Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools
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20.1 Clinching 251
Materials can be joined with an R
m
of up to 1000 Mpa, the general rule for combination being
that the thicker and/or harder semi is placed on the punch side. One constraint is the required
deformability. With low failure strains (below 10%) cracking appears, mainly on the die side
which is under tensile strain.
Some areas where clinching is used include the automobile industry (Figures 20.3 and 20.4),
white goods (household appliances), ventilation and air conditioning, electronics, medical
appliances and sheet processing in general (Figure 20.4).
Figure 20.3 Hardtop BMW 3 series with 125 clinched joints (Eckold GmbH & Co. KG)
Figure 20.4 Roof rack (Eckold GmbH & Co. KG)
252 20 Joining by forming
20.1.3 Tooling and equipment
Figure 20.5 shows active tooling components with a diagram of their accessibility.
Figure 20.5 left: tooling; right: problem with accessibility (BTM Europe GmbH)
Figure 20.6 shows stationary use; a framing (joining) cell with ten clinching mechanisms for
producing automotive bonnets. At 30 í 100 kN, the forces required for clinching are con-
siderably higher than with spot welding; they require sturdier C-frames.
20.1 Clinching 253
Figure 20.6 Joining cell (Eckold GmbH & Co. KG)
20.1.4 Trends in development
Future applications will require advances in quality, e.g.:
– lower tooling and equipment costs (e.g. Figure 20.7 point joined without a shaping die)
– lower joining forces (e.g. clinching with inclined rotating punch)
– better deformability for brittle materials
– combination with other processes (hybrid joining by clinching and adhesive; internal high-
pressure forming and clinching with the fluid as an active joining tool)
Figure 20.7 Dieless clinching (Fraunhofer Institute for Machine Tools and Forming Technology,
IWU, Chemnitz, Germany)
254 20 Joining by forming
20.2 Punch riveting
20.2.1 Principle
Figure 20.8 shows the riveting process with a punch rivet in four steps. First, the parts to be
joined are fixed by the blank holder (view 1). As the punch moves down, the punch rivet pene-
trates both sheet layers and the scrap is ejected (view 2). As the punch moves on, the conical
head of the rivet begins to mould into the upper sheet layer (view 3). Once the head is fully
moulded in, the ring on the face of the die presses the lower layer of sheet material into the
groove in the rivet shank, creating a positive lock (form closure) overlaid by non-positive
radial compressive stresses (view 4).
Figure 20.8 Illustration of the principle of punch riveting (LWF, Paderborn University, Germany)
In Figure 20.9 the groove can be seen in the rivet shank of a punch riveted joint To join differ-
ent total sheet thicknesses, there is a multi-range rivet with several grooves.
Figure 20.9
Cross-section of a punch riveted joint (Fraunhofer IWU,
Chemnitz, Germany)
20.2.2 Application of the process
Figure 20.9 shows the smoothness of the joint at the head area and its flush fit on both sides
These advantages are often a criterion for its use. The main range of application for this joint is
20.2 Punch riveting 255
between s = 1.5 and 5 mm total sheet thickness, the lower layer requiring a minimum thickness
of around 1 mm in order to fill the groove in the shank.
Even materials of R
m
> 1000 MPa can be joined. In the upper layer, very thin materials (< 1
mm) and non-metals can also be used. Deformability is not a major constraint. The shearing
process results in a very restricted local deformation, meaning that materials with low failure
strain, e.g. Magnesium, can also be processed.
The main areas of application for solid punch riveting are the automobile industry (Figure
20.10) and the rail vehicle industry.
Figure 20.10 Punch-riveted thermal shield for the Audi TT (produced by Kerb-Konus-Vertriebs-GmbH)
20.2.3 Tooling and equipment
Figure 20.11 shows a punch-riveting unit consisting of a robot and a C-frame (also known as a
gap frame). The rivets are processed using a magazine; the drive is a hydraulic cylinder.
Figure 20.11
Punch-riveting unit (Kerb-Konus-
Vertriebs-GmbH)
256 20 Joining by forming
20.2.4 Trends in development
Future development is heading in the following directions:
– lower-cost rivets (manufactured by deformation instead of a chip-producing method), tools
and equipment
– FEM calculation (Figure 20.12)
– lower joining forces; accessibility from one side (e.g. high speed)
– wider spectrum of uses (new rivet shapes or tooling concepts, e.g. split dies)
– corrosion-resistant joint (e.g. ceramic rivets)
– combination with other methods (clinching and adhesion).
