Detecting and eliminating collisions in NC machining

Abstract

When a 3D object is milled from a billet by a cutting assembly (tool and an associated cutting head), a common hazard, particularly if the cutting is under numerical control, is that this assembly may collide with (foul) the partly cut billet. Fouling may be detected and eliminated by simulating the changing billet shape and the moving cutting assembly: fouling is detected when billet and cutting assembly models occupy the same model space. The principal method of removing fouling is to modify the billet by removing those parts that would subsequently foul. Alternatively, the cutter assembly may be modified by fitting a longer tool, whose length is calculated to lift the cutter assembly above any part of the billet causing fouling. For premodifying billets, the program records the parts of the billet causing fouling and calculates a cutter path to remove these. Since this preliminary cut may itself cause fouling, the program is applied recursively to the initial toolpath and subsequent defouling toolpaths until a final toolpath is reached that does not cause fouling. These preliminary toolpaths are then executed in the reverse order of their generation. The method assumes that the final shape has a single height above every point of its base, and is therefore limited to shapes that do not have undercutting.