“It’s similar to the Racon point for HSS and cobalt drills,” said Bob Hellinger, the toolmaker’s national sales manager for standards. (A Racon point has a rounded, or curving, point to minimize stress and reduce corner wear or breakage because a rounded cutting edge—similar to a corner radius on Guhring a cutting edge—is To prevent a drill’s less prone to break- corners from ing when cutting breaking when an abrasive mate- machining the rial than one that abrasive ductile cast iron material, comes to a sharp Guhring introduced corner.) “The full the RT 100 R Ratio radius point is al- solid-carbide drill most like a ballnose with a patented full endmill’s rather radius point grind. than the sharp corners on regular drills.” Hellinger added that the full radius point can help reduce burr formation when the tool exits a through-hole and also reduces tool wear. The radius point design creates a smoother transition at the corner edge and reduces the amount of material displaced upon exiting a hole, which in turn reduces the exit burr size. Traditional Racon point designs have an increased tendency for drill wandering and are therefore run with the assistance of a drill bushing. The through-coolant Ratio drill’s specialized web-thinned design eliminates walking by reducing the thickness of the chisel edge. The Ratio drill’s DK 255 F carbide substrate has an average grain size of 0.7 microns and 8 percent cobalt to provide a hard (1,720 HV) yet tough tool, according to Hellinger. “You need a carbide grade that’s forgiving for the inconsistencies in ductile,” he said. Standard ratio drills are 3mm to 20mm in diameter and available for drilling 5 and 7 diameters deep, but Hellinger noted that the smaller diameter tools have flute lengths that allow them to drill holes deeper than 7 diameters, and specials can be ordered for making holes 10 diameters deep or more. For holes up to 5 diameters deep, he said a pilot drill is usually not needed, but he recommended creating a pilot hole about 1 to 2 diameters deep for deeper holes. Drilling significantly deeper holes into ductile iron isn’t unheard of, though. “We have a very large customer manufacturing diesel engines that does 40"-deep holes with a 1" spade drill,” said Allied Machine’s Porter. Cole noted that Sandvik Coromant’s solid-carbide 842 drill has a chamfered instead of radiused corner to better protect the tool when cutting ductile iron. This also reduces burr formation on tool exit. The company’s indexable CoroDrill 880 tool accepts the company’s GT insert, which was designed for cast iron. This tool has a reinforced edge line on the peripheral insert to enable a more secure drilling process. In addition, Cole said the GT insert is ground neutral to toughen the insert
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