USING ROBOTS TO MATERIAL REMOVAL PROCESSES

Fábio de Oliveira Campos 1, Armando Carlos de Pina Filho 2, Aloísio Carlos de Pina 3 1 Universidade Federal do Rio de Janeiro, Mechanical Engineering Department, Polytechnic School, Rio de Janeiro - RJ, Brazil, e-mail: markarm@bol.com.br 2 Universidade Federal do Rio de Janeiro, Urban Engineering Program, Polytechnic School, Rio de Janeiro - RJ, Brazil, e-mail: armando@poli.ufrj.br 3 Universidade Federal do Rio de Janeiro, Civil Engineering Program, COPPE, Rio de Janeiro - RJ, Brazil, e-mail: aloisiopina@bol.com.br Abstract: Robot manipulators can be applied to material removal processes, presenting advantages as compared with CNC machine tools, such as: more flexibility, lower initial cost, lesser waste of material, and better surface finishing on the parts. The objective of this work is present a study about robots used to material removal applications. Keywords: Robotics, manipulators, manufacturing. 1. INTRODUCTION Nowadays, most of the industries utilize CNC machine tools to perform, practically, all the material removal processes in their production lines. But, a few years ago, started to appear a second option that can be much more advantageous for them at some processes: the utilization of robots to perform these processes. These robots are the manipulator robots. However, for that substitution to be really advantageous it s necessary the doing of a study, taking into consideration the process that will be applied, characteristics of the design of the part, the material of which the part is made, among others details. With basis on the information got, the manipulator to perform the operation can be chosen. This work presents a study about these robots specifying details that must be taken into consideration for the correct choose of the robot manipulator. But to understand it, it s necessary to know what is a robot manipulator, which are its basic characteristics and some definitions about it. And that is what this article presents at the first moment. Then it s presented the advantages and disadvantages of the use of manipulators to substitute CNC machine tools. In the end, are presented some specifications that robot manipulators must possess in order to be applied at material removal processes. 2. ROBOT MANIPULATORS As cited above, the robots utilized at material removal processes are the robot manipulators. A manipulator can be defined, according to the RIA (Robotic Industries Association), as a mechanism that is re-programmable, multi-functional and designed to move materials, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks [1]. As their definition says, the robot manipulators are multifunctional, i.e., they are capable of doing a lot of different tasks and for each task there is a specific control program. The definition states as well, that their main objective is the movement of any material around the workspace. Robot manipulators are made of links connected by joints, which can be of rotary type, where one link rotates regarding the other, or of prismatic type, where one link makes a linear movement regarding the other one. The end-effector of a manipulator is a device that is attached to the last link and is the responsible for performing the task on the material and it can be a gripper or a tool. The quantity of links as well as the type of the joints will define the anatomy of the manipulator. A typical robot manipulator has six joints, i.e., six degrees of freedom. The first three are used to place the end-effector at the point wanted and the last three to orient it correctly. Robot manipulators can be classified, according to their anatomy, in four different groups: cartesian robots, cylindrical robots, spherical robots and jointed-arm robots. The cartesian manipulators, also known as gantry robots, have the first three joints of the prismatic type. The cylindrical manipulators have the first joint of the rotary type and the other two of the prismatic type. The spherical robot manipulators group has the first two joints of the rotary type and the third one of the prismatic type. And finally, the jointed-arm manipulators have all the joints of the rotary type. The jointed-arm robots are also known as mechanical arms because their six joints reproduce the human arm movement. In Fig.1, we can see an example of the anatomy of each robot group cited. Workspace of a manipulator is the collection of all points that the end-effector can reach. Each type of robot anatomy has its particular workspace which, evidently, varies according to the dimension of the links and joints limits. Another definition that helps to characterize a manipulator is the payload, which is the maximum weight that the robot can lift while performing a task without losing the precision. 1 Serra Negra, SP - ISSN 2178-3667 1132

Fig. 1. From left to right: a gantry robot, cylindrical robot, polar robot and an arm-jointed robot. The joints of the robot manipulators are activated by actuators that can be of three types: hydraulic, pneumatic and electrical. The first ones are hydraulic pistons and motors that use very viscous oil and are characterized for giving to the manipulator high precision as well as high load capacity. The pneumatic actuators are similar to the hydraulic, but they utilize air as fluid. Because air is very compressible at great pressures, they give to the robot lower precision than the hydraulic. The electrical actuators make use of electric motors to transform electrical energy into mechanical energy. These actuators give to robots great precision and hold the advantage of not taking risks to contaminate the work area. However they have the disadvantage of having lower payload and they can t be used in an environment