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Computer-aided generation of bending sequences for die-bending machines
Journal of Materials Processing Technology, 30 (1992) 1-12 Elsevier
1
Computer-aided generation of bending sequences for die-bending machines M. Hoffmann, U. Geifiler and M. Geiger Lehrstuhl fiir Fertigungstechnologie (LFT), Universit~t Erlangen-Nftrnberg, Erlangen, Germany (Received October 23, 1990; accepted February 26, 1991)
Industrial Summary
The planning of bending sequences and the assigning of tools is a task that has to be done manually at the present time. As part of a computerized process-planning tool for sheet-metal parts, a bending-sequence generator has been developed. The bent part can be developedinto the plane part automatically, technological data and know-how about the bending process and the press brake being included in this tool. The bending sequence generated by the computer can be simulated and altered ad libitum on a graphics screen.
1. Introduction
There are multiple motivations in the use of CAP techniques in manufacturing to close the gap between CAD and CAM. First of all, the generation of control programs for NC-machines is less error-prone when geometrical data can be taken from the CAD model directly: otherwise data already stored in the CAD system have to be re-input manually in a different format [ 1,2 ]. Secondly, the work-intensive step from design to NC-program can be cut short and therefore will be less expensive. A third advantage is the feedback from the planning phase. The designer can be informed immediately of manufacturing problems, as soon as the software detects them. Thus, the quality of design with respect to the manufacturing process is raised and again time is saved. A technique that is surfacing currently will add another motivation for CAP: expert systems will allow the sharing of planning and shop-level know-how between experienced and less experienced personnel [3 ]. C A D / N C integration, especially for the manufacturing of sheet-metal parts, is under research at the authors' institute. Two typical parts are shown in Fig. 1. One aspect from the whole range of planning steps, i.e., the planning of bending sequences, will be presented. 0924-0136/92/$05.00 © 1992Elsevier Science Publishers B.V. All rights reserved.
Fig. 1. Samplesof sheet-metalparts. 2. I n f l u e n c e o f p a r t m o d e l s o n t h e p l a n n i n g p r o c e s s
At the moment CAP systems for the die-bending process are rather scarce and confined to very special cases because of the complex technology involved. At the University of Erlangen a tool has been designed to bridge the gap between design and machining. In this paper the task of defining the sequence of bending will be discussed. Figure 2 shows three possible solutions for planning systems. The first one is used industrially for technologies such as nibbling, turning or milling. This approach has the disadvantage that shop-floor information does not flow back to the designer directly, if at all. All steps are independent from each other, so that errors in one phase have to be reported to the department concerned with the previous step, either on paper or by word of mouth. MANICAP (a CAP system for Modular Automated NC Integration) falls into the second category. The development of MANICAP started in phase one. Initially, MANICAPwas only an intelligent NC-programming tool, bearing in mind that extensive dataflow from and to the design system had to be established in the future. As MANICAP became more developed, the decision was made to put its own modeler on top of it to assure an optimal back-flow of data into the design phase. Still, the use of industrial CAD-systems is possible with MANICAP, the data back-flow and representation, however, having to be included into this C A D -
I DesgiIn '~ Pan/n' g3 I Production
©
System spec. nterface
Standard Interface IGES,STEP
@
@
I Process Plonning
I[ ProcessP,onning 1
E2
Process Planning
Fig. 2. Evolution of part modellers.
system for an ergonomic overall design. A tool is thus provided for the designer that enables him or her to check some aspects of planning before handing the design over to the planning department. The part model not only contains geometrical and topological information but also technological and, to a small extent already, economical data. These are, for instance, the essential operating times and costs for the different production steps. In industrial use there is only a loose coupling between the design and the shop-floor levels. Data is extracted from the CAD data base, converted according to some form of interface definition and processed by post-processors that generate NC-programs for specific machine tools. The only way to get technological data back to the designer is from the output in the conversion steps, and from the shop floor itself. As more and more know-how regarding the manufacturing process itself is included in the CAP model and the planning system, the need arises to present manufacturing-related information to the designer: a one-way coupling is no longer sufficient. The third phase includes an integrated product-model containing every bit of information that is related to the product, from the project to the sales department. This solution will be the base of the real computer-integrated factory. The evolution of planning systems is related directly to the evolution of product models. With MANICAP a back-flow of shop-level data to the designer is possible and, more importantly, there is one single product model that contains all technological data concerning each part. 3. E n v i r o n m e n t o f the s e q u e n c e g e n e r a t o r
The environment in which the sequence generator operates is shown in Fig. 3. The handling processor shown here is still just a graphics tool that enables
CAD-System ]
ITP_Bendingk
Z-Axis Bending Curve Geometr~j
-~3D-CAD-Model
j Contour-Data Generationof ~Cutting ProcessorJ Bending Sequence Bending Sequence"]~']'~ Tool List ,~,~ Handling Processor
J
NC-ProcjrGm
J Postprocessor t Fig. 3. Steps in the computer-assisted planning (CAP) for die-bending machines.
