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A ROBOTIZED SYSTEM FOR PROTOTYPE MANUFACTURING OF CASTINGS AND BILLETS
277
A ROBOTIZED SYSTEM FOR PROTOTYPE MANUFACTURING OF CASTINGS AND BILLETS Mikko Sallinenl, Matti Sirvi6 2
1VTT Electronics, Kaitovayla 1, 90571 Oulu, Finland 2Simtech Systems Inc.oy, Kukkaromfiki 6C5, 02770, Espoo, Finland
ABSTRACT In this paper, we present a new method for manufacturing prototype castings using robot-based system. The contribution of the paper is new methods and tools for managing very different sizes of work pieces. The tools helps and assists the designer for manufacturing pieces which are not a straightforward to program for the robot. The methods are designed for prototype manufacturing which means lotsize is something between one to ten.
KEYWORDS: robot milling, off-line programming, prototype casting
1. INTRODUCTION The models for prototypes of the cast objects has been traditionally made by hands. They are made by wood and the form has been generated into a sand box. Nowadays, the wooden or plastic models are made using a milling machine where the first step towards automation has been taken. However, the disadvantages of the milling machine is the price and flexibility and therefore we have been taken a new approach to use robot as a milling machine Sirvi6 et. al. (2002). Robot-based milling stations have been developed in low intensity over the last number of years. The two main restrictive reasons for use of robot in milling are the rigidity of industrial robots and the difficulty of flexible off-line programming. Rigidity of the robot systems is not anymore such a problem like several years ago. However, in the last years, another problem has been high (or high enough) absolute accuracy of the robot manipulators. The machining of moulds or other prototypes has been made so far using a milling machine Boomenthal (2000). The difficulty of programming 5-axis machining as well as the lack of required accuracy has been forced to use three to five times expensive milling machine instead of a robot. The robotic milling that has been done so far has been concentrated on milling on soft materials like different kind of foams or wood to avoid the problems of rigidity Veergest et. al. (1998). The machining of large work pieces and path planning for material removing has been studied in Jager et. al. (2001).
278 This paper is organized as follows: in chapter 1 an introduction to the topic is given. Chapter 2 describes the prototype manufacturing process, the use of robot system is described in chapter 3. Results from actual tests are in chapter 4. Finally conclusions are given in chapter 5.
2. DESCRIPTION OF THE PROCESS
The principle of the short series production is explained as follows. The geometry of the casting will be machined into the sand. After machining and coating the mould, the work piece will be cast. Depending on the properties of the casting, this process is repeated until a piece with sufficient qualifications has been achieved. This may require one to ten iteration steps. Numerous castings that are produced in small batches, including prototypes and spare parts, they will be produced by substitution of the pattern making process with advanced robotics and utilization of a digital library created from a CAD model, thus economizing in terms of materials and storage space used and time spent. Moreover, patterns will be manufactured by using the same system by only substituting sand mould with plastic material. This will increase end user's productivity over 50% and decrease costs substantially, especially when prototypes are developed. From a technical point of view, new products, of enhanced shapes will be created in extremely short times and in as many items as desired. Finally, the milling and mould making can be totally isolated from the foundry environment, a fact that will decrease dust production substantially. In figure 1, the prototype production could be made without pattern making process.
3. ROBOTIC SYSTEM FOR PROTOTYPE MANUFACTURING The robot system consists of CAM phase, where the milling paths are generated, off-line programming phase, where the robot programs are postprocessed and actual running phase where the milling itself is carried out. 3.1 CAD / C A M
In the system, raw manufacturing paths are generated in CAM software. Raw means here that no machine dependent information is needed, only information of tools. The system is designed to be open such that designer can use any commercial CAD/CAM software he is used to and the output is in APT format. Therefore, the threshold of taking use of the proposed technology is as low as possible. In the CAM phase, user selects the general milling parameters such as tool contact angle and deepness of milling. These can be selected similar way as when using milling machine because
3D-CAD Model
Robotic Path Generation (CAM)
Recycling of sand
~ "
.,,
Grinding of the Sand Mould
Pattern or Sm all-Series' Casting
Pattern or Casting
Dust Removal
Figure 1. Foundry process with robotic sand grinding system
279 the material to be milled is relatively soft for the robot. However, the amount of points have to be reduced due to limited performance of the robot controller. This can be seen as an actual velocity of robot arm.
3.2 Off-line programming 3.2.1 Calibrations of the robot work cell The fundamental problem of robot systems is a requirement for use of one program in several robots. Usually it is not possible even if they are manufactured by the same manufacturer and they should be the same-like, i.e. same model. The reason comes mainly from the manufacturing deviations between the different manipulators. To overcome these inaccuracies the kinematic model and coordinate transformations can be corrected and in that way improve the accuracy of the robot systems. When using robots attached with sensors to different kind of measuring and manipulating task and when they are off-line programmed, following three calibrations should be done: calibration of the sensor internal parameters, hand-eye calibration and calibration of the robot.
