twist drills Oversize Diameters in Drilling Machinery's Handbook, 31st Edition
962
Amount Oversize, Inch
Amount Oversize, Inch
Drill Dia., Inch
Drill Dia., Inch
Average Max.
Mean Average Min.
Average Max.
Mean Average Min.
0.002 0.0045 0.0065
0.0015 0.003 0.004
0.001 0.001 0.0025
0.008 0.008 0.009
0.005 0.005 0.007
0.003 0.003 0.004
1 ⁄ 16
1 ⁄ 2 3 ⁄ 4
1 ⁄ 8 1 ⁄ 4
1
Courtesy of The United States Cutting Tool Institute Some conditions will cause the drilled hole to be undersize. For example, holes drilled in light metals and in other materials having a high coefficient of thermal expansion such as plastics may contract to a size that is smaller than the diameter of the drill as the material surrounding the hole is cooled after having been heated by the drilling. The elastic action of the material surrounding the hole may also cause the drilled hole to be undersize when drilling high-strength materials with a drill that is dull at its outer corner. The accuracy of the drill point has a great effect on the accuracy of the drilled hole. An inaccurately ground twist drill will produce holes that are excessively oversize. The drill point must be symmetrical; i.e., the point angles must be equal, as well as the lip lengths and the axial height of the lips. Any alterations to the lips or to the chisel edge, such as thin ning the web, must be done carefully to preserve the symmetry of the drill point. Adequate relief should be provided behind the chisel edge to prevent heel drag. On conventionally ground drill points, this relief can be estimated by the chisel edge angle. When drilling a hole, as the drill point starts to enter the workpiece, the drill will be unstable and will tend to wander. Then, as the body of the drill enters the hole, the drill will tend to stabilize. The result of this action is a tendency to drill a bellmouth shape in the hole at the entrance and perhaps beyond. Factors contributing to bellmouthing are: an unsymmetrically ground drill point; a large chisel edge length; inadequate relief behind the chisel edge; runout of the spindle and the chuck; using a slender drill that will bend easily; and lack of rigidity of the machine tool, workpiece, or the setup. Correcting these conditions as required will reduce the tendency for bellmouthing to occur and improve the accuracy of the hole diameter and its straightness. Starting the hole with a short stiff drill, such as a center drill, will quickly stabilize the drill that follows and reduce or eliminate bellmouthing; this procedure should always be used when drilling in a lathe, where the work is rotating. Bellmouthing can also be eliminated almost entirely and the accuracy of the hole improved by using a close-fitting drill jig bushing placed close to the workpiece. Although specific recommendations cannot be made, many cutting fluids will help to increase the accuracy of the diameters of drilled holes. Double-margin twist drills, available in the smaller sizes, will drill a more accurate hole than conventional twist drills having only a single margin at the leading edge of the land. The second land, located on the trailing edge of each land, provides greater stability in the drill bushing and in the hole. These drills are especially useful in drilling intersecting off-center holes. Single and double-margin step drills, also available in the smaller sizes, will produce very accurate drilled holes, which are usually less than 0.002 inch (0.051 mm) larger than the drill size. Counterboring.— Counterboring (called spot-facing if the depth is shallow) is the en- largement of a previously formed hole. Counterbores for screw holes are generally made in sets. Each set contains three counterbores: one with the body of the size of the screw head and the pilot the size of the hole to admit the body of the screw; one with the body the size of the head of the screw and the pilot the size of the tap drill; and the third with the body the size of the body of the screw and the pilot the size of the tap drill. Counterbores are usually provided with helical flutes to provide positive effective rake on the cutting edges. The four flutes are so positioned that the end teeth cut ahead of center to provide a shearing action and eliminate chatter in the cut. Three designs are most common: solid, two-piece, and three-piece. Solid designs have the body, cutter, and pilot all in one piece. Two-piece designs have an integral shank and counterbore cutter, with an interchange- able pilot, and provide true concentricity of the cutter diameter with the shank while
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