BendingAxis
AiMiA
+A+M A
N
DifferentAereason the BendingAxis - MAY-Aereo _MUST-Aereo MUSTNot Aereo -
-
MinimumTool MaximumTool Fig. 4. Tool pre-selection.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
v .....................
the operator to program the handling system off-line. However, a project to automate this step has been started.
3.1. Technological aspects The bending-sequence generator is embedded in a software environment that enables it to receive geometrical data from the CAD environment and technological data from a so-called technology processor [4 ]. Additional technological data can be put in manually during the design process. Its outputs are an optimized bending sequence, and tool pre-assignments. Figure 4 shows just one aspect of tool selection. The bending axis can be sub-
divided into three different areas. In a bending area there has to be a tool to perform the bending operation: this is called a "must" area. On the other hand, there are "must not" areas where the presence of a tool would lead to an unwanted deformation. A tool has to cover at least the "must" areas, this tool being called the minimum tool, the maximum tool covering all areas except "must not" areas. All tools in the range between these extremes can be used for the bending operation. The selection of the tool shape is independent of this step and is handled inside the technology processor. To be able to form high-precision parts, the planning has to take into account various items of technological information. Of main interest to sequence generation is the actual geometric shape of the bend, so that length differences of the part before and after the bending process can be calculated. The other value of interest is the minimum angle during over-bending. The technology processor performs its work for one part only once, when the contours of the
Volume Model
Foil Model
Blank
Determin Bending and Tool
Craphic Simulation Fig. 5. Model states during bending-sequence generation.
flat part are calculated. In Fig. 5 the separate model states that are passed through during the sequence generation are displayed. All the additional information that the technology processor provides is stored in the CAP model, where it is attached to the corresponding geometrical shape. Other data generated by this processor are the tool path and the necessary force. 3.2. The MANICAPkernel The kernel of the whole CAP bridge is a part modeller, that has been developed at the authors' institute. Its main advantage over other CAD modellers is its having been dedicated to sheet-metal parts [5,6]. Sheet-metal parts may be transformed to foil bodies, reducing model data to as little as 20 percent of the model data required by a volume modeller (Fig. 6). The material thickness is retained as an attribute to the model so that the original volume model can be reconstructed. Bending zones are represented as edges with attributes attached that contain geometrical and technological data. Collision detection runs considerably faster on this smaller data-base and can be further increased in speed when the general structure of the sheet-metal parts is taken into consideration. The combination of this modeller with various expert-system techniques allows the implementation of a software tool that is capable of generating a feasible bending sequence for a wide variety of die-bending parts. Early experiments have shown that the use of a volume modeller is too timeintensive for the present purposes. A first prototype ran for about 30 minutes for a part with 6 bends. This prototype took a brute-force approach just to find the first bending sequence that was collision-free. A second implementation
36 Points 64 Edges f ] 28 Faces ~ ~
Divisioninto Basic Elements
Foil Generation Determination of Bending Edges
Topology Reduction
Mergingof Faces
Volume Model
32 Points 56 Edges
~-,,, .................................. ,:,,,,.,,:,,,.~ Foil Model Mergingof Partiol Bodies
Generation of Basic Elements
~
I--I
Fig. 6. T r a n s f o r m a t i o n of volume- into foil-models.
