A shop’s production levels are commonly taken into consideration when evaluating different types of cutting tools for a given operation. Consider center drills, which create a hole in the end of a workpiece with a specific diameter, depth and form to receive a lathe or grinding machine’s supporting center. Preliminary center drilling operations must be performed on all long workpieces before they can be turned or ground.
The two most popular types of center drills are HSS and solid carbide. Cost-effective HSS center drills tend to work fine for shops performing just a modest amount of high L:D turning or grinding. Such shops may consume only a few center drills each month. More expensive solid carbide center drills can be attractive to shops that process larger batches, allowing them to drill at higher speeds and feeds and to experience longer tool life. Both types, however, require some sort of measurement routine when changing a worn center drill so that a fresh one drills to the proper depth. (This might be done using a tool presetter or an in-process gage.) Maintaining the specified center hole depth throughout a production run is critical to ensure that a grinding wheel, for example, doesn’t impact a shoulder or some other workpiece feature, scrapping the part.
Taiwan’s Nine9 (exclusively represented in the United States by Everede Tool Company) has developed an alternate type of center drill geared toward high-production environments that eliminates the need to measure replacement drills. Its “i-center” is said to be the industry’s first indexable-insert center drill, combining the performance advantages of solid carbide with the positioning repeatability and quick-change benefits common to indexable-insert tooling. This new type of drill is suited for components such as transmission components, crankshafts and shafts for motion-control applications turned or ground in large volumes.
The i-center tool uses standard insert blanks that offer two drilling cutting edges. Inserts locate on two pins machined into the mating toolholder and are secured via a single Torx screw. Insert edge-to-edge positioning accuracy is 0.05 mm in the axial direction and 0.02 mm in the radial direction, making it unnecessary to re-measure after indexing or changing inserts. The i-center toolholders are made of hardened, high-alloy steel and have internal passages that deliver high-pressure coolant to the tip of the insert. This helps increase cutting speed and extend tool life while preventing chip “bird-nesting.”
Each insert drill tip has two cutting edges as well as reliefs designed to facilitate chip evacuation. This provides a quality hole finish and also helps prevent insert tips from breaking due to excessive heat buildup, which is the most common failure mode for center drills. The i-center inserts have a two-flute-effective design and are offered in diameters ranging from 2 to 10 mm. Standard forms include ANSI 60-degree, DIN332 Form R and DIN332 Form A+B, but special forms are also available. They have a K20F carbide substrate engineered for centering operations and a TiAlN coating, making them suitable for drilling cast iron, carbon/alloy steels, aluminum/aluminum alloys and copper/copper alloys.
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