Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools
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7.9 Hubbing tooling 81
7.9 Hubbing tooling
Hubbing tooling consists of the hub, a reinforcing ring, a retaining ring, a hardened base and
fixing devices. The fixing devices are for keeping the hub and the retaining ring firmly in pla-
ce.
Figure 7.6 Hubbing tooling. 1 retaining ring, 2 armouring, 3 hub support, 4 pressure plate, 5 hub, 7
eccentric ring, 8 die to be hubbed, 9 and 10 pressure plates
Retaining ring (reinforcement, Figure 7.7)
During hubbing, as extreme radial forces occur in the dies to be hubbed, causing high tangen-
tial stresses, the dies must be reinforced in a retaining ring (shrink fit). The pre-stressing of the
retaining ring counteracts the tangential stresses in the tool. The height h of the retaining rings
is equivalent to that of the workpiece. The inner retaining ring a is hardened, with a hardness
of 58 ± 2 HRC. The outer retaining ring is also hardened. Its strength should be around 1200
N/mm
2
. Its external diameter D
1
should be 2.5 D.
Figure 7.7 Layout of reinforcing ring, retaining ring and the workpiece to be hubbed:
a) reinforcement, b) retaining ring, c) die to be hubbed
82 7 Cold hubbing
The tool which actually does the hubbing is the
male hub (Figure 7.8):
– It should not have any sharp edged ridges.
– The head of the hub should be about 20
mm larger than the shank.
– The shank must be finely ground and pol-
ished up to and including the radius of the
junction with the head.
Figure 7.8 Hub
Table 7.3 Hub materials and permissible surface pressure
Material no. p
perm
in N/mm
2
HRC
hardness
1.3343 3200 63
1.2601 3000 62
1.2762 2600 61
7.10 Advantages of cold hubbing
– healthy, unbroken grain structure (Figure 7.9)
– plate-finished smooth surfaces on the hubbed profile
– high dimensional accuracy of the hubbed profile
tolerances: 0.01 – 0.02 mm
– higher tool life due to optimal grain direction and high surface finish
– considerably shorter production times than with production involving chipping.
Figure 7.9 Grain direction: a) with workpiece produced by hubbing í b) by a chip-producing method
7.12 Machines for cold hubbing 83
7.11 Defects during cold hubbing
Table 7.4 Hubbing defects and their causes
Defect Cause Steps to be taken
Workpiece cracks radially Hubbing has taken place without a
reinforcing ring
Reinforce workpiece before hub-
bing and if necessary hub pre-form
which flows more easily.
Work hardening too high
during hubbing
Workpiece has no recesses on the
base and at the sides (Figure 7.4).
Material can not flow outwards.
Make recesses in workpiece.
Hub breaks off. Permissible surface pressure ex-
ceeded.
Hub designed unfavourably.
Carry out intermediate annealing of
workpiece after partial hubbing.
Design the hub head form to flow
better.
7.12 Machines for cold hubbing
Special hydraulic presses are used for hubbing. These machines are characterised by an espe-
cially stable construction and the precision with which the hubbing speed can be controlled.
Figure 7.10 Hubbing press
84 7 Cold hubbing
Table 7.5 Construction measurements of hubbing presses (Sack und Kiesselbach manufacturers)
F
in kN
H
in mm
Piston diameter
in mm
Inside diameter
in mm
Stroke height in
mm
1 600 1 700 210 220 160
3 150 2 000 285 320 250
6 300 2 245 400 415 275
12 500 2 300 570 585 355
25 000 2 700 800 830 380
~ ~ ~ ~ ~
200 000 5 600 – 1 640 400
7.13 Example calculations
Example 1
The aim is to hub a square 15 mm deep into
a press die (Figure 7.11). The square must
have a side width of 12mm.
Material:
X 8 CrMoV 5 Material group I
Find:
hubbing force.
Solution:
Equivalent diameter d
Figure 7.11 Press die
d =
2
1.13 1.13 (15 mm) 16.95 mmA
t/d 12 mm/16.95 mm 0.707
max
p from Table 7.1 for t/d = 0.71 and material group I
max
p = 2710 N/mm
2
F
max max
pA 2710 N/mm
2
· (15 mm)
2
= 609750 N = 609.7 kN
Example 2
The aim is to hub a hexagon 4.2
mm deep into a bolt die made o
f
45 CrMoW5 8. The hexagon
must have a diameter across flats
s = 20 mm.
Find: force and mechanical
work.
7.14 Exercise on Chapter 7 85
Solution:
6
2
ah
A
where 10 mm
2
s
h
2
22
4
a
ah
44
10 11.54 mm
33
ah
Figure 7.12 Hexagon with
measurements
2
11.54 10
6 6 = 346.2 mm
22
ah
A
eq.
