March 2014
217
Tracto-Technik GmbH & Co KG
Article
product, bending geometry and bending process is necessary.
During the project the existing standards has been collected
and evaluated in order to categorise semi-finished products by
their bending-specific parameters. Next was the evaluation of
typical bending geometries, which were likewise categorised
in different quality categories. In the following, the definition of
special criteria for the quality of tube and profile bending parts
are described.
Criteria for bending quality
An important sub-ordinate target on the way to reach the new
standard is the creation of criteria for different categories of
bending work pieces regarding their quality. This quality
standard was defined independently from the production
process. The quality of the bending work pieces was first
defined by a validation of the reached quality of the work pieces
with the quality of the semi-finished part before the bending
process. For this validation, knowledge from the universities
and different bending companies was used
.
A bending component is defined by nine different criteria (see
figure 2). The first four criteria describe the geometry of the
bending part. With the help of the wall factor, the cross section
and the wall thickness deviation, the semi-finished product –
tube or profile – is defined.
The criteria shape and positional tolerances and surface factor
define the complexity of the work piece. Bending components
can be sorted in four categories with a rising complexity in
manufacturing progress.
The geometry of the bending components is critical for the
choice of a bending process. The dimensions of the part have
to be adaptable for the bending machine size. The geometry
of the components can be described by the criteria bending
angle, bending factor, intercept factor and torsion factor. They
are important for the choice of the best process.
The first criterion is the bending angle of the bending arc,
which has three possible categories. Category 1 is from 0°
to 180°. These angles can be reached by most processes
and machines. Work pieces with an angle between 180° and
360° are defined as category 2. Greater bending angles are
classified as category 3. In order to produce bending parts
with bending angles of the categories 2 and 3, additional tools
or machine measurements are necessary. Furthermore, the
kinematics of the machine, the clearance and the free space
should be taken into account in order to prevent a machine
crash.
The maximum strain the material experiences through the
bending process is considered by the bending factor. The
bending factor
B
is defined as the quotient of the average
bending radius
R
and the outer diameter
H
(B = R / H)
[7]
. The
technical limit of the bending factor is determined by the profile
material and the profile cross-section. A minimum bending
factor of B = 0.5 can be achieved geometrically. The bending
factor is subdivided in three categories. Category 1 includes
the production of bending radii, which are five times greater
than the profile outside diameter. At the outer radius, there is
a maximum elongation of 10 per cent. Therefore, almost all
deformable materials can be used for category 1. This usually
requires no separate assessment of the bending method
to prevent wrinkles on the inner arc. Category 2 describes
radii between two and five times the tube outside diameter.
To obtain the cross-section, materials with sufficiently high
uniform elongation must be used and measures are taken to
support the cross-section. Therefore, for closed cross section
geometries mandrels and inner fluid pressures are common
methods for that. Bending factors below the two times outer
diameter of the tube (category 3) call for special precautions
to avoid geometrical differences in the inner arc. The inner
contour of the tube should be supported more effectively.
Furthermore, the choice of the material is limited strongly by
the formability of the material.
The newly defined intercept factor (figure 4) describes the
complexity of the bending geometry. Rays start from the ends
of a component. These rays surround
the limits of 2D bending
parts in one plane of the Cartesian coordinate system. Bending
parts over multiple bending planes should have a pair of rays
in each plane of the Cartesian coordinate system. With the
exception of the angles which are upright to one another in an
Figure 1: Matrix for the choice of a bending process
[5]
Table 1: Criteria for bending parts
[6]
*1: Deformation by torsion the bending part about the axis of gravity
