SIMTech technical reports
Volume 8 Number 2 Apr-Jun 2007
A study into cold rotary forming of precision metal components
C. C. Wong, A. Danno, K. K. Tong, M. Tsuyoshi, K. B. Lim, and M. S. Yong
Abstract – In the work reported in this paper, a
simple flow forming and form rolling facility was
established to investigate the feasibility of forming
thin (thin wall cups) and gear profiles. The results
showed that it is feasible to adopt a two step forming
process, ‘bending’ and flow forming to enable material flow along the mandrel in order to form a thin wall
cup component using two different profiles and
adopting an axial roller movement. Quality of the
cups formed depends on the diameter reduction,
starting disc thickness of the blank and the number of
passes in the flow forming stage. For form rolling of
spur gear profile, the accuracy of the tooth profile
depends on the roller indentation, blank diameter and
matching of the phase angle of the rollers.
chines using simple tool shapes. In addition, tool life
is much improved as compared with forging processes.
Although developments in incremental forming
have expanded manufacturers’ options in the design
of a particular component, a major problem with
almost all incremental rotary deformation processes
has been the high cost of very specialized equipment.
Therefore, the aim of this research is to develop a
more generic incremental cold rotary forming technology utilizing the concept of flow forming and form
rolling for wider applications. Thus, we have adopted
the term ‘cold rotary forming’ which encompasses the
flow forming and forming rolling processes, in this
report.
Keywords: Rotary forming, Flow forming, Form rolling, Gear, Roller path
2
1
OBJECTIVE
The work described in this report represents an
initial stage of this research, which seeks to obtain
fundamental and basic understanding into the cold
rotary forming (flow forming and form rolling) of
axisymmetrical hollow aluminum components profiled, small gears and non ferrous materials. The objectives of this study are to investigate the flow
forming of thin wall cups and the form rolling of spur
gear profile.
BACKGROUND
In recent years, there is a growing demand for
light weight and higher value-add components by the
OEMs of transportation industries. These can be
achieved with lower density materials and new
structural designs. As component shapes are becoming more complicated, machining is not a cost effective process for producing these components and
should be minimized as a production operation. As a
result, precision forging, or net-shape forging, has
become increasingly popular due to savings in material, energy and finishing steps. Precision forging is
sometimes described as close-tolerance forging to
emphasize the goal of achieving, solely through the
forging operation, the dimensional and surface finish
tolerances required in finished parts.
However, as manufacturers strive to reduce
weight and cost, many of the new components, because of their shape complexity and complicated tool
design and high load requirements, are challenging
the current precision forging technologies beyond its
current level of technology. In order to meet this requirement, there is a renewed interest in incremental
forming, especially rotary type incremental forming
processes, such as swaging, cross-wedge rolling, ring
rolling, rotary forging, conventional spinning and
flow forming. As these processes involved plastic
deformation of small volume of the workpiece at a
time, the power and working forces required are reduced significantly, allowing more complicated
components to be produced on relatively small ma-
3
METHODOLOGY
3.1
Flow Forming of Thin Cup Profile
In this work, a Mazak NC lathe was utilized as a
flow forming machine. Only one roller was used in
each experiment. A roller tool was designed and built
to accommodate the lathe tool post, as shown in fig.1.
The mandrel was clamped using the lathe’s chuck and
the workpiece was fixed onto the mandrel and tightened by a bolt. In addition, in order to minimize radial
deflection of the mandrel during flow forming operation, a mandrel holder was designed and fixed onto
the lathe bed. Figure 1 shows the experimental set up.
Two different rollers were used as shown in Fig.
2. Roller A (shown in Fig. 2(a)) has an approach angle
of 60º and the second roller, roller B, has an approach
angle of 20º, shown in Fig. 2(b). In order to reduce the
loading on the machine and prevent severe radial
deflection of the roller tool, an annealed aluminum
alloy, A6061 was used as the workpiece. The hardness of the aluminum disc is approximately 49 HV.
Flat disc blanks of diameter 80 mm and thickness of 5
mm and 10 mm were used as the starting workpiece.
65
C. C. Wong et al
Mandrel holder
Mandrel
initial thicknesses of the workpieces investigated
were 5 mm and 10 mm.
Original workpiece
Roller path
Workpiece
Radius reduction
Mandrel
Formed
disc
blank
Roller
Fig. 1. The experimental set-up of flow forming test.
(a) Bending of flat disc blank
Approach
angle
Noise
radius
Roller
Mandrel
(b) Forming to achieve desired internal diameter and wall
thickness.
Approach
angle
(a) Roller A
Formed cup
Fig. 3. The roller path of the flow forming sequence.
(b) Roller B
3.2
Fig. 2. The designs and geometries of the rollers.