1.13 1.13 346.2 21.0 mmdA
22
max
3
4000 N/mm 346.2 mm
1384.8 kN
10
Fp A
4.2
0.2
21
t
d
1384.8 kN 0.0042 m = 5.8 kNmWFt
7.14 Exercise on Chapter 7
1. What is meant by “cold hubbing”?
2. What is this method used for?
3. Describe the hubbing tooling.
4. What has gone wrong if the workpiece cracks during hubbing?
5. What are the advantages of cold hubbing?
6. What are the advantages of the cold hubbing process compared to a chip-producing manu-
facturing process when producing a bolt die with a hexagon cavity?
8 Coining (stamping)
8.1 Definition
Coining is a cold forming process where certain surface forms are produced with low material
displacement.
8.2 Types and applications of coining processes
8.2.1 Coining
In coining the thickness of the material in the starting stock is altered.
Application:
Coin production (Figure 8.1), indenting impressions into badges, coining components for me-
chanical engineering and the electrical engineering industry (Figure 8.2).
Figure 8.1 Coin production
Figure 8.2 Coined control lever (contact piece)
a) blank, b) finished part
8.2.2 Sizing
This is used to give higher dimensional
accuracy to a blank which has already
been pre-formed. For example, drop
forged connecting rods (Figure 8.3) can
be given higher dimensional precision
by sizing the thickness of the hub and
the spacing of the hub centres.
Figure 8.3 Sizing a connecting rod
8.3 Calculation of force and mechanical work 87
8.2.3 Planishing (straightening)
This is used to straighten twisted or warped stamped parts.
By stamping a grid pattern (Figure 8.4) (rough straightening)
stresses can be relieved and the parts straightened out.
Figure 8.4 Grid pattern on a straightening die,
D
angle of the points, t
spacing
8.3 Calculation of force and mechanical work
8.3.1 Force
In force calculation a difference is made between embossing and impressing lettering, and deep
coining. In deep coining, the depth of the relief, and thus also the deformation stress, k
r
, (Table
8.1) is greater than when embossing.
Table 8.1 k
r
values for coining in N/mm
2
Material R
m
in N/mm
2
k
r
in N/mm
2
Embossing Deep coining
Aluminium, 99 % 80 to 100 50 to 80 80 to 120
Aluminium alloy
180 to 320
150 350
Brass, Ms 63 290 to 410 200 to 300 1500 to 1800
Copper, soft 210 to 240 200 to 300 800 to 1000
Steel 280 to 420 300 to 400 1200 to 1500
Stainless steel 600 to 750 600 to 800 2500 to 3200
punch area A
2
0
4
D
A
S
A in mm
2
punch area
D
0
in mm blank diameter
2
2
(mm )
4
D
A
S
max. coining force F
r
F
kA
Figure 8.5 Basic dimensions in coining.
a) punch, b) blank, c) die
88 8 Coining (stamping)
8.3.2 Coining work
WFhx
I
p
roj
V
h
A
01
hhh
W in Nm coining work
x process factor
V
I
in mm
3
volume of impression
A
proj
in mm
2
projected area of the
coined part
h in mm punch displacement
h
0
in mm blank thickness
h
1
in mm remaining thickness
after deformation
The process factor x can be determined from the force-displacement diagram (Figure 8.6).
m
max
F
x
F
0.5x #
Fig. 8.6 Force-displacement diagram dur-
ing coining
8.4 Tooling
Coining assemblies are usually guided by a
pillar guide frame (Figure 8.7). The raised
sections on the punch are the negative of the
indentations to be produced on the work-
piece. As with forging dies, a difference is
also made here between closed dies and
open dies. Closed dies are used with lower
material displacement, such as when pro-
ducing coins. Open dies, which give rise to
burrs on the workpiece, are used when
coining forgings with large deformation
strains.
Table 8.2 shows common die materials.
Figure 8.7 Closed coining die with pillar guide,
a) blank
8.6 Example 89
Table 8.2 Materials for coining punches and dies
Material Material no. HRC assembly hard-
ness
C 110 W1
1.1550 60
90 Cr 3
1.2056 62
90 Mn V 8
1.2842 62
50 Ni Cr 13
1.2721 58
8.5 Defects during coining (Table 8.3)
Defect Cause Steps to be taken
Impression is incompletely formed Too low a coining force Raise force
Cracking on workpiece Material deformability exceeded Carry out intermediate
annealing
8.6 Example
The aim is to coin an impression as in
Figure 8.8.
Material: Ms 63. (Cu Zn 37)
Find: Coining force and coining
work
Figure 8.8 Coined workpiece
2
r
2
N(80mm)
1500 1500 5024 N 7536000 N 7536 kN
4
mm
FkA
2
r
selected from Table 8.1 1500 N/mmk
I
2
proj
50 mm 10 mm 5 mm
1.56 mm
5024 mm
V
h
A
S
max
7536 kN 0.00156 m 0.5 5.89 kN mWF hx
01
1.56 8 9.56 mm.hhh
90 8 Coining (stamping)
8.7 Exercise on Chapter 8
1. What is meant by “coining”?
2. What methods of coining are there?
3. What are the main elements of coining dies?
4. What is coining used for millions of times in practice?