GRINDING FEEDS AND SPEEDS Machinery's Handbook, 31st Edition
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GRINDING FEEDS AND SPEEDS Grinding data are scarcely available in handbooks, which usually recommend a small range of depths and work speeds at constant wheel speed, including small variations in wheel and work material composition. Wheel life or grinding stiffness are seldom considered. Grinding parameter recommendations typically range as follows: • Wheel speeds are usually recommended in the 1200 to 1800 m/min (4000 to 6000 fpm) range, or in rare cases up to 3600 m/min (12000 fpm) • Work speeds are in the range 20 to 40 m/min (70 to 140 fpm); and, depths of cut of 0.01 to 0.025 mm (0.0004 to 0.001 inch) for roughing, and around 0.005 mm (.0002 in.) for finish grinding. • Grit sizes for roughing are around 46 to 60 for easy-to-grind materials, and for diffi cult-to-grind materials higher such as 80 grit. In finishing, a smaller grit size (higher grit number) is recommended. Internal grinding grit sizes for small holes are approxi mately 100 to 320. • Specific metal removal rate, SMRR , represents the rate of material removal per unit of wheel contact width and are commonly recommended from 200 to 500 mm 3 / mm width/min (0.3 to 0.75 in 3 /inch width/min). • Grinding stiffness is a major variable in determining wheel-life and spark-out time. A typical value of system stiffness in outside-diameter grinding, for 10:1 length/ diameter ratio, is approximately K ST = 30–50 N/ μ m. System stiffness K ST is calculated from the stiffness of the part, K w and the machine and fixtures, K m . Machine values can be obtained from manufacturers, or can be measured using simple equipment along with the part stiffness. • Generally a lower wheel hardness (soft wheel) is recommended when the system stiffness is poor or when a better finish is desired. Basic Rules The wheel speed V and equivalent chip thickness ECT = SMRR ÷ V ÷ 1000 are the pri mary parameters that determine wheel-life, forces and surface finish in grinding. The fol lowing general rules and recommendations, using ECT , are based on extensive laboratory and industry tests both in Europe and the United States. The relationships and shapes of curves pertaining to grinding tool life, grinding time, and cost are similar to those of any metal cutting operation such as turning, milling and drilling. In turning and milling, the ECT theory says that if the product of feed times depth of cut is constant, the tool life is constant no matter how the depth of cut or feed is varied, pro vided that the cutting speed and cutting edge length are maintained constant. In grinding, wheel-life T remains constant for constant cutting speed V , regardless of how depth of cut a r or work speed V w are selected as long as the specific metal removal rate SMRR = V w 3 a r is held constant (neglecting the influence of grinding contact width). ECT is much smaller in grinding than in milling, ranging from about 0.0001 to 0.001 mm (0.000004 to 0.00004 inch). See the section MACHINING ECONOMETRICS starting on page 1196 for a detailed explanation of the role of ECT in conventional machining. Wheel Life and Grinding Ratio.— A commonly used measure of relative wheel-life in grinding is the grinding ratio , which is used to compare grindability when varying grind- ing wheel composition and work material properties under otherwise constant cutting conditions. The grinding ratio is defined as the slope of the wear curve versus metal removal rate: grinding ratio = MRR ÷ W* , where MRR is the metal removal rate, and W* is the volume wheel wear at which the wheel has to be dressed. The grinding ratio is not a measure of wheel-life, but a relationship between grinding ratio and wheel-life T can be obtained
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