Two flow forming steps were proposed in this
experimental study to investigate the feasibility of
forming thin wall cups from flat disc blanks. In the
first step, roller A (Fig. 3(a)) was proposed to ‘bend’
the disc blank to the preset diameter into a cup shape
product. In the second step, roller B (Fig. 3(b)) was
used to flow form the wall of the cup onto the mandrel
to obtain uniform wall thickness, desired internal
diameter and height.
For both forming sequences, the mandrel and the
workpiece were rotating and the roller was fed along
the workpiece parallel to its axis at a preset interference (diameter reduction) for a pre-defined length.
The roller path for both sequences is shown in Fig. 3.
The rotation of the workpiece was fixed at 250 rpm
and the axial feed rate of the roller was set at 1 mm/s
(0.24 mm/rev). Cutting oil was used at the interface
between the roller and the workpiece as well as the
interface between the workpiece and mandrel. The
66
Form Rolling of Spur Gear
For the form rolling of spur gear in this study,
Tsugami rolling machine was used. The diameter of
the roller die and blank was calculated based on the
meshing condition between the bank and the roller.
The workpiece was first placed on a workpiece holder
and subsequently clamped at its center-line portion by
two centering stocks with pneumatic pressure. The
workpiece was then fed into the rotating roller dies
axially. The roller indentation was prescribed by setting the distance between two roller axes. The axial
feed rate of the workpiece is 140 mm/min. Figure 4
shows the external view of working area after rolling
operation.
A study into cold rotary forming of precision metal components
Roller gear
Roller die
Φred = 3% Φred = 12% Φred = 15% Φred = 19% Φred = 22%
(b) Starting wall thickness = 10 mm
Fig. 5. Deformed shape for with diameter reduction for
starting thickness of 5 mm and 10 mm.
Figure 6 shows the variation of cup height and
wall thickness with diameter reduction. It can be seen
from the figure that for both starting disc thicknesses,
cup height increased linearly with increased diameter
reduction. However, for diameter reduction above
19%, the height of the cup increased drastically. This
is because for diameter reduction above 19%, the
material that was being deformed comes in contact
with the mandrel at the beginning of roller axial
translation. This forced the material to flow axially
along the mandrel, thereby elongating the formed cup.
On the other hand, for diameter reduction less than
19%, the cup was practically formed in the ‘air’, i.e.
without any support on the inner walls of the cup (see
Fig. 3(a)), and there is no contact between the inner
wall (internal diameter of the formed cup) and the
mandrel, and the cup formed was parallel to the
horizontal axis of the mandrel. The reason for this
phenomenon is that the rigidity of the cup formed by
the roller is able to withstand the localized deformation that is induced by the roller during the forming
process.
It can also be seen from Fig. 6 that for various
diameter reductions, the variation in wall thickness
for disc thickness of 5 mm and 10 mm is not very
significant compared to cup height. Moreover, taller
cups were produced for starting disc thickness of 10
mm. The taller cups produced using larger starting
disc thickness may be explained by the fact that higher
volume of material was displaced axially compared to
smaller disc thickness. In other words, the height of
the cups is directly affected by the diameter reduction.
On the other hand, the diameter reduction does not
affect the wall thickness. Wall thickness is largely
affected by the nose radius of the roller which determines the amount of plastic deformation induced
along the wall. As a result, the variation in wall
thickness for both starting disc thickness of 5 mm and
10 mm is not significant as the same roller nose radius
was used.
Fig. 4. The external view of working area after rolling
operation.
Before form rolling, the roller die was first rotated to its initial rotational position, i.e. at zero degrees. Then in order to correct the mismatch between
phase angles of both rollers, a very small indentation
was made on the blank by rolling and the mismatch in
the pitching marks induced on the blank was measured approximately by a vernier caliper. The initial
difference in roller phase angle was 0.925º. The material of blank was a low carbon chromium alloy (JIS
SCR415, 0.13~0.18%C, 0.15~0.35%Si, 0.60~0.85%
Mn, 0.90~1.20%Cr) with hardness of around 200 HV
after annealing (850ºC × 4 hr, FC).
4
RESULTS & DISCUSSION
4.1
Flow Forming of Thin Cup Profile
Figures 5(a) and (b) shows the final deformed
shape of the disc blank after the bending process for
starting thicknesses of 5 mm and 10 mm (at different
diameter reductions). For both starting thickness of
the disc blanks, it can be seen that at diameter reduction above 3%, a cup shape component was produced
by simply translating the roller in the axial direction
after a certain diameter reduction was set. For both
starting disc thicknesses, too small a reduction will
result in insignificant cup height and wall thickness.
This is due to the high rigidity of larger wall thickness
which hindered the ‘bending’ mode.
