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Why does my gun drill measure undersize?The Gun Drill R-1 Lowland ContourSingle flute gun drills from most manufacturers in the United States are furnished as standard with an R-1 Lowland Contour (with N-8, 30/20 nose grind). This Contour is a clear choice for a variety of applications where its free cutting action provides the gun drilling benefit of precision, burnished holes to virtually any depth. In difficult materials such as Nickel base, Coppers, Titanium, and others that can wear a drill quickly or close-up on the drill tip, clearance provided by the R-1 Contour make the process more successful. Additional benefit of the R-1 Contour are with short holes for gun drilling (under 15x dia./depth) where any inaccuracy of the drill, miss-alignment, or by the machine itself can lead to oversize holes, bell mouthing, poor finish, or other detraction of hole quality. The R-1 Contour is beneficial in many applications where operators may be unfamiliar with the gun drilling process, and/or conditions are less controlled, and/or the coolant type and pressure is not optimal. This total combination of variables make the R-1 "standard" Contour least likely to cause "issues".Alternate Contours such as R-2 Lowland & Top, R-3 Highland, or R-4 Highland & Top can provide benefits such as drilling more accurate holes over 50 diameters deep, self-support for cross holes, angled entry and exit, and reaming. These Contours are not covered in this writing, but each have their own place for specific applications. Refer to section Deep Hole Gun Drills, then Nose Grind-Contour Combinations for additional detail. To produce the R-1 Lowland Contour, the carbide tip is cylindrically ground to an industry standard diameter of +0, -.0002" (.005mm), and to a standard back taper of .0006"-.0008" (.015-.020mm) per inch. A Contour plus "back taper" is necessary as it allows just the end of the drill tip to cut & burnish and prevent the tip from "seizing" in the bore. At this point in manufacture, it is the last time the tip can be measured by micrometer. The R-1 Contour is a pattern of relief that is ground into the periphery of the tip’s diameter. When ground and viewed from the tip’s end, a "V" channel for oil & chip removal is approximately 115º wide. On the periphery, a "cutting land" that starts from the flat face of the drill tip is on size and about .02"-.04" (.05-1.0mm) wide, depending on drill diameter. "Relief" is ground into the tip’s diameter to about .005" (.12mm) deep and extends from the cutting land to about 70º around the periphery. A full tip diameter "burnishing pad" remains from 70º to about 170º-175º from the cutting edge. The .005" relief returns for the remainder of the tip’s periphery. The burnishing pad provides the fine hole finishes common with gun drilling. As this pad does not extend 180º from the cutting land, measurement by micrometer will indicate a false reading of about .001" to .002" under the gun drills true diameter. The drill is truly on size! A pilot hole or drill bushing to start the gun drill must be sized to the gun drills marked diameter +.0002" to .0005", NEVER undersize. More information about Gun Drill Styles & their uses? Return to Deep Hole Gun Drills |
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