3.2.2 Managing with various size of work pieces Machining of large work pieces is carried out by splitting the paths and CAD models of the work pieces. This is carried out by geometrical information of the robot workcell, i.e. the reachability of the manipulator arm with tools used in the cell. By using this set-up, the full performance of the robot workcell can be used. Using the classical approach, a very large robot would have been chosen. However, the disadvantage of this way is lack of accuracy because the larger robot you choose, the more inaccurate it is. By using medium-size robot, it is a compromise of accuracy but we can still manufacture large work pieces up to several meters. After splitting the paths and CAD information, paths are optimized for proper robot respectively. In the optimization phase, path is converted into form that movements of the robot are minimized between the points in the path. The optimization includes reach and collision check to prevent the unexpected situations.
3.3 Actual machining 3.3.1 Localization of workpiece in robot work cell To be able to manage the split pieces, each piece has to localize very carefully. In the localization, we use methods presented in previous paper Sallinen & Heikkil/i (2000). The idea is to fit the measured points to the reference model of the work piece. Method is fast and flexible to use. The method is open for different surface forms including plane surface, cylinder surface and spherical surface. The localization is here carried out using touch sensor which is rather reliable to use in foundry environment.
3.3.2 Robotic Milling The postprocessor outputs the programs ready-to-run in the robot including commands for running of spindles, tools and tool changers. Depending on the equipments of the robot, the machining parameters such as velocity, rotation speed of the tool and length of the path has to be find out. These depend also from the sand material in a terms of size of the grains and hardening material. To solve these parameters, a certain test-run has to be go trough to find out the best surface quality which is one of the main objectives of the system. The inaccuracies coming from the flexibility of
280 the robot are compensated in a optimizing phase where paths are planned such that movements from one point to another includes minimum amount of movement. This affects a path where movements of the robot are minimized and relative inaccuracy between the points is reduced. The other motivation is the surface quality required in the prototype manufacturing. In those products, typically allowed tolerance is between 6... 10% of the dimensions of the work piece. So in a typical medium size product dimension length of 500 mm, allowed tolerance is between 30...50 mm and that is definitely under performance of the robot system.
4. TESTS IN THE FOUNDRIES
The methods have been tested in simulation and actual production and results are very good. Using the optimization, the paths that was not able to run normally, could be run. The usability of the optimization has been improved based on the comments from the users. Also the splitting of the CAM paths and CAD models was tested with a success. The actual milling process is described in figure 5. The system was built up and tested in two different robot systems: ABB IRB6400 with $4C controller and KUKA KRC 150L 110 with KRC2 controller. Also both electric and air pressure spindles was tested. Both of the robot systems with different spindles and tools was working fine.
5. CONCLUSIONS In this paper, we present methods for manufacturing variety sizes of sand moulds using robot. Using the flexible control of the robot system, a cost-effective production of small series can be achieved. The proposed method consists of three different phases: CAD/CAM, off-line programming and actual milling. The whole system has been tested in actual foundry environment with very promising results.
References Bloomenthal M., Riesenfeld R., Cohen E., Fish R., (2000), An Approach to Rapid Manufacturing with Custom Fixturing, IEEE Int. Conf. on Robotics and Automation, San Francisco, USA, pp. 212219. Jager P. J., Broek J. J., Horvath I., Kooijman A., Smit A. (2001). An Effective Geometric and Kinematical Analysis of Ruled Surface Feature Manufacturability for Rapid Prototypind. Proc. Of DETC'O1. ASME 2001 Eng. Technical Conference and Computers and Information in Engineering Conference. Pittsbourg, PA, Sep. 9-12. 2001. Sallinen M., Heikkilg T., (2000), Flexible Workobject Localisation for CAD -Based Robotics, Proceedings of SPIE Intelligent Robots and Computer Vision XIX: Algorithms, Techniques, and Active Vision. Boston, USA, 7 - 8 Nov. 2000. USA. Vol. 4197 (2000), pp. 130 - 139 Sirvi6 M., Vfiin61fi J., Vapalahti S., Sallinen M. (2002), Automatic Line for Manufacturing Prototype Castings and Billets in Environmentally Friendly Robotic Cell, Proceedings of the International Conference on Machine Automation (ICMA2002), 11-13.9.2002, Tampere, Finland. Veergeest J., Tangelder J., Horvath I., Kovacs Z., Kuczogi G., (1998), Machining large complex shapes using a 7 DOF tool, IFIP SSM'98 Symposium, Chryslr Tech. Center, 9-11 Nov 1998.
