Experimental Investigation on Geometric Accuracy …
211
that decreasing the step size was effective in improving geometric accuracy. Edwards
et al. [9] reported that increase in step size results in reduced spring back. Kumar
et al. [10] investigated the influence of process variables such as tool diameter, step
size and spindle speed on surface characteristics in SPIF process. ˙ It is reported that
surface roughness increase as spindle speed and tool diameter decrease whereas it
decreases with the decrease in step size. Salem et al. [11] investigated thickness
variation in SPIF process. Continuous thickness reduction is observed that a bending
region and thereafter a region of excessive thinning (thinning band) occurs, followed
by a steady-state region of constant thickness in formed part. Behera and Ou [12]
reported that the pre-forming heat treatment enhanced the average roughness while
post-forming heat treatment improved surface and accuracy formed part. Isidore
et al. [13] investigated the role of tool shape and tool size on pillow defect (responsible
for reducing geometric accuracy). It was observed that influence of changing tip shape
of tool hemispherical to flat was more effective than varying tool size. Skjoedt et al.
[14] presented the multistage process which is used to produce a cup with a 90°
wall angle which has not been possible before. Skjoedt et al. [15] observed that the
overall level of strains achieved in multi-stage SPIF process is much higher than
the conventional SPIF process. Li et al. [16] investigated the influence of number of
stages and step increment in wall angle on the formability in ISF process. Uniform
thickness distribution was observed as forming stages increase. Liu et al. [17] reported
at smaller wall angle less thinning of sheet occurs. Kurra et al. [18] reported that
because of springback effect fracture occurred at the corners so it became necessary
for the tool to deform more material in the later stages.
From the literature review, it is found that a multistage SPIF process is a viable
option for forming parts of steeper wall angles with uniform wall thickness along
with the forming depth. Very less research efforts have been made to study multistage
SPIF process. Therefore, in the present work, the influence of the process variables
on the geometric accuracy and the surface roughness of the formed part is investigated. Experiments are designed using Taguchi L 18 orthogonal array. Also, predictive
models for geometry accuracy and surface roughness in multistage incremental sheet
forming are developed.
2 Experimental Plan and Methodology
˙ In present work, experiments are designed with the help of Taguchi L 18 orthogonal
array. Experimental investigation has been done by using a 3–axis CNC milling
machine. Aluminium alloy AA 1080 of 1.22 mm thickness is used sheet material. A
conical frustum with 110 mm base diameter and 70º wall angle with 45 mm upper
bound of height is formed. As shown in Fig. 2, the sheet is held along its edges with
the help of a fixture and the tool is introduced to deform the sheet into the desired
shape. High-speed steel is taken as tool material (Fig. 3).
In multistage forming, number of forming stages and increment angles are important factors influencing geometric accuracy and thickness distribution. Therefore,
211
that decreasing the step size was effective in improving geometric accuracy. Edwards
et al. [9] reported that increase in step size results in reduced spring back. Kumar
et al. [10] investigated the influence of process variables such as tool diameter, step
size and spindle speed on surface characteristics in SPIF process. ˙ It is reported that
surface roughness increase as spindle speed and tool diameter decrease whereas it
decreases with the decrease in step size. Salem et al. [11] investigated thickness
variation in SPIF process. Continuous thickness reduction is observed that a bending
region and thereafter a region of excessive thinning (thinning band) occurs, followed
by a steady-state region of constant thickness in formed part. Behera and Ou [12]
reported that the pre-forming heat treatment enhanced the average roughness while
post-forming heat treatment improved surface and accuracy formed part. Isidore
et al. [13] investigated the role of tool shape and tool size on pillow defect (responsible
for reducing geometric accuracy). It was observed that influence of changing tip shape
of tool hemispherical to flat was more effective than varying tool size. Skjoedt et al.
[14] presented the multistage process which is used to produce a cup with a 90°
wall angle which has not been possible before. Skjoedt et al. [15] observed that the
overall level of strains achieved in multi-stage SPIF process is much higher than
the conventional SPIF process. Li et al. [16] investigated the influence of number of
stages and step increment in wall angle on the formability in ISF process. Uniform
thickness distribution was observed as forming stages increase. Liu et al. [17] reported
at smaller wall angle less thinning of sheet occurs. Kurra et al. [18] reported that
because of springback effect fracture occurred at the corners so it became necessary
for the tool to deform more material in the later stages.
From the literature review, it is found that a multistage SPIF process is a viable
option for forming parts of steeper wall angles with uniform wall thickness along
with the forming depth. Very less research efforts have been made to study multistage
SPIF process. Therefore, in the present work, the influence of the process variables
on the geometric accuracy and the surface roughness of the formed part is investigated. Experiments are designed using Taguchi L 18 orthogonal array. Also, predictive
models for geometry accuracy and surface roughness in multistage incremental sheet
forming are developed.
2 Experimental Plan and Methodology
˙ In present work, experiments are designed with the help of Taguchi L 18 orthogonal
array. Experimental investigation has been done by using a 3–axis CNC milling
machine. Aluminium alloy AA 1080 of 1.22 mm thickness is used sheet material. A
conical frustum with 110 mm base diameter and 70º wall angle with 45 mm upper
bound of height is formed. As shown in Fig. 2, the sheet is held along its edges with
the help of a fixture and the tool is introduced to deform the sheet into the desired
shape. High-speed steel is taken as tool material (Fig. 3).
In multistage forming, number of forming stages and increment angles are important factors influencing geometric accuracy and thickness distribution. Therefore,