Machinery's Handbook, 31st Edition
SPEEDS AND MACHINING POWER 1145 The foregoing speeds are for high-speed steel tools. Tools tipped with tungsten carbide are adapted for cutting various non-metallic products which cannot be machined readily with steel tools, such as slate, marble, synthetic plastic materials, etc. In drilling slate and marble, use flat drills; and for plastic materials, tungsten carbide-tipped twist drills. Cutting speeds ranging from 75 to 150 feet per minute (23–46 m/min) have been used for drilling slate (without coolant) and a feed of 0.025 inch per revolution (0.64 mm/rev) for drills 3 ∕ 4 and 1 inch (19.05 and 25.4 mm) in diameter. Estimating Machining Power Knowledge of the power required to perform machining operations is useful when plan ning new machining operations, for optimizing existing machining operations, and to develop specifications for new machine tools that are to be acquired. The available power on any machine tool places a limit on the size of the cut that it can take. When much metal must be removed from the workpiece it is advisable to estimate the cutting conditions that will utilize the maximum power on the machine. Many machining operations require only light cuts to be taken for which the machine obviously has ample power; in this event, estimating the power required is a wasteful effort. Conditions in different shops may vary and machine tools are not all designed alike, so some variations between the estimated results and those obtained on the job are to be expected. However, by using the methods provided in this section a reasonable estimate of the power required can be made, which will suffice in most practical situations. The measure of power in customary inch units is the horsepower; in SI metric units it is the kilowatt, which is used for both mechanical and electrical power. The power required to cut a material depends upon the rate at which the material is being cut and upon an experimentally determined power constant, K p , which is also called the unit horsepower, unit power, or specific power consumption. The power constant is equal to the horsepower required to cut a material at a rate of one cubic inch per minute; in SI metric units the power constant is equal to the power in kilowatts required to cut a material at a rate of one cubic centimeter per second, or 1000 cubic millimeters per second (1 cm 3 = 1000 mm 3 ). Different values of the power constant are required for inch and for metric units, which are related as follows: to obtain the SI metric power constant, multiply the inch power constant by 2.73; to obtain the inch power constant, divide the SI metric power constant by 2.73. Values of the power constant in Table 1a, and Table 1b can be used for all machining operations except drilling and grinding. Values given are for sharp tools. Table 1a. Power Constants K p Using Sharp Cutting Tools Material Brinell Hardness Number K p Inch Units K p Metric Units Material Brinell Hardness Number K p Inch Units K p Metric Units Ferrous Cast Metals
100-120 0.28 0.76 Malleable Iron 120-1400.350.96 Ferritic
150-175 0.42 1.15
140-160 0.38 1.04 160-180 0.52 1.42 200-220 0.71 1.94 220-240 0.91 2.48 150-175 0.30 0.82 175-200 0.63 1.72 200-250 0.92 2.51
Gray Cast Iron
175-200 0.57 1.56 200-250 0.82 2.24 250-300 1.18 3.22 150-175 0.62 1.69 175-200 0.78 2.13 200-250 0.86 2.35 … … …
180-2000.601.64 Pearlitic
Cast Steel
Alloy Cast Iron
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