~
12 Points 14 Edges / I
Generotionaf CoverFaces
using M A N I C A P for the same problem took about 1 minute on the same hardware--a 68020-based workstation running at 33 MHz. 4. A tool for automatic g e n e r a t i o n of b e n d i n g sequences
The actual sequence generator consists mainly of two parts: the automatic generation tool and the simulation tool. With the help of the automatic generation tool, one "near perfect" bending sequence is generated. This sequence can be examined and altered by the operator via the simulation tool.
4.1. Geometrical analysis of the part For a workpiece with 10 bends there are 10! different bending sequences [6 ]. The computational model would be a balanced tree with 3.6 million leaves. However, certain branches of the tree are cut off, because some bending steps cannot be carried out for one reason or another. Still, there might be a huge number of paths to be worked through, to evaluate each leaf that can be reached. Each bending step that is checked requires collision testing between the workpiece and itself, as well as its surroundings, such as tools and the working area. These collision detections have to be carried out dynamically, checking the whole space the part occupies at any time during the bending process, including the over-bending phase.
4.2. Checking for the most feasible bending sequence Because of the large number of computations the turn-around time is very large and increases very rapidly with the number of bends. Thus an approach should better evaluate the quality of the sequence on its way through the tree, coming up with the "best possible solution" in a single sweep. Even then, collision checking is a very time consuming task. A very fast detection algorithm was developed, therefore, for MANICAPto reduce computation time. In many cases there will be no perfect solution, because there are many influences on the economy of the bending sequence and their relative weight may change from one moment to another. Collisions are most important, but there are a number of other constraints, as shown in Fig. 7. In the present state an algorithmic approach has been made to evaluate bends. This approach shows that feasible sequences can be found by a computerized tool. However, an optimization would be far too time-consuming to be employed in everyday work. The constraints mentioned above are now included in a knowledge base. With the aid of an expert system the most effective bending sequence is picked and presented to the operator in the graphics-simulation step.
Collisions with -Tools ,Machine ,Itself
Part Geometry •Workpiece
Range oWorkpiece Mix
Tools I •Availability I • Number of I f ~ " ~ t "Cost of | U ~ - y ,,Exchangeof I ~_~
of Bending S e ~
I[~Handling "Gripping Possibil. • Gripping Operation • Gripper Exchange ,Programming
Machine
• Working Space| LrT-----TT3 ,Tolerances | • ManufacturingII Time ~1
Fig. 7. Factors influencing the bending sequence.
4.3. Knowledge-based picking of sequences The objective of searching for the best possible solution is, of course, to reduce cost. Influences on cost, however, impose different constraints on the bending sequence: some of them might be even contradictionary. Once a part can be made at all, there are three major sources of cost that can be influenced by the bending sequence: (i) The number of rejects: an optimized bending sequence might be able to shift working tolerances to less important parts of the workpiece [ 7 ]. (ii) Machine time: the actual bending time is independent of the bending sequence, the other part of the essential operating time being used for handling. If, for instance, rotational movement of the workpiece is aspecially time consuming, the sequencer should minimize rotations. (iii) Tools: tool handling is a very expensive task, so the sequencer should avoid the use of non-standard tools and minimize the number of tools. Since some of the knowledge is only diffuse, the best approach available is the use of an expert system (XPS). Figure 8 shows various aspects of the planning phase where knowledge is used. Some of the above knowledge is relevant only to particular press brakes, whilst some of it is common. Thus it will be necessary to split knowledge bases into smaller ones that can be consulted individually. On one hand, this increases the speed of operation at the expert system, but for the generation of handling programmes other data are needed than for the generation of the bending sequence. On the other hand, there will be a significant reduction in work-time if other press brakes or handling systems are included.
