For the past 20-plus years, robots have helped major manufacturers and employees alike by performing dangerous, repetitive tasks with greater accuracy, safety and speed than their human counterparts. The arguments in favor of robots – namely reduced labor costs, changeover and downtime; and improved worker safety, accuracy and throughput – are well-known. At the same time, the ability to reprogram, repurpose and reuse robots in other production lines means that industries that regularly buy and use hard-tooled fixtures for manufacturing automation can greatly reduce tooling costs by switching to flexible robot automation.
In recent years, CNC machine tending has joined the growing base of robot applications. A single robot can tend multiple machines, while making physical measurements, tracking product on the fly, and reporting the results to the plant network. The success of machine tending and the growing acceptance of robotic technology in new markets have prompted some manufacturers to replace expensive CNC machines with robotic routers, drills and mills for machining applications.
Figure 1: Robots can machine to very fine tolerances.
From Material Removal to Machining
Robots have been used for material removal applications for many years, however, direct part machining is a different matter. "Deburring and deflashing are done by robots quite a bit," explains Doug Neibruegge, segment driver for foundry at ABB Robotics (Auburn, Hills, MI). "We get a lot of requests for that kind of work, but when you talk about machining, or what we call pre-machining, that's a developing area."
“Repeatability refers to the robot’s ability to achieve the same point in space in a repeatable manner. Today, robot repeatability is very good and in many cases, robots meet the accuracy requirements to perform robotic machining,” notes Mike Cicco, account manager at FANUC Robotics America.
Accuracy refers to the robot's ability to find a point in 3D space, and in the case of machining, maintain that position while applying force. While a robot's accuracy generally is measured in mills (hundreds of a millimeter), CNC machines regularly create surfaces that vary from the original design by thousandths of an inch or less, although using machine vision, precision fixtures and limiting the range of a robots movement together can improve a robot's overall accuracy, according to Kap Choi, machine tending technology leader at robot manufacturer, Motoman Inc. (West Carrollton, OH). "Introducing [options] like thermal growth calibration software can take a 6-axis arm and turn it into an accurate arm with 250-micron [~10 mils] accuracy," Choi says, "which is plenty good enough for most applications. The myth that robots are not accurate refers to from one far corner of the workcell volume to the other; But in small area, robots can be very accurate. It's more rigidity that 's the limiting factor, rather than accuracy, again assuming that the part is very small."