Machinery's Handbook, 31st Edition
1154
Boring Boring
Boring is an internal turning operation performed with a boring bar to enlarge a previ- ously drilled hole to form an internal shape of required dimensions. Boring operations range from semi-roughing to finishing. An indexable boring bar has three basic elements: indexable cutting insert, shank, and anchor. The designation system for indexable inserts is the same as for turning. The an- chor is the clamping portion of the shank that is held in the tool block. The minimum clamping length is approximately three to four diameters of the shank. The distance the boring bar extends beyond the tool block is the overhang; it determines the cutting depth. The overhang is the unsupported portion of the boring bar. Long overhang causes exces- sive deflection of the shank and generates vibration (chatter), which worsens the surface finish of the bore. Eliminating chatter, especially in long-bore workpieces, is one of the greatest chal- lenges faced by manufacturers and users of boring bars. Deflection of a boring bar de - pends on the mechanical properties of the shank material, the length of the overhang, and the cutting conditions. The following equation is used for calculating deflection ( y ) of a boring bar. (1) where F = the cutting force, lbf or N L = unsupported length of a boring bar (overhang), in. or mm E = the modulus of elasticity (in tension) of a boring bar material, psi or N/mm 2 I = the moment of inertia of a boring bar cross-sectional area, in 4 or mm 4 . Cutting Force.— Cutting force ( F ) is calculated by Formula (2) in customary US units. (2) where d = depth of cut, in. f = a feed rate, ipm K p = a power constant, hp/in 3 /min. The power constant K p data are given on pages 1145–1147 (Table 1a and 1b). The feed rate factors, C , for power constants are given on page 1148 (Table 2). 396,000 = a conversion factor that represents a unit of power equal to 550 in-lb/min Example 1(a): A workpiece is AISI 4140 chromium-molybdenum steel, 220 HB; depth of cut d = 0.080 in.; feed rate f = 0.008 ipr. Calculate the cutting force F in customary US units of measure. Power constant K p = 0.72 hp/ in 3 /min (from Table 1b, page 1147) Feed factor C = 1.08 (from Table 2, page 1148) Adjusted power constant, K pa = K p × C = 0.72 × 1.08 ≅ 0.78.
Calculating: F = 396,000 dfK pa = 396,000 × 0.08 × 0.008 × 0.78 = 197.7 lbf Cutting force ( F ) is calculated by Formula (3) in metric units.
(3)
where d = depth of cut, mm f = feed rate, mm/min.
K p = is a power constant, kW/cm p data are given on pages 1145–1147 (Tables 1a and 1b). The feed rate factors, C , for power constants are given on page 1148 (Table 2). 60,000 = is a conversion factor that represents a unit of power equal to 1 kW × mm/min. 3 /min. The power constant K
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online