MANICAP
Extrac-
Extrac-
Model AnalysisI:==
+
Geometry I Tests ~
Sequencer ~-
I XPS-
q
l Process Plan Bending [~NC-Program GeneratorI NC-Program
Fig. 8. Knowledge-based planning of bending operations. 4.4. Simulation of the bending process The whole machining process can be viewed graphically on the computer screen, e.g. a collision detected by the system can be shown on the screen (Fig. 9). Here the operator can check for conditions that the system is not able to detect or for conditions that cannot be solved because of lack of know-how. The bending process is shown in single steps that can be controlled by the operator. The part and the tools can be viewed from all directions before proceeding to the next step. If any changes are required, the automatic sequencer is restarted in the background, with the change requests applied. If the operator wants to enter sequences that have been checked by the system already, he is told by the system why this sequence was rejected. Tool assignments can be made during the simulation, if necessary. 5. O u t l o o k
The complete automatic generation of general bending sequences is still not a reality. It can be done for a number of parts, but there are some restrictions (Fig. 10). The planner's interaction is still necessary, if the use of standard tools does not enable practicable bending sequences. This aspect, together with the influence of the bending sequence on tolerances are topics of present projects. However, with MANICAP,one large step already has been made. MANICAP can already help during the planning phase, in taking a big load offthe planner.
10
Fig. 9. Simulation of the bending operation.
There might not be any one hundred percent solutions available in the near future, but the day-to-day workload of the planning shop can be reduced significantly.
11
6
6
?
q
Fig. 10. Automatically generated bending sequence of a complex die-bending part.
12 1 G. Koch, Komplizierte Blechteile---automatisch und einbaufertig gebogen, VDI Z., 129 ( 1987 ) 5. 2 J. Fleischer, Rechnerintegrierte Fertigung von Abkantteilen, Paper presented at CAT'88, Stuttgart. 3 R. Ehrismann and J. Reissner, Intelligente Fertigung von Biege-, Stanz- und Laserscheidteilen, Tech. Rundschau, 27 (1987). 4 M. Geiger and U. Gei£1er, Flexibles Blechbearbeitungssystem im interdisziplin~iren Forschungsprojekt PAP, Blech Rohre Profile, 35 (2) (1988) 85-92. 5 M. Hoffmann, MANICAP,ein CAP-System for die Blechbearbeitung, Industrieanzeiger, 112 (8) (1990) 32-33. 6 M. Hoffmann and U. Gei£ler, Automatisierte Fertigungsvorbereitung in der Blechbearbeitung. Teil II: Biegestadienplanung, CAD-CAMRep., 8 (10) (1989) 58-65. 7 J. Reissner and R. Ehrismann, Rechneruntersttitztes Dreipunktbiegen von Vielkantprofilen, Blech Rohre Profile, 33 (1986) 1.
1
Computer-aided generation of bending sequences for die-bending machines M. Hoffmann, U. Geifiler and M. Geiger Lehrstuhl fiir Fertigungstechnologie (LFT), Universit~t Erlangen-Nftrnberg, Erlangen, Germany (Received October 23, 1990; accepted February 26, 1991)
Industrial Summary
The planning of bending sequences and the assigning of tools is a task that has to be done manually at the present time. As part of a computerized process-planning tool for sheet-metal parts, a bending-sequence generator has been developed. The bent part can be developedinto the plane part automatically, technological data and know-how about the bending process and the press brake being included in this tool. The bending sequence generated by the computer can be simulated and altered ad libitum on a graphics screen.
1. Introduction
There are multiple motivations in the use of CAP techniques in manufacturing to close the gap between CAD and CAM. First of all, the generation of control programs for NC-machines is less error-prone when geometrical data can be taken from the CAD model directly: otherwise data already stored in the CAD system have to be re-input manually in a different format [ 1,2 ]. Secondly, the work-intensive step from design to NC-program can be cut short and therefore will be less expensive. A third advantage is the feedback from the planning phase. The designer can be informed immediately of manufacturing problems, as soon as the software detects them. Thus, the quality of design with respect to the manufacturing process is raised and again time is saved. A technique that is surfacing currently will add another motivation for CAP: expert systems will allow the sharing of planning and shop-level know-how between experienced and less experienced personnel [3 ]. C A D / N C integration, especially for the manufacturing of sheet-metal parts, is under research at the authors' institute. Two typical parts are shown in Fig. 1. One aspect from the whole range of planning steps, i.e., the planning of bending sequences, will be presented. 0924-0136/92/$05.00 © 1992Elsevier Science Publishers B.V. All rights reserved.
