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Joining Mechanical joining Table of contents 5 Mechanical
joining
...................................................................................................................
2
5.1 Introduction
.......................................................................................................................
2 5.2 Comparison of properties of some typical mechanical joints
........................................... 4 5.3 Example: Hardtop
Porsche 911
Cabrio............................................................................
5 5.4 Example: Self-piercing riveting of dissimilar materials
..................................................... 6 5.5
Example: Joining by screws
.............................................................................................
7 5.6 Hemming
..........................................................................................................................
8
5.6.1 Process
principle.......................................................................................................
8
5.6.2 Flat and "Rope" hem
.................................................................................................
9 5.7 Bolting
............................................................................................................................
10
5.7.1 General remarks on bolting with fasteners and inserts
.......................................... 10
5.7.2 Press nuts and bolts Features and process
........................................................ 11 5.8
Clinching
.........................................................................................................................
12
5.8.1 Principle methods and application potentials
......................................................... 12
5.8.2 Single-step clinching Process steps
....................................................................
13 5.8.3 Quality-criteria for a connection
..............................................................................
14
5.8.4 Design criteria
.........................................................................................................
15 5.9 Self piercing riveting
.......................................................................................................
16
5.9.1 Self piercing riveting Principle
..............................................................................
16 5.9.2 Design criteria
.........................................................................................................
17
5.9.3 Quality-criteria
.........................................................................................................
19
5.9.4 Self-piercing bolts and nuts
....................................................................................
20
5.9.5 Riveting process Self-piercing nuts
.....................................................................
21 5.9.6 Types of self-piercing bolts and nuts
......................................................................
22
5.10 Blind riveting
.................................................................................................................
23
5.10.1 Blind rivet systems Rivets, Nuts and bolts
......................................................... 23 5.10.2
Blind rivet and riveting process
.............................................................................
24
5.10.3 Blind rivet
types.....................................................................................................
26
5.10.4 Blind riveting bolts and nuts
..................................................................................
27
5.10.5 Riveting process Blind riveting bolts and nuts
................................................... 29 5.10.6 Types
of blind riveting bolts and nuts
...................................................................
31
5.11 Self threading screws
...................................................................................................
32
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5 Mechanical joining
5.1 Introduction Literature:
Hahn, O.; U. Klemens, Fgen durch Umformen - Nieten und
Durchsetzfgen - Innovative Verbindungsverfahren fr die Praxis.
Dokumentation 707, Dsseldorf: Verlag und Vertriebsgesellschaft,
1996, ISBN 3-930621-56-8
Klemens, U. and Hahn, O.: Nietsysteme, 1994, ISBN 3-922293-32-8
Ostermann, F. and 8 Co-authors, Aluminium Materials Technology for
automobile
construction, English translation edited by Roy Woodward, London
Mechanical Engineering Publications Limited, 1993, ISBN 0 85298 880
X
Ostermann, F., Anwendungstechnologie Aluminium, Berlin,
Heidelberg, London, New York, Tokyo: Springer-Verlag, 1998, ISBN
3-540-62706-5
Section of hardtop Porsche 911: blind-riveting nuts and
self-piercing rivets
Source: LWF-Univ. Paderborn
Because of the sensitivity of work-hardened and age-hardened
aluminium alloys for the heat input from fusion and resistance spot
welding and due to the effects of oxide films on RSW electrode life
the use of "non-thermal" joining techniques have gained particular
importance in automotive applications. Mechanical joining
techniques like bolting, self-piercing riveting, blind riveting,
clinching and the combination of these techniques with adhesive
bonding have, therefore, been developed to substitute the
traditional resistance spot welding. The different mechanical
joining methods have found application in the joining of sheet
components and also for use in joining of extrusions and castings.
Some examples are depicted in the figure below.
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Source: ATZ
These methods are described in the following subchapters.
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5.2 Comparison of properties of some typical mechanical
joints
The table below gives a brief survey of static and fatigue
properties of typical mechanical joints in carbody sheet materials.
