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19 International Conference on Production Research
MANUFACTURING FEATURES IN CUTTING SHAPES AND
PUNCHING HOLES IN SHEET METAL
M.Lohtander, J. Varis.
Department of mechanical engineering, Lappeenranta University of Technology,
P.O.Box 20 53851, Lappeenranta, Finland
Abstract
By examining sheet metal parts it is rather evident that each sheet metal shape has its own manufacturing
procedure. There are several manufacturing devices behind each procedure. This means that each shape
can be divided into manufacturing features, which consist of shapes of sheet metal and their manufacturing
methods. When creating rules and structures for manufacturing features, it is possible to prepare simple
guides for engineering designers so that they can design parts which are easy to manufacture.
A manufacturing guide can be a kind of database. A designing engineer may follow the guidelines of the
database. It proposes different kinds of possibilities to produce sheet metal parts. This enables the design of
products which are easy to manufacture. In addition, it allows the designer to diversify his or her knowledge
of manufacturing methods.
In this study, simple sheet metal parts and their manufacturing features are studied. For example, simple
shapes such as holes can be made in several ways. Cutting a hole consists of several attributes which
determine the requirements for manufacturing. This study tries to validate what kinds of factors are needed
in order for the engineering designer to design parts which are simple to manufacture.
Keywords:
Sheet metal, sheet metal manufacturing, sheet metal features.
1 INTRODUCTION
Modern design and manufacture is often distributed to
several actors. For designers, this inevitably leads to a
decrease
in
manufacturing
know-how
because
manufacturing is situated at a distance from design. Thus
the possibilities and limitations involving manufacturing
cannot be taken into consideration in the design of new
products because the activity is distributed. Therefore,
there should be a system that allows the processing of
information between design and manufacturing. Modern
information technology provides ample possibilities for this.
As the design process progresses, the designer can
compare the effect of different shapes on manufacturing
technologies and possibly also on costs. The iteration
rounds are then reduced, which takes the process a step
closer to the “ready at once” principle.
The main responsibility of detailed design is increasingly
being shifted from manufacturers towards businesses
specialising in design services. These design companies
can be located at a distance from the client and the
manufacturing organisation. As the distances grow, the
interaction between different operations becomes difficult.
The importance of the design stage in the product life cycle
cannot be over-emphasised because at that point 70–80%
of the manufacturing costs are established. As open and
innovative an approach as possible in the early design
stage is advisable, even indispensable, in order to obtain
the desired position on the market. An innovative approach
may result in significant savings in material and production
costs, but may also make the product very difficult to
manufacture.
2 CHALLENGES IN DESIGN
Computer-aided design (CAD) of sheet metal components
is rather easy at present. However, the seemingly easy
design process can result in problems in the manufacturing
properties of sheet metal components. This is due to the
fact that design software programmes have a number of
automatic features that facilitate drawing a component, but
do not automatically improve its manufacturability.
Consequently, one should always take into account the
manufacturing technologies used, and consider the
consequences of selecting a certain technology. This is
why designers should have sufficient experience or guides
to help make the product as manufacturing friendly as
possible.
Manufacturing high-quality sheet metal products from the
best materials, with the appropriate manufacturing methods
and at a low cost is very challenging. This can, however,
be seen as a possibility, because each manufacturer faces
the same challenges and the one with the best response to
them will end up the winner on the market. In addition to
the actual designing work, designers must take into
consideration the manufacturability and lead time. There
are many ways to simplify the construction, decrease the
number of components, cut-down on raw material use and
simplify manufacture. One way is to apply manufacturing
features to the design and manufacture.
The feature based model is composed of a variety of
features, which in the case of a sheet metal part include
e.g. material, edge length and width, bending angles and
tolerances. Features also provide a basis for organising
manufacturing information and thus utilising it in the
design, further development or quality assurance of future
products [1] [2].
3 MANUFACTURING BASED ON FEATURES
Product information has traditionally been recorded in
drawings, on paper, in users' manuals and in suppliers'
instructions. Currently, these methods have largely been
replaced by CAD models and a range of databases, which
contain information on e.g. materials, production
management, equipment and tools. Nevertheless,
traditional drawings, CAD models and 3D models are
incapable of presenting and especially transmitting
information between different systems.
Therefore, during the past decade research has been
carried out on how a model should include not only the
product geometry but also other information, such as
manufacturing information. A solution to this is provided,
for instance, by feature based modelling [3] [1]. It allows to
take into account the limitations of a manufacturing
technology and to create manufacturing information
automatically [2]. Regardless of the fact that more
advanced CAD systems use information on features in
internal communication, features cannot, with some
exceptions, be transferred outside the system. Moreover,
the internal feature recognition of the software solutions will
not necessarily function if the transferred product or part is
not modelled with the same software version. Some
attempts have been made to turn information transfer
formats into industrial standards even though they are not
open and, apart from a few attachments, cannot transfer
anything other than geometry [4]. Feature based modelling
is increasingly applied in CAD/CAM software [5], but not
even these systems support open information transfer –
they merely save the information on features in their own,
closed transfer format.
