Machinery's Handbook, 31st Edition
SPEEDS AND FEEDS FOR MILLING 1111 The maximum feed f max is found in Table 15c by multiplying the optimum feed from the speed table by the maximum feed factor that corresponds to the ar / D ratio, which in this instance is 0.02 ∕ 1 = 0.02; the minimum feed f min is found by multiplying the optimum feed by the minimum feed factor. Thus, f max = 4.5 3 0.015 = 0.0675 in/tooth and f min = 3.1 3 0.015 = 0.0465 in/tooth. If a feed between these maximum and minimum values is selected, 0.050 in/tooth for example, then for ar / D = 0.02 and V avg / V opt = 3.0, the feed factors at maximum and minimum feeds are F f 1 = 7.90 and F f 2 = 7.01, respectively, and by interpolation, F f = 7.90 + (0.050 − 0.0465)/(0.0675 − 0.0465) 3 (7.01 − 7.90) = 7.75. The depth of cut factor F d is obtained from Table 15b, using f max from Table 15c instead of the optimum feed f opt for calculating the feed ratio (chosen feed/ optimum feed). In this example, the feed ratio = chosen feed/ f max = 0.050 ∕ 0.0675 = 0.74, so the feed factor is F d = 0.93 for a depth of cut = 1.0 inch and 0 ° lead angle. Therefore, the final cutting speed is 80 3 7.75 3 0.93 = 577 ft/min. Notice that f max obtained from Table 15c was used instead of the optimum feed from the speed table, in determining the feed ratio needed to find F d . Slit Milling.— The tabular data for slit milling is based on an 8-tooth, 10-degree helix angle cutter with a width of 0.4 inch, a diameter D of 4.0 inch, and a depth of cut of 0.6 inch. The given speeds and feeds are valid for any diameters and tool widths, as long as sufficient machine power is available. Adjustments to cutting speeds for other feeds and depths of cut are made using Table 15c or Table 15d, depending on the orientation of the cutter to the work, as illustrated in Case 1 and Case 2 of Fig. 5. The situation illustrated in Case 1 is approximately equivalent to that illustrated in Fig. 3, and Case 2 is approxi mately equivalent to that shown in Fig. 4. Case 1: If the cutter is fed directly into the workpiece, i.e., the feed is perpendicular to the surface of the workpiece, as in cutting off, then Table 15d (face milling) is used to adjust speeds for other feeds. The depth of cut portion of Table 15d is not used in this case ( F d = 1.0), so the adjusted cutting speed V = V opt 3 F f 3 F ar . In determining the factor F ar from Table 15d, the radial depth of cut ar is the length of cut created by the portion of the cutter engaged in the work. Case 2: If the cutter feed is parallel to the surface of the workpiece, as in slotting or side milling, then Table 15c (side milling) is used to adjust the given speeds for other feeds. In Table 15c, the cutting depth (slot depth, for example) is the radial depth of cut ar that is used to determine maximum and minimum allowable feed/tooth and the feed factor F f . These minimum and maximum feeds are determined in the manner described previously, however, the axial depth of cut factor F d is not required. The adjusted cutting speed, valid for cutters of any thickness (width), is given by V = V opt 3 F f .
Slit Mill
f
Case 1
ar
Chip Thickness
Work
ar
Case 2
f
feed/rev, f
Fig. 5. Determination of Radial Depth of Cut or in Slit Milling
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