Φred = 3% Φred = 14%
Φred = 17% Φred = 20% Φred = 22%
(a) Starting wall thickness = 5 mm
67
C. C. Wong et al
Cu p Heig h t & W all T h ickn ess (m m )
40
Cup height, To=5mm
35
Cup height, To=10mm
30
Wall thickness, To=5mm
Wall thickness, To=10mm
25
20
15
10
5
0
0
2
4
6
8
10
12
14
16
18
20
22
Diameter Reduction (%) Inner diameter of
tube touches the
mandrel
24
26
Forming
limit
Fig. 6. Variation of cup height & wall thickness with diameter reduction.
For both starting thicknesses of 5 and 10 mm, it
can be seen from Fig. 7 that a step (difference in internal diameter) is formed along the inner wall of the
cup. This step is more prominent in cups produced
from larger diameter reduction. This is due to the
bending mode of the ‘flange’, which caused the exterior of the flange that is in direct contact with the
roller to flow faster that the interior surface that is
facing the mandrel. This also happen when reduction
is larger than 19% but the size of the step formed was
small due to more axial material flow.
the percentage increase in internal diameter with cup
depth having 5 mm initial disc thickness and diameter
reduction of 22% during the first step. As this step is
similar to the flow forming of cylindrical tubes, the
thickness reduction for the flow forming operation
was recommended to be controlled at 20% to 30% so
as to prevent circumferential flow due to too low a
reduction and ‘bell mouthing’ defects due to too high
a reduction.
From the figure, it can be seen that after the first
pass, the internal diameter of the cup was uneven and
increases along the height of the cup. However, the
accuracy of the internal diameter was improved with
each subsequent pass and the dimension of the internal diameter is tending towards uniformity along the
cup height at about 3rd or 4th pass. It is believed that
the internal diameter will be uniform if the material
can flow along a longer mandrel as compared to the
one used in this study.
Cracking
Fig. 8. Cracking due to large diameter reduction.
Step along inner wall of the cup
when material is flowing in air
Tred=Thickness reduction
Fig. 7. Formation of inner step along the internal diameter of
the cup.
A critical forming limit occurred at diameter reduction of 25%. For both starting disc thicknesses of 5
mm and 10 mm, severe breakage occur during the
initial forming stage for diameter reduction above
25%, as shown in Fig. 8. This may be due to the heavy
material accumulation in front of the roller for high
diameter reduction, resulting in material flowing
predominantly in the radial direction as the roller
moved axially. In addition, the heavy accumulation at
the front of the roller, from high diameter reduction,
gave rise to very high axial stress. This in turn causes
severe bulging which led to instability and ultimately
cracking of the flange in front of the roller.
In order to elongate the cup along the mandrel
and to control the dimension of the formed cup in step
1, flow forming process was proposed as a second
step to obtain the net shape product. Figure 9 shows
68
% in crease in In tern al D iam eter
0.50
0.45
1st pass, Tred=24%
0.40
2nd pass, Tred=29%
0.35
3rd pass, Tred=29%
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
Cup Height (m)
Fig. 9. Percent increase in internal diameter for flow forming of cups for initial starting disc thickness of 5 mm and
diameter reduction of 22%.
4.2
Form Rolling of Spur Gear
Figure 9 shows the form rolled spur gear adopting
the through feed process. In order to examine the
profile of the teeth and to evaluate the filling of the
material in the roller die, the gear was sectioned (cut)
in the middle portion. Of all the samples formed
adopting the experimental conditions mentioned in
the section 3.2, there are cases of under-filling and
over-filling of the teeth.
30
A study into cold rotary forming of precision metal components
meshed between the two diametrically opposite dies.
Adjustments in meshing had to be made to ensure no
backlash between them. Once there is no backlash, the
nut was tightened to fix the gears in place.
The aligned gears were then placed on a centre
work bench, as shown in Fig. 13. To measure the
degree of mismatch, the following steps were carried
out:
1) The width over 5 teeth (Xo and X) was measured
by micrometer; Xo is the thickness for one gear
while X is the total thickness of both gears. The
difference in X and Xo is the mismatch in pitch
phase between the two rollers.
2) The mismatch (in degrees) was then input into the
machine code to adjust the phase angles of the
roller dies.
Figure 14 shows the profile of the spur gear after
adjusting the roller phase angle. It can be seen that
there is no visible lapping effect on the teeth. This
confirmed that lapping defects are largely due to
mismatch of the phase angle of roller die pitch.
Fig. 10. Formed spur gears.
Figure 11 shows a cross-section in the case of
under-filling which is mainly due to insufficient roller
indentation which will give rise to insufficient fill-up
of gear teeth during the forming process. However for
the case of under-filling, no defects were observed.
