Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178 Received for review: 2015-07-20

© 2016 Journal of Mechanical Engineering. All rights reserved. Received revised form: 2015-10-22
DOI:10.5545/sv-jme.2015.2901 Original Scientific Paper Accepted for publication: 2015-12-17

Analysis of the Effect of Process Parameters

on Part Wall Thickness Variation in Conventional Metal Spinning
of Cr-Mn Austenitic Stainless Steels
Šugár, P. – Šugárová, J. – Petrovič, J.
Peter Šugár1 – Jana Šugárová1 – Ján Petrovič2
1 Slovak University of Technology, Faculty of Materials Science and Technology, Slovakia
2 Eiben, Co. Ltd., Slovakia

Metal spinning is one of a number of flexible sheet forming processes which is a cost effective option for the production of parts with a very
high strength to weight ratio. Although the wall thickness of the formed part in conventional spinning is generally considered to be nearly
constant, a non uniform distribution of wall thickness is in fact observed. In this study, the wall thickness variation of a formed part made of
Cr-Mn austenitic stainless steel was analysed. The thickness variation was measured using an optical 3D scanning method and the influence
of mandrel speed, feed ratio and tool path profile (convex, concave and linear) on wall thickness variation was studied. A three-level full
factorial design of the experiment and ANOVA (Analysis of Variance) were used. The results show that the maximal variation in wall thickness is
observed in approximately half of the part wall height (thinning) and on the open end of the part (thickening). Feed ratio and roller path profile
are statistically significant factors governing wall thickness variation. There was no obvious effect from the variation in mandrel speed on the
thickness distribution.
Keywords: metal spinning, stainless steel, 3D optical scanning, wall thickness, ANOVA

Highlights
• Experimental study of conventional metal spinning of Cr-Mn austenitic stainless steel part.
• Analysis of spun part wall thickness distribution by optical 3D scanning method.
• Quantifying the significance of mandrel speed, feed ratio and tool path profile on the wall thickness variation by analysis of
variance (ANOVA).
• Identification of strong statistical significance of the tool path profile and feed ratio effect on wall thickness variation.

0 INTRODUCTION Wall thickness variation is a major defect found

in parts made from sheet metals that influences
Conventional metal spinning is a technology based the intensity of part defects and may cause part
on the gradual shaping of a circular sheet over a failure [7]. The wall thickness of the formed part in
mandrel through the action of a roller that produces conventional spinning is, in general, considered to be
localised pressure and moves axially over the nearly constant [8]. In fact, a non-uniform distribution
outer surface of the sheet to produce a symmetrical of wall thickness is observed and therefore the
product. This technology incorporates conventional spinning process must be optimized in order to
spinning, shear spinning and tube spinning [1] and produce components with a minimal variation in wall
also a group of novel spinning processes, such as non- thickness. Based on experimental studies that have
axisymmetrical spinning, non-circular cross-section been carried out up to date, the spun part thickness
spinning and tooth-shaped spinning [2] and [3]. Due distribution depends mainly on the number of tool
to the development of numerically controlled lathes, passes, the offset of each pass from the other [9], as
spinning is becoming a cost effective option for both well as the tool feed ratio [10], shape of produced
medium and high volume production of parts with part [11], roller noise radius [4] and [12], friction [13]
a very high strength to weight ratio. Moreover, it is and first toolpass trajectory [14] to [16]. Low feed
possible to spin components with tight geometrical rates, large nose radius and multi-pass spinning are
tolerances and to achieve high-quality surface recommended to achieve uniform part wall thickness.
integrity [4] to [6]. The process has a wide variety High offset values tend to reduce wall thickness [4]
of applications including parts for the automotive and [9] to [16].
and aerospace industries, parts such as centrifuges, A strong research effort has been made to study
funnels, tanks, parts for medical and gastronomy the effect of tool path profile on thickness variation.
equipment, as well as parts for musical instruments Kang et al. [14], Liu et al. [15] and Hayama et al. [16]
and pieces of art. reported that the first pass in conventional spinning
*Corr. Author’s Address: Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology, Jana Bottu 25, 917 24 Trnava,
Slovakia, peter.sugar@stuba.sk
171
 Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178