Figure 20.12 a) Cross-section and 2D joining simulation; b) 3D stress simulation for punch-riveted
joint (Fraunhofer IWU, Chemnitz, Germany)
20.3 Self-piercing riveting with semi-tubular rivets 257
End-to-end numerical solutions offer a huge advantage in terms of time and expense. How-
ever, they must cover the individual steps of sheet forming, joining and stressing in all their
complexity and the deformation history produced in each case. The simulation of the joining
method shown in Figure 20.12 (final state) can be calculated axisymmetrically (2D). The shear
tension stress, on the other hand, creates a complex stress condition which must be considered
spatially (3D).
20.3 Self-piercing riveting with semi-tubular rivets
20.3.1 Principle
Figure 20.13 shows the riveting process with a self-piercing semi-tubular rivet in four steps.
First, the parts to be joined are fixed by the blank holder (view 1). As the punch moves
down (view 2) the rivet pierces only the upper sheet layer nearest the punch and the
punched scrap is taken up into the cavity of the semi-tubular rivet. As the punch moves on,
the lower part of the self-piercing semi-tubular rivet splits open and the undercut is formed
(view 3). Again, with this positive lock, radial compressive stresses remain which create the
non-positive lock important for cyclic resilience (view 4).
Figure 20.13 Diagram of the principle of self-piercing semi-tubular riveting (LWF, Paderborn Univer-
sity, Germany)
Figures 20.14 shows a self-piercing semi-tubular rivet joint. The pierced upper sheet layer and
the undercut in the lower layer can be clearly seen. In the lower part, the contour of the die is
clearly visible.
Figure 20.14
Cross-section of a self-piercing semi-tubular rivet
joint (Fraunhofer IWU, Chemnitz, Germany)
258 20 Joining by forming
20.3.2 Application of the process
The self-piercing semi-tubular rivet joint has the highest strength of all such joints, especially
compared with spot welding. This advantage is the reason why these joints are permitted for
use in the parts of an automobile subjected to most stress in a crash.
The joint is mainly used in the range of s = 1.5 í 4 mm total sheet thickness. Materials can be
joined with an R
m
of up to 1000 MPa, the general rule for combination being, unlike clinching,
that the thinner and/or softer semi is placed on the punch side.
As with clinching, the required deformability is a constraint. , meaning that it is better to place
brittle materials such as die-cast aluminium on the punch side.
The main area of application for self-piercing semi-tubular riveting is the automobile industry
(Figure 20.15).
Figure 20.15 left: Audi A2 with 1800 semi-tubular rivet joints, right: close-up (Photographs from
Dresden University of Applied Sciences, Germany)
20.3.3 Tooling and equipment
The active components in Figure 20.16 (blank holder, punch and die) demonstrate the basic
layout generally used for all joining processes without pre-punching.
As well as the hydraulic drives used for riveting systems, which usually feature a positive stop
at the bottom dead centre, electrically-driven spindles are also starting to coming into increas-
ing use in production (Figure 20.17). This completely different concept means that different
types of regulation and monitoring are needed and can be used.
20.3 Self-piercing riveting with semi-tubular rivets 259
Figure 20.16
Schematic cross-section of the active components
during semi-tubular riveting (Böllhoff GmbH)
Figure 20.17 C-frame on knuckle-joint basis and spindle motor (without rivet feed; Böllhoff GmbH
works)
260 20 Joining by forming
20.3.4 Trends in development
Similarly to clinching and punch riveting, again the problems to be solved concern
– cost reduction
– improved joinability for high-strength sheet (use of high-strength rivets, workpiece heat-
ing)
– raised process stability (tolerance for process fluctuations)
– monitoring (monitoring of force is the standard; other possible methods are the analysis of
distance or impact sound)
– realistic calculation of the processes
– reduction of joining force
– accessibility from one side
– raised resistance to corrosion (e.g. new kind of coating in a vacuum)
– combination with other methods (punch riveting and adhesion; internal high-pressure form-
ing and self-piercing semi-tubular riveting with the fluid as an opposing force)
The method described above with clinching, with a single, flat opposing die with no shaping
die, can be used for riveting following the same principle (Figure 20.18) with the following
aims:
– no more concentricity required between punch and die, no outlay for setup, low tooling
wear, high process stability; better accessibility during joining
– bump on underside flatter (good for structural space requirement, appearance, cleaning,
lacquering, hybrid joining with displacement of adhesive in the undercut area)
Figure 20.18 Dieless rivet clinching; left: experiment, right: calculation (Fraunhofer IWU, Chemnitz, Ger-
many)