Products can be classified according to geometrical, nongeometrical, assembly [6] and manufacturing features [7].
Manufacturing features are shapes generated in a
particular sheet metal production method, or characteristics
of a product that demand certain manufacturing measures
[2]. These features are different from e.g. design features
used in designing [7].
Manufacturing features are, to a certain extent, related to
the physical measurements and geometry of a part, and
also to the characteristics required of a sheet metal
product, such as lightening, whether an angle is right and
aligned, and similar features which need yet different
manufacturing technologies. Sometimes the combination of
certain features and shapes results in a new feature. For
example, the size of the blank introduces the need to cut
away the undesired material from around the part. Cutting
is a manufacturing feature, which requires taking physical
measures [2].
Therefore, features contain not only
geometry, but also functional information for e.g.
manufacture or assembly [7].
3.1 Advantages of accurate design
Manufacturing errors resulting from design are often
noticed only when the first batch of the product is being
manufactured. If the first batch is large, the error may
produce considerable costs [2]. No changes should be
made to a product without first considering their effect on
manufacturability. The designer should thus stop to think
about production as well as the product itself [2].
If the manufacturing volumes are great and changes to
them are made gradually, the time spent on designing the
product and its manufacture is rather short compared to
the total manufacturing time of the products [2]. In
conclusion, a great deal of time can be spent on design if
the designing is efficient and reduces manufacturing costs.
3.2 Research objective
The objective of this study is to research the use of feature
classification and its applicability to the manufacture of a
sheet metal part by cutting or punching.
4 CASE EXAMPLE
Manufacturing features include the most common shapes
in sheet metal parts, such as bending, marking and
threads, as well as characteristics, such as size and
material. Figure 1 shows the feature classification of sheet
metal parts. The manufacturing feature class “cutting” is
divided here into two sub-classes, “edge” and “hole,
opening”. The edge and hole are seemingly similar
features, but their manufacturing technologies may differ
significantly. An edge can be cut with a guillotine shear or a
corner punching machine. These processes cannot
manufacture a hole or an opening in a sheet metal part. On
the other hand, many processes for cutting a hole can be
used to cut an edge. These methods include e.g. laser
cutting and nibbling.
.
Figure 1: The manufacturing feature "cutting" is examined here. Cutting is divided into two sub-features, cutting an edge and
cutting a hole or opening.
4.1 Case part
In this study, we will use a fictitious sheet metal part as an
example, Figure 2. The part has 5 edges and 3 holes,
which are countersinked (a cone-like embedding is made
for the screw-head). The part is made of an aluminium
alloy 2 mm thick. With regard to the material, the part can
be considered a regular, uncoated sheet metal product
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19 International Conference on Production Research
In the second stage, it is established whether the openings
are part of a group of whether they are individual ones. The
purpose of a hole group is to ascertain whether only one
punch is required with a so-called serial tool. The diameter
of the openings and especially the distances between them
must be defined accurately. If this is not done, it may be
difficult to distinguish a set of holes close to each other
from an actual group of holes.
The next step is to study the geometry or other information
entered into the model to establish the shape and accuracy
requirements of the holes and edges. An opening in a
sheet metal part may be e.g. a cutout with no specific
requirements in terms of size or tolerances.
Figure 2: The model part used in this study. The part has 5
edges and 3 countersinked holes. The raw material is a 2
mm uncoated aluminium alloy.
The case part is spread out in Figure 3. The countersinking
is performed on the same surface. The spread-out figure is
composed of simple shapes, such as right, 90-degree
angles and circular arches. The figure shows that the part
has two lightenings, which are shaped like a rectangle.
Also a tear-shaped or circular lightening could be used.
Shape identification is needed for the use of standard
tools. Punching tools should be used as much as possible
because punching is often the least expensive way to
manufacture sheet metal parts.
Identifying the shape and accuracy are at the same level in
feature thinking because the manufacturing technology of
the shape has a definitive impact on the accuracy
achieved. For instance, this is the case for the abovementioned cutouts, which the designer has dimensioned
specifically as cutouts. They can be manufactured in a
number of ways, but the least costly alternative may be
punching. Punching can be performed with a single stroke
or by nibbling, the first of which is the least expensive. A
single stroke can be done with tools of different sizes.
However, the size range for the cutouts must be
determined so that the actual size of the opening does not
deviate excessively from the design.
When the part is designed, it is given a number of
attributes in terms of geometry and other features. At this
stage, these features are examined. They can include
material, surface treatment, bending or any other aspect
included in the feature classes in Figure 1.
Figure 3: Case part spread out.
5
SELECTING A MANUFACTURING METHOD BASED
ON MANUFACTURING FEATURES
The case part can be designed with nearly any CAD
system. The only requirement is a spread-out picture of the
model that is saved in VRML format. This is the format
needed to convert the model into X3D form. If the model is
in X3D form, its features can be recognised for instance
with the Pro-FMA software [8]. Features are identified with
the help of two modules in the programme, which its
developers [8] call the X3D edge recognize and X3D face
recognize algorithms. Although the software is developed
for assembly feature recognition, the above-mentioned
algorithms can also be used to identify manufacturing
features because the recognition processes are similar.
