5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT
Guwahati, Assam, India
306-1
Comparison of Dimensional Repeatability and Accuracy for Deformation
Machining Stretching Mode with Sheet Metal Components
Arshpreet Singh1, Anupam Agrawal2*
School of Mechanical, Materials and Energy Engineering, Indian Institute of Technology
Ropar, Rupnagar-140001, India. 1 [email protected],
Abstract
In the present work a comparative study of dimensional repeatability and accuracy for deformation machining
stretching mode and sheet metal components has been performed. Deformation machining enables the creation of
complex structures and geometries, which would rather be difficult or sometimes impossible to manufacture. This
process allows the creation of monolithic components with novel geometries which were earlier
assembled.Experimental studies have been performed for parts created by the DM ‘stretching mode’ process, in
which a thin horizontal floor is machined on the part through high speed machining, and then incrementally formed
into a conical frustum with a single point forming tool.Ten similar components were fabricated by DM stretching
mode, single point increment forming and conventional stretch forming. These components were measured at
various forming depths using a coordinate measuring machine (CMM) and the dimensional repeatability of these
processes was compared.The dimensional repeatability of the DM stretching mode components largely depends
upon the accuracy of the machined floor. Other factors influencing the repeatability of the process are residual
stresses generated during machining, elastic deformation, spring back and highly localised yielding. Keywords: Deformation machining, single point increment forming, thin structure machining.
1 Introduction Deformation machining (DM) is a combination of
two processes-thin structure machining and single point
incremental forming (SPIF). This hybrid process
enables the creation of lighter weight components with
novel and complex geometries which earlier required
complex tooling and equipments. It allows the creation
of monolithic parts which were earlier assembled[Smith
et al. (2007)]. Therefore, enabling cost reduction in
equipment, fabrication and weight of the components.
Thin structure machining is different from
conventional machining due to the lack of stiffness of
machined structure. Therefore, it requires different
machining techniques like use of long slender end mills
[Tlusty et al. (1996)] along with high speed
machining[Smith and Dvorak (1998)]. Single point
incremental forming (SPIF) is a die less forming process
where a hemispherical shaped single point solid tool is
used to deform the thin structure to a desired shape
incrementallyusing computer numeric control.[Jeswiet
et al. (2005)].In this process the thin structure or sheet
metal is deformed locally into plastic stage, enabling
creation of complex shapes according to the tool path
generated by a CNC machining centre.[Malhotra et
al.(2010)]. SPIF hasenabled flexibility in creation of
symmetric, asymmetric and random shapes with
sufficient amount of accuracy.The potential application
of such monolithic parts with complex geometries is in
aerospace industry (e.g. mold lines of fuselage, avionic
shelf, impellers, pressurized bulk heads), biomedical
engineering (cranial plate, bone and joint support,
prosthetics) [Ambrogio et al. (2006)], heat transfer
(irregular, curved fins).
This process can be broadly classified into two –
Deformation machining stretching mode; where the
deformation along the axis of tool resulting in stretching
of the machined thin horizontal structure (thin floor)
(Figure 1) and Deformation machining bending mode;
where the deformation perpendicular to axis of tool
resulting in the bending of thin vertical structure (thin
wall) which was prior machined (Figure 2).
Figure 1 Components manufactured by DM
Stretching Mode process [Smith et al. (2007)]
Figure 2 Components manufactured by DM Bending
Mode process [Smith et al. (2007)]
Comparison of Dimensional Repeatability and Accuracy for Deformation Machining Stretching Mode with Sheet Metal Components
Comparative studies on dimensional repeatability
and fatigue life of DM components
bent components have shown that the DM components
have shown better repeatability than SPIF components
but worse than conventionally bent components
better fatigue life than conventionally bent components
[Agrawal et al. (2012)].
accuracy of the process is mainly attributed to the
influence of residual stresses induced during forming
and spring back effect[Duflou et al
happens when the forming load is removed and the
formed component tries to regain its preformed shape,
thus reducing the overall accuracy of the component.
Geometric accuracy of the formed components can be
improved by providing necessary tool
compensation taking the elas
[Wei et al.(2011)].Bending effect at the beginning of
forming also plays an important role in reducing the
accuracy of formed components.
2 Methodology In the present work ten components each
stretching mode, SPIF and conventional stretch
were fabricated on a 3 axis CNC vertical milling
machine (Make: BFW, Model: VF 30 CNC VS)
inspected for dimensional accuracy and repeatability
using a coordinate measuring machine (CMM) (Ma
Accurate). A fixture holding DM stretching mode
components (Figure 3a) was designed and fabricated
consists of a holding plate and clamps for holding thick
components, a backing plate for supporting the
components during high speed machining, and a
plate for overall stability of the fixture. During forming,
the backing plate (Figure 3b)
fixture and the components were formed through a
circular orifice in the holding plate.
components and conventionally formed compo
same fixture was used by replacing the above mentioned
plates with a set of normal plates with a circular
(Figure 3c).