Values illustrate some effects of type of sheet alloy and typical
thickness on tensile properties and are not meant for design
calculations. The fatigue values are R=0.1
Typical mechanical properties of some mechanical joining
methods
Source: Niedermeier, Alcan
Contrary to screw bolt joints, mechanical joints are generated
by local plastic deformation of either the fastener or the
work-piece or both. For this reason joint properties depend
strongly on the chosen tooling and fastener parameters. Please
contact the tool or fastener supplier for details.
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5.3 Example: Hardtop Porsche 911 Cabrio Description:
Materials:
exterior panel: AA6016-T4, 1.2 mm interior panel: EN AW-5182-O,
1.5 and 2.25 mm
Joining methods: Mechanical joints:
self-piercing rivets: 200, press nuts: 27, screw bolts: 12,
Adhesive tape: 2000 mm (in combination with self-piercing
rivets), Stud welding: 10.
Porsche 911 Hardtop
Courtesy: Porsche AG, LWF Paderborn
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5.4 Example: Self-piercing riveting of dissimilar materials
Aluminium side impact beam with steel sheet stampings joined by
riveting (Opel Astra)
Source: Alcan
Same as above figure: details of joints
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5.5 Example: Joining by screws Combination of aluminium
extrusion with magnesium die castings and die cast / forged
aluminium connecting pieces joined by screw bolts. The figure shows
the instrument panel support of the DaimlerChrysler A-class
model.
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5.6 Hemming
Flat hem of alloy 6016-T4
5.6.1 Process principle Literature:
Baartman, R., Atzema, E.H., Bottema, J.: Optimisation of the
hemming process for AA6016-T4 Aluminium Body Sheet. Proc. ISATA98,
98NM056, (1998)
Hemming, bordering and folding methods are fundamental joining
techniques for the manufacturing of automotive structures. Hemming
belongs to the cold forming processes. The quality of the hem
w.r.t. minimum bend radius depends on:
the aluminium alloy, the heat treatment of the aluminium, the
forming history of the sheet / part, the thickness of the aluminium
sheet and the forming process parameters.
Step 1: bending 90, Step 2: hemming 45, Step 3: folding 180
Source: Anwendungstechnologie Aluminium
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5.6.2 Flat and "Rope" hem The figures below show two possible
designs of hemming:
The type 1 shows the standard geometry of flat hemmed sheets.
The type 2 geometry ("Rope Hem") should only be chosen if the
ductility of the
aluminium sheet is critical.
In order to maintain the bead (rope hem) radius during the final
folding operation, the punch
can be designed with an inclined surface with an angle to the
horizontal which can be varied, depending on the sheet thickness
and the minimum allowable inside bending radius, see figure
below.
Forming a ''Rope Hem''
Source: K. Siegert, TALAT Lecture 3706
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5.7 Bolting
5.7.1 General remarks on bolting with fasteners and inserts See
also:
AAM Joining 1 Fusion welding > Stud welding AAM Joining 5
Mechanical joining > Self piercing riveting AAM Joining 5
Mechanical joining > Blind riveting > Blind riveting bolts
and nuts
Bolted or threaded connections for the attachment of equipment
to aluminium components and structures may be achieved by simply
bolting through the aluminium part in a comparable way as for
conventional blind riveting. It is often necessary to provide
internal support if bolting through a closed section such as for
the engine or suspension attachments to the body front rails. This
support can be with tubes or extrusions fixed inside the section to
prevent the section from collapse under high installation loads.
Bolted connections can also be achieved with threaded studs and
nuts fixed to the aluminium part. Aluminium threads are not
recommended for situations where frequent removal for service is
required, but can be applied for lightly loaded connections for
internal trim, electrical harnesses, equipment attachment etc. Care
must be taken if bolting material combinations are used, which are
critical with respect to galvanic corrosion. Except for stainless
steel, all steel inserts assembled into aluminium parts must be
coated to prevent galvanic corrosion. Insert manufacturers can
supply a range of suitable coatings. Sealants, gaskets or
protective coatings may be required in severe corrosive
environments, whereas simple surface treatment of the steel and/or
the aluminium may be adequate in a dry internal environment.
