A 130 kg. payload Motoman robot is outfitted with a 7.5 HP spindle, ER32 collet-chuck tool holder and cutting tools to create a flexible, affordable machining cell. Patternmaking keeps Supreme's machining robot busy as much as 10 hours per day, for the most part performing three-axis milling on MDF patterns.
Robot-milled patterns are used for forming, among other parts, fiberglass trim components such as wind deflectors (shown here) to help with aerodynamics.
For the last four to five years, pattern-making operations at Supreme Corp. have been supplemented by the firm's six-axis robot, a 130 kg payload model from Motoman outfitted with a 7.5 HP spindle. The spindle holder and cutting tools have transformed the robot into a substitute CNC machining center.
Supreme Corp. Manufacturing Engineer Tom Nowak wondered if the company could achieve higher material removal rates with a dedicated machining center. "Sure, but to get the 10 ft. of horizontal reach and 3 ft. of vertical reach we need would require a very large and very expensive machine," said Nowak. "The robot has proved to be all we need in terms of work envelope as well as flexibility, and it's much more affordable. It paid for itself in about a year and continues to provide dividends daily in our pattern-making shop."
The pattern shop to which Nowak refers makes molds for fiberglass and ABS plastic components. The components are used for specialty vehicles such as box trucks, recreational vehicles, armored vehicles for the U.S. State Department and even mobile command-center vehicles for the Department of Homeland Security.
"When our division in Texas has to outfit a vehicle with armor plating," Nowak said, "we'll custom-build new interior door panels, making sure they are as close to the OEM look as possible. They're of thermoformed ABS plastic, molded from patterns milled by the robot."
Patternmaking keeps Supreme's machining robot busy as much as 10 hours per day, for the most part performing three-axis milling on medium-density fiberboard (MDF) patterns used for fiberglass trim components such as wind deflectors and skirting, to help with aerodynamics. MDF, a resin-impregnated pressed-wood fiber product, is ideal for prototype tooling that is useful for making a set of prototype parts. MDF can also be used to build a heavy fiberglass part that can be then used to build a production tool.
The firm originally acquired the robot to trim molded fiberglass panels for truck bodies but quickly decided that patternmaking made a better fit for Supreme's needs. Part volumes were relatively low, and each part would require its own unique vacuum fixture to locate the parts for robotic trimming, not a very cost effective use of the robot, according to Nowak. Comparatively, the robot can machine patterns in a fraction of time it used to take to manually craft a pattern. For example, machining the pattern for the front end of an RV takes the robot three to four weeks; manually, that same pattern used to take 16 weeks to complete.
The robot tends to its tasks at a 6' by 10' worktable. With a radial reach of 2,650 mm (104.3"), the robot has a big work envelope that allows it to machine molds without repositioning. To machine patterns for extremely large parts, such as an RV front, Nowak breaks the pattern into segments which are then bonded together to fabricate a complete pattern.
No Simple Programming Task
Of course, preparing its robot for multi-axis machining rather than relatively simple contour cutting required a more sophisticated approach to programming, said Nowak. The traditional teach methods don't suffice; for true CNC-type machining, the robot can't be taught its movements since each part program contains hundreds of thousands of points. Instead, to program the robot for CNC-type machining, Nowak employs Robotmaster (from Jabez Technologies), a translator that converts CNC toolpath data from Mastercam into optimized six-axis robot programs. It includes a process simulator that allows users to view machine operation in continuous or step mode and detect collisions.
Programming acumen takes center stage when faced with five-axis programming tasks. "Early on, we learned that the robot's movements are very predictable in a three-axis environment," said Nowak, describing the majority of the robot's tasks. "But for those patterns requiring the robot's entire six degrees of freedom, and where we have to program the fourth and fifth axes of movement, we fight singularities that can cause collisions.
"Singularities arise when we need to machine patterns for RV fronts. In particular, cutting a nice tight profile along the edge of the vehicle's windshield requires a five-axis curve. At first, the robot would try to flip the spindle over and would cause a collision. To overcome those challenges, we worked very closely with Mastercam and Jabez, who provided the training necessary to avoid singularities."
Repeatability Reboot Requires Recalibration
"We rough-machine the MDF blanks with an inserted carbide tool," said Nowak, "and finish machine with solid carbide-either a 1/2-in. ball nose or, for added precision when needed, a 1/4-in. ball nose. We custom-designed and built a tool changer for the cell that allows the robot to move from roughing to finishing in seconds."
Critical to the success of the two-stage robotic machining process is the repeatability of the robot motion from roughing to finishing, "otherwise our patternmakers can struggle to complete a job," said Nowak. "Several months ago we noted some disparity between the tool paths during roughing and finishing. We turned to Motoman to help us recalibrate the robot, which reduced the variance between the two tasks by 75 percent."
Motoman's "Johnny-on-the-spot" to help recalibrate Supreme's hard-working robot was John Patchett, Regional Support Specialist for the Great Lakes region. "It's important that manufacturers understand the difference between accuracy and repeatability," said Patchett, explaining how a difference in the robot axis motions when off just one or two degrees can affect how accurately a robot's tool tip can get to a programmed point in space. "Periodically, robots, in particular robots subjected to high forces such as those found in machining applications, need to be recalibrated. For non-critical applications, manual calibration suffices, but in Supreme's case we needed something more."
That something more was an economical software tool (Motoman's MotoCalV EG) that correlates points in space that are taught by a programmer to the theoretical points, using a software algorithm. "This approach is much more cost effective than using a full-featured software program that works with an external encoder connected to the robot's tool tip," said Patchett. "Using MotocalV EG at Supreme Corp. proved to be all that was needed to get them back on track. Average positioning error was reduced by 90 percent, which in real dimensional terms took position variation between roughing and finishing from 1/8 in. down to just 1/32 in."
For more information contact:
Yaskawa America, Inc.
Motoman Robotics Division
10455 Austin Boulevard
Miamisburg, OH 45342
P.O. Box 463, 46527
2581 East Kercher Road
Goshen, IN 46528