played a decisive role in the final wall thickness systematic investigation of the process using both
variations. They also concluded that the linear tool experimental and theoretical techniques, the possible
path profile is less complicated and helps to reveal causes of thinning in part wall thickness during
deformation characteristics. Concave paths are conventional spinning are still not fully understood
widely used, while convex paths are more suitable and a knowledge gap between academic research
for producing convex cone shapes such as container outcomes and industry requirements exists.
heads. The present paper analyses the results of
Wang and Long [17] applied finite element experimental investigations concerning the wall
analysis to study the effects of tool paths (linear, thickness distribution of spun parts made of Cr-Ni
concave, convex, and combined curve) on the variation austenitic stainless steel. The influence of mandrel
of wall thickness. They suggested that dominant in- speed, feed ratio and tool path profile on the spun
plane tensile radial strains may be the main reason parts wall thickness distribution was determined using
for wall thinning. The authors also reported that in statistical analysis.
a multi-pass conventional spinning process the wall
thickness decreased after each forward roller pass 1 EXPERIMENTAL SETUP AND METHODS
(the roller feeds towards the rim of the workpiece)
and there are almost no thickness changes during the 1.1 Experimental Samples and Set-Up
backward passes. Using a concave roller path tends to
greatly reduce wall thickness of the spun part, while For the production of the experimental samples,
using a convex roller path helps to keep the original whose shape and dimensions are shown in Fig. 1, a
wall thickness unchanged. A greater curvature of the circular blank with an outer diameter of D0 = 200 mm,
concave path would result in more thinning of the prepared by AWJ machining technology, was used.
wall thickness of the part. The sheet used was made of austenitic stainless steel
Essa and Hartley [18] and [19] applied a finite X5CrNi18-10 (DIN 1.4301). The basic mechanical
element model for their study of the thickness properties that define the material plasticity (ultimate
distribution of aluminium cylindrical cups produced tensile strength (Rm), 0.2 % proof strength (Rp0.2)
by conventional spinning. They found that by using and elongation (A80)) of the experimental material is
a roller path profile of an involute curve in the first shown in Table 1.
pass followed by a linear path profile in the second
Table 1. Mechanical properties of the experimental material
pass, minimum sheet thinning and a more uniform
stress distribution could be obtained. Rm [MPa] Rp0.2 [MPa] Rp0.2 / Rm [-] A80 [%]
Polyblank and Alwood in their study [20] showed 604 235 0.39 59.7
that toolpath design in spinning involves finding
a balance between the need for deformation and The experimental samples were produced on a
the avoidance of failure by wrinkling and thinning. DENN spinning machine, type Zenn-80 equipped
They suggest that in order to reduce thinning during with a Sinumeric 840-D CNC control unit (Fig. 2).
the metal spinning process, a greater number of less The mandrel made of Cr-Mo alloy steel
aggressive passes should be used. (X155CrVMo12-1) with a hardness of 56 ± 2 HRC
Although knowledge about the mechanics of and a forming roller also made of Cr-Mo alloy steel
spinning, which helps us to understand the final (X155CrVMo12-1) with a hardness of 54 HRC was
properties of the spun parts, has been developed by used.

Fig. 1. Experimental sample

172 Šugár, P. – Šugárová, J. – Petrovič, J.

 Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178

Table 3. Process parameters and their levels

Parameter Level 1 Level 2 Level 3
Mandrel speed n [rpm] 400 800 1200
Feed ratio f [mm/rev] 0.5 1 1.5
Tool path profile tpp [-] (1) concave (2) linear (3) convex

Fig. 2. Experimental set up

The roller geometrical parameters were: diameter

170 mm, radius R8 and the angle between the axis of
the roller and the axis of the mandrel was 35° (Fig. 3).

Fig. 3. Roller geometry

Three different mandrel speeds and three

different levels of feed ratio (feed rate/mandrel speed)
combined with the off-line designed linear, convex
and concave roller path profiles were applied: nine
movements towards the blank edge (forward passes)
and one forward calibration pass. The detailed
information about the first and last roller path of each
path design is given in Fig. 4 and Table 2.
A review of experimental parameters and their
levels is shown in Table 3. An experimental layout
using full factorial design was used.

Table 2. Polynomial interpolation of roller path profiles

AD: y = –0.006x3 + 0.042x2 – 0.701x + 99.637
Convex
EG: y = –0.038x2 + 1.904x + 58.02
AE: y = 0.004x4 – 0.136x3 + 1.846x2 – 11.709x + 116.999
Concave
FI: y = –0.0009x3 + 0.006x2 + 0.479x – 79.813
AB: y = –1.49x + 103.879 Fig. 4. Roller path profiles (Zero point [0;0]: intersection of
Linear
CD: y = –0.257x + 74.402 mandrel axis with mandrel – workpiece interface plane)

Analysis of the Effect of Process Parameters on Part Wall Thickness Variation in Conventional Metal Spinning of Cr-Mn Austenitic Stainless Steels 173
 Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178