Fig. 1. Samplesof sheet-metalparts. 2. I n f l u e n c e o f p a r t m o d e l s o n t h e p l a n n i n g p r o c e s s
At the moment CAP systems for the die-bending process are rather scarce and confined to very special cases because of the complex technology involved. At the University of Erlangen a tool has been designed to bridge the gap between design and machining. In this paper the task of defining the sequence of bending will be discussed. Figure 2 shows three possible solutions for planning systems. The first one is used industrially for technologies such as nibbling, turning or milling. This approach has the disadvantage that shop-floor information does not flow back to the designer directly, if at all. All steps are independent from each other, so that errors in one phase have to be reported to the department concerned with the previous step, either on paper or by word of mouth. MANICAP (a CAP system for Modular Automated NC Integration) falls into the second category. The development of MANICAP started in phase one. Initially, MANICAPwas only an intelligent NC-programming tool, bearing in mind that extensive dataflow from and to the design system had to be established in the future. As MANICAP became more developed, the decision was made to put its own modeler on top of it to assure an optimal back-flow of data into the design phase. Still, the use of industrial CAD-systems is possible with MANICAP, the data back-flow and representation, however, having to be included into this C A D -
I DesgiIn '~ Pan/n' g3 I Production
©
System spec. nterface
Standard Interface IGES,STEP
@
@
I Process Plonning
I[ ProcessP,onning 1
E2
Process Planning
Fig. 2. Evolution of part modellers.
system for an ergonomic overall design. A tool is thus provided for the designer that enables him or her to check some aspects of planning before handing the design over to the planning department. The part model not only contains geometrical and topological information but also technological and, to a small extent already, economical data. These are, for instance, the essential operating times and costs for the different production steps. In industrial use there is only a loose coupling between the design and the shop-floor levels. Data is extracted from the CAD data base, converted according to some form of interface definition and processed by post-processors that generate NC-programs for specific machine tools. The only way to get technological data back to the designer is from the output in the conversion steps, and from the shop floor itself. As more and more know-how regarding the manufacturing process itself is included in the CAP model and the planning system, the need arises to present manufacturing-related information to the designer: a one-way coupling is no longer sufficient. The third phase includes an integrated product-model containing every bit of information that is related to the product, from the project to the sales department. This solution will be the base of the real computer-integrated factory. The evolution of planning systems is related directly to the evolution of product models. With MANICAP a back-flow of shop-level data to the designer is possible and, more importantly, there is one single product model that contains all technological data concerning each part. 3. E n v i r o n m e n t o f the s e q u e n c e g e n e r a t o r
The environment in which the sequence generator operates is shown in Fig. 3. The handling processor shown here is still just a graphics tool that enables
CAD-System ]
ITP_Bendingk
Z-Axis Bending Curve Geometr~j
-~3D-CAD-Model
j Contour-Data Generationof ~Cutting ProcessorJ Bending Sequence Bending Sequence"]~']'~ Tool List ,~,~ Handling Processor
J
NC-ProcjrGm
J Postprocessor t Fig. 3. Steps in the computer-assisted planning (CAP) for die-bending machines.
BendingAxis
AiMiA
+A+M A
N
DifferentAereason the BendingAxis - MAY-Aereo _MUST-Aereo MUSTNot Aereo -
-
MinimumTool MaximumTool Fig. 4. Tool pre-selection.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
v .....................
the operator to program the handling system off-line. However, a project to automate this step has been started.
3.1. Technological aspects The bending-sequence generator is embedded in a software environment that enables it to receive geometrical data from the CAD environment and technological data from a so-called technology processor [4 ]. Additional technological data can be put in manually during the design process. Its outputs are an optimized bending sequence, and tool pre-assignments. Figure 4 shows just one aspect of tool selection. The bending axis can be sub-
divided into three different areas. In a bending area there has to be a tool to perform the bending operation: this is called a "must" area. On the other hand, there are "must not" areas where the presence of a tool would lead to an unwanted deformation. A tool has to cover at least the "must" areas, this tool being called the minimum tool, the maximum tool covering all areas except "must not" areas. All tools in the range between these extremes can be used for the bending operation. The selection of the tool shape is independent of this step and is handled inside the technology processor. To be able to form high-precision parts, the planning has to take into account various items of technological information. Of main interest to sequence generation is the actual geometric shape of the bend, so that length differences of the part before and after the bending process can be calculated. The other value of interest is the minimum angle during over-bending. The technology processor performs its work for one part only once, when the contours of the
Volume Model
Foil Model
Blank
Determin Bending and Tool
Craphic Simulation Fig. 5. Model states during bending-sequence generation.
