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Cutting Tools 42 MoldMaking Technology April 2015 Tight Areas Toroidal tools are the best choice for cutting tight areas such as helical bores and ribs, or when the cutter diameter is close to the radius of the part. The tool's robust, toroidal shape creates a chip- thinning effect that allows it to cut at faster feed rates. In addition, its radius is smaller than a traditional ball-end tool, so you can increase stepovers and still maintain flat floors without the large cusps typically generated by ball-end tools. Toroidal tools are well-suited for helical bores and ribs because these applications naturally cre- ate a lot of surface contact with the cutter. Using a two-flute cutter with a toroidal shape will minimize surface contact, heat and deflection during cutting. In these two applications, the toroidal tool is usual- ly encapsulated in the cut, therefore, the maximum radial stepover is 25 percent of tool diameter, and the maximum Z depth per pass is 2 percent of tool diameter. When the tool is helically entering the part, or in helical bores, the helical entry angle is 2 to 3 degrees until you get to the 2-percent Z depth. When the tool is not encapsulated in the cut, for example, when it is picking out a corner of a part or cleaning up features, material hardness determines the radial stepover. In 30- to 50-HRc material, the maximum radial stepover is 5 percent of tool diameter. In materials harder than 50 HRc, there is a 2-percent maximum radial stepover and a maxi- mum Z depth of the tool diameter per pass. Straight Walls Bull-nose cutters work best for cutting in wide- open areas with flat floors or straight walls. Tools with four to six flutes, in particular, can excel when profiling the outside shape with straight walls or very wide-open areas. The higher number of flutes allows for faster feed rates, although the programmer still must minimize the amount of contact with the cutter and use light radial-width cuts. Operations on less-rigid machines benefit from using smaller-diameter tools, as they reduce the amount of surface contact. Multi-fluted bull-nose cutters are applied the This diagram illustrates the increased surface area created by a larger cutting tool vs. a smaller cutting tool. In straight-wall machining there is a 37-degree arc of engagement, while a traditional corner cut produces a 127-degree arc of engagement. A variety of tool types are available for cutting hard materials, including (left to right) ball-end, toroidal, bull-nose and square-end tools.