1.2 Method of Wall Thickness Measurement three directions were taken into account for the final
evaluation.
For the experimental measurement of geometrical The 3D optical measurement method has been
accuracy, a non-contact data capture method was validated by the contact measurement method using a
used. The samples were digitized using the optical digital micrometer with pointed measuring faces and
3D system GOM ATOS II TripleScan SO MV 320 a resolution of 0.001 mm. The plot of deviations of
controlled by the software application GOM ATOS measured data shows good agreement between the
Professional v.7.4 [21] and [22]. The sample was results of these measurements (Fig. 6).
scanned on a rotary table (10 scans rotated in 36°
increments) from both the inside and outside. For a

Deviation of measured data [mm]

0.010
correct evaluation of the specimens with respect to the 0.007 0.007
0.008

rolling direction of the blank, a special jig, inserted 0.006

0.005
into a hole at the bottom of the part, was used. The
part surface was sprayed with white powder to provide 0.000
a matt surface, as reflected light negatively influences
the measurement accuracy. Next, non-coded reference -0.005
points with diameters of 3 mm were stuck on the part’s -0.007
-0.008
surface. The cloud of captured points was polygonized -0.010
to obtain a triangulated surface model. Then the digital 5 9 13 17 21 25
Axial distance from the part bottom [mm]
model was exported into an .stl file. The digitized part
was compared with the reference CAD model using Fig. 6. The deviations in measured data of wall thickness obtained
the software GOM Inspect v.8. The shape deviation using a non-contact method (3D optical scanning technology) and
a contact method (micrometer)
between the digitized and CAD (reference) model was
visualized using colour deviation plots (Fig. 5) and the
thickness of part wall was recorded [23]. 2 RESULTS AND DISCUSSION
Thickness variation in this study means any
difference between the part wall thickness and the The results of the experiments were analysed in order
thickness of the original sheet metal. to estimate the contribution of individual parameters
to part wall thickness variation. It was found that
dependent on a combination of process parameters,
the part wall thickness varies from 0.81 mm to 1.15
mm. As shown in Figs. 7 to 9, the maximal reduction
in thickness was observed approximately half way up
the part wall (Region A, situated at a distance of 13
mm from the part bottom) and the maximal increase
in thickness was found at the open end of the formed
part (Region B, situated at a distance of 25 mm from
the part bottom).
In order to quantify the effects of the input factors
studied and the interaction between them on wall
thickness variation, an analysis of variance (ANOVA)
using Minitab v. 17 software was performed. The
Fig. 5. Digitized model of the formed part with color map of
thickness deviations Fisher’s ratio (F-ratio), which is the ratio between
the variance due to the effect of a factor and the
The thickness was measured in 7 places on the variance due to an error term, was used to measure the
wall at distances of (1, 5, 9, 13, 17, 21 and 25) mm significance of the factor at the desired significance
from the part bottom, as shown in Fig. 1. The wall level. If the F-test value is greater than the tabulated
thickness measurement was done in three directions F-test value, the process parameter is considered
related to the rolling direction of the sheet: 0°, 45° and significant.
90°. Due to the negligible differences in the part wall The results of the ANOVA and F-test for part wall
thickness depending on the rolling direction of the thickness measured in the area with maximal thinning
sheet, the mean values of the thickness measured in (axial distance of 13 mm from the cup bottom) and
174 Šugár, P. – Šugárová, J. – Petrovič, J.
 Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178

Fig. 7. Effect of feed ratio on wall thickness variation (Expanded uncertainty of the measure U = 3 μm (k = 2; confidence level 95 %))

Fig. 8. Effect of mandrel speed on wall thickness variation (Expanded uncertainty of the measure U = 3 μm (k = 2; confidence level 95 %))

Fig. 9. Effect of tool path profiles on wall thickness variation (Expanded uncertainty of the measure U = 3 μm (k = 2; confidence level 95 %))

Table 3. ANOVA for thickness variation at a distance of 13 mm Table 4. ANOVA for thickness variation at a distance of 25 mm
from cup bottom from cup bottom
Source Sum of squares DF F-ratio p-value Source Sum of squares DF F-ratio p-value
tpp 371.949 2 533.90 0.000 tpp 211.736 2 196.76 0.000
n 4.776 2 6.85 0.018 n 5.909 2 5.49 0.032
f 148.296 2 212.86 0.000 f 4.807 2 4.47 0.050
tpp×n 14.249 4 10.23 0.003 tpp×n 0.676 4 0.31 0.861
tpp×f 7.616 4 5.47 0.020 tpp×f 16.484 4 7.66 0.008
n×f 2.329 4 1.67 0.248 n×f 1.191 4 0.55 0.703
Error 2.787 8 Error 4.304 8
Total 552.000 26 Total 245.107 26
Tabulated F-test values at 95 % confidence level: Tabulated F-test values at 95 % confidence level:
F (0.05; 2.8) = 4.46; F (0.05; 4.8) = 3.84; R2adj = 94.29 % F (0.05; 2.8) = 4.46; F (0.05; 4.8) = 3.84; R2adj = 98.36 %