5.1 Stages of feature recognition
Figure 4 presents the classification of manufacturing
features applied in this study. Designing a part based on
features starts by identifying the shape of the part and
possible openings within its edges.
At this point, mechanical and thermal methods are
separated because certain requirements may exclude the
use of any thermal methods. In such cases, thermal
methods need not be included in the comparison. A similar
approach is taken to examine the tolerances of the part
and its conditions, which are divided into position and
geometrical tolerances. Geometrical tolerances may refer
to e.g. the alignment of the opening or edge. In addition,
the diameter of each opening is compared to the thickness
of the material. This helps to ensure, for example, that
punching tools are not used to make very small holes.
In the final stage, all of the features above are studied and
compared to different manufacturing processes. At each
stage, information is saved on aspects previously
mentioned. When the process is selected, the information
is compared to the process database. Based on this
comparison, the system selects the processes suitable for
manufacturing the part. There may be one possible
manufacturing method or more. There is also the possibility
that the system cannot recommend a suitable process
based on the prevailing conditions. In such cases, the
shape or feature rejected by the system should be
redesigned.
The system does not take costs into consideration at any
point. Instead, it selects methods based purely on technical
features and limitations. Cost comparison could be
incorporated into the system, which would allow to
compare the costs of each individual feature. This would
enable a detailed optimization of the manufacturability of
each feature with regard to costs.
Figure 4: Main stages of the process selection based on feature classes and features in cutting and punching sheet metal
parts. After the process selection, a cost comparison can be carried out between the processes.
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19 International Conference on Production Research
5.2
Processes for edges and holes and their
parameters
Cutting shapes and punching holes in a sheet metal
product seems a very simple task. From a technical point
of view this is true in many cases, but decisions involving
the manufacturing technology and method applied can be
difficult to make. For example, the size of the hole in
proportion to the material thickness has a significant impact
on the choice of technology. Likewise, tolerance
requirements have an effect on the technology with which
the desired accuracy can be achieved. Requirements
concerning the quality of the shear surface also affect the
choice of technology. If the hole cannot be cone-shaped,
punching will most likely have to be excluded.
If the part has no requirements apart from a certain shape
and non-toleranced shapes, the simple choice between
laser cutting and nibbling may be very difficult. No universal
model for selection exists – the choice is usually casedependent.
In manufacturing holes, just as in cutting edges, the effect
of tolerances and surface treatment must be taken into
consideration. With thermal methods, the coating may be
damaged. The coating may then need to be repaired in the
damaged areas. Cutting and punching holes mechanically
reduces the risk of damaging the surface. Due to the shape
of the part, for instance a corner punching machine could
be an alternative to laser or nibbling. However, in such
cases curves must be cut with a laser or a tool with exactly
the right rotation diameter. In theory, it is also possible to
use a straight tool, but the result is not necessarily very
good.
Another problem may be deciding when to change the tool
manually. This question is often thought to be related to
batch size, but it can also be posed for individually
manufacture parts with certain quality requirements. This is
related to questions on the operating time of the machine,
i.e. how long the process takes overall and how much
individual process stages cost. In other words, is it less
costly to use a slower process or stage in order to achieve
the most economical solution overall?
6 CONCLUSIONS
From the simple observation that behind each shape of a
sheet metal product lies a restricted group of machines, we
can deduce that sheet metal shapes can be classified into
manufacturing features. Thus, each shape or feature
corresponds to a feature class or feature. By preparing
rules and restrictions for the feature classes and features,
the design of sheet metal products can be made more
manufacturing friendly. This study focuses on feature
classification in cutting and punching holes in a sheet metal
part.
The classification of cutting and punching is a seven-stage
process. Each stage involves making a number of choices
regarding the product. These choices should be made
based on shapes and features created in the design stage.
It is thus a step-by-step process towards the selection of a
manufacturing method. All conditions involved are finally
compared to the possibilities of the processes. Based on
the comparison, certain manufacturing processes are
proposed to the user for manufacturing the part in
question. At this point, the choice is based solely on
technological aspects. Cost factors, which play an
important role in the process selection, can be involved in
the comparison after that.
The grounds for rejection and approval should be carefully
examined, because the system is a good servant when it
functions well but a bad one when it does not. It may lead
to choosing the wrong solutions merely because the
system recommends them. On the other hand, the system
may be used very little or not at all if users consider its
proposals irrational.
There are many manufacturing methods for edges and
holes in sheet metal parts. Each process has a number of
variables and parameters that impact manufacturability.
These variables, in turn, are affected by several features of
the part, such as material or the location of bends and
holes. Due to the above, the operation of the system and
its functions and logic should be carefully prepared and
their validity ensured from as many viewpoints as possible.
7
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[3]
[4]
[5]
[6]
[7]
[8]
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