3rd Dimension of Sheet Metal Processing

Commercial CAD systems are not properly suited for complex components
Unquestionably, CAD technology provides a variety of advantages as far as construction is concerned. In the field of mechanical engineering, the use of 2D CAD systems has meanwhile established as a standard. Currently on the march are 3D CAD systems, which offer even more possibilities, such as, for example, reality display or easy assembly of assembly groups.

The production data required by CAM systems can be directly made available, as a by-product, so to speak. Most of the CAD systems, however, do not meet the requirements and are consequently not properly suited for effective construction of sheet metal workpieces, as they lack functionality. This implies, for example, free-cuts of flanges, calculation of the neutral line as well as calculation of the unfolded blank. The lack of functionality can only be compensated by a higher expenditure or by (expensive) adjustments. Consequently, there is an increasing need of software packages that are tailored to the needs of sheet metal construction. With the help of these systems, the time needed for design and production can be considerably reduced.

With the COPRA® MetalBender 3D users have a complex 2 and 3D-solution for the design and production of sheet metal work pieces at their disposal.

Easy design of 3D sheet metal parts with 2D functions
As already mentioned, the use of 2D-CAD systems has meanwhile become a standard. Most of the designers use and highly estimate the respective tools. Effective design, however, requires 3D-CAD systems. The COPRA® MetalBender 3D accounts for both as it provides an integrated 2D-to-3D-conversion module. The sheet metal part can be designed with the help of 2D-functions and can be automatically converted to a 3D-volume model. Designers, who are so far inexperienced with 3D functions have a functional tool at hand, which makes the first steps easy and allows them to work efficiently from the very beginning. Lets take a box as an example to describe the process in the following.

Box design with 2D-functions
The base of the box is drawn as a simple rectangle with a size of 300 mm x 200 mm. From the side walls of the box either the inner or the outer contour is displayed respectively.

You can easily change to an isometric view. Before the contour lines are added to the base, sheet thickness has to be defined. There are three possibilities to add the contour lines to the base. The edges of the base can either equal the external or the internal dimension or they are simply added to the existing base. For 2D-to-3D conversion the intersection curves between the contour lines will be calculated automatically. A gap width can be entered to define the distance between the objects and the number of contour reference points can be increased. This is especially useful for trouble-free production.

Fig. CH01-1: Box design with 2D functions.

Conversion to a 3D volume model
The sides of the base are numbered. Click the base, in order to add a contour line to the base. If only external or internal contours are to be added, the user can define the side at which the sheet thickness is to be added. The selected contour will be added to the respective side whereby direction can be changed from top to bottom and vice versa.

As to each side of the base a different contour or an additional base can be added, almost any box shape can be created automatically. If the contour lines are defined for all sides of the base, the volume model including all interfaces and corner free-cuts is calculated automatically. With the help of the respective functions, bores, punch holes, intersections etc. can be inserted in the 3-dimensional workpiece obtained in this way.

The 2D to 3D conversion option allows to design box shapes within minutes. The example described above will be completed within about five minutes. Design on the basis of the 3D sheet metal functions would take about five to six times longer (although this method is also adjusted to the requirements of sheet metal design).

Fig. CH02-1: automatic conversion to a 3D-volume model

3D construction of sheet metal workpieces
However, not all types of sheet metal workpieces can be designed with the 2D-functions described above. If the 2D-function is not suited, the 3D design functions can be used. They are tailored to created highly complex parts. All design features are based on common Windows standards and are controlled by user-friendly dialogue windows incl. "card index systems".

Before starting with an outline of the workpiece, the default design parameters can be defined. These parameters include sheet thickness, internal bending radius, width and depth of free-cuts of inserted flanges as well as layer control. The entered values are default values which can, however, be adjusted by the user according to his requirements to assure that the standard bending values can be used.

Sheet metal design with the help of special flange functions
Designing starts by entering the base. Only length and width have to be entered; sheet thickness is calculated from the pre-set design parameters. The dimensions of the base can either be taken from the drawing with the help of the cross cursor or entered via the keyboard. For a clearly structured display different views can be created automatically. Outline, left and right lateral view as well as an isometric view can be displayed either individually or in any desired combination, i.e. the often arduous creation of individual views is a thing of the past.

The next step is to add the flanges to the base. As several bases can be contained in a single drawing, the work level has to be determined first. Either the upper or lower part of the base can be defined as work level. The decision for upper or lower part of the base automatically defines the design direction. The different types of flanges can be selected from a card index in the respective dialogue box. Available are non-rounded flanges with an internal radius of 0, rounded flanges, Z-flanges as well as profile and contour flanges.

Fig. CH03-2: Dialogue window: flange construction

The flanges and the individual parameters are added to the dialogue windows as reference drafts. When adding the flanges to the base, there are again three possible positions. The base may define the internal or external dimension or the flange can be added directly to the base. The selected setting will also be displayed in the drafts of the individual flanges, which allows for an easy visual control of the settings. The flange parameters are then entered at the desired positions (either directly or with the help of the cross cursor). The flanges may be flush with the final points of the base or may have an offset.