(a)
Comparison of Dimensional Repeatability and Accuracy for Deformation Machining Stretching Mode with Sheet Metal Components
tudies on dimensional repeatability
and fatigue life of DM components with conventionally
have shown that the DM components
have shown better repeatability than SPIF components
but worse than conventionally bent components and
onventionally bent components
]. Relatively poor geometric
of the process is mainly attributed to the
influence of residual stresses induced during forming
Duflou et al.(2007)]. Spring back
happens when the forming load is removed and the
formed component tries to regain its preformed shape,
thus reducing the overall accuracy of the component.
Geometric accuracy of the formed components can be
improved by providing necessary tool path
taking the elastic spring back into account
Bending effect at the beginning of
forming also plays an important role in reducing the
accuracy of formed components.
In the present work ten components each from DM
stretching mode, SPIF and conventional stretch forming
3 axis CNC vertical milling
machine (Make: BFW, Model: VF 30 CNC VS) and
inspected for dimensional accuracy and repeatability
using a coordinate measuring machine (CMM) (Make:
A fixture holding DM stretching mode
) was designed and fabricated. It
consists of a holding plate and clamps for holding thick
components, a backing plate for supporting the
components during high speed machining, and a base
plate for overall stability of the fixture. During forming,
(Figure 3b) was removed from the
fixture and the components were formed through a
circular orifice in the holding plate. For SPIF
and conventionally formed components
same fixture was used by replacing the above mentioned
plates with a set of normal plates with a circular orifice
(a)
Figure 3 Fixture for holding the components
The workpiece
6101-T6, a commonly used alloy in aerospace, aviation
and marine industry. Table 1 depicts the me
properties of Al 6101
material billet was firstly machined to component size
of 90×90×12 mm to be held in the fixture. Then, floor
of 1.0 mm thickness of diameter 70
by high speed plunge milling technique
components, five were machined within a machining
tolerance of ± 10 µm and the rest five within ±
Thereafter, the
incrementally formed into a conical frustum using a
single point tool with
geometry of the
figure 4.
Table 1
Properties
Density
Melting Point
Poisson’s ratio
Modulus of elasticity
Tensile strength
Yield strength
Figure 4 Geometry
For SPIF and conventionally formed
sheet metal of the same alloy was prepare
150×150 mm. The sheet was
formed using a single point tool and a mandrel of the
size of the conical frustrum to be formed
Contour tool path was employed f
forming. Table 2 show
machining,
Comparison of Dimensional Repeatability and Accuracy for Deformation Machining Stretching Mode with Sheet Metal Components
306-2
(b) (c)
Figure 3 Fixture for holding the components
workpiece material used in the present study is Al
T6, a commonly used alloy in aerospace, aviation
and marine industry. Table 1 depicts the mechanical
properties of Al 6101-T6. For the DM components raw
material billet was firstly machined to component size
f 90×90×12 mm to be held in the fixture. Then, floor
mm thickness of diameter 70mm were machined
by high speed plunge milling technique.Out of the ten
components, five were machined within a machining
tolerance of ± 10 µm and the rest five within ± 25 µm.
Thereafter, the machined floor of desired thickness was
incrementally formed into a conical frustum using a
single point tool with a hemispherical end. The
of the formed conical frustum is shown in
Table 1 Mechanical properties of 6101-T6
Properties Magnitude
Density 2.7 gm/cc
Melting Point 600°C
Poisson’s ratio 0.33
Modulus of elasticity 70 GPa
Tensile strength 97 MPa
Yield strength 76 MPa
Figure 4 Geometry of conical frustum
For SPIF and conventionally formed components,
sheet metal of the same alloy was prepared of size
150×150 mm. The sheet was held in the fixture and
formed using a single point tool and a mandrel of the
size of the conical frustrum to be formed, respectively.
Contour tool path was employed for incremental
forming. Table 2 show various parameters of
incremental and conventional forming.
2
material used in the present study is Al
T6, a commonly used alloy in aerospace, aviation
chanical
. For the DM components raw
material billet was firstly machined to component size
f 90×90×12 mm to be held in the fixture. Then, floor
mm were machined
Out of the ten
components, five were machined within a machining
25 µm.
was
incrementally formed into a conical frustum using a
emispherical end. The
in
components,
d of size
held in the fixture and
formed using a single point tool and a mandrel of the
.
cremental
ious parameters of
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12
Guwahati, Assam, India
Table 2. Parameters for high speed m
and conventional forming
High
Speed
Machining
Tool material Tungsten
Carbide
Tool
Diameter 16mm
Spindle speed 1200 rpm
Transverse
feed (x,y)
400
mm/min
Axial feed (z) 20 mm/min
Cooling/Lubr
ication
Flood
cooling
Figure 5 (a) shows the schematic; (b)
component formed by DM stretching mode.
