Machinery's Handbook, 31st Edition
1062 TOOL WEAR AND SHARPENING 0.010 inch (0.13–0.25 mm) for finish turning and 0.030 to 0.060 inch (0.76–1.52 mm) for rough turning; and for cemented carbides 0.005 to 0.010 inch (0.13–0.25 mm) for finish turning and 0.020 to 0.040 inch (0.51–1.02 mm) for rough turning. Under ideal conditions, which, surprisingly, occur quite frequently, the width of the flank wear land will be very uniform along its entire length. When the depth of cut is uneven, such as when turning out-of-round stock, the bottom edge of the wear land may become somewhat slanted, the wear land being wider toward the nose. A jagged-appearing wear land usually is evidence of chipping at the cutting edge. Sometimes, only one or two sharp depressions of the lower edge of the wear land will appear to indicate that the cutting edge has chipped above these depressions. A deep notch will sometimes occur at the “depth of cut line,” or that part of the cutting opposite the original surface of the work. This can be caused by a hard surface scale on the work, by a work-hardened surface layer on the work, or when machining high-temperature alloys. Often the size of the wear land is enlarged at the nose of the tool. This can be a sign of crater breakthrough near the nose or of chipping in this region. Under certain conditions, when machining with carbides, it can be an indication of deformation of the cutting edge in the region of the nose. When a sharp tool is first used, the initial amount of flank wear is quite large in relation to the subsequent total amount. Under normal operating conditions, the width of the flank wear land will increase at a uniform rate until it reaches a critical size, after which the cut ting edge breaks down completely. This is called catastrophic failure, and the cutting edge should be replaced before this occurs. When cutting at slow speeds with high-speed steel tools, there may be long periods when no increase in the flank wear can be observed. For a given work material and tool material, the rate of flank wear is primarily dependent on the cutting speed and then on the feed rate. Cratering.— A deep crater will sometimes form on the face of the tool, which is easily recognizable. The crater forms at a short distance behind the side cutting edge, leaving a small shelf between the cutting edge and the edge of the crater. This shelf is sometimes covered with the built-up edge and at other times it is uncovered. Often the bottom of the crater is obscured with work material that is welded to the tool in this region. Under nor- mal operating conditions, the crater will gradually enlarge until it breaks through a part of the cutting edge. Usually this occurs on the end cutting edge just behind the nose. When this takes place, the flank wear at the nose increases rapidly and complete tool failure follows shortly. Sometimes cratering cannot be avoided, and a slow increase in the size of the crater is considered normal. However, if the rate of crater growth is rapid, leading to a short tool life, corrective measures must be taken. Cutting Edge Chipping.— Small chips are sometimes broken from the cutting edge, which accelerates tool wear but does not necessarily cause immediate tool failure. Chip ping can be recognized by the appearance of the cutting edge and the flank wear land. A sharp depression in the lower edge of the wear land is a sign of chipping, and, if this edge of the wear land has a jagged appearance, it indicates that a large amount of chipping has taken place. Often the vacancy or cleft in the cutting edge that results from chipping is filled up with work material that is tightly welded in place. This occurs very rapidly when chipping is caused by a built-up edge on the face of the tool. In this manner, the damage to the cutting edge is healed; however, the width of the wear land below the chip is usually increased and the tool life is shortened. Deformation.— Deformation occurs on carbide cutting tools when taking a very heavy cut using a slow cutting speed and a high feed rate. A large section of the cutting edge then becomes very hot, and the heavy cutting pressure compresses the nose of the cutting edge, thereby lowering the face of the tool in the area of the nose. This reduces the relief under the nose, increases the width of the wear land in this region, and shortens tool life.
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