b'Eliminating chatter, especially when boring deep holes, is one of the greatest machining challenges.By Edmund Isakov deep holes, is one of the greatest challenges faced by manufacturers and users of boring B oring is an internal turning operation per- bars. Deflection of a boring bar depends on formed with a boring bar to enlarge a pre- the mechanical properties of the shank mate-viously drilled hole to form an internal shaperial, the length of the overhang and the cutting of specified dimensions. Boring operationsconditions.range from semiroughing to finishing. The following equation is used to calculate A boring bar has three basic elements: andeflection (y) of a boring bar:indexable cutting insert, a shank and an an- y = FL 3 /3EI (1)chor. The designation system for indexable in- Whereserts is the same as for turning. The anchor isF is the cutting force, lbf or N.the clamping portion of the shank that is heldL is the unsupported length of a boring bar in the tool block, and the minimum clamping(overhang), in. or mm.length is approximately three to four diame- E is the modulus of elasticity (in tension) of a ters of the shank. The distance the boring barboring bar material, psi or N/mm 2 .extends beyond the tool block, which is calledI is the moment of inertia of a boring bar overhang, determines the cutting depth. Thecross section area, in. 4or mm 4 .overhang is the unsupported portion of the boring bar. Long overhang causes excessiveCutting Forcedeflection of the shank and generates vibra- Cutting force (F) expressed in customary tion, or chatter, which deteriorates the surfaceU.S. units of measure, calculated by formula (2):finish of the bore. F = 396,000dfK p C (2)Eliminating chatter, especially when boringWhere68JULY 2019 Boring.indd 68 6/17/19 10:14 AM'