May 14, 2013
Many manufacturers that were a bit squeamish about adding robots to their milling operations are finding a new comfort level with the automation.
Robots have been put through their paces. Their reliability has been proven. And, according to Yarek Niedbala, sales director for KUKA Robotics Canada Ltd. in Mississauga, Ont., many manufacturers that were a bit squeamish about adding robots to their milling operations are finding a new comfort level now that the automation has matured into an established technology.
"A robot is no longer a customized solution that needs to be designed from scratch for each situation. It's an off-the-shelf product that you apply to an application. It's been tested, and its reliability is extremely high," Niedbala said. "Standard cells have been developed, and since the engineering has already been done, the acquisition cost can be lower."
Off-the-shelf robotic systems integrate with milling equipment to automatically load material, unload parts, handle pallets, deburr, sort parts, inspect, measure, clean tooling and parts, exchange tooling, or a combination of these tasks. Add a spindle to a robotic arm and, although not as accurate as a conventional machine tool, it can perform elaborate removal of ferrous and nonferrous materials.
Niedbala said that although robots typically are repeatable machines with the ability to go back to taught positions, they do not have great path-accuracy performance based on calculated positions. Path accuracy can vary depending on the robot manufacturer, type of material, process, part size, and temperature. Some manufacturers offer high-accuracy robots that can hold path accuracy to 0.010 inch under certain conditions.
"Milling cycles for large parts can take a while," said Niedbala. "A robot can load a part inside a conventional machine and while it is waiting for that part to be processed, it can perform lower-tolerance milling and drilling operations on the next part. It is adding value rather than sitting idle between loading tasks. With a robot's value-added processes, and since it doesn't need breaks, it can bring machine tool utilization to close to 90 percent."
Flexibility also comes into play. "You can have six-sided manufacturing when you add a robot," said Niedbala. "The robot has the ability to manipulate a part, change its orientation in 6 degrees of freedom. Say you are making a six-sided die. A machine can mill five sides, the robot can reorient the part, and the machine can mill the bottom, all without operator intervention to remove, reorient, and rebottom the part in the chuck.
"A robot could potentially pick up a part using a gripper, place it in a fixture, exchange its tool for a milling head, and begin milling the part," said Niedbala. "Or sometimes the robot can grip the part and present it to a stationary milling head for processing. This increases productivity and lowers cost because the robot never lets go of the part until the milling process is complete."
Existing machines can be linked together with a robot to combine their capabilities into a process flow. "Perhaps you already have a 3-axis milling machine and a turning machine," said Niedbala. "You can add a robot to take parts from one process to the other to automatically complete components that would otherwise require more than three axes without investing in a new machine."
"The image of a good fit for robots used to be making millions of parts—high-volume/low-mix production—but that is changing," said Tom Sipple, material handling product marketing manager, Yaskawa America-Motoman Robotics Div., Miamisburg, Ohio. "Our drawer-style positioner, for example, enables an operator to load and unload parts from the cell while the robot continues to operate. The drawers allow the flexibility for batch manufacturing. You load them with different blanks to run four parts concurrently based on demand."
Pallet handling also fits with robotic automation for job shops. "For a small batch/big parts mix, the robot can pick up standard pallets preloaded by the operator and load the pallet into the machine. That way the part mix is not limited by the gripper," said Niedbala.
"The question," said Sipple, "is if an articulated robot, a Stewart platform, or a milling machine should be used for a given process.
"Articulated robots are serial-link devices with six or seven axes connected together. Each axis has error that's cumulative. They are fine for material removal tasks like deburring, polishing, trimming, or deflashing, but are not a substitute for a rigid and accurate machine tool. A good robot configuration for enabling a machining tool-like process is the Stewart platform."
A Stewart platform has six actuators working together to position a table. It acts like a truss whereby two actuators support each point on the robot table. Sipple said, "The Stewart platform's ±50-micron accuracy and stiff design produce high surface quality, improved tool life, and higher cutting speeds. As a result, it's capable of machining harder materials without deflection and chatter."
The Stewart platform design has been more expensive than an articulated robot design in the past, but now, according to Andy Glaser, vice president of sales for Yaskawa Motoman, Dayton, Ohio, "it's a flexible design that can be sized to meet the customer's specific needs at a price comparable to other designs."
Historically, robots had their own programming language, a unique skill set that needed to be maintained in the user's plant. "Now conversion software packages are available that provide an interface to translate machine tool G-code into the robot language. Programming for machining can be done offline using the part model so there's no need to learn a second language," said Sipple.
In some cases, the language conversion has been eliminated. "We developed a CNC that runs on our robot controller so you are not translating the G-code when you program the robot; you actually use the G-code generated by the CAD/CAM package. You are not constrained by the motion look-ahead of robot controllers," said Niedbala.
If the robot is tending a machine, it can be programmed offline in a simulation package, or by using the programming pendant. Using a pendant now is frequently less complex and more intuitive than in the past. A lot of functionality is stored in menus. Many pendants feature touchscreens and a Windows®-based operating system. Robotic movements can be controlled by built-in pendant buttons, 6-D mouse controls, or joysticks, so knowledge of the Cartesian coordinate system is not necessarily required.
Robot operating system software allows positioning in world or tool coordinates, so jogging of individual joints during teaching is minimized.
"As companies are moving their production back from offshore, they are recognizing that they cannot do things as they have always done them," said Sipple. "The result is a lot of interest in building automation into the capital equipment plans. Companies recognize the benefits of producing while keeping costs as low as possible, and that is a benefit robots bring to a plant."
"When looking for robotic automation, we recommend working with a manufacturer that has a worldwide footprint and can provide global support and access to spare parts and service," Sipple added. "We see customers looking for homogenous designs that can be used in North America now and possibly in India in the future."
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