GRINDING FEEDS AND SPEEDS Machinery's Handbook, 31st Edition
1279
100
ECT = 17 ECT = 33 ECT = 50 ECT = 75 ECT U = 17 ECT U = 33 ECT U = 50 ECT U = 75
Standard V 10 = 2910 for T = 10 minutes
10
T U = 7
1
V U = 1800
10000
1000
V m/min
Fig. 12. Calibration of User Grinding Data to Standard Taylor Lines User Input: V U = 1800 m/min, T U = 7 minutes, ECT = 0.00017 mm
Optimization.— As shown, a global optimum occurs along the G -curve, in selected cases for values of ECT around 0.00075, i.e. at high metal removal rates as in other machining operations. It is recommended to use the simple formula for economic life: T E = 3 3 T V min utes. T V = T RPL + 60 3 C E ÷ H R , minutes, where T RPL is the time required to replace wheel, C E = cost per wheel dressing = wheel cost + cost per dressing, and H R is the hourly rate. In grinding, values of T V range between 2 and 5 minutes in conventional grinders, which means that the economic wheel lives range between 6 and 15 minutes indicating higher metal removal rates than are commonly used. When wheels are sharpened automatically after each stroke as in internal grinding, or when grits are continually replaced as in abra sive grinding (machining), T V may be less than one minute. This translates into wheel lives around one minute in order to achieve minimum cost grinding. Grinding Cost, Optimization and Process Planning: More accurate results are obtained when the firm collects and systemizes the information on wheel lives, wheel and work speeds, and depths of cut from production runs. A computer program can be used to plan the grinding process and apply the rules and formulas presented in this chapter. A complete grinding process planning program, such as that developed by Colding International Corporation, can be used to optimize machine settings for various feed- speed preferences corresponding wheel-life requirements, minimum cost or maximum production rate grinding, required surface finish and sparkout time; machine and fixture requirements based on the grinding forces, torque and power for sharp and worn grinding wheels; and, detailed time and cost analysis per part and per batch including wheel dressing and wheel changing schedules. Table 12 summarizes the time and cost savings per batch as it relates to tool life. The sensitivity of how grinding parameters are selected is obvious. Minimum cost conditions yield a 51 percent reduction of time and 44 percent reduction of cost, while maximum production rate reduces total time by 65 percent but, at the expense of heavy wheel consumption (continuous dressing), cost by only 18 percent. Table 12. Wheel Life versus Cost
Cost per Batch, $ Reduction from Long Life,% Tooling Total Cost Time Cost
Time per Batch, minutes
Preferences
Long Life Economic Life Minimum Cost
2995 2433 1465 1041
39 252 199
2412 2211 1344 1980
— 19 51 65
—
8
44 18
Max Production Rate
1244
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