Boring bars

An ideal solid 20 mm bar with 100 mm unsupported length, 210,000 N/mm² modulus and 100 N transverse end load deflects 0.02021 mm in a static cantilever model. At 120 mm overhang it deflects 0.03492 mm. Check actual clamping, section and dynamic stability separately.

Function and application

A boring bar machines an existing hole. Sandvik distinguishes rough and fine boring and ties selection to diameter, depth, interruptions, tolerance and finish. Sandvik Coromant, Boring holes

In lathe internal turning, relative rotation comes from the workpiece while the bar advances the edge through the bore. Other boring systems rotate a tool assembly. A rotating boring head, its adapter and a stationary lathe bar therefore have different machine-interface requirements, even when they machine a similar internal diameter. This is explanatory synthesis of the process distinction; a shared nominal diameter does not establish interchangeability.

Access is more than shank diameter

The useful dimensions include the minimum bore, reachable cutting depth, shank diameter and overall length. Scientific Cutting Tools lists those as separate columns for its solid-carbide boring-bar family, alongside uncoated and AlTiN+ options. Its B050/B060 tools additionally carry a specific side-clearance note. The catalog illustrates why a shank size alone is not a complete access specification. Scientific Cutting Tools, Boring Bars

A reduced cutting end can enter a hole smaller than the mounting shank, provided the larger section stays outside the restricted region. Conversely, an insert or head can need more room than the supporting bar. These are geometric consequences, not a universal minimum-bore formula. A catalog's maximum depth also does not describe every shoulder, taper or cross-hole obstruction along the insertion path.

Sandvik's internal-turning guidance emphasizes insert clearance in small bores to avoid rubbing against the wall. Insert shape and entering angle affect accessibility and cutting-force direction; positive geometries and smaller nose radii can reduce forces in its longitudinal-turning discussion. Different internal profiles impose different access and edge-strength tradeoffs. Sandvik Coromant, Internal turning

Overhang, stiffness and damping

Overhang is the unsupported projection from the effective support to the cutting region, rather than the tool's full catalog length. The ratio L/D compares that projection with bar diameter. A long overall tool can have a short overhang if sufficiently supported; a short tool can still have a demanding ratio when its diameter is small.

Sandvik's Silent Tools guide gives these recommended maximum overhangs. D is bar diameter:

The guide attributes the more restrictive grooving/threading recommendations to radial forces. It distinguishes carbide's higher static stiffness from the damping mechanism's dynamic benefit. These recommendations are not guaranteed chatter-free limits for unrelated bars, workpieces or holders. Sandvik Coromant, Silent Tools turning application guide

The guide emphasizes clamping contact and center height, split-sleeve support, and permitted overhang ranges. Restrictions on clamping over damping mechanisms mean longer grip is not automatically suitable for every construction. Sandvik, clamping and modification limitations

Chips and the complete cutting system

A bore restricts chip escape and coolant access. Sandvik links poor chip evacuation to insert breakage and degraded surface finish, while long overhang can produce deflection and vibration. Tool geometry, clamping and the workpiece's stability interact; no single diameter or damping label settles the result. Sandvik Coromant, Internal turning

Explanatory synthesis: a blind bottom, intersecting hole or internal shoulder changes the space available for the edge and chips. The insert grade and chip-control geometry still need a workpiece-material and cutting-condition context. A bar fitting the entrance does not establish clearance throughout the cut, and a nominally stiff bar does not eliminate compliance in the turret, holder or workpiece.

Original example: idealized static deflection

For a uniform cantilever under a transverse point load at its free end, small-deflection theory gives:

δ = F × L³ / (3 × E × I).

Here F is force, L unsupported length, E Young's modulus and I the cross-section's second moment of area. MIT's cantilever review derives the end-load relationship. It is a static model, not a machining stability calculation. MIT OpenCourseWare, Lecture 2 appendix, §I, equation 11

For the hypothetical solid circular bar, use I = πD⁴/64 and the stated diameter, length, modulus and end load.

I = π × 20⁴/64 = 7,853.98 mm⁴.

δ = 100 × 100³ / (3 × 210,000 × 7,853.98) ≈ 0.02021 mm.

With unchanged diameter, modulus and force, increasing overhang to 120 mm multiplies deflection by (120/100)³ = 1.728, giving approximately 0.03492 mm. Alternatively, increasing diameter from 20 to 25 mm at the original 100 mm length multiplies deflection by (20/25)⁴ = 0.4096, giving approximately 0.00828 mm.

The comparison explains the strong geometric effects, but not a permissible operating load. Real bars can have flats, coolant passages, joints, reduced necks or internal dampers. Clamping and machine compliance, force direction, dynamic excitation and thermal effects are excluded. The computed transverse displacement is not automatically a measured bore-diameter error or a required compensation value.

Source versions and boundaries

Reviewed 2026-11-02 PST. No separate technical issue dates were verified for the live Sandvik or Scientific Cutting Tools pages. The MIT PDF identifies itself as a Lecture 2 appendix and contains the cantilever derivation, without a separately verified issue date; its hosting course label is not treated as the PDF's publication date. The numerical model supplies no recommended RPM, feed, depth, coolant pressure or universal L/D limit.

Related: Cutting tools and tooling references, tool wear, and internal pockets.