Machinery's Handbook, 31st Edition
1262 GRINDING FEEDS AND SPEEDS from the formula grinding ratio = SMRR 3 T ÷ W* , where SMRR (specific metal removal rate) is determined from MRR = SMRR 3 T or from ECT = SMRR ÷ V ÷ 1000. Thus, grinding ratio = 1000 3 ECT 3 V 3 T ÷ W* , and T = grinding ratio 3 W * ÷ (1000 3 ECT 3 V ), provided that the wheel wear criterion W* is valid for all data combinations. Example 1: If W* in one test is found to be 500 mm 3 for ECT = 0.00033 mm and V = 3600 m/min, and grinding ratio = 10, then wheel-life will vary with measured grinding ratios, wheel speed, and ECT as follows: T =500 3 grinding ratio ÷ ( V 3 ECT ) = 4.2 minutes. In the remainder of this section the grinding ratio will not used, and wheel-life is expressed in terms of ECT or SMRR and wheel speed V . ECT in Grinding.— In turning and milling, ECT is defined as the volume of chips re - moved per unit cutting edge length per revolution of the work or cutter. In milling specif ically, ECT is defined as the ratio of (number of teeth z 3 feed per tooth f z 3 radial depth of cut a r 3 and axial depth of cut a a ) and (cutting edge length CEL divided by π D ), where D is the cutter diameter, thus, ECT CEL Dzf a a z r a π = In grinding, the same definition of ECT applies if we replace the number of teeth with the average number of grits along the wheel periphery, and replace the feed per tooth by the average feed per grit. This definition is not very practical, however, and ECT is better defined by the ratio of the specific metal removal rate SMRR , and the wheel speed V . Thus, ECT = 1000 3 SMRR ÷ V . Keeping ECT constant when varying SMRR requires that the wheel speed must be changed proportionally. In milling and turning ECT can also be redefined in terms of SMRR divided by the work and the cutter speeds, respectively, because SMRR is proportional to the feed rate F R . Work Speed and Depth of Cut Selection: Work speed V w is determined by dividing SMRR by the depth of cut a r , or by using the graph in Fig. 1.
1000
SMRR = 100 SMRR = 200 SMRR = 500 SMRR = 750 SMRR = 1000 SMRR = 1500 SMRR = 2000
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ar, mm
Fig. 1. Work Speed V w versus Depth of Cut a r Referring to Fig. 1 , for depths of cuts of 0.01 and 0.0025 mm, a specific metal removal rate SMRR = 1000 mm 3 /mm width/min is achieved at work speeds of 100 and 400 m/min, respectively, and for SMRR =100 mm 3 /mm width/min at work speeds of 10 and 40 m/min, respectively. Unfortunately, the common use of low values of work speed (20 to 40 m/min) in finishing cause thermal surface damage, disastrous in critical parts such as aircraft components. As the grains slide across the work they generate surface heat and fatigue-type loading may cause residual tensile stresses and severe surface cracks. Proper finish grinding
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