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Cutting Tools 16 MoldMaking Technology —— AUGUST 2016 This comparison shows the strength zone of straight-shank (top), flanged (middle) and shank (bottom) tools. While a flanged tool is an improvement over typical straight-shank tools without a flange, it still does not have the same strength zone as a modular drill that has a larger flange diameter and is pre- loaded to the flange face by the locking screw. This creates some stress in the tool that greatly increases the radial stiffness, making the steel body resistant to bending during plunge roughing or difficult drilling. ensure this is the most economical solution for a particular job, including tool path, material shape, machine power and speed. Some high-feed mills can achieve a metal removal rate of more than 50 cubic ipm, but very few machines can handle such speeds accurately without problems. Success with high-feed milling is also very dependent upon the tool path as well as minimal non-cutting time in order to reduce cycle time. Plunge roughing, on the other hand, is a much more dedicated solution with fewer variables. Its success is impacted only by speed, feed rate and stepover, whereas a traditional milling operation relies on the same, plus program tool path, dynamic feed rates around radii, tool engagement percentage and toolholder radial stiffness. Tool Design Elements Since both indexable drills and most milling tools can be used for these plunge roughing operations, let's examine the unique aspects of each. As indexable drill design has improved over time, they have became better candidates for plunge roughing operations. Solid high speed steel (HSS) and carbide drills are typically not good choices for plunge roughing, as their steep point angles can push the tool off center if less than 60 percent of the cutter diameter is engaged in the part material. Also, solid drills do not handle the shock associated with plunging very well. Carbide, for example, can easily break if exposed to high shock loads. The high-strength steel body of indexable drills, coupled with a complex cutting geometry on the insert, increases the abuse these tools generally can handle. However, not all tools are created equal. Some key design elements for a stable pro- cess are drill stability, chip formation and chip evacuation. For drill stability, the machine, fixturing, tool and toolhold- ing must be considered as one connected system. From a tool standpoint, the goal is to have as stiff of a drill core or web as possible and still be able to smoothly evacuate all chips under all cutting conditions. The core or web of a drill is the area of the tool along the center axis that has no machined flutes. The diameter and longitudinal shape is what creates the radial stiffness of a drill and enables it to resist deflection. A main difference among drills is coolant channel design. A common way to create a coolant channel is to drill one main feed hole through the center of the tool, then two connecting passages to each insert. This is easy and relatively inexpensive to manufacture, however it does reduce the solid material area in the core. Under light drilling conditions, this may not be an issue, but pushing a tool to its physical limits will be notice- able, especially if it is not fully engaged, which adds a radial load component to the tool itself. For maximum drilling performance, other drills are designed with two separate, small coolant channels that are drilled through when the tool is in a rough-machined state during manufacture. The tool body is then heated and twisted to match the helix angle of the fluting, creating a completely solid core of the tool and two independent helical coolant channels that take the coolant flow directly to the cutting edge. From an insert perspective, the cutting edge geometry and position have a huge impact on cutting performance and sta- bility of the drill itself. Since inserts are pressed, they can be given very complex topography that is not possible in grinding solid tools. This, along with a pocket manufacturing method, allows tight control of mounting and position angles, such as the gamma angle that defines the radial angle in relation- ship to the center line of the drill axis. The angle at which the insert sits, the cutting edge design and the contact point of the insert on the material all determine how forces act on the drill. Balanced forces around the drilling axis will create a very stable and smooth drilling operation. When the drill is used as a plunging tool, its diameter is not always fully engaged, which makes this process much more difficult as the force vectors change relative to the tool axis. Most high- performance indexable drills are designed with these issues in mind. The better balanced the drill forces are, even under poor conditions, the more aggressive and difficult applica- tions the drill can perform, including, for example, drilling