flat part are calculated. In Fig. 5 the separate model states that are passed through during the sequence generation are displayed. All the additional information that the technology processor provides is stored in the CAP model, where it is attached to the corresponding geometrical shape. Other data generated by this processor are the tool path and the necessary force. 3.2. The MANICAPkernel The kernel of the whole CAP bridge is a part modeller, that has been developed at the authors' institute. Its main advantage over other CAD modellers is its having been dedicated to sheet-metal parts [5,6]. Sheet-metal parts may be transformed to foil bodies, reducing model data to as little as 20 percent of the model data required by a volume modeller (Fig. 6). The material thickness is retained as an attribute to the model so that the original volume model can be reconstructed. Bending zones are represented as edges with attributes attached that contain geometrical and technological data. Collision detection runs considerably faster on this smaller data-base and can be further increased in speed when the general structure of the sheet-metal parts is taken into consideration. The combination of this modeller with various expert-system techniques allows the implementation of a software tool that is capable of generating a feasible bending sequence for a wide variety of die-bending parts. Early experiments have shown that the use of a volume modeller is too timeintensive for the present purposes. A first prototype ran for about 30 minutes for a part with 6 bends. This prototype took a brute-force approach just to find the first bending sequence that was collision-free. A second implementation
36 Points 64 Edges f ] 28 Faces ~ ~
Divisioninto Basic Elements
Foil Generation Determination of Bending Edges
Topology Reduction
Mergingof Faces
Volume Model
32 Points 56 Edges
~-,,, .................................. ,:,,,,.,,:,,,.~ Foil Model Mergingof Partiol Bodies
Generation of Basic Elements
~
I--I
Fig. 6. T r a n s f o r m a t i o n of volume- into foil-models.
~
12 Points 14 Edges / I
Generotionaf CoverFaces
using M A N I C A P for the same problem took about 1 minute on the same hardware--a 68020-based workstation running at 33 MHz. 4. A tool for automatic g e n e r a t i o n of b e n d i n g sequences
The actual sequence generator consists mainly of two parts: the automatic generation tool and the simulation tool. With the help of the automatic generation tool, one "near perfect" bending sequence is generated. This sequence can be examined and altered by the operator via the simulation tool.
4.1. Geometrical analysis of the part For a workpiece with 10 bends there are 10! different bending sequences [6 ]. The computational model would be a balanced tree with 3.6 million leaves. However, certain branches of the tree are cut off, because some bending steps cannot be carried out for one reason or another. Still, there might be a huge number of paths to be worked through, to evaluate each leaf that can be reached. Each bending step that is checked requires collision testing between the workpiece and itself, as well as its surroundings, such as tools and the working area. These collision detections have to be carried out dynamically, checking the whole space the part occupies at any time during the bending process, including the over-bending phase.
4.2. Checking for the most feasible bending sequence Because of the large number of computations the turn-around time is very large and increases very rapidly with the number of bends. Thus an approach should better evaluate the quality of the sequence on its way through the tree, coming up with the "best possible solution" in a single sweep. Even then, collision checking is a very time consuming task. A very fast detection algorithm was developed, therefore, for MANICAPto reduce computation time. In many cases there will be no perfect solution, because there are many influences on the economy of the bending sequence and their relative weight may change from one moment to another. Collisions are most important, but there are a number of other constraints, as shown in Fig. 7. In the present state an algorithmic approach has been made to evaluate bends. This approach shows that feasible sequences can be found by a computerized tool. However, an optimization would be far too time-consuming to be employed in everyday work. The constraints mentioned above are now included in a knowledge base. With the aid of an expert system the most effective bending sequence is picked and presented to the operator in the graphics-simulation step.
Collisions with -Tools ,Machine ,Itself
Part Geometry •Workpiece
Range oWorkpiece Mix
Tools I •Availability I • Number of I f ~ " ~ t "Cost of | U ~ - y ,,Exchangeof I ~_~
of Bending S e ~
I[~Handling "Gripping Possibil. • Gripping Operation • Gripper Exchange ,Programming
Machine
• Working Space| LrT-----TT3 ,Tolerances | • ManufacturingII Time ~1
Fig. 7. Factors influencing the bending sequence.