Figure 7 Graph of standard
All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12
Table 2. Parameters for high speed machining, SPIF
and conventional forming
High
Speed
Machining
SPIF
Conventio
nal
Forming
ten
Carbide SS 304 SS 304
16mm 10mm Conical
Mandrel
1200 rpm 100 rpm 50 rpm
mm/min
200
mm/min N.A.
20 mm/min 10
mm/min 10 mm/min
Flood
cooling
Mobil
oil-40
Mobil oil-
40
(a) shows the schematic; (b) actual
component formed by DM stretching mode.
Thereafter, all the components fabricated by the
three processes were inspected on the coordinate
measuring machine (CMM). Diameters of the formed
cone at 0 mm
2mm were measured. Figure 6 shows the schematic of
one of the components measured on the CMM showing
diameters at different depths.
Figure 6 Measured component on the CMM
Figure 7 Graph of standard deviation in diameter of ten components of respective processes v/s the forming
depth.
All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT
306-3
Thereafter, all the components fabricated by the
three processes were inspected on the coordinate
measuring machine (CMM). Diameters of the formed
cone at 0 mm up to 22 mm depth with an interval of
2mm were measured. Figure 6 shows the schematic of
one of the components measured on the CMM showing
diameters at different depths.
Figure 6 Measured component on the CMM
deviation in diameter of ten components of respective processes v/s the forming
, 2014, IIT
3
Thereafter, all the components fabricated by the
three processes were inspected on the coordinate
measuring machine (CMM). Diameters of the formed
up to 22 mm depth with an interval of
2mm were measured. Figure 6 shows the schematic of
one of the components measured on the CMM showing
Comparison of Dimensional Repeatability and Accuracy for Deformation Machining Stretching Mode with Sheet Metal Components
306-4
3 Results and discussion 3.1 Dimensional Repeatability
Variation in diameters at different points across the
depth of conical frustum for all the similar components
made by three different processes was recorded.
Standard deviation in the diameter of the ten
components of the respective processes has been plotted
in the figure 7 across the forming depths. From the
graph it is clear that the dimensional repeatability of the
conventionally formed components is the best among all
the processes studied. The standard deviation of
measured diameters for the ten components across the
forming depth varies between 0.033 to 0.053mm with
an average of 0.045 mm. Dimensional repeatability of
the SPIF components is found to be the least among the
three processes. The standard deviation of measured
diameters for the ten components formed by SPIF
across the forming depth varies between 0.229 to
0.259mm with an average of 0.244 mm. Dimensional
repeatability of the DM components has been found
dependant on the machining tolerance of the thin floor
to be formed. The standard deviationof measured
diameters for the five components formed by DM with
machining tolerance of ±10µm varies between 0.074 to
0.097mm with an average of 0.088 and rest five
components with machining tolerance of ±25µm varies
between 0.195 to 0.217mm with an average of 0.204
mm. The results reveal that the dimensional
repeatability of the DM components is comparable with
the conventionally formed components subject to the
machining accuracy of the thin floor to be formed.
The variation in the dimensions of SPIF and DM
components could be further attributed, though
unconclusively, to uneven redistribution of residual
stresses during the incremental forming, compared to
conventional forming. But, redistribution of residual
stresses during high speed machining of the thin floor
for DM components might have a positive influence as
their dimensional repeatability is better than the SPIF
components and comparable to the conventionally
formed components.
3.2 Dimensional Accuracy
Average of the ten diameters at different points
across the depth for all the components formed by the
three processes have been plotted and compared with
the actual required dimensions in figure 8. From the
graph it is evident that the dimensional accuracy of the
conventionally formed components is the best with the
average variation of 1.875mm from the actual required
dimensions. Average variation of the measured diameter
from the required diameter across forming depths for
DM components and SPIF components is comparable
with the magnitude of 6.430mm and 5.982mm
respectively.
Poor dimensional accuracy for DM and SPIF
components is attributed mainly to the bending effect at
the start of the forming (figure 9) and elastic spring back
effect prominent in incremental forming.
Figure 9 Bending effect in DM components
Figure 8. Graph average diameter of the ten components of respective processes v/s the forming depth.
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT
Guwahati, Assam, India
306-5
4 Conclusion Comparative study on dimensional repeatability
and accuracy for deformation machining stretching
mode components with sheet metal components has
been performed. Dimensional repeatability of
conventionally formed sheet metal components is the
better than that of DM components and SPIF sheet
metal components. The poor repeatability of the DM
and SPIF components could be attributed the uneven
redistribution of residual stresses, however this could
not be confirmed conclusively. The role of residual
stresses in incremental forming could be seen as a new
research scope.
Dimensional accuracy ofthe DM components and
SPIF componentsis poorer than the conventionally
formed sheet metal components. This is attributed to the
prominent bending effect and the elastic spring back
effect. Future work goes into developing of a good
strategy to counter these twin problems so as to improve
the overall process accuracy.
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