The tool paths for a plunge roughing routine ought to be straightforward. Indeed, a plunge roughing move is literally straightforward, in that milling in this way involves driving the tool directly into the workpiece along the Z axis. The tool cuts on its face, feeds back out of the work, steps over to make an overlapping plunge and so on.
The stepover is where the strategy lies. Adjacent plunging moves need to overlap to some extent (in part to provide an opening for chips to escape), but how much overlap is enough? What pattern of plunges is the most efficient?
Efficiency, after all, is the point of plunge roughing. Taking advantage of a machine’s stiffness along Z, the technique permits more aggressive cutting of harder materials in cases where that hardness might constrain cutting with conventional X- and Y-axis milling paths. In aircraft part machining, the technique is sometimes used to quickly hog pockets out of titanium. With pockets in these parts getting deeper, personnel with Boeing’s Advanced Manufacturing R&D group in Saint Louis, Missouri, have been experimenting with plunge roughing in search of strategies for optimizing the technique’s effectiveness.
Boeing engineer/scientist Keith Young offers an example of such a strategy. It involves a 2-inch diameter plunge roughing tool from Iscar. When milling titanium with this tool, Boeing personnel achieved the highest productivity through a strategy that uses two different speeds.
The first pattern of plunges is made at 200 rpm, he says. The stepover increment is 1.8 inch—a stepover close enough to the size of the tool that there is little overlap between plunges. The design of the tool, with its clearance for chips, is part of what makes this large spacing possible.
Four-point “stars” are left standing between the widely spaced plunges (as the illustrations show), but these stars can be removed quickly. A follow-up pass takes out these posts at 400 rpm.
Using this two-speed approach, Dr. Young says a pocket can be roughed out of titanium in relatively little time. Boeing typically applies the strategy to deep pockets by plunging away the material in levels of 0.5 inch.