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Cutting Tools 14 MoldMaking Technology —— AUGUST 2016 By Cullen Morrison Images courtesy of KOMET of America. The images above show examples of very difficult drilling conditions for an indexable drill. The design of this particular drill provides excellent stabil- ity, chip formation, chip evacuation and long service life, making it well-suited for plunge roughing. Take the Plunge: Drill vs. Mill R ough milling operations can take hours on large com- ponents and typically remove more than half the raw material by volume. Selecting the proper machining method for the component and machine tool creates a suc- cessful and stable system. The use of plunge roughing, com- monly referred to as plunge milling, has grown recently as cutting tools that can handle the tough requirements of this machining method have become more readily available. Plunge roughing uses a cutting tool in an axial direction for material removal, similar to drilling. This process requires a tool that is designed to handle such axial cutting, such as indexable drills and milling tools, two common tool types that each have their own benefits and limitations. Benefits Breakdown A drawback of plunge roughing is very limited profile control, since using the tool in a plunging tool path will always leave a radius-shaped scallop on the part equal to the tool radius. So, unlike a traditional milling tool path that can be programmed to create any shape, plunge-milling tool paths cannot create smooth contours. Drilling, by nature, typically offers the high- est material-removal rate of all machining operations, so using a drilling operation will create a productive operation with a flexible profile path. Strategic operation planning and cutting tool system optimization can make plunge roughing a productive, stable choice for high-volume material removal. VIDEO Access video at end of article. Consider this example to prove the effectiveness of plunge roughing versus high-feed milling: A section of material equivalent to 325 cubic inches needs to be roughed out from a P20-steel block. The two tool choices are a 2-inch-diameter high-feed mill and a 2-inch-diameter indexable drill. The ini- tial cutting parameters of the high-feed mill, when taking a full-width pass, are 825 surface feet per minute (sfm), 0.0315 inch per tooth (ipt), four cutting edges and a depth of cut of 0.078 inch. This equates to 1,570 rotations per minute (rpm) and 198 inches per minute (ipm) with a material removal rate of 30.9 cubic ipm. So the machining time theoretically will be 10.5 minutes, excluding any non-cutting time. The initial drill cutting parameters are 700 sfm with a feed rate of 0.008 inch per revolution (ipr). This equates to 1,337 rpm at 10.7 ipm and yields a material removal rate of 33.6 cubic ipm for a theoreti- cal time of 9.7 minutes. The mathematics of this example prove that plunge roughing with the indexable drill is favorable in terms of time, however there are many other factors that should also be considered to