4.3. Knowledge-based picking of sequences The objective of searching for the best possible solution is, of course, to reduce cost. Influences on cost, however, impose different constraints on the bending sequence: some of them might be even contradictionary. Once a part can be made at all, there are three major sources of cost that can be influenced by the bending sequence: (i) The number of rejects: an optimized bending sequence might be able to shift working tolerances to less important parts of the workpiece [ 7 ]. (ii) Machine time: the actual bending time is independent of the bending sequence, the other part of the essential operating time being used for handling. If, for instance, rotational movement of the workpiece is aspecially time consuming, the sequencer should minimize rotations. (iii) Tools: tool handling is a very expensive task, so the sequencer should avoid the use of non-standard tools and minimize the number of tools. Since some of the knowledge is only diffuse, the best approach available is the use of an expert system (XPS). Figure 8 shows various aspects of the planning phase where knowledge is used. Some of the above knowledge is relevant only to particular press brakes, whilst some of it is common. Thus it will be necessary to split knowledge bases into smaller ones that can be consulted individually. On one hand, this increases the speed of operation at the expert system, but for the generation of handling programmes other data are needed than for the generation of the bending sequence. On the other hand, there will be a significant reduction in work-time if other press brakes or handling systems are included.
MANICAP
Extrac-
Extrac-
Model AnalysisI:==
+
Geometry I Tests ~
Sequencer ~-
I XPS-
q
l Process Plan Bending [~NC-Program GeneratorI NC-Program
Fig. 8. Knowledge-based planning of bending operations. 4.4. Simulation of the bending process The whole machining process can be viewed graphically on the computer screen, e.g. a collision detected by the system can be shown on the screen (Fig. 9). Here the operator can check for conditions that the system is not able to detect or for conditions that cannot be solved because of lack of know-how. The bending process is shown in single steps that can be controlled by the operator. The part and the tools can be viewed from all directions before proceeding to the next step. If any changes are required, the automatic sequencer is restarted in the background, with the change requests applied. If the operator wants to enter sequences that have been checked by the system already, he is told by the system why this sequence was rejected. Tool assignments can be made during the simulation, if necessary. 5. O u t l o o k
The complete automatic generation of general bending sequences is still not a reality. It can be done for a number of parts, but there are some restrictions (Fig. 10). The planner's interaction is still necessary, if the use of standard tools does not enable practicable bending sequences. This aspect, together with the influence of the bending sequence on tolerances are topics of present projects. However, with MANICAP,one large step already has been made. MANICAP can already help during the planning phase, in taking a big load offthe planner.
10
Fig. 9. Simulation of the bending operation.
There might not be any one hundred percent solutions available in the near future, but the day-to-day workload of the planning shop can be reduced significantly.
11
6
6
?
q
Fig. 10. Automatically generated bending sequence of a complex die-bending part.
12 1 G. Koch, Komplizierte Blechteile---automatisch und einbaufertig gebogen, VDI Z., 129 ( 1987 ) 5. 2 J. Fleischer, Rechnerintegrierte Fertigung von Abkantteilen, Paper presented at CAT'88, Stuttgart. 3 R. Ehrismann and J. Reissner, Intelligente Fertigung von Biege-, Stanz- und Laserscheidteilen, Tech. Rundschau, 27 (1987). 4 M. Geiger and U. Gei£1er, Flexibles Blechbearbeitungssystem im interdisziplin~iren Forschungsprojekt PAP, Blech Rohre Profile, 35 (2) (1988) 85-92. 5 M. Hoffmann, MANICAP,ein CAP-System for die Blechbearbeitung, Industrieanzeiger, 112 (8) (1990) 32-33. 6 M. Hoffmann and U. Gei£ler, Automatisierte Fertigungsvorbereitung in der Blechbearbeitung. Teil II: Biegestadienplanung, CAD-CAMRep., 8 (10) (1989) 58-65. 7 J. Reissner and R. Ehrismann, Rechneruntersttitztes Dreipunktbiegen von Vielkantprofilen, Blech Rohre Profile, 33 (1986) 1.