Aluminium welded studs and nuts are available (s. LINK) that may be
welded directly to the aluminium parts by an electric arc welding
process. Alternatively, steel threaded studs and nuts may be
installed for applications where higher strength or frequent
dismantling may be necessary. The selection of insert type depends
upon the strength (torque) required and whether access is only
possible from one side (blind) or both sides of the aluminium part.
Steel inserts for studs and nuts are available that can be
installed in pre-pierced holes in the aluminium part. The insert
installation can sometimes be incorporated in the press line after
the forming, trimming and piercing operations for a stamped part.
Of course, they can be also installed separately at any stage in
the assembly sequence including in-process and in-service repair.
Fixed studs and nuts are also available that do not require
pre-pierced holes and which can be installed in a single operation
in the press line or as a separate operation. Fixed nut inserts are
available that are sealed to prevent any leakage through the joint.
Some fixed stud and nut inserts leave a raised element on the
opposite side that must be allowed for in the design of subsequent
assembly of the part. Due to the large variety of possible
solutions, no performance values can be quoted here, please refer
to product suppliers for more detail information.
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5.7.2 Press nuts and bolts Features and process
double sided access not necessary pre-punching or drilling
necessary high load bearing capability of screw threads in thin
panels with special blind rivet nuts additional properties, e.g.
gas- and water tightness
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5.8 Clinching
5.8.1 Principle methods and application potentials Clinching
involves the joining of two materials by local forming and does not
require an extra fastener. A punch forms the two materials to be
joined into a die. A button is formed on the underside and provides
an interlock between the sheets. Clinching works with aluminium
combinations, with multi-material combinations, like aluminium and
steel, and also with pre-coated or galvanized materials. Clinching
can also be combined with a sealant or an intermediate layer that
acts as a sound dampener. The clinching process doesn't build any
thermal stresses into the work-piece, so a clinched joint performs
well where there's the potential of thermal fatigue or heat. The
two main principles in clinching are single stroke and double
stroke. Single-stroke clinching requires special tool sets for each
set of parameters, especially sheet thickness. While double-stroke
clinching can adapt to a range of thicknesses, it requires a larger
capital investment and is difficult to integrate into the stamping
press line.
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5.8.2 Single-step clinching Process steps Single-step clinching
is the most commonly used clinching method in automotive joining
operations. The process sequence is illustrated (below) in 4
steps:
Single-step clinching without cutting
Step 1:The punch and blank holder move downward, the work pieces
are clamped and fixed by spring force of the blank holder. Step 2:
By action of the punch the material flows into the bottom die
cavity forming a cup. The process parameters and dimensions of the
punch and die are finely tuned to the sheet thicknesses of the work
pieces. This insures that no material is laterally drawn into the
joint from surrounding area. Step 3: Finally, the thickness of the
cup's bottom is reduced by upsetting and the material forced into
the die groove and in lateral direction, forming the necessary
undercut. Step 4: After reaching a preset maximum force (force
control) or a preset displacement (stroke controlled), the punch is
retracted and the clamping force relieved. The joint connection
requires no finishing.
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5.8.3 Quality-criteria for a connection For round button
non-cutting clinching techniques the strength of the connection is
determined by the magnitude of the undercut and the neck thickness.
These values are influenced by the tool dimensions, such as the
punch diameter and the depth and diameter of the die cavity, as
well as by the setting of the displacement limits for the
upper-die. The residual bottom thickness correlates well with the
joint strength and can be used as a non-destructive quality control
measurement. A larger undercut can be achieved by reducing the
residual bottom thickness. However, to avoid overloading the tools
and work piece due to excessive joining forces, a compromise
between maximum joint strength and tool life is required.
