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INDUSTRY INSIGHTS by Christer Larsson Short-hole drilling is frequently a source
of downtime and bottlenecks--but it doesn't have to be. There is a wide
variety of effective tools to choose from, whether the operation is generating
bolt holes or tapped holes, or more complex cross-, mini- or large diameter
holes. Advanced drilling tools achieve tight tolerances with very good surface
finishes in one setup. The as-machined finish is satisfactory for most applications. Carbide-tipped drills provide the highest tolerances and precision of all the short-hole drills. High-performance drilling tools address the most common problems inherent in short-hole drilling: chip control, deviations from centerline, straightness, roundness, and surface finish. For high-volume runs, indexable tools can replace solid tools, enabling higher feeds and eliminating regrinding. Users also benefit by reducing tool inventories and relying less on regrinding expertise to hone edges. These indexable tools can generally be installed on any machine and for most materials as either stationary or rotating. Seven options When referring to short-hole drilling, including high-speed steel, solid
carbide, and carbide-tipped regrindable drills, and small and mini-diameter
indexable drills are included. These tools cover hole diameters ranging
from 0.098创 to 2.283创 and diameter to depth aspect
ratios of 2 x D to 5 x D. The Coromant U indexable drill is designed for large-volume drilling of single or family components. Typically in drilling, cutting pressure and speed drop as the edge cuts from the workpiece's periphery toward the center. At the initial entry stages, conventional drills press and scrape the material, rather than shear it. This pressure produces high axial forces often causing deflection, vibration, poor finishes, and excessive tool wear. Modern carbide and indexable drilling tools, on the other hand, are freer-cutting. They cut freely all the way to the center of the hole. As a result, they produce holes that are straighter, flatter, rounder, and more cylindrical. Regrindable drills Regrindable drills should be used mostly for small holes ranging from 0.098创 up to 1.181创 diameter. In general, they provide the highest hole and tolerancing precision, mainly because of geometries that create symmetry and well balanced cutting forces. These drills are self-centering. They also generate excellent surface finish. A switch from HSS to carbide drills can decrease throughput by a factor of 5:1 to 10:1. Insert drills When the workpiece surface is convex, concave, inclined or irregular, or when the hole is pre-drilled, or cross-hole drilling is involved, indexable insert drills are your best option. They are particularly cost-beneficial if one has a large volume drilling of single or family components. Combined insert and drill body geometries provide a high positive presentation to the workpiece for sharper cutting, generating small, manageable chips even in very hard alloys. Two internal asymmetric helical flutes provide wider channels for reliable coolant flow to the cutting dome, for better cutting efficiency and chip evacuation. They also provide excellent rigidity to the drill body. Better chipbreaking and disposal protects workpiece surface finish. Indexable drills are equipped with two overlapping inserts, a peripheral, and a center insert. The carbide grades of each are optimized for their location in the drill. By changing out inserts, you adapt the tool to different materials, lessening wear and prolonging tool life. Though faster, one limitation of indexable drills versus solid drills is lower radial precision. Correction for the radial offset is easily accomplished through the machine program or an adjustable holder. Four indexable types Small diameter indexable drills running from 0.500创 to 2.283创 diameter generate hole depths of up to 5 x D. They can be installed on straight shanks or modular quick-change toolholders and are recommended for high-end machines with high to medium stability. An indexable low horsepower drill (Coromant Mini U) for Bridgeport manual, screw, NC short stroke machines, and manual drill presses generates holes ranging from 0.500" to 0.656"diameter. It has the same fluted design as the other indexable short-hole drills, but no coolant holes, since the aspect ratio is 2 x D, which means that external coolant supply is safisfactory. The stack drill is the only choice for drilling stacked individual components. The drill body has a straight flute design with the center insert positioned somewhat in front of the peripheral insert, so that it engages into the cut first, and creates a smaller end disk than a regular indexable drill. This prevents chip jamming. Trepanning drills are used mostly for through-holes in large diameters because trepanning consumes less power than a solid drill when you have to remove a great deal of stock. Instead of breaking chips, these drills remove a solid piece between the hole walls leaving a cylindrically shaped core in the center. Besides these standard tools, custom-made tools can provide special lengths and diameters for geometrically complex workpieces. Prices and deliveries on specials have come down considerably in the last couple of years. They're available at a nominal surcharge and delivery times of four weeks, and development is free.--MKM Sandvik Coromant, Fair Lawn, NJ. http://www.coromant. sandvik.com Circle 468 [metlfax/incl/99mfx.htm] |
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