evaluation of the tool path, which
is able to reproduce an automotive component with the best dimensional accuracy,
the best surface quality, and the lowest sheet thinning (Hussain et al. 2007).
The geometries produced by ISF represent some errors along the oblique walls.
In particular, a sort of distortion is also obtained, generating a curvature on the
expected straight sides. This phenomenon is due to elastic spring back whose effect
is lower in correspondence of the edges, where the geometrical stiffness is higher
than in other areas (Cerro et al. 2006; Filice et al. 2002; Ham and Jeswiet 2007).
The results obtained in geometric accuracy measurements with the process model,
as compared to experimental results obtained by testing in the CNC machine, are
approximately same (Cerro et al. 2006). Azaouzi and Lebaal (2012) gave optimal
solution provided an improvement of about 7 % regarding the sheet thickness
distribution at the maximum forming depth.
Azauzi et al. (2012) found that the forming forces depend largely on the proper
design of the tool path. The forming force is slightly lower than the experimental
values, but results are very good (Cerro et al. 2006). Forming forces obtained by
numerical simulation show good correlation with measured values. However, it was
a slight underestimation of the axial forces during thinning. It is assumed the
influence of grain size and softening on the material behavior (Thibaud et al. 2012).
38 Y. Kumar and S. Kumar

Araghi et al. (2009) investigated that the thinning in ISF depends on the wall
angle and is given by the sine law t1 = t0 sin (90 ). Cerro et al. (2006)
investigated that the results obtained in thickness measurements with the process
model, in comparison with experimental results obtained by testing in the CNC
machine, are approximately same. Increasing stretching and shear perpendicular to
the tool direction account for differences between the sine law and measured wall
thickness for SPIF and TPIF (Jackson and Allwood 2009). The prediction of
thickness distribution is close to that obtained on the real part (Thibaud et al. 2012).
The literature review reveals that ISF method is well investigated, but there are
still some issues not well understood in incremental forming like strain distribution
on the sheet over the length of deformation, effect of step size, tool size, etc., in ISF.

2 Simulation of Single-Point Incremental Forming Process

In deforming sheets by incremental forming, methodology of deformation plays an

important role. The current literature survey in the area of ISF has been kept into
consideration in order to adopt a suitable forming methodology. To carry out a
dedicated simulation study of the process, a blank holding arrangement for ISF has
been prepared as shown in Fig. 9, using DEFORM 3D.
For the successful implementation of ISF, a proper forming methodology has to
be followed as shown in Fig. 10. The rst step in ISF is to identify need for the
components. As soon as the need analysis is done, the geometrical dimensions of
the product to be manufactured are decided. Based on geometrical dimensions, a 3D

Fig. 9 Blank and blank holding arrangement

Incremental Sheet Forming (ISF) 39

Fig. 10 Forming methodology of 3D components by ISF

CAD model of the product is developed using CAD/CAM package. Moving for-
ward with the help of CAD/CAM package, the tool path suitable for movement of
the tool is generated. In the next step, the workpiece is held in the proper blank
holding, and test pieces are produced using the generated tool path. The accuracy of
test pieces is tested against the desired prole.

Fig. 11 CAD model of single-point ISF machine

40 Y. Kumar and S. Kumar

The metals as well as alloys can be used for the implementation of ISF process.
For the current investigations, the initial blank of material brass is selected for nding
the capabilities of ISF process brass has good formability at room temperature. The
sheet having dimensions 80 mm 80 mm 5 mm has been used for current case
study. A CAD model of the single-point ISF can is represented in Fig. 11.

2.1 Simulation Results

Based on deforming methodology, a product as shown in Fig. 12 is produced using

simulation run under ISF process. The stresses are found to be distributed in
normal. The experimental results have also validated same. The effective stresses

Fig. 12 Defect-free product

(wall angle = 30)

Fig. 13 Variation of effective

strain with time
Incremental Sheet Forming (ISF) 41

Fig. 14 Force prediction in

X-direction

Fig. 15 Force prediction in

Y-direction

Fig. 16 Force prediction in

Z-direction
42 Y. Kumar and S. Kumar

are found increase as the DT path going to nish, i.e., the stresses increase with
respect to time. This is obvious due to the strain hardening. Figure 13 shows the
evolution of effective strain during the process, which is increasing with time.
The forming forces in X- and Y-direction are repetitive in nature. The forces in
X- and Y-directions were also increased gradually. As the tool moves in Z-direc-
tion, the deforming force in Z-direction is found increasing. The force predicted in
X-, Y-, and Z-directions are represented in Figs. 14, 15, and 16, respectively.