For inserted flanges, width and depth of the free-cut can thereby be defined. The free-cut is required to prevent the sheet metal from tearing during bending. An built-in preview function allows for a collision check of the flange. The preview also allows you to change the flange’s direction in case you have selected the wrong side of the base. Of course, other flange types can be added to the created flange etc.

Fig. CH04-1: sheet metal workpiece

Powerful modification functions
In general, it is a long way from the rough draft to the perfectly designed part. Often, the sheet metal workpiece has to be modified to be able to meet the requirements. For this reason, the COPRA® MetalBender 3D offers a variety of 3-dimensional features for editing, modification and adjustment. The most important pre-requisite for a simple modification is to edit the individual bending zones as individual components and independently of each other. This allows to delete and replace individual parts of the model. This would, however, not be possible, if the workpiece had been combined already during construction.

The flange parameters can be changed subsequently, even if the design was not parameter-based. For example, height, width, radius and angles of a flange can be easily modified. Flange surfaces can be extended to other surfaces whereby the position of the target surface will automatically be considered.

Automatic corner link function
A further advantage is the automatic corner link function. There are several possibilities of how the corners between flanges can be linked (e.g. edge to edge, overlapping or rounded).

Punch holes, intersections and bore holes
After completion of the basic design, punch holes, bore holes, intersections or punch holes have often to be added. Tailor-made functions are provided to meet these requirements. Standardised hole shapes, e. g. bore holes, slots or rectangular punch holes are available in special libraries. Furthermore, any desired hole shape can be generated by the designer. He only has to draw the contour which will then be saved in especially compiled tool libraries, together with the tool information. This allows designers to create, save and reload frequently used tools.

Quick and comfortable calculation of the unfolded blank
The heart of COPRA® MetalBender 3D is the calculation of the unfolded blank which is controlled by an independent solver. From the CAD system only the data of the sheet metal part are extracted and transferred to the solver. The solver, which is especially adjusted to the needs of metal bending, offers a variety of functions.

Fig. CH05-2: Dialogue window: Calculation of the unfolded blank

For example, the sheet thickness can either be modified or can be directly taken from the workpiece. For calculation of the neutral line different methods are available.

In addition to DIN 6935, the contracting factors can be taken from a material data table, machine-specific allowance values can be used or the position of the neutral line can be estimated on the basis of experience.

There are several possibilities to calculate the unfolded blank. Unfolding can be based on the position of the neutral line. The result would be a flat pattern. It might, however, also occur, that another part, a so-called intersection part intersects the workpiece. In order to obtain the intersection line, calculation of the unfolded blank has to consider both top and bottom part of the workpiece. Each side of this object has different intersection lines. Optionally, the maximum required intersection line can be indicated, provided that the cut is < 90° as compared to the flat pattern.

Intersection lines are not only resulting from calculation of the maximally required cutting edge of intersecting parts but also if, for example, the punch holes are within the bending zones. The intersection lines normally consist of many short straight lines, which are, however, not well-suited for CNC-based machines. For this reason, the number of reference points for the intersection lines can be reduced to a minimum by defining a maximum offset from the actual intersection line, e.g. 0,1mm. The number of reference points can be reduced linearly or can be approximated with the help of arc segments within the range of the defined offset. Moreover, the reference points can be changed to arc segments so that a tangential overlap will be the result.

The unfolded blank is calculated within a few seconds depending on how complex the sheet metal is. It includes bending lines, bending zones, all intersections, bore holes and punvh holrd as well as information on the punching tools used. The contour of the unfolded blank is a closed line, which can be used directly for manufacture.

Fig. CH06-1: unfolded workpiece

Bending simulation for manuacturing order
The bending simulation allows you to easily unfold and fold bending zones by a simple mouse click. In this way it is possible to simulate any intermediate step of production, which can be positioned within the tool. This allows to perform a feasibility analysis.

Optimization and production interfaces
The contours of the calculated unfolded blank can be optimised via a nesting algorithm and assigned to the available sheets. For production, you can furthermore, create automatic transfer files for NC-programming. The respective interfaces for machine controlling software are provided, too.

Laser-supported measurement of bending angles
The bending angle measuring systems was especially designed to measure bending angles at forging-bending presses. It uses a non-contacting sensor, which protects it against damage and wear. Its design allows many different presses to be retrofitted. For trouble-free operation tools have not to be modified. The measurement of the bending angle is based on a line laser projection on a leg of the bent sheet. The position of the line is thereby measured with the help of a camera, which focuses diagonally to the sheet surface. On the basis of the angle included between laser and camera axis the bent angle can recalculated with an accuracy of 0,1°.

For measuring recoil, pressure sensors with analogue output signal can be connected. In this way, recoil is measured on the basis of 2 pairs of values, only. The system can be operated without keyboard, mouse or monitor. All required operating and display functions are integrated in the sensor.

Stefan Freitag was born in 1960.
After having completed his studies in mechanical engineering at the Technical University of Munich, he had been working in the field of sheet metal processing at Duesseldorf.
In 1987, he and his partners founded the data M GmbH at Taufkirchen, a company which offers software solutions for the sheet-metal working industry, especially in the field of chamfering, wing bending and roll forming. Here, he is the managing director.