Tabletop gamers often use hot wire cutters to turn chunks of foam into elaborate miniature landscapes, but this pair of robot arms turns hot wire carving into an intricate art form. By coordinating their movements, they can recreate even the most complex curves of a 3D model by bending and warping a flexible metal rod as it slices through foam.
Traditionally, hot wire cutters use straight wires that are only capable of making straight cuts in polystyrene foam. Recreating curves or more organic shapes requires crude contours to be sliced first, and then later refined and smoothed using files, rasps, or other tools. The process can be expedited using robots, but researchers from the Computational Robotics Lab at ETH Zurich’s Department of Computer Science have further improved that approach by teaching a pair of robot arms to work together so that the carving process is no longer limited to straight cuts.
The robot, a YuMi IRB 14000, has two arms that can each move with seven degrees of freedom. Each arm’s gripper holds onto an end of an “inextensible elastic metal rod” which means that while it can be bent and warped, it can’t be stretched so its length never changes. That’s important because, as detailed in a paper titled “RoboCut: Hot-wire Cutting with Robot-controlled Flexible Rods,” in order for this idea to work, the researchers had to be able to calculate and model the exact shape of the wire as the two arms move, and any unwanted and unplanned stretching would have made it all but impossible to accurately predict the shape as the rod is warped.
Mastering the robot arms’ movements and being able to recreate very specific sweeps of the metal rod was just one part of the problem. The rod can only be bent and warped to a certain degree before damage occurs, so the researchers had to determine its limitations, and then plan their cuts within those boundaries. As demonstrated in the video above, carving the famous Stanford bunny 3D test model using this technique can’t be done in just a single pass. The researchers had to develop tools that would analyze a 3D model and then calculate a series of sweeping cuts that remove pieces of foam in a specific sequence to eventually reveal the animal.
Those planned sequences also had to take into account the complexity of the model so that, for example, a sweeping cut designed to carve out the bunny’s tail didn’t accidentally slice off the bunny’s ears in the process. The research takes a unique subtractive approach to 3D printing, which is traditionally accomplished by building up materials layer by layer over time in an additive process, instead of here where an existing block of material is whittled away to reveal a 3D model. Could it work on other materials? It doesn’t take much effort to carve through polystyrene foam, but metal, stone, and even wood would pose a bigger challenge. A chainsaw blade that can bend and warp like a metal rod simply doesn’t exist yet. However, if you’ve ever marveled at the elaborate butter sculptures on display at your local state fair, this research could potentially take those artworks to another level.