A ROBOTIZED SYSTEM FOR PROTOTYPE MANUFACTURING OF CASTINGS AND BILLETS Mikko Sallinenl, Matti Sirvi6 2
1VTT Electronics, Kaitovayla 1, 90571 Oulu, Finland 2Simtech Systems Inc.oy, Kukkaromfiki 6C5, 02770, Espoo, Finland
ABSTRACT In this paper, we present a new method for manufacturing prototype castings using robot-based system. The contribution of the paper is new methods and tools for managing very different sizes of work pieces. The tools helps and assists the designer for manufacturing pieces which are not a straightforward to program for the robot. The methods are designed for prototype manufacturing which means lotsize is something between one to ten.
KEYWORDS: robot milling, off-line programming, prototype casting
1. INTRODUCTION The models for prototypes of the cast objects has been traditionally made by hands. They are made by wood and the form has been generated into a sand box. Nowadays, the wooden or plastic models are made using a milling machine where the first step towards automation has been taken. However, the disadvantages of the milling machine is the price and flexibility and therefore we have been taken a new approach to use robot as a milling machine Sirvi6 et. al. (2002). Robot-based milling stations have been developed in low intensity over the last number of years. The two main restrictive reasons for use of robot in milling are the rigidity of industrial robots and the difficulty of flexible off-line programming. Rigidity of the robot systems is not anymore such a problem like several years ago. However, in the last years, another problem has been high (or high enough) absolute accuracy of the robot manipulators. The machining of moulds or other prototypes has been made so far using a milling machine Boomenthal (2000). The difficulty of programming 5-axis machining as well as the lack of required accuracy has been forced to use three to five times expensive milling machine instead of a robot. The robotic milling that has been done so far has been concentrated on milling on soft materials like different kind of foams or wood to avoid the problems of rigidity Veergest et. al. (1998). The machining of large work pieces and path planning for material removing has been studied in Jager et. al. (2001).
278 This paper is organized as follows: in chapter 1 an introduction to the topic is given. Chapter 2 describes the prototype manufacturing process, the use of robot system is described in chapter 3. Results from actual tests are in chapter 4. Finally conclusions are given in chapter 5.
2. DESCRIPTION OF THE PROCESS
The principle of the short series production is explained as follows. The geometry of the casting will be machined into the sand. After machining and coating the mould, the work piece will be cast. Depending on the properties of the casting, this process is repeated until a piece with sufficient qualifications has been achieved. This may require one to ten iteration steps. Numerous castings that are produced in small batches, including prototypes and spare parts, they will be produced by substitution of the pattern making process with advanced robotics and utilization of a digital library created from a CAD model, thus economizing in terms of materials and storage space used and time spent. Moreover, patterns will be manufactured by using the same system by only substituting sand mould with plastic material. This will increase end user's productivity over 50% and decrease costs substantially, especially when prototypes are developed. From a technical point of view, new products, of enhanced shapes will be created in extremely short times and in as many items as desired. Finally, the milling and mould making can be totally isolated from the foundry environment, a fact that will decrease dust production substantially. In figure 1, the prototype production could be made without pattern making process.
3. ROBOTIC SYSTEM FOR PROTOTYPE MANUFACTURING The robot system consists of CAM phase, where the milling paths are generated, off-line programming phase, where the robot programs are postprocessed and actual running phase where the milling itself is carried out. 3.1 CAD / C A M
In the system, raw manufacturing paths are generated in CAM software. Raw means here that no machine dependent information is needed, only information of tools. The system is designed to be open such that designer can use any commercial CAD/CAM software he is used to and the output is in APT format. Therefore, the threshold of taking use of the proposed technology is as low as possible. In the CAM phase, user selects the general milling parameters such as tool contact angle and deepness of milling. These can be selected similar way as when using milling machine because
3D-CAD Model
Robotic Path Generation (CAM)
Recycling of sand
~ "
.,,
Grinding of the Sand Mould
Pattern or Sm all-Series' Casting
Pattern or Casting
Dust Removal
Figure 1. Foundry process with robotic sand grinding system
279 the material to be milled is relatively soft for the robot. However, the amount of points have to be reduced due to limited performance of the robot controller. This can be seen as an actual velocity of robot arm.
3.2 Off-line programming 3.2.1 Calibrations of the robot work cell The fundamental problem of robot systems is a requirement for use of one program in several robots. Usually it is not possible even if they are manufactured by the same manufacturer and they should be the same-like, i.e. same model. The reason comes mainly from the manufacturing deviations between the different manipulators. To overcome these inaccuracies the kinematic model and coordinate transformations can be corrected and in that way improve the accuracy of the robot systems. When using robots attached with sensors to different kind of measuring and manipulating task and when they are off-line programmed, following three calibrations should be done: calibration of the sensor internal parameters, hand-eye calibration and calibration of the robot.