Quality criteria for a clinched connection
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5.8.4 Design criteria For best results when dissimilar materials
are being joined the rivet is generally applied from the direction
of the thick sheet into the thin sheet, or from the high strength
material into the low strength material. Various suppliers offer
tooling designs which can be optimised for the special automotive
applications. The strength of a clinched joint depends basically on
four main factors:
the type of aluminium alloy of the work pieces, the sheet
thickness, the clinch button size (the diameter should be as large
as possible), the surface condition of the material a completely
dry, grease free surface will give
a stronger joint then if the surface is oily or wet (a minimum
lubrication, however, avoids adhesion and significantly improves
the tool life).
Rules of joining direction for different materials and
materials' thickness:
"thick sheet into thin sheet" or
"high strength into low strength" The flange width "D" -
distance from the edge where the clinch spot is to be placed - must
be sufficient to ensure that there is material to contain the
deformed clinch spot and sheet.
The button may otherwise burst out of the edge of the flange or
cause distortion in the joint. Proper overlap of the layers to be
joined and a correct flange width will also help ensure proper
alignment between the work-piece, punch and die. A pre-clamping
step may be helpful if joining a flange width close to the minimum
width is to be undertaken. The clinch spots should be spaced to
avoid contact with previously driven clinch spots or the strained
area immediately around them. Placing several clinch spots too near
to each other may cause distortion or some bending of the joint. A
pre-clamping step can help to minimise this. A sufficient number of
clinching spots must, however, be used to guarantee the overall
design strength of a section. A precise relationship between part
fit-up, alignment and joint quality is not easy to quantify.
However, good control of these two variables will help ensure that
the layers of material to be joined are drawn together properly as
the clinch spots are driven and set. In addition, force will not be
diverted into pressing parts.
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5.9 Self piercing riveting
5.9.1 Self piercing riveting Principle Self-piercing riveting
requires no pre-drilling of holes into the mating work pieces. A
punch and die are used to complete the joining operation in a
single step. With solid rivets, the punch drives the rivet which
pierces the sheet plies completely. Using semi-tubular rivets, the
punch drives the rivet which pierces the top sheet and is set into
the work-piece by partially piercing the bottom layer. A shaped die
on the underside reacts to the setting force and causes the rivet
tail to flare within the bottom sheet. This produces a mechanical
interlock which includes the added rivet joining element and
creates a button in the bottom sheet. The length of the rivet tail,
hole diameter and hole depth to shank diameter, and the design of
the tooling mainly determine the final shape of the rivet and of
the button on the underside of the joint. There is a wide choice of
rivet forms. The rivet is generally semi-tubular but may also be
solid. The self piercing rivets are generally made of steel coated
with AlZn powder to prevent galvanic corrosion. In addition, also
aluminium self piercing rivets can be used in special cases.
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5.9.2 Design criteria Countersunk heads can provide a flush
finish in the top sheet and even ensure colour matching to
organic-coated or pre-painted material. For best results when
dissimilar materials are being joined the rivet is generally
applied from the direction of the thin sheet into the thick sheet,
or from the low strength material into the high strength material.
Various suppliers have developed rivet and tooling designs which
can be tailored to the special automotive application. Also
materials such as organic-coated and pre-painted aluminium sheets
which are usually non-weldable can be joined by self-piercing
riveting. Riveting of pre-finished material can eliminate the need
for post-joining painting of parts. The ability to join dissimilar
materials such as aluminium, steel, plastics and composites opens
up new exciting opportunities for this process. Rules for
self-piercing riveting of different materials and thickness:
"thin sheet into thick sheet" or
"low strength into high strength". Estimating the length L of a
semi tubular rivet:
3 mm rivet diameter: L3 = thickness of sheet plies + 2.5mm
5 mm rivet diameter: L5 = thickness of sheet plies + 3.5mm The
flange width D - i.e. distance from the edge to where the rivet is
to be placed - must be sufficient to ensure that there is enough
material to contain the deformed rivet and sheet. The button may
otherwise break out of the edge of the flange or cause distortion.
Proper overlap of the layers to be joined and a correct flange
width will also help ensure proper alignment between the
work-piece, punch and die. A pre-clamping step may be helpful if
joining a flange width close to the minimum width. Rivets should be
spaced to avoid contact with neighbouring rivets or the strained
area immediately around them.
