A boring bar is a cantilevered cutting tool used on a lathe to perform internal turning operations. Where external turning tools face outward and cut the outside surface of a rotating workpiece, a boring bar reaches inside the bore of the workpiece and cuts the internal surface. This makes boring bars essential for finishing drilled holes to precise diameters, enlarging existing bores, cutting internal grooves and undercuts, and threading the inside of threaded bores.
Buyohlic boring bars cover the range of sizes and styles used in American machine shops for both production and jobbing lathe work. Carbide insert boring bars accept standard insert cartridges for fast cutting speeds and consistent geometry across multiple insert changes. HSS ground boring bars allow resharpening for economical use in low-volume operations. Specialty boring bars for internal threading, internal grooving, and very small bore diameters round out the collection for complete internal turning capability.
Boring Bar Design Principles
The fundamental challenge in boring bar design is stiffness. Because the bar extends into the bore as a cantilever supported only at the mounting end, any lack of rigidity shows up as vibration, chatter, and dimensional error in the finished bore. The three design factors that maximize boring bar stiffness are: maximizing the bar diameter relative to bore size, minimizing the projection length (overhang) beyond the bar holder, and using the stiffest available bar material.
Standard steel boring bars work well for projections up to 3 or 4 times the bar diameter. For deeper bores requiring longer projections, tungsten carbide boring bars (solid carbide, not just carbide-tipped) provide three times the stiffness of steel at the same diameter, significantly reducing chatter in deep bore operations. For the deepest and most demanding bores, heavy metal (tungsten alloy) boring bars with damping mechanisms can achieve stable cutting at projections up to 10 times the bar diameter.
Boring Bar Sizes and Selection
- Micro boring bars (under 6mm shank): For small bores down to 5mm diameter in watch parts, medical devices, and miniature components
- Small boring bars (6mm to 12mm shank): For bore diameters from 8mm to 25mm, covering most small part internal operations
- Medium boring bars (16mm to 20mm shank): For bore diameters from 20mm to 50mm, the most common range for general machining work
- Large boring bars (25mm and above shank): For larger bores in heavy parts, flanges, and large diameter components
For boring heads for milling machines, see our boring heads collection. For a complete range of internal and external lathe tooling, see turning tool holders and lathe tools and accessories.
Frequently Asked Questions - Boring Bars
A boring bar is used on a lathe to perform internal turning operations: enlarging and truing up drilled holes, cutting internal grooves and recesses, cutting internal threads, and finishing bore diameters to precise dimensions for bearing and bushing fits. The bar mounts in the tool post and extends into the rotating workpiece bore, cutting the inside surface with a single-point tool tip.
The boring bar should be just long enough to reach from the tool holder to the bottom of the bore with a small clearance, plus the minimum amount needed for the holder to grip the bar securely. Every millimeter of unnecessary overhang reduces stiffness and increases chatter risk. As a rule, keep the bar projection below 4 times the bar diameter for standard steel bars. For deeper bores, switch to a larger diameter bar or a carbide bar to maintain adequate stiffness.
A boring bar is the shank component that holds the cutting tool and provides the reach into the bore. A boring tool refers to the complete cutting unit, which includes the bar, the insert or tool bit, and any holding hardware. In common usage, "boring bar" and "boring tool" are often used interchangeably to mean the complete assembly used for internal turning operations.
Boring bar chatter is caused by the bar vibrating at its natural frequency during cutting. The main causes are: too much overhang for the bar diameter, insufficient clamping force in the tool holder, spindle speed at a resonant frequency, or excessive depth of cut for the bar rigidity. Minimize overhang, ensure the holder is tight, change spindle speed by 15 to 20 percent in either direction, and reduce depth of cut to eliminate chatter.