3.2.2 Managing with various size of work pieces Machining of large work pieces is carried out by splitting the paths and CAD models of the work pieces. This is carried out by geometrical information of the robot workcell, i.e. the reachability of the manipulator arm with tools used in the cell. By using this set-up, the full performance of the robot workcell can be used. Using the classical approach, a very large robot would have been chosen. However, the disadvantage of this way is lack of accuracy because the larger robot you choose, the more inaccurate it is. By using medium-size robot, it is a compromise of accuracy but we can still manufacture large work pieces up to several meters. After splitting the paths and CAD information, paths are optimized for proper robot respectively. In the optimization phase, path is converted into form that movements of the robot are minimized between the points in the path. The optimization includes reach and collision check to prevent the unexpected situations.
3.3 Actual machining 3.3.1 Localization of workpiece in robot work cell To be able to manage the split pieces, each piece has to localize very carefully. In the localization, we use methods presented in previous paper Sallinen & Heikkil/i (2000). The idea is to fit the measured points to the reference model of the work piece. Method is fast and flexible to use. The method is open for different surface forms including plane surface, cylinder surface and spherical surface. The localization is here carried out using touch sensor which is rather reliable to use in foundry environment.
3.3.2 Robotic Milling The postprocessor outputs the programs ready-to-run in the robot including commands for running of spindles, tools and tool changers. Depending on the equipments of the robot, the machining parameters such as velocity, rotation speed of the tool and length of the path has to be find out. These depend also from the sand material in a terms of size of the grains and hardening material. To solve these parameters, a certain test-run has to be go trough to find out the best surface quality which is one of the main objectives of the system. The inaccuracies coming from the flexibility of
280 the robot are compensated in a optimizing phase where paths are planned such that movements from one point to another includes minimum amount of movement. This affects a path where movements of the robot are minimized and relative inaccuracy between the points is reduced. The other motivation is the surface quality required in the prototype manufacturing. In those products, typically allowed tolerance is between 6... 10% of the dimensions of the work piece. So in a typical medium size product dimension length of 500 mm, allowed tolerance is between 30...50 mm and that is definitely under performance of the robot system.
4. TESTS IN THE FOUNDRIES
The methods have been tested in simulation and actual production and results are very good. Using the optimization, the paths that was not able to run normally, could be run. The usability of the optimization has been improved based on the comments from the users. Also the splitting of the CAM paths and CAD models was tested with a success. The actual milling process is described in figure 5. The system was built up and tested in two different robot systems: ABB IRB6400 with $4C controller and KUKA KRC 150L 110 with KRC2 controller. Also both electric and air pressure spindles was tested. Both of the robot systems with different spindles and tools was working fine.
5. CONCLUSIONS In this paper, we present methods for manufacturing variety sizes of sand moulds using robot. Using the flexible control of the robot system, a cost-effective production of small series can be achieved. The proposed method consists of three different phases: CAD/CAM, off-line programming and actual milling. The whole system has been tested in actual foundry environment with very promising results.
References Bloomenthal M., Riesenfeld R., Cohen E., Fish R., (2000), An Approach to Rapid Manufacturing with Custom Fixturing, IEEE Int. Conf. on Robotics and Automation, San Francisco, USA, pp. 212219. Jager P. J., Broek J. J., Horvath I., Kooijman A., Smit A. (2001). An Effective Geometric and Kinematical Analysis of Ruled Surface Feature Manufacturability for Rapid Prototypind. Proc. Of DETC'O1. ASME 2001 Eng. Technical Conference and Computers and Information in Engineering Conference. Pittsbourg, PA, Sep. 9-12. 2001. Sallinen M., Heikkilg T., (2000), Flexible Workobject Localisation for CAD -Based Robotics, Proceedings of SPIE Intelligent Robots and Computer Vision XIX: Algorithms, Techniques, and Active Vision. Boston, USA, 7 - 8 Nov. 2000. USA. Vol. 4197 (2000), pp. 130 - 139 Sirvi6 M., Vfiin61fi J., Vapalahti S., Sallinen M. (2002), Automatic Line for Manufacturing Prototype Castings and Billets in Environmentally Friendly Robotic Cell, Proceedings of the International Conference on Machine Automation (ICMA2002), 11-13.9.2002, Tampere, Finland. Veergeest J., Tangelder J., Horvath I., Kovacs Z., Kuczogi G., (1998), Machining large complex shapes using a 7 DOF tool, IFIP SSM'98 Symposium, Chryslr Tech. Center, 9-11 Nov 1998.