Since the rivets are made of harder material than the work
pieces, riveting over an existing joint may result in serious
damage to the tooling. Placing several rivets too near to each
other may cause distortion or some bending of the joint. A
pre-clamping step can help to minimise this. Poor fit-up and
alignment may reduce joint performance and accelerate tool
wear.
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Recommended Joint Design
Flange width should be sufficient to contain deformed rivet and
sheet Ensure adequate spacing between rivets Ensure good fit up of
stampings.
A precise relationship between part fit-up, alignment and joint
quality is not easy to quantify. However, good control of these two
variables will help ensure that the layers of material to be joined
are drawn together properly as the rivets are driven and set.
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5.9.3 Quality-criteria In self-piercing riveting with
semi-tubular rivets as well as with solid rivets, the strength of
the joint is determined by the amount of undercutting as shown in
the figures below. Since the tool and rivet dimensions are
carefully tuned to each other and to the joint thickness, the
amount of interlock is determined by the "compression measure" (s.
figures below), which can serve as a non-destructive quality
criterion when compensated for the position of the rivet head
within the joint.
The figures below show quality relevant measures for joints with
semi-tubular and solid rivets.
Semi-tubular rivet
Solid rivet
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5.9.4 Self-piercing bolts and nuts Characteristic features:
double sided access necessary without pre-punching or drilling
necessary high load bearing capability of screw threads in thin
panels great resistance to static and dynamic loads no damage of
coatings
The figures show terms of a self piercing nut and bolt and the
geometrical parameters.
Terms of self-piercing nuts and self-piercing bolts
Geometrical parameters
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5.9.5 Riveting process Self-piercing nuts The riveting process
for self-piercing nuts is illustrated in the figure below. The
process for self-piercing bolts works in analogy.
Riveting process: self-piercing nuts
Step 1: Blank and self-piercing nut are both positioned. Step 2:
In a single-step mounting process, the joining element punches a
hole through the sheet blank and in combination with the die
generates a positive connection with the material of the sheet
blank. In the process, material flows into a circumferential
groove. Step 3: The punched slug is pressed out of the sheet blank
by an ejector. Step 4: The joint has been created. As a rule, the
geometry of the nut does not undergo any alteration. The joint is
flush on one side.
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5.9.6 Types of self-piercing bolts and nuts The standard form of
the self-piercing nut has a rectangular geometry, which ensures a
torsion protected joint via a positive connection. Depending on the
design type of the nut elements, the sheet blank is to be prepared
with or without a bead. Rotational symmetrical self-piercing nuts
require grooves and/or a beam for a positive connection for a
torsionally strong seat in the sheet blank. Round shoulder nuts are
used to attach dynamically highly-loaded components. Positive and
non-positive connections, which can be loaded from both directions,
are achieved via beading. Self-piercing bolts are joined in one
self-punching step in a manner such that a plane bolting surface
results. Acting forces from operating loads can be equally well
accepted in both traction and compression directions.
Types of self-piercing bolts and nuts
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5.10 Blind riveting
5.10.1 Blind rivet systems Rivets, Nuts and bolts See also:
AAM Joining 5 Mechanical joining > Self piercing riveting
Literature:
Grandt, J.: Blindniettechnik, 1994, ISBN 3-478-93115-0 Grandt,
J.: Blindnietgewindesysteme: Typen, Verarbeitung,
Einsatzbereiche.
Landsberg/Lech: Verl. Moderne Industrie, 1998. (Die Bibliothek
der Technik; Bd. 159) ISBN 3-478-93174-6
Only rivet systems amenable to fully mechanised fabrication are
preferably applied in the automotive industry. Rivet technologies
can be subdivided into two groups: 1. rivet systems requiring
pre-punched holes. 2. self piercing systems which require no
pre-punched holes. The first category includes standard (upsetting)
rivet systems. However, blind riveting fasteners, which can be
applied from one side only, are or greater importance in automotive
applications. For information of self-piercing riveting systems,
see Links. Blind riveting is particularly useful joining technique
for repair work on aluminium automotive structures, often combined
with adhesive bonding. Characteristic features of blind riveting
systems:
double sided access not necessary, pre-punching or drilling
necessary, high load bearing capability of screw threads in thin
panels, with special blind rivet nuts additional properties, e.g.
gas and water tightness.