with fire risks. Control a system means to get from it the wanted answer. There are two methods of controlling a system: open-loop and close-loop. In the open-loop control it s not given to the controller information about the evolution of the system. Therefore, if happens any disturb with the answer of the system, the controller will continue sending the same stimulation that was sending before thinking that the answer still is the wanted one. In the close-loop control, sensors are utilized to send information to the controller about the answer that the system is giving. If any disturb changes the answer, the controller sends a stimulation that will correct it. There are three basic types of control that are utilized with robot manipulators. The first one is called limitedsequence or bang-bang. This type of control has the objective to move the end-effector from one point to another and these points are given by the mechanical characteristics of the actuator. The limited-sequence control operates in open-loop and it doesn t give much precision to the robot. The second type of control is the point-to-point control where the end-effector goes from one point to another, passing or not through intermediate points, as long as these intermediate points are preprogrammed. This type of control operates in close-loop with position sensors. The last type of control is called controlled path where the end-effector makes the movement between two points following a preprogrammed path. It operates in close-loop with position and speed sensors. There are two essential concepts that qualify the work of a robot manipulator. One of them is the repeatability concept that measures the ability of the robot to return to the same position over and over. The other concept is precision of the robot, that is the ability of it to go to a specific point of its workspace. A robot can have a great repeatability, but low precision or the opposite can happen as well. 3. ROBOTS X CNC MACHINE TOOLS Arc and spot welding, palletizing, pink and place, assembly, painting, among other processes are examples of some applications in industries for robot manipulators. However, it s been several years that the utilization of these robots for the automation of material removal processes is increasing. Industries have already realized that, for some of the material removal processes, they present a lot of advantages as compared with CNC machine tools. The first obvious advantage of these robots is the number of axis. Usually, conventional material removal CNC machine tools have three to five axis, whereas robot manipulators used in material removal processes have six axis. That means they have one to three degrees of freedom more than the machine tools. That difference even the smaller one gives to the robot a much bigger workspace, resulting in higher flexibility at the operation moment. Besides, robot manipulator anatomies also make their work a lot more flexible as compared with the CNC machine tools. There are some processes in which it s necessary to stop the CNC tool to change the work tool or to change the position of the part that is being manufactured, so the machine tool can reach a side of it that wasn t before. Robot manipulators can reach all sides of the part and finish the work all at once. That results in higher throughput in much less time because the manipulator stays more time working than the CNC machine tool. For an example, in an automotive factory it s needed to create an automotive hood scoop mold. First, the CNC tool would remove the unwanted material of one plane and then would be needed to turn the part 90º, so that it could remove the material of the other plane [2]. A manipulator would be capable of removing all unwanted material making fewer operations and wouldn t be needed to move the part. After removing the material in one plane, it would place the end-effector on the other plane and would remove the rest of the material. It would result in less time to manufacture the mold and, consequently, much higher number of products manufactured in a 2 Serra Negra, SP - ISSN 2178-3667 1133

determined period of time. Also, it would eliminate the chance of happen a manufacturing error because of the bad placing of the part. Another characteristic that makes manipulators much more flexible than the CNC machine tools is the fact that there isn t any manipulator designed to perform only one task. All robot manipulators are designed with specific characteristics that allow it to perform different tasks. For that to happen, it s only needed for them to change the end-effector. For that change to happen, it s not necessary that an operative do it. The change just needs to be in the control program of the robot. For an example of this flexibility, the Fig. 2 shows Kuka s KR 140 robot. It can be used at a lot of material removal processes, but it can be used, as well, at palletizing, assembly, spot welding, material handling, coating and much other operations. USING ROBOTS TO MATERIAL REMOVAL PROCESSES Fig. 3. Motoman s robot. Fig. 2. Kuka s KR 140 robot. Another example of robot manipulator that can be used at a lot of material removal processes is the Motoman s Robot (Fig. 3). Besides the fact that they have higher flexibility, another essential advantage of them is the initial cost for the installation of the equipment. While the cost of the implementation of a big five-axis milling is between $750,000 and a million dollars, the cost of a six-axis manipulator is between $175,000 and $300,000, i.e., a reduction of costs of 82.5% at best. In the next two figures we can see another advantage of the robot manipulators over CNC machine tools. Fig. 4 shows the top view of the Fast Service s Gantry G1890 