Machinery's Handbook, 31st Edition
TOOL WEAR AND SHARPENING 1063 Surface Finish.— The finish on the machined surface does not necessarily indicate poor cutting tool performance unless there is a rapid deterioration. A good surface finish is, however, sometimes a requirement. The principal cause of a poor surface finish is the built-up edge that forms along the edge of the cutting tool. The elimination of the built-up edge will always result in an improvement of the surface finish. The most effective way to eliminate the built-up edge is to increase cutting speed. When cutting speed is increased beyond a certain critical limit, there will be a rather sudden and large improvement in the surface finish. Cemented carbide tools can operate successfully at higher cutting speeds, where the built-up edge does not occur and where a good surface finish is obtained. When- ever possible, cemented carbide tools should be operated at cutting speeds where a good surface finish will result. There are times when such speeds are not possible. Also, high- speed tools cannot be operated at the speed where the built-up edge does not form. In these conditions, the most effective method of obtaining a good surface finish is to employ a cutting fluid that has active sulphur or chlorine additives. Cutting tool materials that do not alloy readily with the work material are also effective in obtaining an improved surface finish. Straight titanium carbide and diamond are the two principal tool materials that fall into this category. The presence of feed marks can mar an otherwise good surface finish, and attention must be paid to the feed rate and the nose radius of the tool if a good surface finish is desired. Changes in tool geometry can also be helpful. A small “flat,” or secondary cutting edge, ground on the end cutting edge behind the nose will sometimes provide the desired surface finish. When the tool is in operation, the flank wear should not be allowed to become too large, particularly in the region of the nose where the finished surface is produced. Sharpening Twist Drills.— Twist drills are cutting tools designed to perform concur rently several functions, such as penetrating directly into solid material, ejecting the re- moved chips outside the cutting area, maintaining the essentially straight direction of the advance movement and controlling the size of the drilled hole. The geometry needed for these multiple functions is incorporated into the design of the twist drill in such a manner that it can be retained even after repeated sharpening operations. Twist drills are resharp ened many times during their service life, with the practically complete restitution of their original operational characteristics. However, in order to obtain all the benefits which the design of the twist drill is capable of providing, the surfaces generated in the sharpening process must agree with the original form of the tool’s operating surfaces, unless a change of shape is required for use on a different work material. The principal elements of tool geometry essential for the adequate cutting performance of twist drills are shown in Fig. 1. The generally used values for these dimensions are the following: Point angle: Commonly 118 ° , except for high strength steels, 118 ° to 135 ° ; aluminum alloys, 90 ° to 140 ° ; and magnesium alloys, 70 ° to 118 ° . Helix angle: Commonly 24 ° to 32 ° , except for magnesium and copper alloys, 10 ° to 30 ° . Lip relief angle: Commonly 10 ° to 15 ° , except for high strength or tough steels, 7 ° to 12 ° . The lower values of these angle ranges are used for drills of larger diameter, the higher values for the smaller diameters. For drills of diameters less than 1 ⁄ 4 inch (6.35 mm), the lip relief angles are increased beyond the listed maximum values up to 24 ° . For soft and free machining materials, 12 ° to 18 ° except for diameters less than 1 ⁄ 4 inch (6.35 mm), 20 ° to 26 ° . Relief Grinding of the Tool Flanks.— In sharpening twist drills the tool flanks contain ing the two cutting edges are ground. Each flank consists of a curved surface, which provides the relief needed for the easy penetration and free cutting of the tool edges. In grinding the flanks, Fig. 2, the drill is swung around the axis A of an imaginary cone while resting in a support that holds the drill at one-half the point angle B with respect to the face of the grinding wheel. Feed f for stock removal is in the direction of the drill axis.
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