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5.10.2 Blind rivet and riveting process Literature:
Grandt, J.: Blindniettechnik, 1994, ISBN 3-478-93115-0 The blind
rivet consists of the rivet sleeve equipped with a tool pin. While
the shaft of the tool pin is discarded after setting, the pin head
remains permanently attached, see figures below.
Typical blind rivet details and forms
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Blind riveting process
The blind rivets are classified in categories A, B, C and D (s.
Lit.). For structural purposes the C and D categories have to be
chosen.
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5.10.3 Blind rivet types Blind rivets are available in different
designs. The selection depends on the respective requirements, e.g.
component material and strength, examples are:
Standard blind rivet, Multirange blind rivet, Impervious blind
rivet, Split blind rivet, Press-tab blind rivet, Pull-through blind
rivet and Drive-in blind rivet.
Blind rivet types
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5.10.4 Blind riveting bolts and nuts Blind-riveting nuts and
bolts are thread-bearing insert fasteners. They are inserted into a
pre-punched hole from one side and efficiently and rapidly set with
a processing tool. The figures show terms and geometrical
parameters of blind rivet nuts and bolts.
Terms of blind-riveting nuts and bolts
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Geometrical parameters
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5.10.5 Riveting process Blind riveting bolts and nuts Setting of
blind riveting bolts and nuts proceeds in analogy to the setting of
blind rivets, but requires a rotary action to release the inserted
fastener from the chuck or mandrel.
Riveting process: blind rivet bolts
Source: LWF, University of Paderborn
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Riveting process: blind rivet nuts
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5.10.6 Types of blind riveting bolts and nuts Blind-riveting
nuts and bolts are set from one side without the need for
additional finishing work. The figure shows different types of
bolts and nuts.
Types of blind rivet bolts and nuts
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5.11 Self threading screws Due to the lightweight philosophy of
aluminium automotive structures, the single aluminium components,
i.e. stamped sheets, die castings and extrusions are designed with
thin gauge dimensions. This typically means for
aluminium sheets: 1 1.5 mm aluminium die casts: 1.5 2.5 mm
aluminium extrusions: 1.8 2.5 mm
Regarding screw joints this thin gauge design causes a limited
load bearing length of the screw. Different methods are used to
increase the load bearing strength:
creating a collared hole by plastic forming of the sheet: -
flowing drill screw, flow hole forming, collar forming
use of additional fastening elements like - pressed nuts, spring
nuts, etc.
5 Mechanical joining5.1 Introduction5.2 Comparison of properties
of some typical mechanical joints5.3 Example: Hardtop Porsche 911
Cabrio5.4 Example: Self-piercing riveting of dissimilar
materials5.5 Example: Joining by screws5.6 Hemming5.6.1 Process
principle5.6.2 Flat and "Rope" hem
5.7 Bolting5.7.1 General remarks on bolting with fasteners and
inserts5.7.2 Press nuts and bolts Features and process
5.8 Clinching5.8.1 Principle methods and application
potentials5.8.2 Single-step clinching Process steps5.8.3
Quality-criteria for a connection5.8.4 Design criteria
5.9 Self piercing riveting5.9.1 Self piercing riveting
Principle5.9.2 Design criteria5.9.3 Quality-criteria5.9.4
Self-piercing bolts and nuts5.9.5 Riveting process Self-piercing
nuts5.9.6 Types of self-piercing bolts and nuts
5.10 Blind riveting5.10.1 Blind rivet systems Rivets, Nuts and
bolts5.10.2 Blind rivet and riveting process5.10.3 Blind rivet
types5.10.4 Blind riveting bolts and nuts5.10.5 Riveting process
Blind riveting bolts and nuts5.10.6 Types of blind riveting bolts
and nuts
5.11 Self threading screws