Machinery's Handbook, 31st Edition
TOOL WEAR AND SHARPENING 1067 50 Concentration: In the finer grit sizes of 220, 240, 320, 400, and 500, a 50 concentra tion is recommended for offhand grinding with resinoid-bonded cup-wheels. 25 Concentration: A low concentration of 25 is recommended for offhand grinding with resinoid-bonded cup-wheels with grit sizes of 100, 120 and 150. Depth of Diamond Section: The radial depth of the diamond section usually varies from 1 ⁄ 16 to 1 ⁄ 4 inch (1.6 to 6.4 mm). The depth varies somewhat according to the wheel size and type of bond. Dry Versus Wet Grinding of Carbide Tools.— In using silicon carbide wheels, grind- ing should be done either absolutely dry or with enough coolant to flood the wheel and tool. Satisfactory results may be obtained either by the wet or dry method. However, dry grinding is the most prevalent usually because, in wet grinding, operators tend to use an inadequate supply of coolant to obtain better visibility of the grinding operation and avoid getting wet; hence checking or cracking in many cases is more likely to occur in wet grind ing than in dry grinding. Wet Grinding with Silicon Carbide Wheels: One advantage commonly cited in connec tion with wet grinding is that an ample supply of coolant permits using wheels about one grade harder than in dry grinding, thus increasing the wheel life. Plenty of coolant also prevents thermal stresses and the resulting cracks, and there is less tendency for the wheel to load. A dust exhaust system also is unnecessary. Wet Grinding with Diamond Wheels: In grinding with diamond wheels, the general prac tice is to use a coolant to keep the wheel face clean and promote free cutting. The amount of coolant may vary from a small stream to a coating applied to the wheel face by a felt pad. Coolants for Carbide Tool Grinding.— In grinding either with silicon carbide or dia mond wheels, a coolant that is used extensively consists of water plus a small amount ei- ther of soluble oil, sal soda, or soda ash to prevent corrosion. One prominent manufacturer recommends for silicon carbide wheels about 1 ounce of soda ash per gallon of water, and for diamond wheels kerosene. The use of kerosene is quite general for diamond wheels and is usually applied to the wheel face by a felt pad. Another coolant recommended for diamond wheels consists of 80 percent water and 20 percent soluble oil. Peripheral Versus Flat Side Grinding.— In grinding single-point carbide tools with sili con carbide wheels, the roughing preparatory to finishing with diamond wheels may be done either by using the flat face of a cup-shaped wheel (side grinding) or the periphery of a “straight” or disk-shaped wheel. Even where side grinding is preferred, the periphery of a straight wheel may be used for heavy roughing as in grinding back-chipped or broken tools (see left-hand diagram). Reasons for preferring peripheral grinding include faster cutting with less danger of localized heating and checking especially in grinding broad surfaces. The advantages usually claimed for side grinding are that proper rake or relief angles are easier to obtain and the relief or land is ground flat. The diamond wheels used for tool sharpening are designed for side grinding. (See right-hand diagram.) Tungsten Carbide Tip 15° Grind Carbide Only
18 ” or 20 ” Diam.
Cut Steel Back to 15° Angle
Steel Cleared 15°
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