milling. We can see that the equipment occupy, approximately, 11 m² of floor space. Fig. 4. Top view of the Fast Service s Gantry G1890 milling. In Fig.5 we can see the top view of Motoman s HP80-50 manipulator which can be used at a lot of removal material processes, among many others different tasks. The factory floor space necessary for its implementation is, approximately, 0.5 m², i.e., despite having much bigger workspace, robot manipulators are more compact, needing much less floor space for the installation. Industries have opted each time more for the utilization of robot manipulators at material removal processes not only because of their smaller initial cost, but also because of their smaller operations cost. They present less expenditure with energy and less waste material, what is very important since both energy and material costs are constantly increasing [3]. They reduce costs with workforce as well. Clearly, the automation of these 3 Serra Negra, SP - ISSN 2178-3667 1134

processes brings a direct benefit for humans because it keeps him away of noxious environments and hazardous situations that some material removal processes present. An illustration of that tendency of the industries is the robots sales numbers. For example, according to RIA Robotics Industries Association in the first semester of 2006 the robots selling in North America fell 38% regarding to the same period of the year before. However, whereas most of robotics sectors presented a decline on their sales, the sector of the material removal robots increased 33% [4]. That robot disadvantage happens, mostly, because of their anatomy. They are built in a way that all links are linked by joints, from the base (fixed anywhere) to the end-effector (material removal tools), i.e., each link needs to support by itself the entire load that is made on it [6]. An element that made robots take more time to get into the material removal process market, once and for all, was the language program of the robots control program. CNC machine tools use a language called G-code. With this language, they can make use of CAD/CAM (Computer Aided Design/Computer Aided Manufacturing) system of manufacturing. With this system we can design a 3D model of a part on the computer screen and, using a software, CNC machine tools can learn the exact path that they need to make to manufacture that part. Robot manipulators didn t have a language that read G-code programs, but each manufacturer created his own software that is capable of converting G-codes to its own language. So manipulators can be taught the same way CNC machine tools do and make use of CAD/CAM system [2]. In Fig.6 and 7 we can see an example of manufacturing using CAD/CAM system by an ABB manipulator robot. In the Fig. 6 we can see the 3D model of the part made with IRBCAM software created by ABB. With this program, the points that the manipulator needs to follow to make the part are programmed. Fig. 7 shows the real part already manufactured. Fig. 5. Top view of the Motoman s HP80-50 robot. Despite all these advantages presented above, the application of robot manipulators to substitute CNC machine tools presents a crucial disadvantage responsible for the no total domination of robots over CNC machines. One of the mains, if not the greater, characteristic that an equipment must have to perform a good material removal process is rigidity mechanical stiffness under load. For material removal processes on plastic or foam, current robots present enough rigidity to perform the operation properly, but the forces generated at operations with steel, for example, are much higher [5]. Anyway, there are some manipulators that are used at material removal processes at parts made with materials that require a higher rigidity, but it depends of the precision that the part design specifies. Robots that present higher rigidity are very robust and can t operate with as much precision as CNC machine tools. Fig. 6. The 3D CAD design of a part. Fig. 7. The real part, already manufactured by milling. 4 Serra Negra, SP - ISSN 2178-3667 1135

4. SPECIFICATIONS OF THE ROBOTS According to Brumson [3], material removal robots can be divided, basically, in five big groups. The first one represents processes where the robot manipulator removes material of the perimeter of the parts, like cutting and trimming. The second large group includes surface finishing operations and polishing, where the robot has the objective of removing rough edges or smoothing surfaces of finished parts. Another group of material removal robot is plastic edge finishing, where it s removed any rough edges of the mold. The fourth group is stripping and cleaning. At these operations the robot removes a superficial coat of a specific part, so that the coat can be reapplied on the part or can be done any other operation on it. The last and newer robotic material removal group is of milling operations. At this process, the robot creates a specific shape out of a block made of some material. A quick research through robot manipulators available on the market at the greatest manufactures of robots shows that material removal robots are, mostly, of the jointed-arm type, i.e., they have six axis (that means six degrees of freedom) and all of their joints are of the rotary type. Another kind of anatomy found, but very rarely, is of the gantry type. The use of jointed-arm manipulators is very obvious because it is the most flexible type of anatomy and it can be applied at almost every operation. At material removal processes, is very important that the tool follows an exact path on the surface of the part that is being manufactured. For that reason, is essential that the control type utilized on these robots of the controlled