Analysis of the Effect of Process Parameters on Part Wall Thickness Variation in Conventional Metal Spinning of Cr-Mn Austenitic Stainless Steels 175
 Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178

maximal thickening (axial distance of 25 mm from the The results further showed that for part wall
cup bottom) are shown in Tables 3 and 4. thinning there is a significant interaction among the
The results of ANOVA show that all of the input parameters, which is the tool path profile – feed
main input factors have statistically significant ratio (p-value: 0.01 to 0.05), and a very significant
(p-value: 0.01 to 0.05) or extremely significant interaction among the input parameters, which is the
(p-value < 0.0001) influence on wall thickness tool path profile – mandrel speed (p-value: 0.001 to
variation. The most significant effect of tool path 0.01). For part wall thickening there is only one very
profile on the wall thickness variation is clearly seen significant interactive influence: the tool path profile
in both measured areas. The thinning of the part – feed ratio.
wall is also affected by feed rate, mandrel speed has The determination coefficients, R2adj = 94.29 %
minimal effect. The thickening of the part wall is also for part wall thinning and R2adj = 98.36 % for part
influenced not only by the tool path profile, but also wall thickening, demonstrate that the models are well
by the mandrel speed and feed ratio. fitted.

a a
-8
10

Thickness variation [%]

Thickness variation [%]

-10

8
-12
7.31
-12.79 -13.37 6.79
6.76
6 6.17
-14
-13.53 -13.78

-16
4

-18
400 800 1200 400 800 1200
Mandrel speed [rpm] Mandrel speed [rpm]
b b
-8
10
Thickness variation [%]
Thickness variation [%]

-10
-10.91
8
-12 7.27
-12.67 6.76 6.76
-13.37
6.23
-14 6

-16
-16.52 4

-18
0.5 1.5 2.0
0.5 1.5 2.0
Feed ratio [mm/rev] Feed ratio [mm/rev]
c c
-8
-8.58
10
9.76
Thickness variation [%]

Thickness variation [%]

-10

8
-12 7.48
6.76
-13.37

-14 -13.90 6

-16
4
-17.62 3.02
-18
1 2 3 1 2 3
Tool path profile Tool path profile
Fig. 10. The main effects of parameters on part wall thinning: Fig. 11. The main effects of parameters on part wall thickening:
a) mandrel speed, b) feed ratio and c) tool path profile a) mandrel speed, b) feed ratio and c) tool path profile

176 Šugár, P. – Šugárová, J. – Petrovič, J.

 Strojniški vestnik - Journal of Mechanical Engineering 62(2016)3, 171-178

Fig. 10 shows graphs of the effects of the input 5. The finding related to the tool path profile and
parameters on the part wall thickness in the area of feed ratio influence on wall thickness variation
maximal thinning. It is clear that maximal thinning agrees with the results of other authors who
was observed when the concave tool path profile was have studied this problem experimentally and by
used. The convex tool path profile leads to minimal applying FE analysis for other types of spun part
thinning. The graphs also show that higher values materials (mild steel, aluminium) [12], [14], [17]
of feed ratio decrease the part wall thinning. There and [24].
is a specific situation in the case of mandrel speed. 6. The influence of mandrel speed on wall thickness
Minimal thinning is reached when the mandrel speed variation in both regions of the part wall is only
is set to a value of 800 min–1. In both cases, when minimal. The findings can be expressed thus:
the mandrel speed value is higher or lower, a higher mandrel speed has negligible influence on axial
degree of part wall thinning is observed. and radial force components [10], therefore radial
The effects of the main input parameters on part and thickness strain is minimal.
wall thickness in the area of maximal thickening 7. For minimal variation in spun part wall thickness
are shown in Fig. 11. It can be seen that dominant it is recommended to apply a convex tool path in
thickening has been achieved for the convex tool combination with higher values of the feed ratio.
path profile. The minimum value of thickening was
recorded in the case where the concave tool path 4 ACKNOWLEDGEMENTS
profile was applied. From the feed ratio point of view,
minimal thickening was observed in the case where The authors wish to acknowledge the financial support
the maximal value of the feed ratio was applied and provided by the Ministry of Education, Science,
the maximal value of thickening was reported for the Research and Sport of the Slovak Republic project
middle value of the feed ratio. VEGA 1/0669/15 and the research project Manunet:
FormTool MANUNET-2014-11283. The authors also
3 CONCLUSIONS wish to thank MSc. E. Eiben and MSc. G. Eiben of
Eiben Co. Ltd. for their technical assistance.
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