For certain multi-axis milling applications, using a traditional CNC machining center might be overkill. These include milling materials such as clay, foam or wax for new product design, styling and rapid prototyping projects, as well as traditional materials in which dimensional tolerances are low. In these cases, a robot could be more affordable than a machining center and also accommodate larger workpieces, such as scale automobile bodies or full-size automotive components. According to Mark Evans, president of Direct Controls (Provo, Utah), the robot’s controller can limit how effectively a robot maneuvers a cutting tool through its cuts.
Direct Controls has developed the Robo-Mill robotic milling system, which combines a Denso robot with the company’s proprietary direct-from-CAM controller. The DMAC (direct machining and control) PC-based controller is able to drive robot movement directly from a CAM model without requiring a postprocessor to generate machining code (think in terms of a computer driving a printer). This can offer significant time savings and also allow any changes made to cut data to be saved directly within the CAM model. The DMAC controller is capable of performing NURBS interpolation and is currently compatible with Catia, GibbsCAM and Unigraphics CAM packages.
The robot controller provides far-reaching lookahead capability similar to a machine tool CNC, so maximum jerk and acceleration values are not exceeded when a tool must quickly change direction. Similarly, the controller is able to anticipate robot singularities. These are situations in which an infinite number of robot joint angle combinations is possible or when an instantaneous large-angle move is necessary to attain a particular tool position. Typically, when a singularity is reached, a robot will stop and the programmer must re-program its path. In anticipating singularities, the DMAC control automatically shifts its approach so that the robot moves in a controlled manner through the singularity without lifting the cutting tool off the workpiece.
In addition, the control calculates the robot inverse kinematics on the fly, which is the process of determining the robot’s joint displacements given position and orientation constraints on the robot’s end-effector (which is the cutter, in the case of milling). This is typically done during post processing, but because the toolpath data is taken directly from the CAM system, there is no need to post to every robot that will be milling that particular job to account for the inverse kinematics.
The base Robo-Mill model uses a Denso robot mounted on a Lamerson worktable. The Robo-Mill currently has maximum reach from 0.85 meter to 3.5 meters, maintaining ±0.02-mm repeatability. Its maximum spindle speed is 14,000 rpm and its collet toolholder accepts cutting tools up to 3/8 inch in diameter. The company is developing systems using larger robots, which may open up opportunities for light metal cutting duties, as long as the robot can provide sufficient rigidity and payload capacity.