3 Development of Single-Point Incremental Forming

Machine

The major elements of ISF are identied as follows: (i) a sheet metal blank, (ii) a
blank holder, (iii) a single-point forming tool or DT, and (iv) CNC machine. The
path of the DT is responsible for the shape, size and accuracy of the nal product.
Thus, the quality of the product depends on the proper tool path planning. The
development of different parts of ISF machine has been discussed as below:

3.1 Sheet Metal Blank

The ISF machine is basically used for deforming product from metal sheets. The
current research is mainly focused on the development of ISF process for light
alloy. The material for metal sheets can be aluminum alloys, brass, titanium alloys,
etc. For testing the machine developed at IIT (BHU), a brass sheet of blank size
100 mm 100 mm 0.5 mm has been used for current experimental study.
The shape, size, and accuracy of the nished product are compared with the
simulation model. The shape and size of the processed model as compared to the
simulation model has been found approximately same.

3.2 Blank Holder or Blank Holding Arrangement

Blank holder is the second most important element of the ISF machine. The proper
blank holding arrangement is necessary for properly holding the blank sheet.
A CAD model of the blank holding arrangement as shown in Fig. 11 is developed
for simulation study. Also based on the simulation study, a modied blank holding
arrangement and a single-point incremental forming machine have been developed
as shown in Figs. 17 and 18. The metals as well as alloys can be deformed easily by
ISF process. For the current investigations, the initial blank of material brass is
selected for nding the capabilities of ISF process because brass is having good
formability at room temperature.
Incremental Sheet Forming (ISF) 43

Fig. 17 Blank and blank

holding arrangement 1. Tool
2. Blank
3. Screw
4. Holding Plate
5. Back Plate
6. Solid Rod
7. Base Plate

Fig. 18 Single-point
incremental forming machine

3.3 Single-Point Forming Tool or Deforming Tool

The single-point forming tool also known as DTis another major element or most
important element of the ISF machine. The DT may be spherical or elliptical or
conical in shape. For the current research, a conical tool, having hemispherical
shape at tip, has been used as shown in Fig. 17.

3.4 CNC Machine

The CNC machine is needed in order to control the tool path. A GALIL-make
motion card having capability of controlling 8 independent axes has been used.
A 3-axis in-house CNC machine has been developed for controlling the tool path.
The completed experimental setup of single-point incremental forming machine has
been represented in Fig. 18.
44 Y. Kumar and S. Kumar

Fig. 19 Defect-free product

(brass)

Fig. 20 Grid pattern on the

bottom surface

4 Experimental Results

In order to investigate the strain distribution in the deformed product, the grid
pattern was prepared on the bottom surface of sheet as shown in Fig. 20 having
resolution of 1 mm. The metal sheet was divided into the 5 portions. Each portion is
having 5 grids, i.e., 5 mm in length. The cone having top circle radius R = 20 mm
and bottom circle radius = 5 mm has been successfully produced by ISF (Fig. 19).
For the analysis of strain distribution in the sheet deformed by ISF process, the
following 5 regions were identied on the deformed sheet:
Region A: Starting from center point (Point 1) of bottom circle next 5 grids (Point 2)
were identied as the region A.
Region B: Starting from Point 2 of next 5 grids, i.e., up to Point 3 were identied as
the region B.
Region C: Starting from Point 3 of next 5 grids, i.e., up to Point 4 were identied as
the region C.
Region D: Starting from Point 4 of next 5 grids, i.e., up to Point 5 were identied as
the region D.
Incremental Sheet Forming (ISF) 45

Fig. 21 Strain distribution

Fig. 22 Product formed by dieless forming machine (top row all aluminum and bottom row from
left to right 1st brass, 2nd and 3rd aluminum, and 4th titanium)

Region E: The portion beyond Point 5 was identied as the region E.

The strain was found to be distributed normally as shown in Fig. 21.
Based on the simulation study, the experiments were carried out to get the
defect-free products as shown in Fig. 22.

5 Conclusions

The ISF process is a flexible forming process, and it can be easily used for
producing 3D complex shapes. The process can be used for larger forming angles
with proper forming methodology. The laboratory setup of single-point ISF
machine has been developed using motion card to control the 3 servo motors giving
46 Y. Kumar and S. Kumar

capability of individual 3-axis controlling. The defect-free products of brass,

aluminum, and titanium have been produced using the ISF. The strain distribution
over the length of deformation has been computed, and it has been found to be
distributed over the length of the deformation.

Acknowledgments The authors acknowledge the help in running the simulation at IIT Kanpur.

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