Underfill
Fig. 11. The cross-section of form rolled gear teeth with
under-fill.
However, in cases of over-filling, the lapping
defects occurred at the root and along the flank of the
teeth, as shown in Figs. 12a) and 12b). These defects
are largely due to the miss-match of roller phase angle.
In addition, lapping at tooth root is also due to excessive indentation of the roller die.
Fig. 13. The setup of a new instrument of 2-gear system to measure the mismatch between the phase angles of two rollers at the initial stage.
X
a) At tooth flank
b) At tooth root
Xo
Lapping defects
Fig. 14. Measuring of a mismatch between 2 roller phases.
Fig. 12. The cross-section of form rolled gear teeth with
over-fill and defect.
Figure 15 shows the flank line error on the left
and right side of the formed gear. From the flank line
profile, a crowning of flank line on both sides of the
teeth flank can be seen as well as a slight taper in teeth
thickness. This is due to the low forming load during
the initial and final stage of rolling as the axial contact
length between blank and roller dies decreases in the
initial and final stage of rolling.
To eliminate lapping defects, a new instrument of
2-gear system was designed to measure more accurately the mismatch between the phase angles of two
rollers at the initial stage (see Fig. 13). The 2-gear
system is clamped between the front and tail stock
(similar to clamping of the blank). To check the
mismatch of the roller die, the gear system set and
69
C. C. Wong et al
•
Fig. 15. The formed spur gear after adjusting the roller
pitch.
In addition to flank line error (Fig. 16), the tooth
thickness and pitch error were also measured. The
results indicated that the errors plotted are generally
Sine-like or Cosine-like curves which indicate the
error is mainly due to eccentricity of teeth frank in
reference to the centre of gear. Nevertheless, most of
the errors measured are within the limits of JIS 10
class standards.
Fig. 16. The flank line error.
5
CONCLUSIONS
In this work, the feasibility of forming a thin wall
component by a two step flow forming process, using
multi-pass flow forming in the second step has been
demonstrated. In addition, form rolling of spur gear
profile was investigated. Based on the outcomes, the
following conclusions may be drawn:
Flow forming of thin cup profile:
• A roller with an approach angle of 60° can be
used to produce an initial cup shape from a flat
disc blank.
• In the first step, reduction above 25% will result
in severe cracking of the disc at roller contact
area for starting disc thickness of 5 mm and 10
mm.
• In the first step, diameter reduction above 19%
will allow the material to flow axially along the
mandrel rather than radially, thereby achieving
greater height.
• In first step, wall thickness of the wall depends on
nose radius of the roller and cup height depends
on the initial diameter reduction.
• The diameter of the mandrel has to be changed
accordingly to the required internal diameter for
second step so as to prevent unnecessary flow
forming passes, which will lead to galling effects
due to excessive work hardening.
70
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Multi-pass flow forming in second step can improve the dimensional accuracy and the uniformity of the internal diameter.
Form rolling of gear profile:
• The amount of roller indentation is important to
prevent under-filling or over-filling defects in the
formed gear.
• Matching of the phase angle between two diametrically opposite rollers is important to prevent
lapping defects.
• With the correct parameters, formed spur gear
accuracy can match those produced by machining.
6
INDUSTRIAL SIGNIFICANCE
Although developments in various types of rotary
forming processes have expanded manufacturers’
options in the design of a particular component, a
major problem with almost all incremental rotary
deformation processes has been the high cost of very
specialized equipment. This study has showed that the
combination of flow forming and form rolling processes can be employed to produce higher added value
components towards net shape. Advantages of this
development are:
• Use as an additional operation (replacing secondary machining) to existing forming route.
• Replacement of complicated fabrications (e.g.
forming and welding) by single piece parts with
consequent improvement in mechanical properties and reduced cost.
• Generation of complicated shapes using simple
and cheap tool forms programmed to move in
complex paths. Thus the final shape of the
product depends on tool path rather than expensive tools in the case of parts with complex geometries.
• Net or near net shape could be obtain because the
formed component is not depend on the toolmaker but on the tool path program.
REFERENCES
[1] M. Jahazi, G. Ebrahimi, “The influence of flow forming
parameters and microstructure on the quality of a D6ac
steel, J. Mater. Process. Technol., vol. 103 (1-3), pp.
362-366, 1997.
[2] C.C. Wong, J. Lin, T.A. Dean, “Effects of roller path
and geometry on the flow forming of solid cylindrical
components”, J. Mater. Process. Technol., vol. 167, pp.
344-353, 2005.
[3] J. Yao, M. Makoto, “An experimental study in paraxial
spinning of one tube end”, J. Mater. Process. Technol.,
vol. 128(1-3), pp. 324-329, 2002.