path type where the end-effector makes an accurate and predetermined trajectory and it s used sensors to guarantee that the operation will be done right. Another factor of great importance is the robot repeatability. Manipulators must present great repeatability because the higher it is the greater will be the constancy of the robot movements which is essential for a good removal material process. While for another applications, there are robots with repeatability of ±0,60 mm, for material removal applications the ideal is that repeatability stays between ±0,02 mm, for small and low payload robots, and ±0,40 mm, for very large robots [3]. Also it s necessary the use of sensors at robotic material removal processes. First, it s essential that the robot have a position sensor so it can know if the tool is at the right point. It s also necessary that the robot have a speed sensor because the speed cut of the tool is one of the most important variables at material removal process. It s extremely important too that the robot has sensors for force control. These sensors can sense the force on the work piece and adjust it so that the tool does the expected force on the part. Force control is essential for the final quality of the part. Without it, the manipulator can remove too much or not enough material from the part, the tool can break, among others consequences [7]. Due to their high flexibility, there aren t any specific characteristics that allow robot manipulators to perform a determined material removal process. They are designed in a way that most of them can be used at a lot of different processes. The only operation that requires some specific characteristics is the milling process, but only when the part that will be manufactured is made of very hard material so the manipulator must present a greater rigidity. 5. CONCLUSION The utilization of robot manipulators at material removal processes is already a reality in industries around the world and has been developed quickly with the implementation with new technologies. When correctly chosen, robot manipulator presents a lot of advantages as compared with CNC machine tools. They can be perfectly utilized at material removal processes on parts made of plastic, foam and others materials that aren t so tough, as well as at operations on parts that don t require small tolerance. Robots still aren t capable of performing material removal processes with such small tolerance as CNC machine tools can. They also don t present sufficient rigidity to perform the operation on steel parts, for an example, with small tolerance. The development of the technology of force control is one of the factors that can make possible the use of these robots at processes that require more accuracy and at material removal processes on tougher materials parts. There are some manufacturers of manipulator robots that believe on designing a specific robot to do these processes on tougher materials. They would be much more robust and would have the necessary rigidity to perform these operations on such materials, but they would lose the capability of doing others tasks. ACKNOWLEDGMENTS The author Aloísio Carlos de Pina would like to thank to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro) for financial support in the course of this work. REFERENCES [1] F. Pazos, Automação de Sistemas & Robótica, First Edition, Axcel Books, Rio de Janeiro, Brazil, 2002. [2] G. Webb, and M. K. Morel, Robots: The Lower Cost, More Flexible Process Improvement Alternative to CNC Machine Tools, Robotic Industries Association, <http://www.robotics.org/>. Path: More Feature Articles; Application; Material Removal & Cutting, 19 Sep. 2005. [3] B. Brumson, Robots on the Grindstone: Material Removal Robots, Robotic Industries Association, <http://www.robotics.org/>. Path: More Feature Articles; Application; Material Removal & Cutting, 6 Aug. 2008. [4] RIA, Robot Sales Fall 38% in North America, Robotic Industries Ass., <http://www.robotics.org/>. Path: Industry Statistics, 3 March 2006. [5] B. Brumson, What Material Removal Process Is Right for You?, Robotic Industries Association, <http://www.robotics.org/>, 16 May 2005. 5 Serra Negra, SP - ISSN 2178-3667 1136

[6] M. Tolinsky, Robots Step Up to Machining, AllBusiness.com Inc.,, http://www.allbusiness.com/ manufacturing/4083401-1.html, 1 Sep. 2006. [7] B. Brumson, Take it Off: Robotic Material Removal Applications, Robotic Industries Association, <http://www.robotics.org/>. Path: More Feature Articles; Application; Material Removal & Cutting, 2 May 2002. [8] B. Brumson, Robot Removal: Parts Finishing Is Big Business for Robots, Robotic Industries Association, <http://www.robotics.org/>. Path: More Feature Articles; Application; Material Removal & Cutting, 17 Aug. 2007. [9] Fast Service, < http:// www.fastservice.ind.br/imgs_ textos/_fastservice/13_15101114118495648.jpg>, 12 March 2009. [10] E. P. Ferreira, Robótica Básica, Preliminary Edition, V EBAI, Rio de Janeiro, Brazil, 1991. [11] Kuka Robot Group, < http://www.kukarobotics.com/ en/products/industrial_robots/high/kr140_comp/start.htm>, 12 March 2009. [12] B. Morel, Robotic Material Removal and Machining, Manufacturing Engineering, FindArticles.com, <http://findarticles.com/p/articles/mi_qa3618/is_200 711/ai_n21137862>, Nov. 2007. [13] RobotWorx,, <http://www.robots.com/showimages.php?type=robots&tag=476&index=2>, 12 March 2009. [14] Manufacturing Engineering, What s Up With Robotic Milling, Manufacturing Engineering, FindArticles.com, < http://findarticles.com/p/articles/ mi_qa3618/is_200509/ai_n15352480>, Sep. 2005. [15] IRBCAM, < http://www.irbcam.com/pictures.asp>, 12 March 2009. USING ROBOTS TO MATERIAL REMOVAL PROCESSES 6 Serra Negra, SP - ISSN 2178-3667 1137