Metal cutting on less modern machining centres has been seriously challenged by robotic machining. Automation suppliers and robotic system integrators are gaining popularity with the latest robotic hardware and associated software. Robots are now used in a range of tasks that were previously only possible with machine tools for material removal.
Robotic material removal applications include grinding, deburring, trimming and polishing. Laser and waterjet cutting processes are also handled by robots. When robots are compared to traditional machine tools, the advantage is significantly more room for manoeuvre. In addition to robotic milling of softer materials such as foam, plastics, wood, sand casting and aluminium, material removal applications also include robotic control of laser and waterjet cutting processes. According to Robert Gian, professor of mechanical engineering at the Lee-Ming Institute of Technology in Taipei, robots have a significant advantage over conventional machine tools, as their working area is much larger.
Gian says that manufacturers can use KR60 robots from Kuka Roboter GmbH (Augsburg, Germany) equipped with Kuka’s CAMRob software and Delcam’s CAM PowerMILL software to machine very large parts and mill shaped parts from softer materials thanks to the robots’ large working area (about 27.24 m3) and 185° swivel range. According to Gian, the robots have a very high degree of flexibility, up to eight or more axes, and a repeatability of 0.15 mm that can be used in many applications.
Robots are increasingly used in aerospace and automotive applications. Instead of creating patterns for casting large metal parts, some shops are now using robots to make huge sand castings. According to Tom Bentley, president of Programming Plus Inc, cutting the sand directly is preferable to creating a pattern.
If smaller volume items are produced, typically for steel foundries making larger single components, it saves a lot of money. Making a pattern for a single item weighing 13,500 kg (30,000 lb) costs $40,000, especially if large parts are being created. You simply cut the sand without creating a pattern. It works quite well.
Kuka’s Bentley says robotic cutting of wood, urethanes and sand castings is very common. He notes that the accuracy of robotic systems is a deterrent to the use of robots in milling. Bentley says the repeatability achieved with Kuka robots is the best he has ever tested. He adds, “They are used in cutouts and a lot of architectural sculpting work,” citing the example of customer Garner Holt Productions (San Bernardino, CA), which used Kuka robots on an RMC100, a seven-axis robotic milling cell (RMC) from PPI’s sister company, Robotic Solutions Inc. (New Berlin, WI), to produce realistic animatronic figures for theme parks, entertainment centres and exhibition spaces.
Robots must be much stiffer than a standard robot to be able to cut harder metals. One parallel-link robot that can cut carbon steel is called the Fanuc F-200iB.
For use in the automotive industry, Fanuc has created a robot that can cut 1/4 [6.4 mm] sheet metal using its parallel-linked F-200iB machine. Fanuc CEO: “We are planning to transition to the parallel-link robot when we move to cold-rolled steel”. The robot will be used to cut low-carbon steel for door handles and car bumpers.
Notes Virgil Wilson, senior material removal engineer at Fanuc Robotics America Inc (Rochester Hills, MI). The robot’s bases are hooked together to give it the maximum rigidity possible for a parallel-link robot. The fact that the robot naturally deflects when working with harder materials, he says, is at the core of what prevents serial robots from performing all types of machining.
Increasing the robot’s accuracy is another goal of robot designers, especially to meet the needs of the aerospace industry. Wilson says that robot precision is another issue. The inherent accuracy of a robot is useful in milling-type applications because they are typically coded offline, downloaded to the robot and then told to move to a particular location. Fanuc has been developing methods to improve that accuracy for 15 years. Depending on the level of accuracy required by the application, we have a variety of tools to address it.
According to Wilson, Fanuc offers this functionality as a software and hardware package that customers can purchase. “That’s one level, and then there’s another level where we use a Leica tracker to measure the robot in space, go through a number of different locations, and then we recalculate all the kinematics, or what we call DH parameters.
Robots with the latest encoder technology can meet the precision demands of aircraft builders. According to Wilson, customers include large aerospace OEMs, particularly for drilling and riveting applications. He says, “We’ve had a good number of requests for milling, and we kind of see it as the next step. The next step we need to take is to evolve to milling, which would involve a path accuracy issue.
According to Greg Garmann of Motoman Inc, robots are often used in material removal applications. Thanks to the company’s software, users can easily program the robots using the same techniques as machine tool programming. According to Garmann, “You can operate the robot just as you would on a production floor using a teach pendant. Motoman’s EH80 robot and its heavier-duty DX1350N model are capable of a wide variety of jobs. Deburring applications in the aerospace sector are among the systems installed with Motoman customers.
ABB’s main application has been using robots to drill holes in composite materials for aircraft fuselages and wings. Drilling holes in composite and titanium parts is done with heavy-duty robots, such as the 500 kg ABB 7600 robot.
The articulated arm sensors and software controls provided by ABB‘s Force Control for Machining help regulate the force applied to the tool at the end of the arm. The robot will change its path but maintain surface force by applying force-controlled machining if the component has a deflection or is not positioned exactly in one position.
Photo: With its seven-axis Kuka robot, Garner Holt Productions produces animated characters used in theme parks, casinos and museums.