Machinery's Handbook, 31st Edition
1068 TOOL WEAR AND SHARPENING Lapping Carbide Tools.— Carbide tools may be finished by lapping, especially if an ex- ceptionally fine finish is required on the work as, for example, with tools used for precision boring or turning nonferrous metals. If the finishing is done by using a diamond wheel of very fine grit (such as 240, 320, or 400), the operation is often called “lapping.” A second lapping method is by means of a power-driven lapping disk charged with diamond dust, Norbide powder, or silicon carbide finishing compound. A third method is by using a hand lap or hone usually of 320 or 400 grit. In many plants, the finishes obtained with carbide tools meet requirements without a special lapping operation. In all cases, any feather edge which may be left on tools should be removed, and it is good practice to bevel the edges of roughing tools at 45 degrees to leave a chamfer 0.005 to 0.010 inch wide (0.127 − 0.254 mm). This is done by hand honing, and the object is to prevent crumbling or flaking off at the edges when hard scale or heavy chip pressure is encountered. Hand Honing: The cutting edge of carbide tools, and tools made from other tool materi als, is sometimes hand honed before it is used in order to strengthen the cutting edge. When interrupted cuts or heavy roughing cuts are to be taken, or when the grade of carbide is slightly too hard, hand honing is beneficial because it will prevent chipping or even possibly, breakage of the cutting edge. Whenever chipping is encountered, hand honing the cutting edge before use will be helpful. It is important, however, to hone the edge lightly and only when necessary. Heavy honing will always cause a reduction in tool life. Normally, removing 0.002 to 0.004 inch (0.051 − 0.102 mm) from the cutting edge is sufficient. When indexable inserts are used, the use of pre-honed inserts is preferred to hand honing, although sometimes an additional amount of honing is required. Hand honing of carbide tools in between cuts is sometimes done to defer grinding or to increase the life of a cutting edge on an indexable insert. If correctly done, so as not to change the relief angle, this procedure is sometimes helpful. If improperly done, it can result in a reduction in tool life. Chipbreaker Grinding.— For this operation a straight diamond wheel is used on a uni versal tool and cutter grinder, a small surface grinder, or a special chipbreaker grinder. A resinoid-bonded wheel of grade J or N commonly is used, and the tool is held rigidly in an adjustable holder or vise. The width of the diamond wheel usually varies from 1 ⁄ 8 to 1 ⁄ 4 inch (3.2 − 6.4 mm). A vitrified bond may be used for wheels as thick as 1 ⁄ 4 inch (6.35 mm), and a resinoid bond for relatively narrow wheels. Summary of Miscellaneous Points.— In grinding a single-point carbide tool, traverse it across the wheel face continuously to avoid localized heating. This traverse movement should be quite rapid in using silicon carbide wheels and comparatively slow with dia mond wheels. A hand traversing and feeding movement, whenever practicable, is gener ally recommended because of greater sensitivity. In grinding, maintain a constant, mod- erate pressure. Manipulating the tool so as to keep the contact area with the wheel as small as possible will reduce heating and increase the rate of stock removal. Never cool a hot tool by dipping it in a liquid, as this may crack the tip. Wheel rotation should preferably be against the cutting edge or from the front face toward the back. If the grinder is driven by a reversing motor, opposite sides of a cup wheel can be used for grinding right-and left-hand tools and with rotation against the cutting edge. If it is necessary to grind the top face of a single-point tool, this should precede the grinding of the side and front relief, and top-face grinding should be minimized to maintain the tip thickness. In machine grinding with a diamond wheel, limit the feed per traverse to 0.001 inch (0.025 mm) for 100 to 120 grit; 0.0005 inch (0.013 mm) for 150 to 240 grit; and 0.0002 inch (0.005 mm) for 320 grit and finer. Meshes, Sieves, and Screens Sieving or screening is a method of categorizing powder particle size by running the powder through a specific size screen or screens. Powder can be separated into two or more size fractions by stacking the screens (Fig. 1), thereby determining the powder
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