Machinery's Handbook, 31st Edition
TOOL WEAR AND SHARPENING 1065 Both sharpening operations—the relieved face grinding and the point thinning—should be carried out in special drill-grinding machines or with twist drill-grinding fixtures mounted on general-purpose tool-grinding machines designed to assure the essential accuracy of the required tool geometry. Off-hand grinding may be used for the important web thinning when a special machine is not available; however, such operations require skill and experience. Improperly sharpened twist drills, e.g. those with unequal edge length or asymmetrical point angle, will tend to produce holes with poor diameter and directional control. For deep holes and for drilling into stainless steel, titanium alloys, high temperature alloys, nickel alloys, very high-strength materials and in some cases tool steels, split- point grinding, resulting in a “crankshaft” type drill point, is recommended. In this type of pointing, see Fig. 4, the chisel edge is entirely eliminated, extending the positive rake cutting edges to the center of the drill, thereby greatly reducing the required thrust in drilling. Points on modified-point drills must be restored after sharpening to maintain their increased drilling efficiency. Sharpening Carbide Tools.— Cemented carbide indexable inserts are usually not re- sharpened, but sometimes they require a special grind in order to form a contour on the cutting edge to suit a special purpose. Brazed-type carbide cutting tools are resharpened after the cutting edge has become worn. On brazed carbide tools, the cutting-edge wear should not be allowed to become excessive before the tool is re-sharpened. One method of determining when brazed carbide tools need resharpening is by periodic inspection of the flank wear and the condition of the face. Another method is to determine the amount of production normally obtained before excessive wear has taken place, or to determine the equivalent period of time. One disadvantage of this method is that slight variations in the work material will often cause the wear rate not to be uniform and the number of parts machined before regrinding will not be the same each time. Usually, sharpening should not require removal of more than 0.005 to 0.010 inch (0.13 − 0.25 mm) of carbide. General Procedure in Carbide-Tool Grinding: The general procedure depends upon the kind of grinding operation required. If the operation is to resharpen a dull tool, a diamond wheel of 100- to 120-grain size is recommended, although a finer wheel—up to 150-grain size—is sometimes used to obtain a better finish. If the tool is new or is a “standard” design and changes in shape are necessary, a 100-grit diamond wheel is recommended for roughing and a finer grit diamond wheel can be used for finishing. Some shops prefer to rough-grind the carbide with a vitrified silicon carbide wheel, the finish grinding being done with a diamond wheel. A final operation commonly designated as lapping may or may not be employed for obtaining an extra-fine finish. Wheel Speeds: The speed of silicon carbide wheels usually is about 5000 feet per minute (25.4 m/s). The speeds of diamond wheels generally range from 5000 − 6000 fpm (25.4 − 30.5 m/s); yet lower speeds (550 − 3000 fpm or 2.8 − 15.2 m/s) can be effective. Offhand Grinding: In grinding single-point tools (excepting chipbreakers), the common practice is to hold the tool by hand, press it against the wheel face and traverse it continu ously across the wheel face while the tool is supported on the machine rest or table, which is adjusted to the required angle. This is known as “offhand grinding” to distinguish it from the machine grinding of cutters as in regular cutter-grinding practice. The selection of wheels adapted to carbide-tool grinding is very important. Silicon Carbide Wheels.— The green-colored silicon carbide wheels generally are pre ferred to the dark gray or gray-black variety, although the latter are sometimes used. Grain or Grit Sizes: For roughing, a grain size of 60 is very generally used. For finish grinding with silicon carbide wheels, a finer grain size of 100 or 120 is common. A silicon carbide wheel such as C60-I-7V may be used for grinding both the steel shank and carbide tip. However, for under-cutting steel shanks up to the carbide tip, it may be advantageous to use an aluminum oxide wheel suitable for grinding softer, carbon steel.
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